mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-10-30 08:02:30 +00:00
fb95aae6e6
As diffstat shows we've had again a lot of works done for this cycle: majority of changes are the continued componentization and code refactoring in ASoC, the tree-wide PCM API updates and cleanups and SOF updates while a few ASoC driver updates are seen, too. Here we go, some highlights: Core: - Finally y2038 support landed to ALSA ABI; some ioctls have been extended and lots of tricks were applied - Applying the new managed PCM buffer API to all drivers; the API itself was already merged in 5.5 - The already deprecated dimension support in ALSA control API is dropped completely now - Verification of ALSA control elements to catch API misuses ASoC: - Further code refactorings and moving things to the component level - Lots of updates and improvements on SOF / Intel drivers; now including common HDMI driver and SoundWire support - New driver support for Ingenic JZ4770, Mediatek MT6660, Qualcomm WCD934x and WSA881x, and Realtek RT700, RT711, RT715, RT1011, RT1015 and RT1308 HD-audio: - Improved ring-buffer communications using waitqueue - Drop the superfluous buffer preallocation on x86 Others: - Many code cleanups, mostly constifications over the whole tree - USB-audio: quirks for MOTU, Corsair Virtuoso, Line6 Helix - FireWire: code refactoring for oxfw and dice drivers -----BEGIN PGP SIGNATURE----- iQJCBAABCAAsFiEEIXTw5fNLNI7mMiVaLtJE4w1nLE8FAl4v7qsOHHRpd2FpQHN1 c2UuZGUACgkQLtJE4w1nLE9s7hAAoeqwRF+WffBaSMZKShDyFD5L7Z/YeXxh2b81 ErnVaSbXmWMhzgx7G7dj3bchkJWYsqAH//j3/AHYCF22slJdeTof0cpqaRgC0qEv eIj9ALu2Hh+z5jJHOlbpcYevtK89frY9zu4Su/5YAfZloUNLqAbl59SNOiS99/hu SR5rF0UCGngHf+pjiBpruQv2hahD5Y0a4rIGMaZB/GAa3p7eQ42koMmIEje/rjF0 bSpiYmmAwGJ62RlTUFCBNlj9w78eUfqrf8SHr91d7zNHUZpHR9GoFswmLGM0UtGS 5gIm+6UkBmTQcqgKhYsLl2eT/PSLVHpbYUeABjP62EqxWpksOl2/fQfrGuVT1Vjm QVoc345dKoKcNxVH1iuVg+/JYsmuPIpSmviFCKbH5IUlTWWyOYzWxZVdrZ/gIyXh fY/vDL2yOdR9mTnHYAfsJ8IB2ABY4pDahC6Dbvh5JoN/vmsND2Wv2l+HIIdLTarj R/n7+Rn6HLQmCJvgIWLniG6FxYxx4gqd/VVVaL7eJKYrcqvQq6VNZ/36Fgh3Ahdi HnKxyer3K7vC8CGM3cNH/Dq/iGTNgTcX6pnhVepl/elSZTMDrxZcWbbA0nPhBe8J 5SWGEstv11OBi/JL3vGdTmV4ceZ/yIBg9pIGg4j0rH3hKT83G7O5E++PixKCmmMc fZPCeAg= =PGgQ -----END PGP SIGNATURE----- Merge tag 'sound-5.6-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound Pull sound updates from Takashi Iwai: "As the diffstat shows we've had again a lot of works done for this cycle: the majority of changes are the continued componentization and code refactoring in ASoC, the tree-wide PCM API updates and cleanups and SOF updates while a few ASoC driver updates are seen, too. Here we go, some highlights: Core: - Finally y2038 support landed to ALSA ABI; some ioctls have been extended and lots of tricks were applied - Applying the new managed PCM buffer API to all drivers; the API itself was already merged in 5.5 - The already deprecated dimension support in ALSA control API is dropped completely now - Verification of ALSA control elements to catch API misuses ASoC: - Further code refactorings and moving things to the component level - Lots of updates and improvements on SOF / Intel drivers; now including common HDMI driver and SoundWire support - New driver support for Ingenic JZ4770, Mediatek MT6660, Qualcomm WCD934x and WSA881x, and Realtek RT700, RT711, RT715, RT1011, RT1015 and RT1308 HD-audio: - Improved ring-buffer communications using waitqueue - Drop the superfluous buffer preallocation on x86 Others: - Many code cleanups, mostly constifications over the whole tree - USB-audio: quirks for MOTU, Corsair Virtuoso, Line6 Helix - FireWire: code refactoring for oxfw and dice drivers" * tag 'sound-5.6-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound: (638 commits) ALSA: usb-audio: add quirks for Line6 Helix devices fw>=2.82 ALSA: hda: Add Clevo W65_67SB the power_save blacklist ASoC: soc-core: remove null_snd_soc_ops ASoC: soc-pcm: add soc_rtd_trigger() ASoC: soc-pcm: add soc_rtd_hw_free() ASoC: soc-pcm: add soc_rtd_hw_params() ASoC: soc-pcm: add soc_rtd_prepare() ASoC: soc-pcm: add soc_rtd_shutdown() ASoC: soc-pcm: add soc_rtd_startup() ASoC: rt1015: add rt1015 amplifier driver ASoC: madera: Correct some kernel doc ASoC: topology: fix soc_tplg_fe_link_create() - link->dobj initialization order ASoC: Intel: skl_hda_dsp_common: Fix global-out-of-bounds bug ASoC: madera: Correct DMIC only input hook ups ALSA: cs46xx: fix spelling mistake "to" -> "too" ALSA: hda - Add docking station support for Lenovo Thinkpad T420s ASoC: Add MediaTek MT6660 Speaker Amp Driver ASoC: dt-bindings: rt5645: add suppliers ASoC: max98090: fix deadlock in max98090_dapm_put_enum_double() ASoC: dapm: add snd_soc_dapm_put_enum_double_locked ...
4322 lines
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ReStructuredText
4322 lines
142 KiB
ReStructuredText
======================
|
||
Writing an ALSA Driver
|
||
======================
|
||
|
||
:Author: Takashi Iwai <tiwai@suse.de>
|
||
|
||
Preface
|
||
=======
|
||
|
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This document describes how to write an `ALSA (Advanced Linux Sound
|
||
Architecture) <http://www.alsa-project.org/>`__ driver. The document
|
||
focuses mainly on PCI soundcards. In the case of other device types, the
|
||
API might be different, too. However, at least the ALSA kernel API is
|
||
consistent, and therefore it would be still a bit help for writing them.
|
||
|
||
This document targets people who already have enough C language skills
|
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and have basic linux kernel programming knowledge. This document doesn't
|
||
explain the general topic of linux kernel coding and doesn't cover
|
||
low-level driver implementation details. It only describes the standard
|
||
way to write a PCI sound driver on ALSA.
|
||
|
||
This document is still a draft version. Any feedback and corrections,
|
||
please!!
|
||
|
||
File Tree Structure
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||
===================
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||
|
||
General
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||
-------
|
||
|
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The file tree structure of ALSA driver is depicted below.
|
||
|
||
::
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||
|
||
sound
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||
/core
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||
/oss
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||
/seq
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||
/oss
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||
/include
|
||
/drivers
|
||
/mpu401
|
||
/opl3
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||
/i2c
|
||
/synth
|
||
/emux
|
||
/pci
|
||
/(cards)
|
||
/isa
|
||
/(cards)
|
||
/arm
|
||
/ppc
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||
/sparc
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||
/usb
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/pcmcia /(cards)
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/soc
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||
/oss
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||
|
||
|
||
core directory
|
||
--------------
|
||
|
||
This directory contains the middle layer which is the heart of ALSA
|
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drivers. In this directory, the native ALSA modules are stored. The
|
||
sub-directories contain different modules and are dependent upon the
|
||
kernel config.
|
||
|
||
core/oss
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||
~~~~~~~~
|
||
|
||
The codes for PCM and mixer OSS emulation modules are stored in this
|
||
directory. The rawmidi OSS emulation is included in the ALSA rawmidi
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||
code since it's quite small. The sequencer code is stored in
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``core/seq/oss`` directory (see `below <#core-seq-oss>`__).
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||
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||
core/seq
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~~~~~~~~
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||
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||
This directory and its sub-directories are for the ALSA sequencer. This
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directory contains the sequencer core and primary sequencer modules such
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like snd-seq-midi, snd-seq-virmidi, etc. They are compiled only when
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||
``CONFIG_SND_SEQUENCER`` is set in the kernel config.
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||
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||
core/seq/oss
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||
~~~~~~~~~~~~
|
||
|
||
This contains the OSS sequencer emulation codes.
|
||
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||
include directory
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||
-----------------
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||
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This is the place for the public header files of ALSA drivers, which are
|
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to be exported to user-space, or included by several files at different
|
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directories. Basically, the private header files should not be placed in
|
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this directory, but you may still find files there, due to historical
|
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reasons :)
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||
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drivers directory
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-----------------
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||
|
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This directory contains code shared among different drivers on different
|
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architectures. They are hence supposed not to be architecture-specific.
|
||
For example, the dummy pcm driver and the serial MIDI driver are found
|
||
in this directory. In the sub-directories, there is code for components
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which are independent from bus and cpu architectures.
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||
|
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drivers/mpu401
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||
~~~~~~~~~~~~~~
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||
|
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The MPU401 and MPU401-UART modules are stored here.
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||
|
||
drivers/opl3 and opl4
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||
~~~~~~~~~~~~~~~~~~~~~
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||
|
||
The OPL3 and OPL4 FM-synth stuff is found here.
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||
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||
i2c directory
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||
-------------
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||
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This contains the ALSA i2c components.
|
||
|
||
Although there is a standard i2c layer on Linux, ALSA has its own i2c
|
||
code for some cards, because the soundcard needs only a simple operation
|
||
and the standard i2c API is too complicated for such a purpose.
|
||
|
||
synth directory
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||
---------------
|
||
|
||
This contains the synth middle-level modules.
|
||
|
||
So far, there is only Emu8000/Emu10k1 synth driver under the
|
||
``synth/emux`` sub-directory.
|
||
|
||
pci directory
|
||
-------------
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||
|
||
This directory and its sub-directories hold the top-level card modules
|
||
for PCI soundcards and the code specific to the PCI BUS.
|
||
|
||
The drivers compiled from a single file are stored directly in the pci
|
||
directory, while the drivers with several source files are stored on
|
||
their own sub-directory (e.g. emu10k1, ice1712).
|
||
|
||
isa directory
|
||
-------------
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||
|
||
This directory and its sub-directories hold the top-level card modules
|
||
for ISA soundcards.
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||
|
||
arm, ppc, and sparc directories
|
||
-------------------------------
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||
|
||
They are used for top-level card modules which are specific to one of
|
||
these architectures.
|
||
|
||
usb directory
|
||
-------------
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||
|
||
This directory contains the USB-audio driver. In the latest version, the
|
||
USB MIDI driver is integrated in the usb-audio driver.
|
||
|
||
pcmcia directory
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||
----------------
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||
|
||
The PCMCIA, especially PCCard drivers will go here. CardBus drivers will
|
||
be in the pci directory, because their API is identical to that of
|
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standard PCI cards.
|
||
|
||
soc directory
|
||
-------------
|
||
|
||
This directory contains the codes for ASoC (ALSA System on Chip)
|
||
layer including ASoC core, codec and machine drivers.
|
||
|
||
oss directory
|
||
-------------
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||
|
||
Here contains OSS/Lite codes.
|
||
All codes have been deprecated except for dmasound on m68k as of
|
||
writing this.
|
||
|
||
|
||
Basic Flow for PCI Drivers
|
||
==========================
|
||
|
||
Outline
|
||
-------
|
||
|
||
The minimum flow for PCI soundcards is as follows:
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||
|
||
- define the PCI ID table (see the section `PCI Entries`_).
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||
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- create ``probe`` callback.
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||
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||
- create ``remove`` callback.
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||
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- create a :c:type:`struct pci_driver <pci_driver>` structure
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||
containing the three pointers above.
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||
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- create an ``init`` function just calling the
|
||
:c:func:`pci_register_driver()` to register the pci_driver
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||
table defined above.
|
||
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||
- create an ``exit`` function to call the
|
||
:c:func:`pci_unregister_driver()` function.
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||
|
||
Full Code Example
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||
-----------------
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||
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The code example is shown below. Some parts are kept unimplemented at
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this moment but will be filled in the next sections. The numbers in the
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comment lines of the :c:func:`snd_mychip_probe()` function refer
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to details explained in the following section.
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::
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#include <linux/init.h>
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#include <linux/pci.h>
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#include <linux/slab.h>
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#include <sound/core.h>
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#include <sound/initval.h>
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/* module parameters (see "Module Parameters") */
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/* SNDRV_CARDS: maximum number of cards supported by this module */
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static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
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static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
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static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
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/* definition of the chip-specific record */
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struct mychip {
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struct snd_card *card;
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/* the rest of the implementation will be in section
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* "PCI Resource Management"
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*/
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};
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/* chip-specific destructor
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* (see "PCI Resource Management")
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*/
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static int snd_mychip_free(struct mychip *chip)
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{
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.... /* will be implemented later... */
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}
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||
|
||
/* component-destructor
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* (see "Management of Cards and Components")
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*/
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static int snd_mychip_dev_free(struct snd_device *device)
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||
{
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||
return snd_mychip_free(device->device_data);
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}
|
||
|
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/* chip-specific constructor
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||
* (see "Management of Cards and Components")
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*/
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static int snd_mychip_create(struct snd_card *card,
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struct pci_dev *pci,
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struct mychip **rchip)
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||
{
|
||
struct mychip *chip;
|
||
int err;
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||
static const struct snd_device_ops ops = {
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.dev_free = snd_mychip_dev_free,
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||
};
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||
|
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*rchip = NULL;
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||
|
||
/* check PCI availability here
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* (see "PCI Resource Management")
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*/
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....
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||
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/* allocate a chip-specific data with zero filled */
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chip = kzalloc(sizeof(*chip), GFP_KERNEL);
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if (chip == NULL)
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return -ENOMEM;
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||
|
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chip->card = card;
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||
|
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/* rest of initialization here; will be implemented
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* later, see "PCI Resource Management"
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*/
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....
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err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
|
||
if (err < 0) {
|
||
snd_mychip_free(chip);
|
||
return err;
|
||
}
|
||
|
||
*rchip = chip;
|
||
return 0;
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||
}
|
||
|
||
/* constructor -- see "Driver Constructor" sub-section */
|
||
static int snd_mychip_probe(struct pci_dev *pci,
|
||
const struct pci_device_id *pci_id)
|
||
{
|
||
static int dev;
|
||
struct snd_card *card;
|
||
struct mychip *chip;
|
||
int err;
|
||
|
||
/* (1) */
|
||
if (dev >= SNDRV_CARDS)
|
||
return -ENODEV;
|
||
if (!enable[dev]) {
|
||
dev++;
|
||
return -ENOENT;
|
||
}
|
||
|
||
/* (2) */
|
||
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
|
||
0, &card);
|
||
if (err < 0)
|
||
return err;
|
||
|
||
/* (3) */
|
||
err = snd_mychip_create(card, pci, &chip);
|
||
if (err < 0)
|
||
goto error;
|
||
|
||
/* (4) */
|
||
strcpy(card->driver, "My Chip");
|
||
strcpy(card->shortname, "My Own Chip 123");
|
||
sprintf(card->longname, "%s at 0x%lx irq %i",
|
||
card->shortname, chip->port, chip->irq);
|
||
|
||
/* (5) */
|
||
.... /* implemented later */
|
||
|
||
/* (6) */
|
||
err = snd_card_register(card);
|
||
if (err < 0)
|
||
goto error;
|
||
|
||
/* (7) */
|
||
pci_set_drvdata(pci, card);
|
||
dev++;
|
||
return 0;
|
||
|
||
error:
|
||
snd_card_free(card);
|
||
return err;
|
||
}
|
||
|
||
/* destructor -- see the "Destructor" sub-section */
|
||
static void snd_mychip_remove(struct pci_dev *pci)
|
||
{
|
||
snd_card_free(pci_get_drvdata(pci));
|
||
}
|
||
|
||
|
||
|
||
Driver Constructor
|
||
------------------
|
||
|
||
The real constructor of PCI drivers is the ``probe`` callback. The
|
||
``probe`` callback and other component-constructors which are called
|
||
from the ``probe`` callback cannot be used with the ``__init`` prefix
|
||
because any PCI device could be a hotplug device.
|
||
|
||
In the ``probe`` callback, the following scheme is often used.
|
||
|
||
1) Check and increment the device index.
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int dev;
|
||
....
|
||
if (dev >= SNDRV_CARDS)
|
||
return -ENODEV;
|
||
if (!enable[dev]) {
|
||
dev++;
|
||
return -ENOENT;
|
||
}
|
||
|
||
|
||
where ``enable[dev]`` is the module option.
|
||
|
||
Each time the ``probe`` callback is called, check the availability of
|
||
the device. If not available, simply increment the device index and
|
||
returns. dev will be incremented also later (`step 7
|
||
<#set-the-pci-driver-data-and-return-zero>`__).
|
||
|
||
2) Create a card instance
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
struct snd_card *card;
|
||
int err;
|
||
....
|
||
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
|
||
0, &card);
|
||
|
||
|
||
The details will be explained in the section `Management of Cards and
|
||
Components`_.
|
||
|
||
3) Create a main component
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
In this part, the PCI resources are allocated.
|
||
|
||
::
|
||
|
||
struct mychip *chip;
|
||
....
|
||
err = snd_mychip_create(card, pci, &chip);
|
||
if (err < 0)
|
||
goto error;
|
||
|
||
The details will be explained in the section `PCI Resource
|
||
Management`_.
|
||
|
||
When something goes wrong, the probe function needs to deal with the
|
||
error. In this example, we have a single error handling path placed
|
||
at the end of the function.
|
||
|
||
::
|
||
|
||
error:
|
||
snd_card_free(card);
|
||
return err;
|
||
|
||
Since each component can be properly freed, the single
|
||
:c:func:`snd_card_free()` call should suffice in most cases.
|
||
|
||
|
||
4) Set the driver ID and name strings.
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
strcpy(card->driver, "My Chip");
|
||
strcpy(card->shortname, "My Own Chip 123");
|
||
sprintf(card->longname, "%s at 0x%lx irq %i",
|
||
card->shortname, chip->port, chip->irq);
|
||
|
||
The driver field holds the minimal ID string of the chip. This is used
|
||
by alsa-lib's configurator, so keep it simple but unique. Even the
|
||
same driver can have different driver IDs to distinguish the
|
||
functionality of each chip type.
|
||
|
||
The shortname field is a string shown as more verbose name. The longname
|
||
field contains the information shown in ``/proc/asound/cards``.
|
||
|
||
5) Create other components, such as mixer, MIDI, etc.
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
Here you define the basic components such as `PCM <#PCM-Interface>`__,
|
||
mixer (e.g. `AC97 <#API-for-AC97-Codec>`__), MIDI (e.g.
|
||
`MPU-401 <#MIDI-MPU401-UART-Interface>`__), and other interfaces.
|
||
Also, if you want a `proc file <#Proc-Interface>`__, define it here,
|
||
too.
|
||
|
||
6) Register the card instance.
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
err = snd_card_register(card);
|
||
if (err < 0)
|
||
goto error;
|
||
|
||
Will be explained in the section `Management of Cards and
|
||
Components`_, too.
|
||
|
||
7) Set the PCI driver data and return zero.
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
pci_set_drvdata(pci, card);
|
||
dev++;
|
||
return 0;
|
||
|
||
In the above, the card record is stored. This pointer is used in the
|
||
remove callback and power-management callbacks, too.
|
||
|
||
Destructor
|
||
----------
|
||
|
||
The destructor, remove callback, simply releases the card instance. Then
|
||
the ALSA middle layer will release all the attached components
|
||
automatically.
|
||
|
||
It would be typically just :c:func:`calling snd_card_free()`:
|
||
|
||
::
|
||
|
||
static void snd_mychip_remove(struct pci_dev *pci)
|
||
{
|
||
snd_card_free(pci_get_drvdata(pci));
|
||
}
|
||
|
||
|
||
The above code assumes that the card pointer is set to the PCI driver
|
||
data.
|
||
|
||
Header Files
|
||
------------
|
||
|
||
For the above example, at least the following include files are
|
||
necessary.
|
||
|
||
::
|
||
|
||
#include <linux/init.h>
|
||
#include <linux/pci.h>
|
||
#include <linux/slab.h>
|
||
#include <sound/core.h>
|
||
#include <sound/initval.h>
|
||
|
||
where the last one is necessary only when module options are defined
|
||
in the source file. If the code is split into several files, the files
|
||
without module options don't need them.
|
||
|
||
In addition to these headers, you'll need ``<linux/interrupt.h>`` for
|
||
interrupt handling, and ``<linux/io.h>`` for I/O access. If you use the
|
||
:c:func:`mdelay()` or :c:func:`udelay()` functions, you'll need
|
||
to include ``<linux/delay.h>`` too.
|
||
|
||
The ALSA interfaces like the PCM and control APIs are defined in other
|
||
``<sound/xxx.h>`` header files. They have to be included after
|
||
``<sound/core.h>``.
|
||
|
||
Management of Cards and Components
|
||
==================================
|
||
|
||
Card Instance
|
||
-------------
|
||
|
||
For each soundcard, a “card” record must be allocated.
|
||
|
||
A card record is the headquarters of the soundcard. It manages the whole
|
||
list of devices (components) on the soundcard, such as PCM, mixers,
|
||
MIDI, synthesizer, and so on. Also, the card record holds the ID and the
|
||
name strings of the card, manages the root of proc files, and controls
|
||
the power-management states and hotplug disconnections. The component
|
||
list on the card record is used to manage the correct release of
|
||
resources at destruction.
|
||
|
||
As mentioned above, to create a card instance, call
|
||
:c:func:`snd_card_new()`.
|
||
|
||
::
|
||
|
||
struct snd_card *card;
|
||
int err;
|
||
err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);
|
||
|
||
|
||
The function takes six arguments: the parent device pointer, the
|
||
card-index number, the id string, the module pointer (usually
|
||
``THIS_MODULE``), the size of extra-data space, and the pointer to
|
||
return the card instance. The extra_size argument is used to allocate
|
||
card->private_data for the chip-specific data. Note that these data are
|
||
allocated by :c:func:`snd_card_new()`.
|
||
|
||
The first argument, the pointer of struct :c:type:`struct device
|
||
<device>`, specifies the parent device. For PCI devices, typically
|
||
``&pci->`` is passed there.
|
||
|
||
Components
|
||
----------
|
||
|
||
After the card is created, you can attach the components (devices) to
|
||
the card instance. In an ALSA driver, a component is represented as a
|
||
:c:type:`struct snd_device <snd_device>` object. A component
|
||
can be a PCM instance, a control interface, a raw MIDI interface, etc.
|
||
Each such instance has one component entry.
|
||
|
||
A component can be created via :c:func:`snd_device_new()`
|
||
function.
|
||
|
||
::
|
||
|
||
snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
|
||
|
||
This takes the card pointer, the device-level (``SNDRV_DEV_XXX``), the
|
||
data pointer, and the callback pointers (``&ops``). The device-level
|
||
defines the type of components and the order of registration and
|
||
de-registration. For most components, the device-level is already
|
||
defined. For a user-defined component, you can use
|
||
``SNDRV_DEV_LOWLEVEL``.
|
||
|
||
This function itself doesn't allocate the data space. The data must be
|
||
allocated manually beforehand, and its pointer is passed as the
|
||
argument. This pointer (``chip`` in the above example) is used as the
|
||
identifier for the instance.
|
||
|
||
Each pre-defined ALSA component such as ac97 and pcm calls
|
||
:c:func:`snd_device_new()` inside its constructor. The destructor
|
||
for each component is defined in the callback pointers. Hence, you don't
|
||
need to take care of calling a destructor for such a component.
|
||
|
||
If you wish to create your own component, you need to set the destructor
|
||
function to the dev_free callback in the ``ops``, so that it can be
|
||
released automatically via :c:func:`snd_card_free()`. The next
|
||
example will show an implementation of chip-specific data.
|
||
|
||
Chip-Specific Data
|
||
------------------
|
||
|
||
Chip-specific information, e.g. the I/O port address, its resource
|
||
pointer, or the irq number, is stored in the chip-specific record.
|
||
|
||
::
|
||
|
||
struct mychip {
|
||
....
|
||
};
|
||
|
||
|
||
In general, there are two ways of allocating the chip record.
|
||
|
||
1. Allocating via :c:func:`snd_card_new()`.
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
As mentioned above, you can pass the extra-data-length to the 5th
|
||
argument of :c:func:`snd_card_new()`, i.e.
|
||
|
||
::
|
||
|
||
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
|
||
sizeof(struct mychip), &card);
|
||
|
||
:c:type:`struct mychip <mychip>` is the type of the chip record.
|
||
|
||
In return, the allocated record can be accessed as
|
||
|
||
::
|
||
|
||
struct mychip *chip = card->private_data;
|
||
|
||
With this method, you don't have to allocate twice. The record is
|
||
released together with the card instance.
|
||
|
||
2. Allocating an extra device.
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
After allocating a card instance via :c:func:`snd_card_new()`
|
||
(with ``0`` on the 4th arg), call :c:func:`kzalloc()`.
|
||
|
||
::
|
||
|
||
struct snd_card *card;
|
||
struct mychip *chip;
|
||
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
|
||
0, &card);
|
||
.....
|
||
chip = kzalloc(sizeof(*chip), GFP_KERNEL);
|
||
|
||
The chip record should have the field to hold the card pointer at least,
|
||
|
||
::
|
||
|
||
struct mychip {
|
||
struct snd_card *card;
|
||
....
|
||
};
|
||
|
||
|
||
Then, set the card pointer in the returned chip instance.
|
||
|
||
::
|
||
|
||
chip->card = card;
|
||
|
||
Next, initialize the fields, and register this chip record as a
|
||
low-level device with a specified ``ops``,
|
||
|
||
::
|
||
|
||
static const struct snd_device_ops ops = {
|
||
.dev_free = snd_mychip_dev_free,
|
||
};
|
||
....
|
||
snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
|
||
|
||
:c:func:`snd_mychip_dev_free()` is the device-destructor
|
||
function, which will call the real destructor.
|
||
|
||
::
|
||
|
||
static int snd_mychip_dev_free(struct snd_device *device)
|
||
{
|
||
return snd_mychip_free(device->device_data);
|
||
}
|
||
|
||
where :c:func:`snd_mychip_free()` is the real destructor.
|
||
|
||
The demerit of this method is the obviously more amount of codes.
|
||
The merit is, however, you can trigger the own callback at registering
|
||
and disconnecting the card via setting in snd_device_ops.
|
||
About the registering and disconnecting the card, see the subsections
|
||
below.
|
||
|
||
|
||
Registration and Release
|
||
------------------------
|
||
|
||
After all components are assigned, register the card instance by calling
|
||
:c:func:`snd_card_register()`. Access to the device files is
|
||
enabled at this point. That is, before
|
||
:c:func:`snd_card_register()` is called, the components are safely
|
||
inaccessible from external side. If this call fails, exit the probe
|
||
function after releasing the card via :c:func:`snd_card_free()`.
|
||
|
||
For releasing the card instance, you can call simply
|
||
:c:func:`snd_card_free()`. As mentioned earlier, all components
|
||
are released automatically by this call.
|
||
|
||
For a device which allows hotplugging, you can use
|
||
:c:func:`snd_card_free_when_closed()`. This one will postpone
|
||
the destruction until all devices are closed.
|
||
|
||
PCI Resource Management
|
||
=======================
|
||
|
||
Full Code Example
|
||
-----------------
|
||
|
||
In this section, we'll complete the chip-specific constructor,
|
||
destructor and PCI entries. Example code is shown first, below.
|
||
|
||
::
|
||
|
||
struct mychip {
|
||
struct snd_card *card;
|
||
struct pci_dev *pci;
|
||
|
||
unsigned long port;
|
||
int irq;
|
||
};
|
||
|
||
static int snd_mychip_free(struct mychip *chip)
|
||
{
|
||
/* disable hardware here if any */
|
||
.... /* (not implemented in this document) */
|
||
|
||
/* release the irq */
|
||
if (chip->irq >= 0)
|
||
free_irq(chip->irq, chip);
|
||
/* release the I/O ports & memory */
|
||
pci_release_regions(chip->pci);
|
||
/* disable the PCI entry */
|
||
pci_disable_device(chip->pci);
|
||
/* release the data */
|
||
kfree(chip);
|
||
return 0;
|
||
}
|
||
|
||
/* chip-specific constructor */
|
||
static int snd_mychip_create(struct snd_card *card,
|
||
struct pci_dev *pci,
|
||
struct mychip **rchip)
|
||
{
|
||
struct mychip *chip;
|
||
int err;
|
||
static const struct snd_device_ops ops = {
|
||
.dev_free = snd_mychip_dev_free,
|
||
};
|
||
|
||
*rchip = NULL;
|
||
|
||
/* initialize the PCI entry */
|
||
err = pci_enable_device(pci);
|
||
if (err < 0)
|
||
return err;
|
||
/* check PCI availability (28bit DMA) */
|
||
if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
|
||
pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
|
||
printk(KERN_ERR "error to set 28bit mask DMA\n");
|
||
pci_disable_device(pci);
|
||
return -ENXIO;
|
||
}
|
||
|
||
chip = kzalloc(sizeof(*chip), GFP_KERNEL);
|
||
if (chip == NULL) {
|
||
pci_disable_device(pci);
|
||
return -ENOMEM;
|
||
}
|
||
|
||
/* initialize the stuff */
|
||
chip->card = card;
|
||
chip->pci = pci;
|
||
chip->irq = -1;
|
||
|
||
/* (1) PCI resource allocation */
|
||
err = pci_request_regions(pci, "My Chip");
|
||
if (err < 0) {
|
||
kfree(chip);
|
||
pci_disable_device(pci);
|
||
return err;
|
||
}
|
||
chip->port = pci_resource_start(pci, 0);
|
||
if (request_irq(pci->irq, snd_mychip_interrupt,
|
||
IRQF_SHARED, KBUILD_MODNAME, chip)) {
|
||
printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
|
||
snd_mychip_free(chip);
|
||
return -EBUSY;
|
||
}
|
||
chip->irq = pci->irq;
|
||
card->sync_irq = chip->irq;
|
||
|
||
/* (2) initialization of the chip hardware */
|
||
.... /* (not implemented in this document) */
|
||
|
||
err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
|
||
if (err < 0) {
|
||
snd_mychip_free(chip);
|
||
return err;
|
||
}
|
||
|
||
*rchip = chip;
|
||
return 0;
|
||
}
|
||
|
||
/* PCI IDs */
|
||
static struct pci_device_id snd_mychip_ids[] = {
|
||
{ PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
|
||
PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
|
||
....
|
||
{ 0, }
|
||
};
|
||
MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
|
||
|
||
/* pci_driver definition */
|
||
static struct pci_driver driver = {
|
||
.name = KBUILD_MODNAME,
|
||
.id_table = snd_mychip_ids,
|
||
.probe = snd_mychip_probe,
|
||
.remove = snd_mychip_remove,
|
||
};
|
||
|
||
/* module initialization */
|
||
static int __init alsa_card_mychip_init(void)
|
||
{
|
||
return pci_register_driver(&driver);
|
||
}
|
||
|
||
/* module clean up */
|
||
static void __exit alsa_card_mychip_exit(void)
|
||
{
|
||
pci_unregister_driver(&driver);
|
||
}
|
||
|
||
module_init(alsa_card_mychip_init)
|
||
module_exit(alsa_card_mychip_exit)
|
||
|
||
EXPORT_NO_SYMBOLS; /* for old kernels only */
|
||
|
||
Some Hafta's
|
||
------------
|
||
|
||
The allocation of PCI resources is done in the ``probe`` function, and
|
||
usually an extra :c:func:`xxx_create()` function is written for this
|
||
purpose.
|
||
|
||
In the case of PCI devices, you first have to call the
|
||
:c:func:`pci_enable_device()` function before allocating
|
||
resources. Also, you need to set the proper PCI DMA mask to limit the
|
||
accessed I/O range. In some cases, you might need to call
|
||
:c:func:`pci_set_master()` function, too.
|
||
|
||
Suppose the 28bit mask, and the code to be added would be like:
|
||
|
||
::
|
||
|
||
err = pci_enable_device(pci);
|
||
if (err < 0)
|
||
return err;
|
||
if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
|
||
pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
|
||
printk(KERN_ERR "error to set 28bit mask DMA\n");
|
||
pci_disable_device(pci);
|
||
return -ENXIO;
|
||
}
|
||
|
||
|
||
Resource Allocation
|
||
-------------------
|
||
|
||
The allocation of I/O ports and irqs is done via standard kernel
|
||
functions. These resources must be released in the destructor
|
||
function (see below).
|
||
|
||
Now assume that the PCI device has an I/O port with 8 bytes and an
|
||
interrupt. Then :c:type:`struct mychip <mychip>` will have the
|
||
following fields:
|
||
|
||
::
|
||
|
||
struct mychip {
|
||
struct snd_card *card;
|
||
|
||
unsigned long port;
|
||
int irq;
|
||
};
|
||
|
||
|
||
For an I/O port (and also a memory region), you need to have the
|
||
resource pointer for the standard resource management. For an irq, you
|
||
have to keep only the irq number (integer). But you need to initialize
|
||
this number as -1 before actual allocation, since irq 0 is valid. The
|
||
port address and its resource pointer can be initialized as null by
|
||
:c:func:`kzalloc()` automatically, so you don't have to take care of
|
||
resetting them.
|
||
|
||
The allocation of an I/O port is done like this:
|
||
|
||
::
|
||
|
||
err = pci_request_regions(pci, "My Chip");
|
||
if (err < 0) {
|
||
kfree(chip);
|
||
pci_disable_device(pci);
|
||
return err;
|
||
}
|
||
chip->port = pci_resource_start(pci, 0);
|
||
|
||
It will reserve the I/O port region of 8 bytes of the given PCI device.
|
||
The returned value, ``chip->res_port``, is allocated via
|
||
:c:func:`kmalloc()` by :c:func:`request_region()`. The pointer
|
||
must be released via :c:func:`kfree()`, but there is a problem with
|
||
this. This issue will be explained later.
|
||
|
||
The allocation of an interrupt source is done like this:
|
||
|
||
::
|
||
|
||
if (request_irq(pci->irq, snd_mychip_interrupt,
|
||
IRQF_SHARED, KBUILD_MODNAME, chip)) {
|
||
printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
|
||
snd_mychip_free(chip);
|
||
return -EBUSY;
|
||
}
|
||
chip->irq = pci->irq;
|
||
|
||
where :c:func:`snd_mychip_interrupt()` is the interrupt handler
|
||
defined `later <#pcm-interface-interrupt-handler>`__. Note that
|
||
``chip->irq`` should be defined only when :c:func:`request_irq()`
|
||
succeeded.
|
||
|
||
On the PCI bus, interrupts can be shared. Thus, ``IRQF_SHARED`` is used
|
||
as the interrupt flag of :c:func:`request_irq()`.
|
||
|
||
The last argument of :c:func:`request_irq()` is the data pointer
|
||
passed to the interrupt handler. Usually, the chip-specific record is
|
||
used for that, but you can use what you like, too.
|
||
|
||
I won't give details about the interrupt handler at this point, but at
|
||
least its appearance can be explained now. The interrupt handler looks
|
||
usually like the following:
|
||
|
||
::
|
||
|
||
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
|
||
{
|
||
struct mychip *chip = dev_id;
|
||
....
|
||
return IRQ_HANDLED;
|
||
}
|
||
|
||
After requesting the IRQ, you can passed it to ``card->sync_irq``
|
||
field:
|
||
::
|
||
|
||
card->irq = chip->irq;
|
||
|
||
This allows PCM core automatically performing
|
||
:c:func:`synchronize_irq()` at the necessary timing like ``hw_free``.
|
||
See the later section `sync_stop callback`_ for details.
|
||
|
||
Now let's write the corresponding destructor for the resources above.
|
||
The role of destructor is simple: disable the hardware (if already
|
||
activated) and release the resources. So far, we have no hardware part,
|
||
so the disabling code is not written here.
|
||
|
||
To release the resources, the “check-and-release” method is a safer way.
|
||
For the interrupt, do like this:
|
||
|
||
::
|
||
|
||
if (chip->irq >= 0)
|
||
free_irq(chip->irq, chip);
|
||
|
||
Since the irq number can start from 0, you should initialize
|
||
``chip->irq`` with a negative value (e.g. -1), so that you can check
|
||
the validity of the irq number as above.
|
||
|
||
When you requested I/O ports or memory regions via
|
||
:c:func:`pci_request_region()` or
|
||
:c:func:`pci_request_regions()` like in this example, release the
|
||
resource(s) using the corresponding function,
|
||
:c:func:`pci_release_region()` or
|
||
:c:func:`pci_release_regions()`.
|
||
|
||
::
|
||
|
||
pci_release_regions(chip->pci);
|
||
|
||
When you requested manually via :c:func:`request_region()` or
|
||
:c:func:`request_mem_region()`, you can release it via
|
||
:c:func:`release_resource()`. Suppose that you keep the resource
|
||
pointer returned from :c:func:`request_region()` in
|
||
chip->res_port, the release procedure looks like:
|
||
|
||
::
|
||
|
||
release_and_free_resource(chip->res_port);
|
||
|
||
Don't forget to call :c:func:`pci_disable_device()` before the
|
||
end.
|
||
|
||
And finally, release the chip-specific record.
|
||
|
||
::
|
||
|
||
kfree(chip);
|
||
|
||
We didn't implement the hardware disabling part in the above. If you
|
||
need to do this, please note that the destructor may be called even
|
||
before the initialization of the chip is completed. It would be better
|
||
to have a flag to skip hardware disabling if the hardware was not
|
||
initialized yet.
|
||
|
||
When the chip-data is assigned to the card using
|
||
:c:func:`snd_device_new()` with ``SNDRV_DEV_LOWLELVEL`` , its
|
||
destructor is called at the last. That is, it is assured that all other
|
||
components like PCMs and controls have already been released. You don't
|
||
have to stop PCMs, etc. explicitly, but just call low-level hardware
|
||
stopping.
|
||
|
||
The management of a memory-mapped region is almost as same as the
|
||
management of an I/O port. You'll need three fields like the
|
||
following:
|
||
|
||
::
|
||
|
||
struct mychip {
|
||
....
|
||
unsigned long iobase_phys;
|
||
void __iomem *iobase_virt;
|
||
};
|
||
|
||
and the allocation would be like below:
|
||
|
||
::
|
||
|
||
err = pci_request_regions(pci, "My Chip");
|
||
if (err < 0) {
|
||
kfree(chip);
|
||
return err;
|
||
}
|
||
chip->iobase_phys = pci_resource_start(pci, 0);
|
||
chip->iobase_virt = ioremap(chip->iobase_phys,
|
||
pci_resource_len(pci, 0));
|
||
|
||
and the corresponding destructor would be:
|
||
|
||
::
|
||
|
||
static int snd_mychip_free(struct mychip *chip)
|
||
{
|
||
....
|
||
if (chip->iobase_virt)
|
||
iounmap(chip->iobase_virt);
|
||
....
|
||
pci_release_regions(chip->pci);
|
||
....
|
||
}
|
||
|
||
Of course, a modern way with :c:func:`pci_iomap()` will make things a
|
||
bit easier, too.
|
||
|
||
::
|
||
|
||
err = pci_request_regions(pci, "My Chip");
|
||
if (err < 0) {
|
||
kfree(chip);
|
||
return err;
|
||
}
|
||
chip->iobase_virt = pci_iomap(pci, 0, 0);
|
||
|
||
which is paired with :c:func:`pci_iounmap()` at destructor.
|
||
|
||
|
||
PCI Entries
|
||
-----------
|
||
|
||
So far, so good. Let's finish the missing PCI stuff. At first, we need a
|
||
:c:type:`struct pci_device_id <pci_device_id>` table for
|
||
this chipset. It's a table of PCI vendor/device ID number, and some
|
||
masks.
|
||
|
||
For example,
|
||
|
||
::
|
||
|
||
static struct pci_device_id snd_mychip_ids[] = {
|
||
{ PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
|
||
PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
|
||
....
|
||
{ 0, }
|
||
};
|
||
MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
|
||
|
||
The first and second fields of the :c:type:`struct pci_device_id
|
||
<pci_device_id>` structure are the vendor and device IDs. If you
|
||
have no reason to filter the matching devices, you can leave the
|
||
remaining fields as above. The last field of the :c:type:`struct
|
||
pci_device_id <pci_device_id>` struct contains private data
|
||
for this entry. You can specify any value here, for example, to define
|
||
specific operations for supported device IDs. Such an example is found
|
||
in the intel8x0 driver.
|
||
|
||
The last entry of this list is the terminator. You must specify this
|
||
all-zero entry.
|
||
|
||
Then, prepare the :c:type:`struct pci_driver <pci_driver>`
|
||
record:
|
||
|
||
::
|
||
|
||
static struct pci_driver driver = {
|
||
.name = KBUILD_MODNAME,
|
||
.id_table = snd_mychip_ids,
|
||
.probe = snd_mychip_probe,
|
||
.remove = snd_mychip_remove,
|
||
};
|
||
|
||
The ``probe`` and ``remove`` functions have already been defined in
|
||
the previous sections. The ``name`` field is the name string of this
|
||
device. Note that you must not use a slash “/” in this string.
|
||
|
||
And at last, the module entries:
|
||
|
||
::
|
||
|
||
static int __init alsa_card_mychip_init(void)
|
||
{
|
||
return pci_register_driver(&driver);
|
||
}
|
||
|
||
static void __exit alsa_card_mychip_exit(void)
|
||
{
|
||
pci_unregister_driver(&driver);
|
||
}
|
||
|
||
module_init(alsa_card_mychip_init)
|
||
module_exit(alsa_card_mychip_exit)
|
||
|
||
Note that these module entries are tagged with ``__init`` and ``__exit``
|
||
prefixes.
|
||
|
||
That's all!
|
||
|
||
PCM Interface
|
||
=============
|
||
|
||
General
|
||
-------
|
||
|
||
The PCM middle layer of ALSA is quite powerful and it is only necessary
|
||
for each driver to implement the low-level functions to access its
|
||
hardware.
|
||
|
||
For accessing to the PCM layer, you need to include ``<sound/pcm.h>``
|
||
first. In addition, ``<sound/pcm_params.h>`` might be needed if you
|
||
access to some functions related with hw_param.
|
||
|
||
Each card device can have up to four pcm instances. A pcm instance
|
||
corresponds to a pcm device file. The limitation of number of instances
|
||
comes only from the available bit size of the Linux's device numbers.
|
||
Once when 64bit device number is used, we'll have more pcm instances
|
||
available.
|
||
|
||
A pcm instance consists of pcm playback and capture streams, and each
|
||
pcm stream consists of one or more pcm substreams. Some soundcards
|
||
support multiple playback functions. For example, emu10k1 has a PCM
|
||
playback of 32 stereo substreams. In this case, at each open, a free
|
||
substream is (usually) automatically chosen and opened. Meanwhile, when
|
||
only one substream exists and it was already opened, the successful open
|
||
will either block or error with ``EAGAIN`` according to the file open
|
||
mode. But you don't have to care about such details in your driver. The
|
||
PCM middle layer will take care of such work.
|
||
|
||
Full Code Example
|
||
-----------------
|
||
|
||
The example code below does not include any hardware access routines but
|
||
shows only the skeleton, how to build up the PCM interfaces.
|
||
|
||
::
|
||
|
||
#include <sound/pcm.h>
|
||
....
|
||
|
||
/* hardware definition */
|
||
static struct snd_pcm_hardware snd_mychip_playback_hw = {
|
||
.info = (SNDRV_PCM_INFO_MMAP |
|
||
SNDRV_PCM_INFO_INTERLEAVED |
|
||
SNDRV_PCM_INFO_BLOCK_TRANSFER |
|
||
SNDRV_PCM_INFO_MMAP_VALID),
|
||
.formats = SNDRV_PCM_FMTBIT_S16_LE,
|
||
.rates = SNDRV_PCM_RATE_8000_48000,
|
||
.rate_min = 8000,
|
||
.rate_max = 48000,
|
||
.channels_min = 2,
|
||
.channels_max = 2,
|
||
.buffer_bytes_max = 32768,
|
||
.period_bytes_min = 4096,
|
||
.period_bytes_max = 32768,
|
||
.periods_min = 1,
|
||
.periods_max = 1024,
|
||
};
|
||
|
||
/* hardware definition */
|
||
static struct snd_pcm_hardware snd_mychip_capture_hw = {
|
||
.info = (SNDRV_PCM_INFO_MMAP |
|
||
SNDRV_PCM_INFO_INTERLEAVED |
|
||
SNDRV_PCM_INFO_BLOCK_TRANSFER |
|
||
SNDRV_PCM_INFO_MMAP_VALID),
|
||
.formats = SNDRV_PCM_FMTBIT_S16_LE,
|
||
.rates = SNDRV_PCM_RATE_8000_48000,
|
||
.rate_min = 8000,
|
||
.rate_max = 48000,
|
||
.channels_min = 2,
|
||
.channels_max = 2,
|
||
.buffer_bytes_max = 32768,
|
||
.period_bytes_min = 4096,
|
||
.period_bytes_max = 32768,
|
||
.periods_min = 1,
|
||
.periods_max = 1024,
|
||
};
|
||
|
||
/* open callback */
|
||
static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
|
||
{
|
||
struct mychip *chip = snd_pcm_substream_chip(substream);
|
||
struct snd_pcm_runtime *runtime = substream->runtime;
|
||
|
||
runtime->hw = snd_mychip_playback_hw;
|
||
/* more hardware-initialization will be done here */
|
||
....
|
||
return 0;
|
||
}
|
||
|
||
/* close callback */
|
||
static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
|
||
{
|
||
struct mychip *chip = snd_pcm_substream_chip(substream);
|
||
/* the hardware-specific codes will be here */
|
||
....
|
||
return 0;
|
||
|
||
}
|
||
|
||
/* open callback */
|
||
static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
|
||
{
|
||
struct mychip *chip = snd_pcm_substream_chip(substream);
|
||
struct snd_pcm_runtime *runtime = substream->runtime;
|
||
|
||
runtime->hw = snd_mychip_capture_hw;
|
||
/* more hardware-initialization will be done here */
|
||
....
|
||
return 0;
|
||
}
|
||
|
||
/* close callback */
|
||
static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
|
||
{
|
||
struct mychip *chip = snd_pcm_substream_chip(substream);
|
||
/* the hardware-specific codes will be here */
|
||
....
|
||
return 0;
|
||
}
|
||
|
||
/* hw_params callback */
|
||
static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
|
||
struct snd_pcm_hw_params *hw_params)
|
||
{
|
||
/* the hardware-specific codes will be here */
|
||
....
|
||
return 0;
|
||
}
|
||
|
||
/* hw_free callback */
|
||
static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
|
||
{
|
||
/* the hardware-specific codes will be here */
|
||
....
|
||
return 0;
|
||
}
|
||
|
||
/* prepare callback */
|
||
static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
|
||
{
|
||
struct mychip *chip = snd_pcm_substream_chip(substream);
|
||
struct snd_pcm_runtime *runtime = substream->runtime;
|
||
|
||
/* set up the hardware with the current configuration
|
||
* for example...
|
||
*/
|
||
mychip_set_sample_format(chip, runtime->format);
|
||
mychip_set_sample_rate(chip, runtime->rate);
|
||
mychip_set_channels(chip, runtime->channels);
|
||
mychip_set_dma_setup(chip, runtime->dma_addr,
|
||
chip->buffer_size,
|
||
chip->period_size);
|
||
return 0;
|
||
}
|
||
|
||
/* trigger callback */
|
||
static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
|
||
int cmd)
|
||
{
|
||
switch (cmd) {
|
||
case SNDRV_PCM_TRIGGER_START:
|
||
/* do something to start the PCM engine */
|
||
....
|
||
break;
|
||
case SNDRV_PCM_TRIGGER_STOP:
|
||
/* do something to stop the PCM engine */
|
||
....
|
||
break;
|
||
default:
|
||
return -EINVAL;
|
||
}
|
||
}
|
||
|
||
/* pointer callback */
|
||
static snd_pcm_uframes_t
|
||
snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
|
||
{
|
||
struct mychip *chip = snd_pcm_substream_chip(substream);
|
||
unsigned int current_ptr;
|
||
|
||
/* get the current hardware pointer */
|
||
current_ptr = mychip_get_hw_pointer(chip);
|
||
return current_ptr;
|
||
}
|
||
|
||
/* operators */
|
||
static struct snd_pcm_ops snd_mychip_playback_ops = {
|
||
.open = snd_mychip_playback_open,
|
||
.close = snd_mychip_playback_close,
|
||
.hw_params = snd_mychip_pcm_hw_params,
|
||
.hw_free = snd_mychip_pcm_hw_free,
|
||
.prepare = snd_mychip_pcm_prepare,
|
||
.trigger = snd_mychip_pcm_trigger,
|
||
.pointer = snd_mychip_pcm_pointer,
|
||
};
|
||
|
||
/* operators */
|
||
static struct snd_pcm_ops snd_mychip_capture_ops = {
|
||
.open = snd_mychip_capture_open,
|
||
.close = snd_mychip_capture_close,
|
||
.hw_params = snd_mychip_pcm_hw_params,
|
||
.hw_free = snd_mychip_pcm_hw_free,
|
||
.prepare = snd_mychip_pcm_prepare,
|
||
.trigger = snd_mychip_pcm_trigger,
|
||
.pointer = snd_mychip_pcm_pointer,
|
||
};
|
||
|
||
/*
|
||
* definitions of capture are omitted here...
|
||
*/
|
||
|
||
/* create a pcm device */
|
||
static int snd_mychip_new_pcm(struct mychip *chip)
|
||
{
|
||
struct snd_pcm *pcm;
|
||
int err;
|
||
|
||
err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
|
||
if (err < 0)
|
||
return err;
|
||
pcm->private_data = chip;
|
||
strcpy(pcm->name, "My Chip");
|
||
chip->pcm = pcm;
|
||
/* set operators */
|
||
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
|
||
&snd_mychip_playback_ops);
|
||
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
|
||
&snd_mychip_capture_ops);
|
||
/* pre-allocation of buffers */
|
||
/* NOTE: this may fail */
|
||
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
|
||
&chip->pci->dev,
|
||
64*1024, 64*1024);
|
||
return 0;
|
||
}
|
||
|
||
|
||
PCM Constructor
|
||
---------------
|
||
|
||
A pcm instance is allocated by the :c:func:`snd_pcm_new()`
|
||
function. It would be better to create a constructor for pcm, namely,
|
||
|
||
::
|
||
|
||
static int snd_mychip_new_pcm(struct mychip *chip)
|
||
{
|
||
struct snd_pcm *pcm;
|
||
int err;
|
||
|
||
err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
|
||
if (err < 0)
|
||
return err;
|
||
pcm->private_data = chip;
|
||
strcpy(pcm->name, "My Chip");
|
||
chip->pcm = pcm;
|
||
....
|
||
return 0;
|
||
}
|
||
|
||
The :c:func:`snd_pcm_new()` function takes four arguments. The
|
||
first argument is the card pointer to which this pcm is assigned, and
|
||
the second is the ID string.
|
||
|
||
The third argument (``index``, 0 in the above) is the index of this new
|
||
pcm. It begins from zero. If you create more than one pcm instances,
|
||
specify the different numbers in this argument. For example, ``index =
|
||
1`` for the second PCM device.
|
||
|
||
The fourth and fifth arguments are the number of substreams for playback
|
||
and capture, respectively. Here 1 is used for both arguments. When no
|
||
playback or capture substreams are available, pass 0 to the
|
||
corresponding argument.
|
||
|
||
If a chip supports multiple playbacks or captures, you can specify more
|
||
numbers, but they must be handled properly in open/close, etc.
|
||
callbacks. When you need to know which substream you are referring to,
|
||
then it can be obtained from :c:type:`struct snd_pcm_substream
|
||
<snd_pcm_substream>` data passed to each callback as follows:
|
||
|
||
::
|
||
|
||
struct snd_pcm_substream *substream;
|
||
int index = substream->number;
|
||
|
||
|
||
After the pcm is created, you need to set operators for each pcm stream.
|
||
|
||
::
|
||
|
||
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
|
||
&snd_mychip_playback_ops);
|
||
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
|
||
&snd_mychip_capture_ops);
|
||
|
||
The operators are defined typically like this:
|
||
|
||
::
|
||
|
||
static struct snd_pcm_ops snd_mychip_playback_ops = {
|
||
.open = snd_mychip_pcm_open,
|
||
.close = snd_mychip_pcm_close,
|
||
.hw_params = snd_mychip_pcm_hw_params,
|
||
.hw_free = snd_mychip_pcm_hw_free,
|
||
.prepare = snd_mychip_pcm_prepare,
|
||
.trigger = snd_mychip_pcm_trigger,
|
||
.pointer = snd_mychip_pcm_pointer,
|
||
};
|
||
|
||
All the callbacks are described in the Operators_ subsection.
|
||
|
||
After setting the operators, you probably will want to pre-allocate the
|
||
buffer and set up the managed allocation mode.
|
||
For that, simply call the following:
|
||
|
||
::
|
||
|
||
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
|
||
&chip->pci->dev,
|
||
64*1024, 64*1024);
|
||
|
||
It will allocate a buffer up to 64kB as default. Buffer management
|
||
details will be described in the later section `Buffer and Memory
|
||
Management`_.
|
||
|
||
Additionally, you can set some extra information for this pcm in
|
||
``pcm->info_flags``. The available values are defined as
|
||
``SNDRV_PCM_INFO_XXX`` in ``<sound/asound.h>``, which is used for the
|
||
hardware definition (described later). When your soundchip supports only
|
||
half-duplex, specify like this:
|
||
|
||
::
|
||
|
||
pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
|
||
|
||
|
||
... And the Destructor?
|
||
-----------------------
|
||
|
||
The destructor for a pcm instance is not always necessary. Since the pcm
|
||
device will be released by the middle layer code automatically, you
|
||
don't have to call the destructor explicitly.
|
||
|
||
The destructor would be necessary if you created special records
|
||
internally and needed to release them. In such a case, set the
|
||
destructor function to ``pcm->private_free``:
|
||
|
||
::
|
||
|
||
static void mychip_pcm_free(struct snd_pcm *pcm)
|
||
{
|
||
struct mychip *chip = snd_pcm_chip(pcm);
|
||
/* free your own data */
|
||
kfree(chip->my_private_pcm_data);
|
||
/* do what you like else */
|
||
....
|
||
}
|
||
|
||
static int snd_mychip_new_pcm(struct mychip *chip)
|
||
{
|
||
struct snd_pcm *pcm;
|
||
....
|
||
/* allocate your own data */
|
||
chip->my_private_pcm_data = kmalloc(...);
|
||
/* set the destructor */
|
||
pcm->private_data = chip;
|
||
pcm->private_free = mychip_pcm_free;
|
||
....
|
||
}
|
||
|
||
|
||
|
||
Runtime Pointer - The Chest of PCM Information
|
||
----------------------------------------------
|
||
|
||
When the PCM substream is opened, a PCM runtime instance is allocated
|
||
and assigned to the substream. This pointer is accessible via
|
||
``substream->runtime``. This runtime pointer holds most information you
|
||
need to control the PCM: the copy of hw_params and sw_params
|
||
configurations, the buffer pointers, mmap records, spinlocks, etc.
|
||
|
||
The definition of runtime instance is found in ``<sound/pcm.h>``. Here
|
||
are the contents of this file:
|
||
|
||
::
|
||
|
||
struct _snd_pcm_runtime {
|
||
/* -- Status -- */
|
||
struct snd_pcm_substream *trigger_master;
|
||
snd_timestamp_t trigger_tstamp; /* trigger timestamp */
|
||
int overrange;
|
||
snd_pcm_uframes_t avail_max;
|
||
snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */
|
||
snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
|
||
|
||
/* -- HW params -- */
|
||
snd_pcm_access_t access; /* access mode */
|
||
snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */
|
||
snd_pcm_subformat_t subformat; /* subformat */
|
||
unsigned int rate; /* rate in Hz */
|
||
unsigned int channels; /* channels */
|
||
snd_pcm_uframes_t period_size; /* period size */
|
||
unsigned int periods; /* periods */
|
||
snd_pcm_uframes_t buffer_size; /* buffer size */
|
||
unsigned int tick_time; /* tick time */
|
||
snd_pcm_uframes_t min_align; /* Min alignment for the format */
|
||
size_t byte_align;
|
||
unsigned int frame_bits;
|
||
unsigned int sample_bits;
|
||
unsigned int info;
|
||
unsigned int rate_num;
|
||
unsigned int rate_den;
|
||
|
||
/* -- SW params -- */
|
||
struct timespec tstamp_mode; /* mmap timestamp is updated */
|
||
unsigned int period_step;
|
||
unsigned int sleep_min; /* min ticks to sleep */
|
||
snd_pcm_uframes_t start_threshold;
|
||
snd_pcm_uframes_t stop_threshold;
|
||
snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
|
||
noise is nearest than this */
|
||
snd_pcm_uframes_t silence_size; /* Silence filling size */
|
||
snd_pcm_uframes_t boundary; /* pointers wrap point */
|
||
|
||
snd_pcm_uframes_t silenced_start;
|
||
snd_pcm_uframes_t silenced_size;
|
||
|
||
snd_pcm_sync_id_t sync; /* hardware synchronization ID */
|
||
|
||
/* -- mmap -- */
|
||
volatile struct snd_pcm_mmap_status *status;
|
||
volatile struct snd_pcm_mmap_control *control;
|
||
atomic_t mmap_count;
|
||
|
||
/* -- locking / scheduling -- */
|
||
spinlock_t lock;
|
||
wait_queue_head_t sleep;
|
||
struct timer_list tick_timer;
|
||
struct fasync_struct *fasync;
|
||
|
||
/* -- private section -- */
|
||
void *private_data;
|
||
void (*private_free)(struct snd_pcm_runtime *runtime);
|
||
|
||
/* -- hardware description -- */
|
||
struct snd_pcm_hardware hw;
|
||
struct snd_pcm_hw_constraints hw_constraints;
|
||
|
||
/* -- timer -- */
|
||
unsigned int timer_resolution; /* timer resolution */
|
||
|
||
/* -- DMA -- */
|
||
unsigned char *dma_area; /* DMA area */
|
||
dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */
|
||
size_t dma_bytes; /* size of DMA area */
|
||
|
||
struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */
|
||
|
||
#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
|
||
/* -- OSS things -- */
|
||
struct snd_pcm_oss_runtime oss;
|
||
#endif
|
||
};
|
||
|
||
|
||
For the operators (callbacks) of each sound driver, most of these
|
||
records are supposed to be read-only. Only the PCM middle-layer changes
|
||
/ updates them. The exceptions are the hardware description (hw) DMA
|
||
buffer information and the private data. Besides, if you use the
|
||
standard managed buffer allocation mode, you don't need to set the
|
||
DMA buffer information by yourself.
|
||
|
||
In the sections below, important records are explained.
|
||
|
||
Hardware Description
|
||
~~~~~~~~~~~~~~~~~~~~
|
||
|
||
The hardware descriptor (:c:type:`struct snd_pcm_hardware
|
||
<snd_pcm_hardware>`) contains the definitions of the fundamental
|
||
hardware configuration. Above all, you'll need to define this in the
|
||
`PCM open callback`_. Note that the runtime instance holds the copy of
|
||
the descriptor, not the pointer to the existing descriptor. That is,
|
||
in the open callback, you can modify the copied descriptor
|
||
(``runtime->hw``) as you need. For example, if the maximum number of
|
||
channels is 1 only on some chip models, you can still use the same
|
||
hardware descriptor and change the channels_max later:
|
||
|
||
::
|
||
|
||
struct snd_pcm_runtime *runtime = substream->runtime;
|
||
...
|
||
runtime->hw = snd_mychip_playback_hw; /* common definition */
|
||
if (chip->model == VERY_OLD_ONE)
|
||
runtime->hw.channels_max = 1;
|
||
|
||
Typically, you'll have a hardware descriptor as below:
|
||
|
||
::
|
||
|
||
static struct snd_pcm_hardware snd_mychip_playback_hw = {
|
||
.info = (SNDRV_PCM_INFO_MMAP |
|
||
SNDRV_PCM_INFO_INTERLEAVED |
|
||
SNDRV_PCM_INFO_BLOCK_TRANSFER |
|
||
SNDRV_PCM_INFO_MMAP_VALID),
|
||
.formats = SNDRV_PCM_FMTBIT_S16_LE,
|
||
.rates = SNDRV_PCM_RATE_8000_48000,
|
||
.rate_min = 8000,
|
||
.rate_max = 48000,
|
||
.channels_min = 2,
|
||
.channels_max = 2,
|
||
.buffer_bytes_max = 32768,
|
||
.period_bytes_min = 4096,
|
||
.period_bytes_max = 32768,
|
||
.periods_min = 1,
|
||
.periods_max = 1024,
|
||
};
|
||
|
||
- The ``info`` field contains the type and capabilities of this
|
||
pcm. The bit flags are defined in ``<sound/asound.h>`` as
|
||
``SNDRV_PCM_INFO_XXX``. Here, at least, you have to specify whether
|
||
the mmap is supported and which interleaved format is
|
||
supported. When the hardware supports mmap, add the
|
||
``SNDRV_PCM_INFO_MMAP`` flag here. When the hardware supports the
|
||
interleaved or the non-interleaved formats,
|
||
``SNDRV_PCM_INFO_INTERLEAVED`` or ``SNDRV_PCM_INFO_NONINTERLEAVED``
|
||
flag must be set, respectively. If both are supported, you can set
|
||
both, too.
|
||
|
||
In the above example, ``MMAP_VALID`` and ``BLOCK_TRANSFER`` are
|
||
specified for the OSS mmap mode. Usually both are set. Of course,
|
||
``MMAP_VALID`` is set only if the mmap is really supported.
|
||
|
||
The other possible flags are ``SNDRV_PCM_INFO_PAUSE`` and
|
||
``SNDRV_PCM_INFO_RESUME``. The ``PAUSE`` bit means that the pcm
|
||
supports the “pause” operation, while the ``RESUME`` bit means that
|
||
the pcm supports the full “suspend/resume” operation. If the
|
||
``PAUSE`` flag is set, the ``trigger`` callback below must handle
|
||
the corresponding (pause push/release) commands. The suspend/resume
|
||
trigger commands can be defined even without the ``RESUME``
|
||
flag. See `Power Management`_ section for details.
|
||
|
||
When the PCM substreams can be synchronized (typically,
|
||
synchronized start/stop of a playback and a capture streams), you
|
||
can give ``SNDRV_PCM_INFO_SYNC_START``, too. In this case, you'll
|
||
need to check the linked-list of PCM substreams in the trigger
|
||
callback. This will be described in the later section.
|
||
|
||
- ``formats`` field contains the bit-flags of supported formats
|
||
(``SNDRV_PCM_FMTBIT_XXX``). If the hardware supports more than one
|
||
format, give all or'ed bits. In the example above, the signed 16bit
|
||
little-endian format is specified.
|
||
|
||
- ``rates`` field contains the bit-flags of supported rates
|
||
(``SNDRV_PCM_RATE_XXX``). When the chip supports continuous rates,
|
||
pass ``CONTINUOUS`` bit additionally. The pre-defined rate bits are
|
||
provided only for typical rates. If your chip supports
|
||
unconventional rates, you need to add the ``KNOT`` bit and set up
|
||
the hardware constraint manually (explained later).
|
||
|
||
- ``rate_min`` and ``rate_max`` define the minimum and maximum sample
|
||
rate. This should correspond somehow to ``rates`` bits.
|
||
|
||
- ``channel_min`` and ``channel_max`` define, as you might already
|
||
expected, the minimum and maximum number of channels.
|
||
|
||
- ``buffer_bytes_max`` defines the maximum buffer size in
|
||
bytes. There is no ``buffer_bytes_min`` field, since it can be
|
||
calculated from the minimum period size and the minimum number of
|
||
periods. Meanwhile, ``period_bytes_min`` and define the minimum and
|
||
maximum size of the period in bytes. ``periods_max`` and
|
||
``periods_min`` define the maximum and minimum number of periods in
|
||
the buffer.
|
||
|
||
The “period” is a term that corresponds to a fragment in the OSS
|
||
world. The period defines the size at which a PCM interrupt is
|
||
generated. This size strongly depends on the hardware. Generally,
|
||
the smaller period size will give you more interrupts, that is,
|
||
more controls. In the case of capture, this size defines the input
|
||
latency. On the other hand, the whole buffer size defines the
|
||
output latency for the playback direction.
|
||
|
||
- There is also a field ``fifo_size``. This specifies the size of the
|
||
hardware FIFO, but currently it is neither used in the driver nor
|
||
in the alsa-lib. So, you can ignore this field.
|
||
|
||
PCM Configurations
|
||
~~~~~~~~~~~~~~~~~~
|
||
|
||
Ok, let's go back again to the PCM runtime records. The most
|
||
frequently referred records in the runtime instance are the PCM
|
||
configurations. The PCM configurations are stored in the runtime
|
||
instance after the application sends ``hw_params`` data via
|
||
alsa-lib. There are many fields copied from hw_params and sw_params
|
||
structs. For example, ``format`` holds the format type chosen by the
|
||
application. This field contains the enum value
|
||
``SNDRV_PCM_FORMAT_XXX``.
|
||
|
||
One thing to be noted is that the configured buffer and period sizes
|
||
are stored in “frames” in the runtime. In the ALSA world, ``1 frame =
|
||
channels \* samples-size``. For conversion between frames and bytes,
|
||
you can use the :c:func:`frames_to_bytes()` and
|
||
:c:func:`bytes_to_frames()` helper functions.
|
||
|
||
::
|
||
|
||
period_bytes = frames_to_bytes(runtime, runtime->period_size);
|
||
|
||
Also, many software parameters (sw_params) are stored in frames, too.
|
||
Please check the type of the field. ``snd_pcm_uframes_t`` is for the
|
||
frames as unsigned integer while ``snd_pcm_sframes_t`` is for the
|
||
frames as signed integer.
|
||
|
||
DMA Buffer Information
|
||
~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
The DMA buffer is defined by the following four fields, ``dma_area``,
|
||
``dma_addr``, ``dma_bytes`` and ``dma_private``. The ``dma_area``
|
||
holds the buffer pointer (the logical address). You can call
|
||
:c:func:`memcpy()` from/to this pointer. Meanwhile, ``dma_addr`` holds
|
||
the physical address of the buffer. This field is specified only when
|
||
the buffer is a linear buffer. ``dma_bytes`` holds the size of buffer
|
||
in bytes. ``dma_private`` is used for the ALSA DMA allocator.
|
||
|
||
If you use either the managed buffer allocation mode or the standard
|
||
API function :c:func:`snd_pcm_lib_malloc_pages()` for allocating the buffer,
|
||
these fields are set by the ALSA middle layer, and you should *not*
|
||
change them by yourself. You can read them but not write them. On the
|
||
other hand, if you want to allocate the buffer by yourself, you'll
|
||
need to manage it in hw_params callback. At least, ``dma_bytes`` is
|
||
mandatory. ``dma_area`` is necessary when the buffer is mmapped. If
|
||
your driver doesn't support mmap, this field is not
|
||
necessary. ``dma_addr`` is also optional. You can use dma_private as
|
||
you like, too.
|
||
|
||
Running Status
|
||
~~~~~~~~~~~~~~
|
||
|
||
The running status can be referred via ``runtime->status``. This is
|
||
the pointer to the :c:type:`struct snd_pcm_mmap_status
|
||
<snd_pcm_mmap_status>` record. For example, you can get the current
|
||
DMA hardware pointer via ``runtime->status->hw_ptr``.
|
||
|
||
The DMA application pointer can be referred via ``runtime->control``,
|
||
which points to the :c:type:`struct snd_pcm_mmap_control
|
||
<snd_pcm_mmap_control>` record. However, accessing directly to
|
||
this value is not recommended.
|
||
|
||
Private Data
|
||
~~~~~~~~~~~~
|
||
|
||
You can allocate a record for the substream and store it in
|
||
``runtime->private_data``. Usually, this is done in the `PCM open
|
||
callback`_. Don't mix this with ``pcm->private_data``. The
|
||
``pcm->private_data`` usually points to the chip instance assigned
|
||
statically at the creation of PCM, while the ``runtime->private_data``
|
||
points to a dynamic data structure created at the PCM open
|
||
callback.
|
||
|
||
::
|
||
|
||
static int snd_xxx_open(struct snd_pcm_substream *substream)
|
||
{
|
||
struct my_pcm_data *data;
|
||
....
|
||
data = kmalloc(sizeof(*data), GFP_KERNEL);
|
||
substream->runtime->private_data = data;
|
||
....
|
||
}
|
||
|
||
|
||
The allocated object must be released in the `close callback`_.
|
||
|
||
Operators
|
||
---------
|
||
|
||
OK, now let me give details about each pcm callback (``ops``). In
|
||
general, every callback must return 0 if successful, or a negative
|
||
error number such as ``-EINVAL``. To choose an appropriate error
|
||
number, it is advised to check what value other parts of the kernel
|
||
return when the same kind of request fails.
|
||
|
||
The callback function takes at least the argument with :c:type:`struct
|
||
snd_pcm_substream <snd_pcm_substream>` pointer. To retrieve the chip
|
||
record from the given substream instance, you can use the following
|
||
macro.
|
||
|
||
::
|
||
|
||
int xxx() {
|
||
struct mychip *chip = snd_pcm_substream_chip(substream);
|
||
....
|
||
}
|
||
|
||
The macro reads ``substream->private_data``, which is a copy of
|
||
``pcm->private_data``. You can override the former if you need to
|
||
assign different data records per PCM substream. For example, the
|
||
cmi8330 driver assigns different ``private_data`` for playback and
|
||
capture directions, because it uses two different codecs (SB- and
|
||
AD-compatible) for different directions.
|
||
|
||
PCM open callback
|
||
~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_open(struct snd_pcm_substream *substream);
|
||
|
||
This is called when a pcm substream is opened.
|
||
|
||
At least, here you have to initialize the ``runtime->hw``
|
||
record. Typically, this is done by like this:
|
||
|
||
::
|
||
|
||
static int snd_xxx_open(struct snd_pcm_substream *substream)
|
||
{
|
||
struct mychip *chip = snd_pcm_substream_chip(substream);
|
||
struct snd_pcm_runtime *runtime = substream->runtime;
|
||
|
||
runtime->hw = snd_mychip_playback_hw;
|
||
return 0;
|
||
}
|
||
|
||
where ``snd_mychip_playback_hw`` is the pre-defined hardware
|
||
description.
|
||
|
||
You can allocate a private data in this callback, as described in
|
||
`Private Data`_ section.
|
||
|
||
If the hardware configuration needs more constraints, set the hardware
|
||
constraints here, too. See Constraints_ for more details.
|
||
|
||
close callback
|
||
~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_close(struct snd_pcm_substream *substream);
|
||
|
||
|
||
Obviously, this is called when a pcm substream is closed.
|
||
|
||
Any private instance for a pcm substream allocated in the ``open``
|
||
callback will be released here.
|
||
|
||
::
|
||
|
||
static int snd_xxx_close(struct snd_pcm_substream *substream)
|
||
{
|
||
....
|
||
kfree(substream->runtime->private_data);
|
||
....
|
||
}
|
||
|
||
ioctl callback
|
||
~~~~~~~~~~~~~~
|
||
|
||
This is used for any special call to pcm ioctls. But usually you can
|
||
leave it as NULL, then PCM core calls the generic ioctl callback
|
||
function :c:func:`snd_pcm_lib_ioctl()`. If you need to deal with the
|
||
unique setup of channel info or reset procedure, you can pass your own
|
||
callback function here.
|
||
|
||
hw_params callback
|
||
~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
|
||
struct snd_pcm_hw_params *hw_params);
|
||
|
||
This is called when the hardware parameter (``hw_params``) is set up
|
||
by the application, that is, once when the buffer size, the period
|
||
size, the format, etc. are defined for the pcm substream.
|
||
|
||
Many hardware setups should be done in this callback, including the
|
||
allocation of buffers.
|
||
|
||
Parameters to be initialized are retrieved by
|
||
:c:func:`params_xxx()` macros.
|
||
|
||
When you set up the managed buffer allocation mode for the substream,
|
||
a buffer is already allocated before this callback gets
|
||
called. Alternatively, you can call a helper function below for
|
||
allocating the buffer, too.
|
||
|
||
::
|
||
|
||
snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
|
||
|
||
:c:func:`snd_pcm_lib_malloc_pages()` is available only when the
|
||
DMA buffers have been pre-allocated. See the section `Buffer Types`_
|
||
for more details.
|
||
|
||
Note that this and ``prepare`` callbacks may be called multiple times
|
||
per initialization. For example, the OSS emulation may call these
|
||
callbacks at each change via its ioctl.
|
||
|
||
Thus, you need to be careful not to allocate the same buffers many
|
||
times, which will lead to memory leaks! Calling the helper function
|
||
above many times is OK. It will release the previous buffer
|
||
automatically when it was already allocated.
|
||
|
||
Another note is that this callback is non-atomic (schedulable) as
|
||
default, i.e. when no ``nonatomic`` flag set. This is important,
|
||
because the ``trigger`` callback is atomic (non-schedulable). That is,
|
||
mutexes or any schedule-related functions are not available in
|
||
``trigger`` callback. Please see the subsection Atomicity_ for
|
||
details.
|
||
|
||
hw_free callback
|
||
~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
|
||
|
||
This is called to release the resources allocated via
|
||
``hw_params``.
|
||
|
||
This function is always called before the close callback is called.
|
||
Also, the callback may be called multiple times, too. Keep track
|
||
whether the resource was already released.
|
||
|
||
When you have set up the managed buffer allocation mode for the PCM
|
||
substream, the allocated PCM buffer will be automatically released
|
||
after this callback gets called. Otherwise you'll have to release the
|
||
buffer manually. Typically, when the buffer was allocated from the
|
||
pre-allocated pool, you can use the standard API function
|
||
:c:func:`snd_pcm_lib_malloc_pages()` like:
|
||
|
||
::
|
||
|
||
snd_pcm_lib_free_pages(substream);
|
||
|
||
prepare callback
|
||
~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_prepare(struct snd_pcm_substream *substream);
|
||
|
||
This callback is called when the pcm is “prepared”. You can set the
|
||
format type, sample rate, etc. here. The difference from ``hw_params``
|
||
is that the ``prepare`` callback will be called each time
|
||
:c:func:`snd_pcm_prepare()` is called, i.e. when recovering after
|
||
underruns, etc.
|
||
|
||
Note that this callback is now non-atomic. You can use
|
||
schedule-related functions safely in this callback.
|
||
|
||
In this and the following callbacks, you can refer to the values via
|
||
the runtime record, ``substream->runtime``. For example, to get the
|
||
current rate, format or channels, access to ``runtime->rate``,
|
||
``runtime->format`` or ``runtime->channels``, respectively. The
|
||
physical address of the allocated buffer is set to
|
||
``runtime->dma_area``. The buffer and period sizes are in
|
||
``runtime->buffer_size`` and ``runtime->period_size``, respectively.
|
||
|
||
Be careful that this callback will be called many times at each setup,
|
||
too.
|
||
|
||
trigger callback
|
||
~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
|
||
|
||
This is called when the pcm is started, stopped or paused.
|
||
|
||
Which action is specified in the second argument,
|
||
``SNDRV_PCM_TRIGGER_XXX`` in ``<sound/pcm.h>``. At least, the ``START``
|
||
and ``STOP`` commands must be defined in this callback.
|
||
|
||
::
|
||
|
||
switch (cmd) {
|
||
case SNDRV_PCM_TRIGGER_START:
|
||
/* do something to start the PCM engine */
|
||
break;
|
||
case SNDRV_PCM_TRIGGER_STOP:
|
||
/* do something to stop the PCM engine */
|
||
break;
|
||
default:
|
||
return -EINVAL;
|
||
}
|
||
|
||
When the pcm supports the pause operation (given in the info field of
|
||
the hardware table), the ``PAUSE_PUSH`` and ``PAUSE_RELEASE`` commands
|
||
must be handled here, too. The former is the command to pause the pcm,
|
||
and the latter to restart the pcm again.
|
||
|
||
When the pcm supports the suspend/resume operation, regardless of full
|
||
or partial suspend/resume support, the ``SUSPEND`` and ``RESUME``
|
||
commands must be handled, too. These commands are issued when the
|
||
power-management status is changed. Obviously, the ``SUSPEND`` and
|
||
``RESUME`` commands suspend and resume the pcm substream, and usually,
|
||
they are identical to the ``STOP`` and ``START`` commands, respectively.
|
||
See the `Power Management`_ section for details.
|
||
|
||
As mentioned, this callback is atomic as default unless ``nonatomic``
|
||
flag set, and you cannot call functions which may sleep. The
|
||
``trigger`` callback should be as minimal as possible, just really
|
||
triggering the DMA. The other stuff should be initialized
|
||
``hw_params`` and ``prepare`` callbacks properly beforehand.
|
||
|
||
sync_stop callback
|
||
~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_sync_stop(struct snd_pcm_substream *substream);
|
||
|
||
This callback is optional, and NULL can be passed. It's called after
|
||
the PCM core stops the stream and changes the stream state
|
||
``prepare``, ``hw_params`` or ``hw_free``.
|
||
Since the IRQ handler might be still pending, we need to wait until
|
||
the pending task finishes before moving to the next step; otherwise it
|
||
might lead to a crash due to resource conflicts or access to the freed
|
||
resources. A typical behavior is to call a synchronization function
|
||
like :c:func:`synchronize_irq()` here.
|
||
|
||
For majority of drivers that need only a call of
|
||
:c:func:`synchronize_irq()`, there is a simpler setup, too.
|
||
While keeping NULL to ``sync_stop`` PCM callback, the driver can set
|
||
``card->sync_irq`` field to store the valid interrupt number after
|
||
requesting an IRQ, instead. Then PCM core will look call
|
||
:c:func:`synchronize_irq()` with the given IRQ appropriately.
|
||
|
||
If the IRQ handler is released at the card destructor, you don't need
|
||
to clear ``card->sync_irq``, as the card itself is being released.
|
||
So, usually you'll need to add just a single line for assigning
|
||
``card->sync_irq`` in the driver code unless the driver re-acquires
|
||
the IRQ. When the driver frees and re-acquires the IRQ dynamically
|
||
(e.g. for suspend/resume), it needs to clear and re-set
|
||
``card->sync_irq`` again appropriately.
|
||
|
||
pointer callback
|
||
~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
|
||
|
||
This callback is called when the PCM middle layer inquires the current
|
||
hardware position on the buffer. The position must be returned in
|
||
frames, ranging from 0 to ``buffer_size - 1``.
|
||
|
||
This is called usually from the buffer-update routine in the pcm
|
||
middle layer, which is invoked when :c:func:`snd_pcm_period_elapsed()`
|
||
is called in the interrupt routine. Then the pcm middle layer updates
|
||
the position and calculates the available space, and wakes up the
|
||
sleeping poll threads, etc.
|
||
|
||
This callback is also atomic as default.
|
||
|
||
copy_user, copy_kernel and fill_silence ops
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
These callbacks are not mandatory, and can be omitted in most cases.
|
||
These callbacks are used when the hardware buffer cannot be in the
|
||
normal memory space. Some chips have their own buffer on the hardware
|
||
which is not mappable. In such a case, you have to transfer the data
|
||
manually from the memory buffer to the hardware buffer. Or, if the
|
||
buffer is non-contiguous on both physical and virtual memory spaces,
|
||
these callbacks must be defined, too.
|
||
|
||
If these two callbacks are defined, copy and set-silence operations
|
||
are done by them. The detailed will be described in the later section
|
||
`Buffer and Memory Management`_.
|
||
|
||
ack callback
|
||
~~~~~~~~~~~~
|
||
|
||
This callback is also not mandatory. This callback is called when the
|
||
``appl_ptr`` is updated in read or write operations. Some drivers like
|
||
emu10k1-fx and cs46xx need to track the current ``appl_ptr`` for the
|
||
internal buffer, and this callback is useful only for such a purpose.
|
||
|
||
This callback is atomic as default.
|
||
|
||
page callback
|
||
~~~~~~~~~~~~~
|
||
|
||
This callback is optional too. The mmap calls this callback to get the
|
||
page fault address.
|
||
|
||
Since the recent changes, you need no special callback any longer for
|
||
the standard SG-buffer or vmalloc-buffer. Hence this callback should
|
||
be rarely used.
|
||
|
||
mmap calllback
|
||
~~~~~~~~~~~~~~
|
||
|
||
This is another optional callback for controlling mmap behavior.
|
||
Once when defined, PCM core calls this callback when a page is
|
||
memory-mapped instead of dealing via the standard helper.
|
||
If you need special handling (due to some architecture or
|
||
device-specific issues), implement everything here as you like.
|
||
|
||
|
||
PCM Interrupt Handler
|
||
---------------------
|
||
|
||
The rest of pcm stuff is the PCM interrupt handler. The role of PCM
|
||
interrupt handler in the sound driver is to update the buffer position
|
||
and to tell the PCM middle layer when the buffer position goes across
|
||
the prescribed period size. To inform this, call the
|
||
:c:func:`snd_pcm_period_elapsed()` function.
|
||
|
||
There are several types of sound chips to generate the interrupts.
|
||
|
||
Interrupts at the period (fragment) boundary
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
This is the most frequently found type: the hardware generates an
|
||
interrupt at each period boundary. In this case, you can call
|
||
:c:func:`snd_pcm_period_elapsed()` at each interrupt.
|
||
|
||
:c:func:`snd_pcm_period_elapsed()` takes the substream pointer as
|
||
its argument. Thus, you need to keep the substream pointer accessible
|
||
from the chip instance. For example, define ``substream`` field in the
|
||
chip record to hold the current running substream pointer, and set the
|
||
pointer value at ``open`` callback (and reset at ``close`` callback).
|
||
|
||
If you acquire a spinlock in the interrupt handler, and the lock is used
|
||
in other pcm callbacks, too, then you have to release the lock before
|
||
calling :c:func:`snd_pcm_period_elapsed()`, because
|
||
:c:func:`snd_pcm_period_elapsed()` calls other pcm callbacks
|
||
inside.
|
||
|
||
Typical code would be like:
|
||
|
||
::
|
||
|
||
|
||
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
|
||
{
|
||
struct mychip *chip = dev_id;
|
||
spin_lock(&chip->lock);
|
||
....
|
||
if (pcm_irq_invoked(chip)) {
|
||
/* call updater, unlock before it */
|
||
spin_unlock(&chip->lock);
|
||
snd_pcm_period_elapsed(chip->substream);
|
||
spin_lock(&chip->lock);
|
||
/* acknowledge the interrupt if necessary */
|
||
}
|
||
....
|
||
spin_unlock(&chip->lock);
|
||
return IRQ_HANDLED;
|
||
}
|
||
|
||
|
||
|
||
High frequency timer interrupts
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
This happens when the hardware doesn't generate interrupts at the period
|
||
boundary but issues timer interrupts at a fixed timer rate (e.g. es1968
|
||
or ymfpci drivers). In this case, you need to check the current hardware
|
||
position and accumulate the processed sample length at each interrupt.
|
||
When the accumulated size exceeds the period size, call
|
||
:c:func:`snd_pcm_period_elapsed()` and reset the accumulator.
|
||
|
||
Typical code would be like the following.
|
||
|
||
::
|
||
|
||
|
||
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
|
||
{
|
||
struct mychip *chip = dev_id;
|
||
spin_lock(&chip->lock);
|
||
....
|
||
if (pcm_irq_invoked(chip)) {
|
||
unsigned int last_ptr, size;
|
||
/* get the current hardware pointer (in frames) */
|
||
last_ptr = get_hw_ptr(chip);
|
||
/* calculate the processed frames since the
|
||
* last update
|
||
*/
|
||
if (last_ptr < chip->last_ptr)
|
||
size = runtime->buffer_size + last_ptr
|
||
- chip->last_ptr;
|
||
else
|
||
size = last_ptr - chip->last_ptr;
|
||
/* remember the last updated point */
|
||
chip->last_ptr = last_ptr;
|
||
/* accumulate the size */
|
||
chip->size += size;
|
||
/* over the period boundary? */
|
||
if (chip->size >= runtime->period_size) {
|
||
/* reset the accumulator */
|
||
chip->size %= runtime->period_size;
|
||
/* call updater */
|
||
spin_unlock(&chip->lock);
|
||
snd_pcm_period_elapsed(substream);
|
||
spin_lock(&chip->lock);
|
||
}
|
||
/* acknowledge the interrupt if necessary */
|
||
}
|
||
....
|
||
spin_unlock(&chip->lock);
|
||
return IRQ_HANDLED;
|
||
}
|
||
|
||
|
||
|
||
On calling :c:func:`snd_pcm_period_elapsed()`
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
In both cases, even if more than one period are elapsed, you don't have
|
||
to call :c:func:`snd_pcm_period_elapsed()` many times. Call only
|
||
once. And the pcm layer will check the current hardware pointer and
|
||
update to the latest status.
|
||
|
||
Atomicity
|
||
---------
|
||
|
||
One of the most important (and thus difficult to debug) problems in
|
||
kernel programming are race conditions. In the Linux kernel, they are
|
||
usually avoided via spin-locks, mutexes or semaphores. In general, if a
|
||
race condition can happen in an interrupt handler, it has to be managed
|
||
atomically, and you have to use a spinlock to protect the critical
|
||
session. If the critical section is not in interrupt handler code and if
|
||
taking a relatively long time to execute is acceptable, you should use
|
||
mutexes or semaphores instead.
|
||
|
||
As already seen, some pcm callbacks are atomic and some are not. For
|
||
example, the ``hw_params`` callback is non-atomic, while ``trigger``
|
||
callback is atomic. This means, the latter is called already in a
|
||
spinlock held by the PCM middle layer. Please take this atomicity into
|
||
account when you choose a locking scheme in the callbacks.
|
||
|
||
In the atomic callbacks, you cannot use functions which may call
|
||
:c:func:`schedule()` or go to :c:func:`sleep()`. Semaphores and
|
||
mutexes can sleep, and hence they cannot be used inside the atomic
|
||
callbacks (e.g. ``trigger`` callback). To implement some delay in such a
|
||
callback, please use :c:func:`udelay()` or :c:func:`mdelay()`.
|
||
|
||
All three atomic callbacks (trigger, pointer, and ack) are called with
|
||
local interrupts disabled.
|
||
|
||
The recent changes in PCM core code, however, allow all PCM operations
|
||
to be non-atomic. This assumes that the all caller sides are in
|
||
non-atomic contexts. For example, the function
|
||
:c:func:`snd_pcm_period_elapsed()` is called typically from the
|
||
interrupt handler. But, if you set up the driver to use a threaded
|
||
interrupt handler, this call can be in non-atomic context, too. In such
|
||
a case, you can set ``nonatomic`` filed of :c:type:`struct snd_pcm
|
||
<snd_pcm>` object after creating it. When this flag is set, mutex
|
||
and rwsem are used internally in the PCM core instead of spin and
|
||
rwlocks, so that you can call all PCM functions safely in a non-atomic
|
||
context.
|
||
|
||
Constraints
|
||
-----------
|
||
|
||
If your chip supports unconventional sample rates, or only the limited
|
||
samples, you need to set a constraint for the condition.
|
||
|
||
For example, in order to restrict the sample rates in the some supported
|
||
values, use :c:func:`snd_pcm_hw_constraint_list()`. You need to
|
||
call this function in the open callback.
|
||
|
||
::
|
||
|
||
static unsigned int rates[] =
|
||
{4000, 10000, 22050, 44100};
|
||
static struct snd_pcm_hw_constraint_list constraints_rates = {
|
||
.count = ARRAY_SIZE(rates),
|
||
.list = rates,
|
||
.mask = 0,
|
||
};
|
||
|
||
static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
|
||
{
|
||
int err;
|
||
....
|
||
err = snd_pcm_hw_constraint_list(substream->runtime, 0,
|
||
SNDRV_PCM_HW_PARAM_RATE,
|
||
&constraints_rates);
|
||
if (err < 0)
|
||
return err;
|
||
....
|
||
}
|
||
|
||
|
||
|
||
There are many different constraints. Look at ``sound/pcm.h`` for a
|
||
complete list. You can even define your own constraint rules. For
|
||
example, let's suppose my_chip can manage a substream of 1 channel if
|
||
and only if the format is ``S16_LE``, otherwise it supports any format
|
||
specified in the :c:type:`struct snd_pcm_hardware
|
||
<snd_pcm_hardware>` structure (or in any other
|
||
constraint_list). You can build a rule like this:
|
||
|
||
::
|
||
|
||
static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
|
||
struct snd_pcm_hw_rule *rule)
|
||
{
|
||
struct snd_interval *c = hw_param_interval(params,
|
||
SNDRV_PCM_HW_PARAM_CHANNELS);
|
||
struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
|
||
struct snd_interval ch;
|
||
|
||
snd_interval_any(&ch);
|
||
if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
|
||
ch.min = ch.max = 1;
|
||
ch.integer = 1;
|
||
return snd_interval_refine(c, &ch);
|
||
}
|
||
return 0;
|
||
}
|
||
|
||
|
||
Then you need to call this function to add your rule:
|
||
|
||
::
|
||
|
||
snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
|
||
hw_rule_channels_by_format, NULL,
|
||
SNDRV_PCM_HW_PARAM_FORMAT, -1);
|
||
|
||
The rule function is called when an application sets the PCM format, and
|
||
it refines the number of channels accordingly. But an application may
|
||
set the number of channels before setting the format. Thus you also need
|
||
to define the inverse rule:
|
||
|
||
::
|
||
|
||
static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
|
||
struct snd_pcm_hw_rule *rule)
|
||
{
|
||
struct snd_interval *c = hw_param_interval(params,
|
||
SNDRV_PCM_HW_PARAM_CHANNELS);
|
||
struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
|
||
struct snd_mask fmt;
|
||
|
||
snd_mask_any(&fmt); /* Init the struct */
|
||
if (c->min < 2) {
|
||
fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
|
||
return snd_mask_refine(f, &fmt);
|
||
}
|
||
return 0;
|
||
}
|
||
|
||
|
||
... and in the open callback:
|
||
|
||
::
|
||
|
||
snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
|
||
hw_rule_format_by_channels, NULL,
|
||
SNDRV_PCM_HW_PARAM_CHANNELS, -1);
|
||
|
||
One typical usage of the hw constraints is to align the buffer size
|
||
with the period size. As default, ALSA PCM core doesn't enforce the
|
||
buffer size to be aligned with the period size. For example, it'd be
|
||
possible to have a combination like 256 period bytes with 999 buffer
|
||
bytes.
|
||
|
||
Many device chips, however, require the buffer to be a multiple of
|
||
periods. In such a case, call
|
||
:c:func:`snd_pcm_hw_constraint_integer()` for
|
||
``SNDRV_PCM_HW_PARAM_PERIODS``.
|
||
|
||
::
|
||
|
||
snd_pcm_hw_constraint_integer(substream->runtime,
|
||
SNDRV_PCM_HW_PARAM_PERIODS);
|
||
|
||
This assures that the number of periods is integer, hence the buffer
|
||
size is aligned with the period size.
|
||
|
||
The hw constraint is a very much powerful mechanism to define the
|
||
preferred PCM configuration, and there are relevant helpers.
|
||
I won't give more details here, rather I would like to say, “Luke, use
|
||
the source.”
|
||
|
||
Control Interface
|
||
=================
|
||
|
||
General
|
||
-------
|
||
|
||
The control interface is used widely for many switches, sliders, etc.
|
||
which are accessed from user-space. Its most important use is the mixer
|
||
interface. In other words, since ALSA 0.9.x, all the mixer stuff is
|
||
implemented on the control kernel API.
|
||
|
||
ALSA has a well-defined AC97 control module. If your chip supports only
|
||
the AC97 and nothing else, you can skip this section.
|
||
|
||
The control API is defined in ``<sound/control.h>``. Include this file
|
||
if you want to add your own controls.
|
||
|
||
Definition of Controls
|
||
----------------------
|
||
|
||
To create a new control, you need to define the following three
|
||
callbacks: ``info``, ``get`` and ``put``. Then, define a
|
||
:c:type:`struct snd_kcontrol_new <snd_kcontrol_new>` record, such as:
|
||
|
||
::
|
||
|
||
|
||
static struct snd_kcontrol_new my_control = {
|
||
.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
|
||
.name = "PCM Playback Switch",
|
||
.index = 0,
|
||
.access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
|
||
.private_value = 0xffff,
|
||
.info = my_control_info,
|
||
.get = my_control_get,
|
||
.put = my_control_put
|
||
};
|
||
|
||
|
||
The ``iface`` field specifies the control type,
|
||
``SNDRV_CTL_ELEM_IFACE_XXX``, which is usually ``MIXER``. Use ``CARD``
|
||
for global controls that are not logically part of the mixer. If the
|
||
control is closely associated with some specific device on the sound
|
||
card, use ``HWDEP``, ``PCM``, ``RAWMIDI``, ``TIMER``, or ``SEQUENCER``,
|
||
and specify the device number with the ``device`` and ``subdevice``
|
||
fields.
|
||
|
||
The ``name`` is the name identifier string. Since ALSA 0.9.x, the
|
||
control name is very important, because its role is classified from
|
||
its name. There are pre-defined standard control names. The details
|
||
are described in the `Control Names`_ subsection.
|
||
|
||
The ``index`` field holds the index number of this control. If there
|
||
are several different controls with the same name, they can be
|
||
distinguished by the index number. This is the case when several
|
||
codecs exist on the card. If the index is zero, you can omit the
|
||
definition above.
|
||
|
||
The ``access`` field contains the access type of this control. Give
|
||
the combination of bit masks, ``SNDRV_CTL_ELEM_ACCESS_XXX``,
|
||
there. The details will be explained in the `Access Flags`_
|
||
subsection.
|
||
|
||
The ``private_value`` field contains an arbitrary long integer value
|
||
for this record. When using the generic ``info``, ``get`` and ``put``
|
||
callbacks, you can pass a value through this field. If several small
|
||
numbers are necessary, you can combine them in bitwise. Or, it's
|
||
possible to give a pointer (casted to unsigned long) of some record to
|
||
this field, too.
|
||
|
||
The ``tlv`` field can be used to provide metadata about the control;
|
||
see the `Metadata`_ subsection.
|
||
|
||
The other three are `Control Callbacks`_.
|
||
|
||
Control Names
|
||
-------------
|
||
|
||
There are some standards to define the control names. A control is
|
||
usually defined from the three parts as “SOURCE DIRECTION FUNCTION”.
|
||
|
||
The first, ``SOURCE``, specifies the source of the control, and is a
|
||
string such as “Master”, “PCM”, “CD” and “Line”. There are many
|
||
pre-defined sources.
|
||
|
||
The second, ``DIRECTION``, is one of the following strings according to
|
||
the direction of the control: “Playback”, “Capture”, “Bypass Playback”
|
||
and “Bypass Capture”. Or, it can be omitted, meaning both playback and
|
||
capture directions.
|
||
|
||
The third, ``FUNCTION``, is one of the following strings according to
|
||
the function of the control: “Switch”, “Volume” and “Route”.
|
||
|
||
The example of control names are, thus, “Master Capture Switch” or “PCM
|
||
Playback Volume”.
|
||
|
||
There are some exceptions:
|
||
|
||
Global capture and playback
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
“Capture Source”, “Capture Switch” and “Capture Volume” are used for the
|
||
global capture (input) source, switch and volume. Similarly, “Playback
|
||
Switch” and “Playback Volume” are used for the global output gain switch
|
||
and volume.
|
||
|
||
Tone-controls
|
||
~~~~~~~~~~~~~
|
||
|
||
tone-control switch and volumes are specified like “Tone Control - XXX”,
|
||
e.g. “Tone Control - Switch”, “Tone Control - Bass”, “Tone Control -
|
||
Center”.
|
||
|
||
3D controls
|
||
~~~~~~~~~~~
|
||
|
||
3D-control switches and volumes are specified like “3D Control - XXX”,
|
||
e.g. “3D Control - Switch”, “3D Control - Center”, “3D Control - Space”.
|
||
|
||
Mic boost
|
||
~~~~~~~~~
|
||
|
||
Mic-boost switch is set as “Mic Boost” or “Mic Boost (6dB)”.
|
||
|
||
More precise information can be found in
|
||
``Documentation/sound/designs/control-names.rst``.
|
||
|
||
Access Flags
|
||
------------
|
||
|
||
The access flag is the bitmask which specifies the access type of the
|
||
given control. The default access type is
|
||
``SNDRV_CTL_ELEM_ACCESS_READWRITE``, which means both read and write are
|
||
allowed to this control. When the access flag is omitted (i.e. = 0), it
|
||
is considered as ``READWRITE`` access as default.
|
||
|
||
When the control is read-only, pass ``SNDRV_CTL_ELEM_ACCESS_READ``
|
||
instead. In this case, you don't have to define the ``put`` callback.
|
||
Similarly, when the control is write-only (although it's a rare case),
|
||
you can use the ``WRITE`` flag instead, and you don't need the ``get``
|
||
callback.
|
||
|
||
If the control value changes frequently (e.g. the VU meter),
|
||
``VOLATILE`` flag should be given. This means that the control may be
|
||
changed without `Change notification`_. Applications should poll such
|
||
a control constantly.
|
||
|
||
When the control is inactive, set the ``INACTIVE`` flag, too. There are
|
||
``LOCK`` and ``OWNER`` flags to change the write permissions.
|
||
|
||
Control Callbacks
|
||
-----------------
|
||
|
||
info callback
|
||
~~~~~~~~~~~~~
|
||
|
||
The ``info`` callback is used to get detailed information on this
|
||
control. This must store the values of the given :c:type:`struct
|
||
snd_ctl_elem_info <snd_ctl_elem_info>` object. For example,
|
||
for a boolean control with a single element:
|
||
|
||
::
|
||
|
||
|
||
static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
|
||
struct snd_ctl_elem_info *uinfo)
|
||
{
|
||
uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
|
||
uinfo->count = 1;
|
||
uinfo->value.integer.min = 0;
|
||
uinfo->value.integer.max = 1;
|
||
return 0;
|
||
}
|
||
|
||
|
||
|
||
The ``type`` field specifies the type of the control. There are
|
||
``BOOLEAN``, ``INTEGER``, ``ENUMERATED``, ``BYTES``, ``IEC958`` and
|
||
``INTEGER64``. The ``count`` field specifies the number of elements in
|
||
this control. For example, a stereo volume would have count = 2. The
|
||
``value`` field is a union, and the values stored are depending on the
|
||
type. The boolean and integer types are identical.
|
||
|
||
The enumerated type is a bit different from others. You'll need to set
|
||
the string for the currently given item index.
|
||
|
||
::
|
||
|
||
static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
|
||
struct snd_ctl_elem_info *uinfo)
|
||
{
|
||
static char *texts[4] = {
|
||
"First", "Second", "Third", "Fourth"
|
||
};
|
||
uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
|
||
uinfo->count = 1;
|
||
uinfo->value.enumerated.items = 4;
|
||
if (uinfo->value.enumerated.item > 3)
|
||
uinfo->value.enumerated.item = 3;
|
||
strcpy(uinfo->value.enumerated.name,
|
||
texts[uinfo->value.enumerated.item]);
|
||
return 0;
|
||
}
|
||
|
||
The above callback can be simplified with a helper function,
|
||
:c:func:`snd_ctl_enum_info()`. The final code looks like below.
|
||
(You can pass ``ARRAY_SIZE(texts)`` instead of 4 in the third argument;
|
||
it's a matter of taste.)
|
||
|
||
::
|
||
|
||
static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
|
||
struct snd_ctl_elem_info *uinfo)
|
||
{
|
||
static char *texts[4] = {
|
||
"First", "Second", "Third", "Fourth"
|
||
};
|
||
return snd_ctl_enum_info(uinfo, 1, 4, texts);
|
||
}
|
||
|
||
|
||
Some common info callbacks are available for your convenience:
|
||
:c:func:`snd_ctl_boolean_mono_info()` and
|
||
:c:func:`snd_ctl_boolean_stereo_info()`. Obviously, the former
|
||
is an info callback for a mono channel boolean item, just like
|
||
:c:func:`snd_myctl_mono_info()` above, and the latter is for a
|
||
stereo channel boolean item.
|
||
|
||
get callback
|
||
~~~~~~~~~~~~
|
||
|
||
This callback is used to read the current value of the control and to
|
||
return to user-space.
|
||
|
||
For example,
|
||
|
||
::
|
||
|
||
|
||
static int snd_myctl_get(struct snd_kcontrol *kcontrol,
|
||
struct snd_ctl_elem_value *ucontrol)
|
||
{
|
||
struct mychip *chip = snd_kcontrol_chip(kcontrol);
|
||
ucontrol->value.integer.value[0] = get_some_value(chip);
|
||
return 0;
|
||
}
|
||
|
||
|
||
|
||
The ``value`` field depends on the type of control as well as on the
|
||
info callback. For example, the sb driver uses this field to store the
|
||
register offset, the bit-shift and the bit-mask. The ``private_value``
|
||
field is set as follows:
|
||
|
||
::
|
||
|
||
.private_value = reg | (shift << 16) | (mask << 24)
|
||
|
||
and is retrieved in callbacks like
|
||
|
||
::
|
||
|
||
static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
|
||
struct snd_ctl_elem_value *ucontrol)
|
||
{
|
||
int reg = kcontrol->private_value & 0xff;
|
||
int shift = (kcontrol->private_value >> 16) & 0xff;
|
||
int mask = (kcontrol->private_value >> 24) & 0xff;
|
||
....
|
||
}
|
||
|
||
In the ``get`` callback, you have to fill all the elements if the
|
||
control has more than one elements, i.e. ``count > 1``. In the example
|
||
above, we filled only one element (``value.integer.value[0]``) since
|
||
it's assumed as ``count = 1``.
|
||
|
||
put callback
|
||
~~~~~~~~~~~~
|
||
|
||
This callback is used to write a value from user-space.
|
||
|
||
For example,
|
||
|
||
::
|
||
|
||
|
||
static int snd_myctl_put(struct snd_kcontrol *kcontrol,
|
||
struct snd_ctl_elem_value *ucontrol)
|
||
{
|
||
struct mychip *chip = snd_kcontrol_chip(kcontrol);
|
||
int changed = 0;
|
||
if (chip->current_value !=
|
||
ucontrol->value.integer.value[0]) {
|
||
change_current_value(chip,
|
||
ucontrol->value.integer.value[0]);
|
||
changed = 1;
|
||
}
|
||
return changed;
|
||
}
|
||
|
||
|
||
|
||
As seen above, you have to return 1 if the value is changed. If the
|
||
value is not changed, return 0 instead. If any fatal error happens,
|
||
return a negative error code as usual.
|
||
|
||
As in the ``get`` callback, when the control has more than one
|
||
elements, all elements must be evaluated in this callback, too.
|
||
|
||
Callbacks are not atomic
|
||
~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
All these three callbacks are basically not atomic.
|
||
|
||
Control Constructor
|
||
-------------------
|
||
|
||
When everything is ready, finally we can create a new control. To create
|
||
a control, there are two functions to be called,
|
||
:c:func:`snd_ctl_new1()` and :c:func:`snd_ctl_add()`.
|
||
|
||
In the simplest way, you can do like this:
|
||
|
||
::
|
||
|
||
err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
|
||
if (err < 0)
|
||
return err;
|
||
|
||
where ``my_control`` is the :c:type:`struct snd_kcontrol_new
|
||
<snd_kcontrol_new>` object defined above, and chip is the object
|
||
pointer to be passed to kcontrol->private_data which can be referred
|
||
to in callbacks.
|
||
|
||
:c:func:`snd_ctl_new1()` allocates a new :c:type:`struct
|
||
snd_kcontrol <snd_kcontrol>` instance, and
|
||
:c:func:`snd_ctl_add()` assigns the given control component to the
|
||
card.
|
||
|
||
Change Notification
|
||
-------------------
|
||
|
||
If you need to change and update a control in the interrupt routine, you
|
||
can call :c:func:`snd_ctl_notify()`. For example,
|
||
|
||
::
|
||
|
||
snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
|
||
|
||
This function takes the card pointer, the event-mask, and the control id
|
||
pointer for the notification. The event-mask specifies the types of
|
||
notification, for example, in the above example, the change of control
|
||
values is notified. The id pointer is the pointer of :c:type:`struct
|
||
snd_ctl_elem_id <snd_ctl_elem_id>` to be notified. You can
|
||
find some examples in ``es1938.c`` or ``es1968.c`` for hardware volume
|
||
interrupts.
|
||
|
||
Metadata
|
||
--------
|
||
|
||
To provide information about the dB values of a mixer control, use on of
|
||
the ``DECLARE_TLV_xxx`` macros from ``<sound/tlv.h>`` to define a
|
||
variable containing this information, set the ``tlv.p`` field to point to
|
||
this variable, and include the ``SNDRV_CTL_ELEM_ACCESS_TLV_READ`` flag
|
||
in the ``access`` field; like this:
|
||
|
||
::
|
||
|
||
static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
|
||
|
||
static struct snd_kcontrol_new my_control = {
|
||
...
|
||
.access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
|
||
SNDRV_CTL_ELEM_ACCESS_TLV_READ,
|
||
...
|
||
.tlv.p = db_scale_my_control,
|
||
};
|
||
|
||
|
||
The :c:func:`DECLARE_TLV_DB_SCALE()` macro defines information
|
||
about a mixer control where each step in the control's value changes the
|
||
dB value by a constant dB amount. The first parameter is the name of the
|
||
variable to be defined. The second parameter is the minimum value, in
|
||
units of 0.01 dB. The third parameter is the step size, in units of 0.01
|
||
dB. Set the fourth parameter to 1 if the minimum value actually mutes
|
||
the control.
|
||
|
||
The :c:func:`DECLARE_TLV_DB_LINEAR()` macro defines information
|
||
about a mixer control where the control's value affects the output
|
||
linearly. The first parameter is the name of the variable to be defined.
|
||
The second parameter is the minimum value, in units of 0.01 dB. The
|
||
third parameter is the maximum value, in units of 0.01 dB. If the
|
||
minimum value mutes the control, set the second parameter to
|
||
``TLV_DB_GAIN_MUTE``.
|
||
|
||
API for AC97 Codec
|
||
==================
|
||
|
||
General
|
||
-------
|
||
|
||
The ALSA AC97 codec layer is a well-defined one, and you don't have to
|
||
write much code to control it. Only low-level control routines are
|
||
necessary. The AC97 codec API is defined in ``<sound/ac97_codec.h>``.
|
||
|
||
Full Code Example
|
||
-----------------
|
||
|
||
::
|
||
|
||
struct mychip {
|
||
....
|
||
struct snd_ac97 *ac97;
|
||
....
|
||
};
|
||
|
||
static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
|
||
unsigned short reg)
|
||
{
|
||
struct mychip *chip = ac97->private_data;
|
||
....
|
||
/* read a register value here from the codec */
|
||
return the_register_value;
|
||
}
|
||
|
||
static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
|
||
unsigned short reg, unsigned short val)
|
||
{
|
||
struct mychip *chip = ac97->private_data;
|
||
....
|
||
/* write the given register value to the codec */
|
||
}
|
||
|
||
static int snd_mychip_ac97(struct mychip *chip)
|
||
{
|
||
struct snd_ac97_bus *bus;
|
||
struct snd_ac97_template ac97;
|
||
int err;
|
||
static struct snd_ac97_bus_ops ops = {
|
||
.write = snd_mychip_ac97_write,
|
||
.read = snd_mychip_ac97_read,
|
||
};
|
||
|
||
err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
|
||
if (err < 0)
|
||
return err;
|
||
memset(&ac97, 0, sizeof(ac97));
|
||
ac97.private_data = chip;
|
||
return snd_ac97_mixer(bus, &ac97, &chip->ac97);
|
||
}
|
||
|
||
|
||
AC97 Constructor
|
||
----------------
|
||
|
||
To create an ac97 instance, first call :c:func:`snd_ac97_bus()`
|
||
with an ``ac97_bus_ops_t`` record with callback functions.
|
||
|
||
::
|
||
|
||
struct snd_ac97_bus *bus;
|
||
static struct snd_ac97_bus_ops ops = {
|
||
.write = snd_mychip_ac97_write,
|
||
.read = snd_mychip_ac97_read,
|
||
};
|
||
|
||
snd_ac97_bus(card, 0, &ops, NULL, &pbus);
|
||
|
||
The bus record is shared among all belonging ac97 instances.
|
||
|
||
And then call :c:func:`snd_ac97_mixer()` with an :c:type:`struct
|
||
snd_ac97_template <snd_ac97_template>` record together with
|
||
the bus pointer created above.
|
||
|
||
::
|
||
|
||
struct snd_ac97_template ac97;
|
||
int err;
|
||
|
||
memset(&ac97, 0, sizeof(ac97));
|
||
ac97.private_data = chip;
|
||
snd_ac97_mixer(bus, &ac97, &chip->ac97);
|
||
|
||
where chip->ac97 is a pointer to a newly created ``ac97_t``
|
||
instance. In this case, the chip pointer is set as the private data,
|
||
so that the read/write callback functions can refer to this chip
|
||
instance. This instance is not necessarily stored in the chip
|
||
record. If you need to change the register values from the driver, or
|
||
need the suspend/resume of ac97 codecs, keep this pointer to pass to
|
||
the corresponding functions.
|
||
|
||
AC97 Callbacks
|
||
--------------
|
||
|
||
The standard callbacks are ``read`` and ``write``. Obviously they
|
||
correspond to the functions for read and write accesses to the
|
||
hardware low-level codes.
|
||
|
||
The ``read`` callback returns the register value specified in the
|
||
argument.
|
||
|
||
::
|
||
|
||
static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
|
||
unsigned short reg)
|
||
{
|
||
struct mychip *chip = ac97->private_data;
|
||
....
|
||
return the_register_value;
|
||
}
|
||
|
||
Here, the chip can be cast from ``ac97->private_data``.
|
||
|
||
Meanwhile, the ``write`` callback is used to set the register
|
||
value
|
||
|
||
::
|
||
|
||
static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
|
||
unsigned short reg, unsigned short val)
|
||
|
||
|
||
These callbacks are non-atomic like the control API callbacks.
|
||
|
||
There are also other callbacks: ``reset``, ``wait`` and ``init``.
|
||
|
||
The ``reset`` callback is used to reset the codec. If the chip
|
||
requires a special kind of reset, you can define this callback.
|
||
|
||
The ``wait`` callback is used to add some waiting time in the standard
|
||
initialization of the codec. If the chip requires the extra waiting
|
||
time, define this callback.
|
||
|
||
The ``init`` callback is used for additional initialization of the
|
||
codec.
|
||
|
||
Updating Registers in The Driver
|
||
--------------------------------
|
||
|
||
If you need to access to the codec from the driver, you can call the
|
||
following functions: :c:func:`snd_ac97_write()`,
|
||
:c:func:`snd_ac97_read()`, :c:func:`snd_ac97_update()` and
|
||
:c:func:`snd_ac97_update_bits()`.
|
||
|
||
Both :c:func:`snd_ac97_write()` and
|
||
:c:func:`snd_ac97_update()` functions are used to set a value to
|
||
the given register (``AC97_XXX``). The difference between them is that
|
||
:c:func:`snd_ac97_update()` doesn't write a value if the given
|
||
value has been already set, while :c:func:`snd_ac97_write()`
|
||
always rewrites the value.
|
||
|
||
::
|
||
|
||
snd_ac97_write(ac97, AC97_MASTER, 0x8080);
|
||
snd_ac97_update(ac97, AC97_MASTER, 0x8080);
|
||
|
||
:c:func:`snd_ac97_read()` is used to read the value of the given
|
||
register. For example,
|
||
|
||
::
|
||
|
||
value = snd_ac97_read(ac97, AC97_MASTER);
|
||
|
||
:c:func:`snd_ac97_update_bits()` is used to update some bits in
|
||
the given register.
|
||
|
||
::
|
||
|
||
snd_ac97_update_bits(ac97, reg, mask, value);
|
||
|
||
Also, there is a function to change the sample rate (of a given register
|
||
such as ``AC97_PCM_FRONT_DAC_RATE``) when VRA or DRA is supported by the
|
||
codec: :c:func:`snd_ac97_set_rate()`.
|
||
|
||
::
|
||
|
||
snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
|
||
|
||
|
||
The following registers are available to set the rate:
|
||
``AC97_PCM_MIC_ADC_RATE``, ``AC97_PCM_FRONT_DAC_RATE``,
|
||
``AC97_PCM_LR_ADC_RATE``, ``AC97_SPDIF``. When ``AC97_SPDIF`` is
|
||
specified, the register is not really changed but the corresponding
|
||
IEC958 status bits will be updated.
|
||
|
||
Clock Adjustment
|
||
----------------
|
||
|
||
In some chips, the clock of the codec isn't 48000 but using a PCI clock
|
||
(to save a quartz!). In this case, change the field ``bus->clock`` to
|
||
the corresponding value. For example, intel8x0 and es1968 drivers have
|
||
their own function to read from the clock.
|
||
|
||
Proc Files
|
||
----------
|
||
|
||
The ALSA AC97 interface will create a proc file such as
|
||
``/proc/asound/card0/codec97#0/ac97#0-0`` and ``ac97#0-0+regs``. You
|
||
can refer to these files to see the current status and registers of
|
||
the codec.
|
||
|
||
Multiple Codecs
|
||
---------------
|
||
|
||
When there are several codecs on the same card, you need to call
|
||
:c:func:`snd_ac97_mixer()` multiple times with ``ac97.num=1`` or
|
||
greater. The ``num`` field specifies the codec number.
|
||
|
||
If you set up multiple codecs, you either need to write different
|
||
callbacks for each codec or check ``ac97->num`` in the callback
|
||
routines.
|
||
|
||
MIDI (MPU401-UART) Interface
|
||
============================
|
||
|
||
General
|
||
-------
|
||
|
||
Many soundcards have built-in MIDI (MPU401-UART) interfaces. When the
|
||
soundcard supports the standard MPU401-UART interface, most likely you
|
||
can use the ALSA MPU401-UART API. The MPU401-UART API is defined in
|
||
``<sound/mpu401.h>``.
|
||
|
||
Some soundchips have a similar but slightly different implementation of
|
||
mpu401 stuff. For example, emu10k1 has its own mpu401 routines.
|
||
|
||
MIDI Constructor
|
||
----------------
|
||
|
||
To create a rawmidi object, call :c:func:`snd_mpu401_uart_new()`.
|
||
|
||
::
|
||
|
||
struct snd_rawmidi *rmidi;
|
||
snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
|
||
irq, &rmidi);
|
||
|
||
|
||
The first argument is the card pointer, and the second is the index of
|
||
this component. You can create up to 8 rawmidi devices.
|
||
|
||
The third argument is the type of the hardware, ``MPU401_HW_XXX``. If
|
||
it's not a special one, you can use ``MPU401_HW_MPU401``.
|
||
|
||
The 4th argument is the I/O port address. Many backward-compatible
|
||
MPU401 have an I/O port such as 0x330. Or, it might be a part of its own
|
||
PCI I/O region. It depends on the chip design.
|
||
|
||
The 5th argument is a bitflag for additional information. When the I/O
|
||
port address above is part of the PCI I/O region, the MPU401 I/O port
|
||
might have been already allocated (reserved) by the driver itself. In
|
||
such a case, pass a bit flag ``MPU401_INFO_INTEGRATED``, and the
|
||
mpu401-uart layer will allocate the I/O ports by itself.
|
||
|
||
When the controller supports only the input or output MIDI stream, pass
|
||
the ``MPU401_INFO_INPUT`` or ``MPU401_INFO_OUTPUT`` bitflag,
|
||
respectively. Then the rawmidi instance is created as a single stream.
|
||
|
||
``MPU401_INFO_MMIO`` bitflag is used to change the access method to MMIO
|
||
(via readb and writeb) instead of iob and outb. In this case, you have
|
||
to pass the iomapped address to :c:func:`snd_mpu401_uart_new()`.
|
||
|
||
When ``MPU401_INFO_TX_IRQ`` is set, the output stream isn't checked in
|
||
the default interrupt handler. The driver needs to call
|
||
:c:func:`snd_mpu401_uart_interrupt_tx()` by itself to start
|
||
processing the output stream in the irq handler.
|
||
|
||
If the MPU-401 interface shares its interrupt with the other logical
|
||
devices on the card, set ``MPU401_INFO_IRQ_HOOK`` (see
|
||
`below <#MIDI-Interrupt-Handler>`__).
|
||
|
||
Usually, the port address corresponds to the command port and port + 1
|
||
corresponds to the data port. If not, you may change the ``cport``
|
||
field of :c:type:`struct snd_mpu401 <snd_mpu401>` manually afterward.
|
||
However, :c:type:`struct snd_mpu401 <snd_mpu401>` pointer is
|
||
not returned explicitly by :c:func:`snd_mpu401_uart_new()`. You
|
||
need to cast ``rmidi->private_data`` to :c:type:`struct snd_mpu401
|
||
<snd_mpu401>` explicitly,
|
||
|
||
::
|
||
|
||
struct snd_mpu401 *mpu;
|
||
mpu = rmidi->private_data;
|
||
|
||
and reset the ``cport`` as you like:
|
||
|
||
::
|
||
|
||
mpu->cport = my_own_control_port;
|
||
|
||
The 6th argument specifies the ISA irq number that will be allocated. If
|
||
no interrupt is to be allocated (because your code is already allocating
|
||
a shared interrupt, or because the device does not use interrupts), pass
|
||
-1 instead. For a MPU-401 device without an interrupt, a polling timer
|
||
will be used instead.
|
||
|
||
MIDI Interrupt Handler
|
||
----------------------
|
||
|
||
When the interrupt is allocated in
|
||
:c:func:`snd_mpu401_uart_new()`, an exclusive ISA interrupt
|
||
handler is automatically used, hence you don't have anything else to do
|
||
than creating the mpu401 stuff. Otherwise, you have to set
|
||
``MPU401_INFO_IRQ_HOOK``, and call
|
||
:c:func:`snd_mpu401_uart_interrupt()` explicitly from your own
|
||
interrupt handler when it has determined that a UART interrupt has
|
||
occurred.
|
||
|
||
In this case, you need to pass the private_data of the returned rawmidi
|
||
object from :c:func:`snd_mpu401_uart_new()` as the second
|
||
argument of :c:func:`snd_mpu401_uart_interrupt()`.
|
||
|
||
::
|
||
|
||
snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
|
||
|
||
|
||
RawMIDI Interface
|
||
=================
|
||
|
||
Overview
|
||
--------
|
||
|
||
The raw MIDI interface is used for hardware MIDI ports that can be
|
||
accessed as a byte stream. It is not used for synthesizer chips that do
|
||
not directly understand MIDI.
|
||
|
||
ALSA handles file and buffer management. All you have to do is to write
|
||
some code to move data between the buffer and the hardware.
|
||
|
||
The rawmidi API is defined in ``<sound/rawmidi.h>``.
|
||
|
||
RawMIDI Constructor
|
||
-------------------
|
||
|
||
To create a rawmidi device, call the :c:func:`snd_rawmidi_new()`
|
||
function:
|
||
|
||
::
|
||
|
||
struct snd_rawmidi *rmidi;
|
||
err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
|
||
if (err < 0)
|
||
return err;
|
||
rmidi->private_data = chip;
|
||
strcpy(rmidi->name, "My MIDI");
|
||
rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
|
||
SNDRV_RAWMIDI_INFO_INPUT |
|
||
SNDRV_RAWMIDI_INFO_DUPLEX;
|
||
|
||
The first argument is the card pointer, the second argument is the ID
|
||
string.
|
||
|
||
The third argument is the index of this component. You can create up to
|
||
8 rawmidi devices.
|
||
|
||
The fourth and fifth arguments are the number of output and input
|
||
substreams, respectively, of this device (a substream is the equivalent
|
||
of a MIDI port).
|
||
|
||
Set the ``info_flags`` field to specify the capabilities of the
|
||
device. Set ``SNDRV_RAWMIDI_INFO_OUTPUT`` if there is at least one
|
||
output port, ``SNDRV_RAWMIDI_INFO_INPUT`` if there is at least one
|
||
input port, and ``SNDRV_RAWMIDI_INFO_DUPLEX`` if the device can handle
|
||
output and input at the same time.
|
||
|
||
After the rawmidi device is created, you need to set the operators
|
||
(callbacks) for each substream. There are helper functions to set the
|
||
operators for all the substreams of a device:
|
||
|
||
::
|
||
|
||
snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
|
||
snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
|
||
|
||
The operators are usually defined like this:
|
||
|
||
::
|
||
|
||
static struct snd_rawmidi_ops snd_mymidi_output_ops = {
|
||
.open = snd_mymidi_output_open,
|
||
.close = snd_mymidi_output_close,
|
||
.trigger = snd_mymidi_output_trigger,
|
||
};
|
||
|
||
These callbacks are explained in the `RawMIDI Callbacks`_ section.
|
||
|
||
If there are more than one substream, you should give a unique name to
|
||
each of them:
|
||
|
||
::
|
||
|
||
struct snd_rawmidi_substream *substream;
|
||
list_for_each_entry(substream,
|
||
&rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
|
||
list {
|
||
sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
|
||
}
|
||
/* same for SNDRV_RAWMIDI_STREAM_INPUT */
|
||
|
||
RawMIDI Callbacks
|
||
-----------------
|
||
|
||
In all the callbacks, the private data that you've set for the rawmidi
|
||
device can be accessed as ``substream->rmidi->private_data``.
|
||
|
||
If there is more than one port, your callbacks can determine the port
|
||
index from the struct snd_rawmidi_substream data passed to each
|
||
callback:
|
||
|
||
::
|
||
|
||
struct snd_rawmidi_substream *substream;
|
||
int index = substream->number;
|
||
|
||
RawMIDI open callback
|
||
~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_open(struct snd_rawmidi_substream *substream);
|
||
|
||
|
||
This is called when a substream is opened. You can initialize the
|
||
hardware here, but you shouldn't start transmitting/receiving data yet.
|
||
|
||
RawMIDI close callback
|
||
~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static int snd_xxx_close(struct snd_rawmidi_substream *substream);
|
||
|
||
Guess what.
|
||
|
||
The ``open`` and ``close`` callbacks of a rawmidi device are
|
||
serialized with a mutex, and can sleep.
|
||
|
||
Rawmidi trigger callback for output substreams
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
|
||
|
||
|
||
This is called with a nonzero ``up`` parameter when there is some data
|
||
in the substream buffer that must be transmitted.
|
||
|
||
To read data from the buffer, call
|
||
:c:func:`snd_rawmidi_transmit_peek()`. It will return the number
|
||
of bytes that have been read; this will be less than the number of bytes
|
||
requested when there are no more data in the buffer. After the data have
|
||
been transmitted successfully, call
|
||
:c:func:`snd_rawmidi_transmit_ack()` to remove the data from the
|
||
substream buffer:
|
||
|
||
::
|
||
|
||
unsigned char data;
|
||
while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
|
||
if (snd_mychip_try_to_transmit(data))
|
||
snd_rawmidi_transmit_ack(substream, 1);
|
||
else
|
||
break; /* hardware FIFO full */
|
||
}
|
||
|
||
If you know beforehand that the hardware will accept data, you can use
|
||
the :c:func:`snd_rawmidi_transmit()` function which reads some
|
||
data and removes them from the buffer at once:
|
||
|
||
::
|
||
|
||
while (snd_mychip_transmit_possible()) {
|
||
unsigned char data;
|
||
if (snd_rawmidi_transmit(substream, &data, 1) != 1)
|
||
break; /* no more data */
|
||
snd_mychip_transmit(data);
|
||
}
|
||
|
||
If you know beforehand how many bytes you can accept, you can use a
|
||
buffer size greater than one with the
|
||
:c:func:`snd_rawmidi_transmit\*()` functions.
|
||
|
||
The ``trigger`` callback must not sleep. If the hardware FIFO is full
|
||
before the substream buffer has been emptied, you have to continue
|
||
transmitting data later, either in an interrupt handler, or with a
|
||
timer if the hardware doesn't have a MIDI transmit interrupt.
|
||
|
||
The ``trigger`` callback is called with a zero ``up`` parameter when
|
||
the transmission of data should be aborted.
|
||
|
||
RawMIDI trigger callback for input substreams
|
||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
|
||
|
||
|
||
This is called with a nonzero ``up`` parameter to enable receiving data,
|
||
or with a zero ``up`` parameter do disable receiving data.
|
||
|
||
The ``trigger`` callback must not sleep; the actual reading of data
|
||
from the device is usually done in an interrupt handler.
|
||
|
||
When data reception is enabled, your interrupt handler should call
|
||
:c:func:`snd_rawmidi_receive()` for all received data:
|
||
|
||
::
|
||
|
||
void snd_mychip_midi_interrupt(...)
|
||
{
|
||
while (mychip_midi_available()) {
|
||
unsigned char data;
|
||
data = mychip_midi_read();
|
||
snd_rawmidi_receive(substream, &data, 1);
|
||
}
|
||
}
|
||
|
||
|
||
drain callback
|
||
~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
|
||
|
||
|
||
This is only used with output substreams. This function should wait
|
||
until all data read from the substream buffer have been transmitted.
|
||
This ensures that the device can be closed and the driver unloaded
|
||
without losing data.
|
||
|
||
This callback is optional. If you do not set ``drain`` in the struct
|
||
snd_rawmidi_ops structure, ALSA will simply wait for 50 milliseconds
|
||
instead.
|
||
|
||
Miscellaneous Devices
|
||
=====================
|
||
|
||
FM OPL3
|
||
-------
|
||
|
||
The FM OPL3 is still used in many chips (mainly for backward
|
||
compatibility). ALSA has a nice OPL3 FM control layer, too. The OPL3 API
|
||
is defined in ``<sound/opl3.h>``.
|
||
|
||
FM registers can be directly accessed through the direct-FM API, defined
|
||
in ``<sound/asound_fm.h>``. In ALSA native mode, FM registers are
|
||
accessed through the Hardware-Dependent Device direct-FM extension API,
|
||
whereas in OSS compatible mode, FM registers can be accessed with the
|
||
OSS direct-FM compatible API in ``/dev/dmfmX`` device.
|
||
|
||
To create the OPL3 component, you have two functions to call. The first
|
||
one is a constructor for the ``opl3_t`` instance.
|
||
|
||
::
|
||
|
||
struct snd_opl3 *opl3;
|
||
snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
|
||
integrated, &opl3);
|
||
|
||
The first argument is the card pointer, the second one is the left port
|
||
address, and the third is the right port address. In most cases, the
|
||
right port is placed at the left port + 2.
|
||
|
||
The fourth argument is the hardware type.
|
||
|
||
When the left and right ports have been already allocated by the card
|
||
driver, pass non-zero to the fifth argument (``integrated``). Otherwise,
|
||
the opl3 module will allocate the specified ports by itself.
|
||
|
||
When the accessing the hardware requires special method instead of the
|
||
standard I/O access, you can create opl3 instance separately with
|
||
:c:func:`snd_opl3_new()`.
|
||
|
||
::
|
||
|
||
struct snd_opl3 *opl3;
|
||
snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
|
||
|
||
Then set ``command``, ``private_data`` and ``private_free`` for the
|
||
private access function, the private data and the destructor. The
|
||
``l_port`` and ``r_port`` are not necessarily set. Only the command
|
||
must be set properly. You can retrieve the data from the
|
||
``opl3->private_data`` field.
|
||
|
||
After creating the opl3 instance via :c:func:`snd_opl3_new()`,
|
||
call :c:func:`snd_opl3_init()` to initialize the chip to the
|
||
proper state. Note that :c:func:`snd_opl3_create()` always calls
|
||
it internally.
|
||
|
||
If the opl3 instance is created successfully, then create a hwdep device
|
||
for this opl3.
|
||
|
||
::
|
||
|
||
struct snd_hwdep *opl3hwdep;
|
||
snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
|
||
|
||
The first argument is the ``opl3_t`` instance you created, and the
|
||
second is the index number, usually 0.
|
||
|
||
The third argument is the index-offset for the sequencer client assigned
|
||
to the OPL3 port. When there is an MPU401-UART, give 1 for here (UART
|
||
always takes 0).
|
||
|
||
Hardware-Dependent Devices
|
||
--------------------------
|
||
|
||
Some chips need user-space access for special controls or for loading
|
||
the micro code. In such a case, you can create a hwdep
|
||
(hardware-dependent) device. The hwdep API is defined in
|
||
``<sound/hwdep.h>``. You can find examples in opl3 driver or
|
||
``isa/sb/sb16_csp.c``.
|
||
|
||
The creation of the ``hwdep`` instance is done via
|
||
:c:func:`snd_hwdep_new()`.
|
||
|
||
::
|
||
|
||
struct snd_hwdep *hw;
|
||
snd_hwdep_new(card, "My HWDEP", 0, &hw);
|
||
|
||
where the third argument is the index number.
|
||
|
||
You can then pass any pointer value to the ``private_data``. If you
|
||
assign a private data, you should define the destructor, too. The
|
||
destructor function is set in the ``private_free`` field.
|
||
|
||
::
|
||
|
||
struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
|
||
hw->private_data = p;
|
||
hw->private_free = mydata_free;
|
||
|
||
and the implementation of the destructor would be:
|
||
|
||
::
|
||
|
||
static void mydata_free(struct snd_hwdep *hw)
|
||
{
|
||
struct mydata *p = hw->private_data;
|
||
kfree(p);
|
||
}
|
||
|
||
The arbitrary file operations can be defined for this instance. The file
|
||
operators are defined in the ``ops`` table. For example, assume that
|
||
this chip needs an ioctl.
|
||
|
||
::
|
||
|
||
hw->ops.open = mydata_open;
|
||
hw->ops.ioctl = mydata_ioctl;
|
||
hw->ops.release = mydata_release;
|
||
|
||
And implement the callback functions as you like.
|
||
|
||
IEC958 (S/PDIF)
|
||
---------------
|
||
|
||
Usually the controls for IEC958 devices are implemented via the control
|
||
interface. There is a macro to compose a name string for IEC958
|
||
controls, :c:func:`SNDRV_CTL_NAME_IEC958()` defined in
|
||
``<include/asound.h>``.
|
||
|
||
There are some standard controls for IEC958 status bits. These controls
|
||
use the type ``SNDRV_CTL_ELEM_TYPE_IEC958``, and the size of element is
|
||
fixed as 4 bytes array (value.iec958.status[x]). For the ``info``
|
||
callback, you don't specify the value field for this type (the count
|
||
field must be set, though).
|
||
|
||
“IEC958 Playback Con Mask” is used to return the bit-mask for the IEC958
|
||
status bits of consumer mode. Similarly, “IEC958 Playback Pro Mask”
|
||
returns the bitmask for professional mode. They are read-only controls,
|
||
and are defined as MIXER controls (iface =
|
||
``SNDRV_CTL_ELEM_IFACE_MIXER``).
|
||
|
||
Meanwhile, “IEC958 Playback Default” control is defined for getting and
|
||
setting the current default IEC958 bits. Note that this one is usually
|
||
defined as a PCM control (iface = ``SNDRV_CTL_ELEM_IFACE_PCM``),
|
||
although in some places it's defined as a MIXER control.
|
||
|
||
In addition, you can define the control switches to enable/disable or to
|
||
set the raw bit mode. The implementation will depend on the chip, but
|
||
the control should be named as “IEC958 xxx”, preferably using the
|
||
:c:func:`SNDRV_CTL_NAME_IEC958()` macro.
|
||
|
||
You can find several cases, for example, ``pci/emu10k1``,
|
||
``pci/ice1712``, or ``pci/cmipci.c``.
|
||
|
||
Buffer and Memory Management
|
||
============================
|
||
|
||
Buffer Types
|
||
------------
|
||
|
||
ALSA provides several different buffer allocation functions depending on
|
||
the bus and the architecture. All these have a consistent API. The
|
||
allocation of physically-contiguous pages is done via
|
||
:c:func:`snd_malloc_xxx_pages()` function, where xxx is the bus
|
||
type.
|
||
|
||
The allocation of pages with fallback is
|
||
:c:func:`snd_malloc_xxx_pages_fallback()`. This function tries
|
||
to allocate the specified pages but if the pages are not available, it
|
||
tries to reduce the page sizes until enough space is found.
|
||
|
||
The release the pages, call :c:func:`snd_free_xxx_pages()`
|
||
function.
|
||
|
||
Usually, ALSA drivers try to allocate and reserve a large contiguous
|
||
physical space at the time the module is loaded for the later use. This
|
||
is called “pre-allocation”. As already written, you can call the
|
||
following function at pcm instance construction time (in the case of PCI
|
||
bus).
|
||
|
||
::
|
||
|
||
snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
|
||
&pci->dev, size, max);
|
||
|
||
where ``size`` is the byte size to be pre-allocated and the ``max`` is
|
||
the maximum size to be changed via the ``prealloc`` proc file. The
|
||
allocator will try to get an area as large as possible within the
|
||
given size.
|
||
|
||
The second argument (type) and the third argument (device pointer) are
|
||
dependent on the bus. For normal devices, pass the device pointer
|
||
(typically identical as ``card->dev``) to the third argument with
|
||
``SNDRV_DMA_TYPE_DEV`` type. For the continuous buffer unrelated to the
|
||
bus can be pre-allocated with ``SNDRV_DMA_TYPE_CONTINUOUS`` type.
|
||
You can pass NULL to the device pointer in that case, which is the
|
||
default mode implying to allocate with ``GFP_KRENEL`` flag.
|
||
If you need a different GFP flag, you can pass it by encoding the flag
|
||
into the device pointer via a special macro
|
||
:c:func:`snd_dma_continuous_data()`.
|
||
For the scatter-gather buffers, use ``SNDRV_DMA_TYPE_DEV_SG`` with the
|
||
device pointer (see the `Non-Contiguous Buffers`_ section).
|
||
|
||
Once the buffer is pre-allocated, you can use the allocator in the
|
||
``hw_params`` callback:
|
||
|
||
::
|
||
|
||
snd_pcm_lib_malloc_pages(substream, size);
|
||
|
||
Note that you have to pre-allocate to use this function.
|
||
|
||
Most of drivers use, though, rather the newly introduced "managed
|
||
buffer allocation mode" instead of the manual allocation or release.
|
||
This is done by calling :c:func:`snd_pcm_set_managed_buffer_all()`
|
||
instead of :c:func:`snd_pcm_lib_preallocate_pages_for_all()`.
|
||
|
||
::
|
||
|
||
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
|
||
&pci->dev, size, max);
|
||
|
||
where passed arguments are identical in both functions.
|
||
The difference in the managed mode is that PCM core will call
|
||
:c:func:`snd_pcm_lib_malloc_pages()` internally already before calling
|
||
the PCM ``hw_params`` callback, and call :c:func:`snd_pcm_lib_free_pages()`
|
||
after the PCM ``hw_free`` callback automatically. So the driver
|
||
doesn't have to call these functions explicitly in its callback any
|
||
longer. This made many driver code having NULL ``hw_params`` and
|
||
``hw_free`` entries.
|
||
|
||
External Hardware Buffers
|
||
-------------------------
|
||
|
||
Some chips have their own hardware buffers and the DMA transfer from the
|
||
host memory is not available. In such a case, you need to either 1)
|
||
copy/set the audio data directly to the external hardware buffer, or 2)
|
||
make an intermediate buffer and copy/set the data from it to the
|
||
external hardware buffer in interrupts (or in tasklets, preferably).
|
||
|
||
The first case works fine if the external hardware buffer is large
|
||
enough. This method doesn't need any extra buffers and thus is more
|
||
effective. You need to define the ``copy_user`` and ``copy_kernel``
|
||
callbacks for the data transfer, in addition to ``fill_silence``
|
||
callback for playback. However, there is a drawback: it cannot be
|
||
mmapped. The examples are GUS's GF1 PCM or emu8000's wavetable PCM.
|
||
|
||
The second case allows for mmap on the buffer, although you have to
|
||
handle an interrupt or a tasklet to transfer the data from the
|
||
intermediate buffer to the hardware buffer. You can find an example in
|
||
the vxpocket driver.
|
||
|
||
Another case is when the chip uses a PCI memory-map region for the
|
||
buffer instead of the host memory. In this case, mmap is available only
|
||
on certain architectures like the Intel one. In non-mmap mode, the data
|
||
cannot be transferred as in the normal way. Thus you need to define the
|
||
``copy_user``, ``copy_kernel`` and ``fill_silence`` callbacks as well,
|
||
as in the cases above. The examples are found in ``rme32.c`` and
|
||
``rme96.c``.
|
||
|
||
The implementation of the ``copy_user``, ``copy_kernel`` and
|
||
``silence`` callbacks depends upon whether the hardware supports
|
||
interleaved or non-interleaved samples. The ``copy_user`` callback is
|
||
defined like below, a bit differently depending whether the direction
|
||
is playback or capture:
|
||
|
||
::
|
||
|
||
static int playback_copy_user(struct snd_pcm_substream *substream,
|
||
int channel, unsigned long pos,
|
||
void __user *src, unsigned long count);
|
||
static int capture_copy_user(struct snd_pcm_substream *substream,
|
||
int channel, unsigned long pos,
|
||
void __user *dst, unsigned long count);
|
||
|
||
In the case of interleaved samples, the second argument (``channel``) is
|
||
not used. The third argument (``pos``) points the current position
|
||
offset in bytes.
|
||
|
||
The meaning of the fourth argument is different between playback and
|
||
capture. For playback, it holds the source data pointer, and for
|
||
capture, it's the destination data pointer.
|
||
|
||
The last argument is the number of bytes to be copied.
|
||
|
||
What you have to do in this callback is again different between playback
|
||
and capture directions. In the playback case, you copy the given amount
|
||
of data (``count``) at the specified pointer (``src``) to the specified
|
||
offset (``pos``) on the hardware buffer. When coded like memcpy-like
|
||
way, the copy would be like:
|
||
|
||
::
|
||
|
||
my_memcpy_from_user(my_buffer + pos, src, count);
|
||
|
||
For the capture direction, you copy the given amount of data (``count``)
|
||
at the specified offset (``pos``) on the hardware buffer to the
|
||
specified pointer (``dst``).
|
||
|
||
::
|
||
|
||
my_memcpy_to_user(dst, my_buffer + pos, count);
|
||
|
||
Here the functions are named as ``from_user`` and ``to_user`` because
|
||
it's the user-space buffer that is passed to these callbacks. That
|
||
is, the callback is supposed to copy from/to the user-space data
|
||
directly to/from the hardware buffer.
|
||
|
||
Careful readers might notice that these callbacks receive the
|
||
arguments in bytes, not in frames like other callbacks. It's because
|
||
it would make coding easier like the examples above, and also it makes
|
||
easier to unify both the interleaved and non-interleaved cases, as
|
||
explained in the following.
|
||
|
||
In the case of non-interleaved samples, the implementation will be a bit
|
||
more complicated. The callback is called for each channel, passed by
|
||
the second argument, so totally it's called for N-channels times per
|
||
transfer.
|
||
|
||
The meaning of other arguments are almost same as the interleaved
|
||
case. The callback is supposed to copy the data from/to the given
|
||
user-space buffer, but only for the given channel. For the detailed
|
||
implementations, please check ``isa/gus/gus_pcm.c`` or
|
||
"pci/rme9652/rme9652.c" as examples.
|
||
|
||
The above callbacks are the copy from/to the user-space buffer. There
|
||
are some cases where we want copy from/to the kernel-space buffer
|
||
instead. In such a case, ``copy_kernel`` callback is called. It'd
|
||
look like:
|
||
|
||
::
|
||
|
||
static int playback_copy_kernel(struct snd_pcm_substream *substream,
|
||
int channel, unsigned long pos,
|
||
void *src, unsigned long count);
|
||
static int capture_copy_kernel(struct snd_pcm_substream *substream,
|
||
int channel, unsigned long pos,
|
||
void *dst, unsigned long count);
|
||
|
||
As found easily, the only difference is that the buffer pointer is
|
||
without ``__user`` prefix; that is, a kernel-buffer pointer is passed
|
||
in the fourth argument. Correspondingly, the implementation would be
|
||
a version without the user-copy, such as:
|
||
|
||
::
|
||
|
||
my_memcpy(my_buffer + pos, src, count);
|
||
|
||
Usually for the playback, another callback ``fill_silence`` is
|
||
defined. It's implemented in a similar way as the copy callbacks
|
||
above:
|
||
|
||
::
|
||
|
||
static int silence(struct snd_pcm_substream *substream, int channel,
|
||
unsigned long pos, unsigned long count);
|
||
|
||
The meanings of arguments are the same as in the ``copy_user`` and
|
||
``copy_kernel`` callbacks, although there is no buffer pointer
|
||
argument. In the case of interleaved samples, the channel argument has
|
||
no meaning, as well as on ``copy_*`` callbacks.
|
||
|
||
The role of ``fill_silence`` callback is to set the given amount
|
||
(``count``) of silence data at the specified offset (``pos``) on the
|
||
hardware buffer. Suppose that the data format is signed (that is, the
|
||
silent-data is 0), and the implementation using a memset-like function
|
||
would be like:
|
||
|
||
::
|
||
|
||
my_memset(my_buffer + pos, 0, count);
|
||
|
||
In the case of non-interleaved samples, again, the implementation
|
||
becomes a bit more complicated, as it's called N-times per transfer
|
||
for each channel. See, for example, ``isa/gus/gus_pcm.c``.
|
||
|
||
Non-Contiguous Buffers
|
||
----------------------
|
||
|
||
If your hardware supports the page table as in emu10k1 or the buffer
|
||
descriptors as in via82xx, you can use the scatter-gather (SG) DMA. ALSA
|
||
provides an interface for handling SG-buffers. The API is provided in
|
||
``<sound/pcm.h>``.
|
||
|
||
For creating the SG-buffer handler, call
|
||
:c:func:`snd_pcm_set_managed_buffer()` or
|
||
:c:func:`snd_pcm_set_managed_buffer_all()` with
|
||
``SNDRV_DMA_TYPE_DEV_SG`` in the PCM constructor like other PCI
|
||
pre-allocator. You need to pass ``&pci->dev``, where pci is
|
||
the :c:type:`struct pci_dev <pci_dev>` pointer of the chip as
|
||
well.
|
||
|
||
::
|
||
|
||
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV_SG,
|
||
&pci->dev, size, max);
|
||
|
||
The ``struct snd_sg_buf`` instance is created as
|
||
``substream->dma_private`` in turn. You can cast the pointer like:
|
||
|
||
::
|
||
|
||
struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
|
||
|
||
Then in :c:func:`snd_pcm_lib_malloc_pages()` call, the common SG-buffer
|
||
handler will allocate the non-contiguous kernel pages of the given size
|
||
and map them onto the virtually contiguous memory. The virtual pointer
|
||
is addressed in runtime->dma_area. The physical address
|
||
(``runtime->dma_addr``) is set to zero, because the buffer is
|
||
physically non-contiguous. The physical address table is set up in
|
||
``sgbuf->table``. You can get the physical address at a certain offset
|
||
via :c:func:`snd_pcm_sgbuf_get_addr()`.
|
||
|
||
If you need to release the SG-buffer data explicitly, call the
|
||
standard API function :c:func:`snd_pcm_lib_free_pages()` as usual.
|
||
|
||
Vmalloc'ed Buffers
|
||
------------------
|
||
|
||
It's possible to use a buffer allocated via :c:func:`vmalloc()`, for
|
||
example, for an intermediate buffer. In the recent version of kernel,
|
||
you can simply allocate it via standard
|
||
:c:func:`snd_pcm_lib_malloc_pages()` and co after setting up the
|
||
buffer preallocation with ``SNDRV_DMA_TYPE_VMALLOC`` type.
|
||
|
||
::
|
||
|
||
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_VMALLOC,
|
||
NULL, 0, 0);
|
||
|
||
The NULL is passed to the device pointer argument, which indicates
|
||
that the default pages (GFP_KERNEL and GFP_HIGHMEM) will be
|
||
allocated.
|
||
|
||
Also, note that zero is passed to both the size and the max size
|
||
arguments here. Since each vmalloc call should succeed at any time,
|
||
we don't need to pre-allocate the buffers like other continuous
|
||
pages.
|
||
|
||
If you need the 32bit DMA allocation, pass the device pointer encoded
|
||
by :c:func:`snd_dma_continuous_data()` with ``GFP_KERNEL|__GFP_DMA32``
|
||
argument.
|
||
|
||
::
|
||
|
||
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_VMALLOC,
|
||
snd_dma_continuous_data(GFP_KERNEL | __GFP_DMA32), 0, 0);
|
||
|
||
Proc Interface
|
||
==============
|
||
|
||
ALSA provides an easy interface for procfs. The proc files are very
|
||
useful for debugging. I recommend you set up proc files if you write a
|
||
driver and want to get a running status or register dumps. The API is
|
||
found in ``<sound/info.h>``.
|
||
|
||
To create a proc file, call :c:func:`snd_card_proc_new()`.
|
||
|
||
::
|
||
|
||
struct snd_info_entry *entry;
|
||
int err = snd_card_proc_new(card, "my-file", &entry);
|
||
|
||
where the second argument specifies the name of the proc file to be
|
||
created. The above example will create a file ``my-file`` under the
|
||
card directory, e.g. ``/proc/asound/card0/my-file``.
|
||
|
||
Like other components, the proc entry created via
|
||
:c:func:`snd_card_proc_new()` will be registered and released
|
||
automatically in the card registration and release functions.
|
||
|
||
When the creation is successful, the function stores a new instance in
|
||
the pointer given in the third argument. It is initialized as a text
|
||
proc file for read only. To use this proc file as a read-only text file
|
||
as it is, set the read callback with a private data via
|
||
:c:func:`snd_info_set_text_ops()`.
|
||
|
||
::
|
||
|
||
snd_info_set_text_ops(entry, chip, my_proc_read);
|
||
|
||
where the second argument (``chip``) is the private data to be used in
|
||
the callbacks. The third parameter specifies the read buffer size and
|
||
the fourth (``my_proc_read``) is the callback function, which is
|
||
defined like
|
||
|
||
::
|
||
|
||
static void my_proc_read(struct snd_info_entry *entry,
|
||
struct snd_info_buffer *buffer);
|
||
|
||
In the read callback, use :c:func:`snd_iprintf()` for output
|
||
strings, which works just like normal :c:func:`printf()`. For
|
||
example,
|
||
|
||
::
|
||
|
||
static void my_proc_read(struct snd_info_entry *entry,
|
||
struct snd_info_buffer *buffer)
|
||
{
|
||
struct my_chip *chip = entry->private_data;
|
||
|
||
snd_iprintf(buffer, "This is my chip!\n");
|
||
snd_iprintf(buffer, "Port = %ld\n", chip->port);
|
||
}
|
||
|
||
The file permissions can be changed afterwards. As default, it's set as
|
||
read only for all users. If you want to add write permission for the
|
||
user (root as default), do as follows:
|
||
|
||
::
|
||
|
||
entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
|
||
|
||
and set the write buffer size and the callback
|
||
|
||
::
|
||
|
||
entry->c.text.write = my_proc_write;
|
||
|
||
For the write callback, you can use :c:func:`snd_info_get_line()`
|
||
to get a text line, and :c:func:`snd_info_get_str()` to retrieve
|
||
a string from the line. Some examples are found in
|
||
``core/oss/mixer_oss.c``, core/oss/and ``pcm_oss.c``.
|
||
|
||
For a raw-data proc-file, set the attributes as follows:
|
||
|
||
::
|
||
|
||
static const struct snd_info_entry_ops my_file_io_ops = {
|
||
.read = my_file_io_read,
|
||
};
|
||
|
||
entry->content = SNDRV_INFO_CONTENT_DATA;
|
||
entry->private_data = chip;
|
||
entry->c.ops = &my_file_io_ops;
|
||
entry->size = 4096;
|
||
entry->mode = S_IFREG | S_IRUGO;
|
||
|
||
For the raw data, ``size`` field must be set properly. This specifies
|
||
the maximum size of the proc file access.
|
||
|
||
The read/write callbacks of raw mode are more direct than the text mode.
|
||
You need to use a low-level I/O functions such as
|
||
:c:func:`copy_from/to_user()` to transfer the data.
|
||
|
||
::
|
||
|
||
static ssize_t my_file_io_read(struct snd_info_entry *entry,
|
||
void *file_private_data,
|
||
struct file *file,
|
||
char *buf,
|
||
size_t count,
|
||
loff_t pos)
|
||
{
|
||
if (copy_to_user(buf, local_data + pos, count))
|
||
return -EFAULT;
|
||
return count;
|
||
}
|
||
|
||
If the size of the info entry has been set up properly, ``count`` and
|
||
``pos`` are guaranteed to fit within 0 and the given size. You don't
|
||
have to check the range in the callbacks unless any other condition is
|
||
required.
|
||
|
||
Power Management
|
||
================
|
||
|
||
If the chip is supposed to work with suspend/resume functions, you need
|
||
to add power-management code to the driver. The additional code for
|
||
power-management should be ifdef-ed with ``CONFIG_PM``, or annotated
|
||
with __maybe_unused attribute; otherwise the compiler will complain
|
||
you.
|
||
|
||
If the driver *fully* supports suspend/resume that is, the device can be
|
||
properly resumed to its state when suspend was called, you can set the
|
||
``SNDRV_PCM_INFO_RESUME`` flag in the pcm info field. Usually, this is
|
||
possible when the registers of the chip can be safely saved and restored
|
||
to RAM. If this is set, the trigger callback is called with
|
||
``SNDRV_PCM_TRIGGER_RESUME`` after the resume callback completes.
|
||
|
||
Even if the driver doesn't support PM fully but partial suspend/resume
|
||
is still possible, it's still worthy to implement suspend/resume
|
||
callbacks. In such a case, applications would reset the status by
|
||
calling :c:func:`snd_pcm_prepare()` and restart the stream
|
||
appropriately. Hence, you can define suspend/resume callbacks below but
|
||
don't set ``SNDRV_PCM_INFO_RESUME`` info flag to the PCM.
|
||
|
||
Note that the trigger with SUSPEND can always be called when
|
||
:c:func:`snd_pcm_suspend_all()` is called, regardless of the
|
||
``SNDRV_PCM_INFO_RESUME`` flag. The ``RESUME`` flag affects only the
|
||
behavior of :c:func:`snd_pcm_resume()`. (Thus, in theory,
|
||
``SNDRV_PCM_TRIGGER_RESUME`` isn't needed to be handled in the trigger
|
||
callback when no ``SNDRV_PCM_INFO_RESUME`` flag is set. But, it's better
|
||
to keep it for compatibility reasons.)
|
||
|
||
In the earlier version of ALSA drivers, a common power-management layer
|
||
was provided, but it has been removed. The driver needs to define the
|
||
suspend/resume hooks according to the bus the device is connected to. In
|
||
the case of PCI drivers, the callbacks look like below:
|
||
|
||
::
|
||
|
||
static int __maybe_unused snd_my_suspend(struct device *dev)
|
||
{
|
||
.... /* do things for suspend */
|
||
return 0;
|
||
}
|
||
static int __maybe_unused snd_my_resume(struct device *dev)
|
||
{
|
||
.... /* do things for suspend */
|
||
return 0;
|
||
}
|
||
|
||
The scheme of the real suspend job is as follows.
|
||
|
||
1. Retrieve the card and the chip data.
|
||
|
||
2. Call :c:func:`snd_power_change_state()` with
|
||
``SNDRV_CTL_POWER_D3hot`` to change the power status.
|
||
|
||
3. If AC97 codecs are used, call :c:func:`snd_ac97_suspend()` for
|
||
each codec.
|
||
|
||
4. Save the register values if necessary.
|
||
|
||
5. Stop the hardware if necessary.
|
||
|
||
A typical code would be like:
|
||
|
||
::
|
||
|
||
static int __maybe_unused mychip_suspend(struct device *dev)
|
||
{
|
||
/* (1) */
|
||
struct snd_card *card = dev_get_drvdata(dev);
|
||
struct mychip *chip = card->private_data;
|
||
/* (2) */
|
||
snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
|
||
/* (3) */
|
||
snd_ac97_suspend(chip->ac97);
|
||
/* (4) */
|
||
snd_mychip_save_registers(chip);
|
||
/* (5) */
|
||
snd_mychip_stop_hardware(chip);
|
||
return 0;
|
||
}
|
||
|
||
|
||
The scheme of the real resume job is as follows.
|
||
|
||
1. Retrieve the card and the chip data.
|
||
|
||
2. Re-initialize the chip.
|
||
|
||
3. Restore the saved registers if necessary.
|
||
|
||
4. Resume the mixer, e.g. calling :c:func:`snd_ac97_resume()`.
|
||
|
||
5. Restart the hardware (if any).
|
||
|
||
6. Call :c:func:`snd_power_change_state()` with
|
||
``SNDRV_CTL_POWER_D0`` to notify the processes.
|
||
|
||
A typical code would be like:
|
||
|
||
::
|
||
|
||
static int __maybe_unused mychip_resume(struct pci_dev *pci)
|
||
{
|
||
/* (1) */
|
||
struct snd_card *card = dev_get_drvdata(dev);
|
||
struct mychip *chip = card->private_data;
|
||
/* (2) */
|
||
snd_mychip_reinit_chip(chip);
|
||
/* (3) */
|
||
snd_mychip_restore_registers(chip);
|
||
/* (4) */
|
||
snd_ac97_resume(chip->ac97);
|
||
/* (5) */
|
||
snd_mychip_restart_chip(chip);
|
||
/* (6) */
|
||
snd_power_change_state(card, SNDRV_CTL_POWER_D0);
|
||
return 0;
|
||
}
|
||
|
||
Note that, at the time this callback gets called, the PCM stream has
|
||
been already suspended via its own PM ops calling
|
||
:c:func:`snd_pcm_suspend_all()` internally.
|
||
|
||
OK, we have all callbacks now. Let's set them up. In the initialization
|
||
of the card, make sure that you can get the chip data from the card
|
||
instance, typically via ``private_data`` field, in case you created the
|
||
chip data individually.
|
||
|
||
::
|
||
|
||
static int snd_mychip_probe(struct pci_dev *pci,
|
||
const struct pci_device_id *pci_id)
|
||
{
|
||
....
|
||
struct snd_card *card;
|
||
struct mychip *chip;
|
||
int err;
|
||
....
|
||
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
|
||
0, &card);
|
||
....
|
||
chip = kzalloc(sizeof(*chip), GFP_KERNEL);
|
||
....
|
||
card->private_data = chip;
|
||
....
|
||
}
|
||
|
||
When you created the chip data with :c:func:`snd_card_new()`, it's
|
||
anyway accessible via ``private_data`` field.
|
||
|
||
::
|
||
|
||
static int snd_mychip_probe(struct pci_dev *pci,
|
||
const struct pci_device_id *pci_id)
|
||
{
|
||
....
|
||
struct snd_card *card;
|
||
struct mychip *chip;
|
||
int err;
|
||
....
|
||
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
|
||
sizeof(struct mychip), &card);
|
||
....
|
||
chip = card->private_data;
|
||
....
|
||
}
|
||
|
||
If you need a space to save the registers, allocate the buffer for it
|
||
here, too, since it would be fatal if you cannot allocate a memory in
|
||
the suspend phase. The allocated buffer should be released in the
|
||
corresponding destructor.
|
||
|
||
And next, set suspend/resume callbacks to the pci_driver.
|
||
|
||
::
|
||
|
||
static SIMPLE_DEV_PM_OPS(snd_my_pm_ops, mychip_suspend, mychip_resume);
|
||
|
||
static struct pci_driver driver = {
|
||
.name = KBUILD_MODNAME,
|
||
.id_table = snd_my_ids,
|
||
.probe = snd_my_probe,
|
||
.remove = snd_my_remove,
|
||
.driver.pm = &snd_my_pm_ops,
|
||
};
|
||
|
||
Module Parameters
|
||
=================
|
||
|
||
There are standard module options for ALSA. At least, each module should
|
||
have the ``index``, ``id`` and ``enable`` options.
|
||
|
||
If the module supports multiple cards (usually up to 8 = ``SNDRV_CARDS``
|
||
cards), they should be arrays. The default initial values are defined
|
||
already as constants for easier programming:
|
||
|
||
::
|
||
|
||
static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
|
||
static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
|
||
static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
|
||
|
||
If the module supports only a single card, they could be single
|
||
variables, instead. ``enable`` option is not always necessary in this
|
||
case, but it would be better to have a dummy option for compatibility.
|
||
|
||
The module parameters must be declared with the standard
|
||
``module_param()``, ``module_param_array()`` and
|
||
:c:func:`MODULE_PARM_DESC()` macros.
|
||
|
||
The typical coding would be like below:
|
||
|
||
::
|
||
|
||
#define CARD_NAME "My Chip"
|
||
|
||
module_param_array(index, int, NULL, 0444);
|
||
MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
|
||
module_param_array(id, charp, NULL, 0444);
|
||
MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
|
||
module_param_array(enable, bool, NULL, 0444);
|
||
MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
|
||
|
||
Also, don't forget to define the module description and the license.
|
||
Especially, the recent modprobe requires to define the
|
||
module license as GPL, etc., otherwise the system is shown as “tainted”.
|
||
|
||
::
|
||
|
||
MODULE_DESCRIPTION("Sound driver for My Chip");
|
||
MODULE_LICENSE("GPL");
|
||
|
||
|
||
How To Put Your Driver Into ALSA Tree
|
||
=====================================
|
||
|
||
General
|
||
-------
|
||
|
||
So far, you've learned how to write the driver codes. And you might have
|
||
a question now: how to put my own driver into the ALSA driver tree? Here
|
||
(finally :) the standard procedure is described briefly.
|
||
|
||
Suppose that you create a new PCI driver for the card “xyz”. The card
|
||
module name would be snd-xyz. The new driver is usually put into the
|
||
alsa-driver tree, ``sound/pci`` directory in the case of PCI
|
||
cards.
|
||
|
||
In the following sections, the driver code is supposed to be put into
|
||
Linux kernel tree. The two cases are covered: a driver consisting of a
|
||
single source file and one consisting of several source files.
|
||
|
||
Driver with A Single Source File
|
||
--------------------------------
|
||
|
||
1. Modify sound/pci/Makefile
|
||
|
||
Suppose you have a file xyz.c. Add the following two lines
|
||
|
||
::
|
||
|
||
snd-xyz-objs := xyz.o
|
||
obj-$(CONFIG_SND_XYZ) += snd-xyz.o
|
||
|
||
2. Create the Kconfig entry
|
||
|
||
Add the new entry of Kconfig for your xyz driver. config SND_XYZ
|
||
tristate "Foobar XYZ" depends on SND select SND_PCM help Say Y here
|
||
to include support for Foobar XYZ soundcard. To compile this driver
|
||
as a module, choose M here: the module will be called snd-xyz. the
|
||
line, select SND_PCM, specifies that the driver xyz supports PCM. In
|
||
addition to SND_PCM, the following components are supported for
|
||
select command: SND_RAWMIDI, SND_TIMER, SND_HWDEP,
|
||
SND_MPU401_UART, SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB,
|
||
SND_AC97_CODEC. Add the select command for each supported
|
||
component.
|
||
|
||
Note that some selections imply the lowlevel selections. For example,
|
||
PCM includes TIMER, MPU401_UART includes RAWMIDI, AC97_CODEC
|
||
includes PCM, and OPL3_LIB includes HWDEP. You don't need to give
|
||
the lowlevel selections again.
|
||
|
||
For the details of Kconfig script, refer to the kbuild documentation.
|
||
|
||
Drivers with Several Source Files
|
||
---------------------------------
|
||
|
||
Suppose that the driver snd-xyz have several source files. They are
|
||
located in the new subdirectory, sound/pci/xyz.
|
||
|
||
1. Add a new directory (``sound/pci/xyz``) in ``sound/pci/Makefile``
|
||
as below
|
||
|
||
::
|
||
|
||
obj-$(CONFIG_SND) += sound/pci/xyz/
|
||
|
||
|
||
2. Under the directory ``sound/pci/xyz``, create a Makefile
|
||
|
||
::
|
||
|
||
snd-xyz-objs := xyz.o abc.o def.o
|
||
obj-$(CONFIG_SND_XYZ) += snd-xyz.o
|
||
|
||
3. Create the Kconfig entry
|
||
|
||
This procedure is as same as in the last section.
|
||
|
||
|
||
Useful Functions
|
||
================
|
||
|
||
:c:func:`snd_printk()` and friends
|
||
----------------------------------
|
||
|
||
.. note:: This subsection describes a few helper functions for
|
||
decorating a bit more on the standard :c:func:`printk()` & co.
|
||
However, in general, the use of such helpers is no longer recommended.
|
||
If possible, try to stick with the standard functions like
|
||
:c:func:`dev_err()` or :c:func:`pr_err()`.
|
||
|
||
ALSA provides a verbose version of the :c:func:`printk()` function.
|
||
If a kernel config ``CONFIG_SND_VERBOSE_PRINTK`` is set, this function
|
||
prints the given message together with the file name and the line of the
|
||
caller. The ``KERN_XXX`` prefix is processed as well as the original
|
||
:c:func:`printk()` does, so it's recommended to add this prefix,
|
||
e.g. snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\\n");
|
||
|
||
There are also :c:func:`printk()`'s for debugging.
|
||
:c:func:`snd_printd()` can be used for general debugging purposes.
|
||
If ``CONFIG_SND_DEBUG`` is set, this function is compiled, and works
|
||
just like :c:func:`snd_printk()`. If the ALSA is compiled without
|
||
the debugging flag, it's ignored.
|
||
|
||
:c:func:`snd_printdd()` is compiled in only when
|
||
``CONFIG_SND_DEBUG_VERBOSE`` is set.
|
||
|
||
:c:func:`snd_BUG()`
|
||
-------------------
|
||
|
||
It shows the ``BUG?`` message and stack trace as well as
|
||
:c:func:`snd_BUG_ON()` at the point. It's useful to show that a
|
||
fatal error happens there.
|
||
|
||
When no debug flag is set, this macro is ignored.
|
||
|
||
:c:func:`snd_BUG_ON()`
|
||
----------------------
|
||
|
||
:c:func:`snd_BUG_ON()` macro is similar with
|
||
:c:func:`WARN_ON()` macro. For example, snd_BUG_ON(!pointer); or
|
||
it can be used as the condition, if (snd_BUG_ON(non_zero_is_bug))
|
||
return -EINVAL;
|
||
|
||
The macro takes an conditional expression to evaluate. When
|
||
``CONFIG_SND_DEBUG``, is set, if the expression is non-zero, it shows
|
||
the warning message such as ``BUG? (xxx)`` normally followed by stack
|
||
trace. In both cases it returns the evaluated value.
|
||
|
||
Acknowledgments
|
||
===============
|
||
|
||
I would like to thank Phil Kerr for his help for improvement and
|
||
corrections of this document.
|
||
|
||
Kevin Conder reformatted the original plain-text to the DocBook format.
|
||
|
||
Giuliano Pochini corrected typos and contributed the example codes in
|
||
the hardware constraints section.
|