mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-27 19:29:37 +00:00
![Nicolas Kaiser](/assets/img/avatar_default.png)
Fixed typos. v2: Incorporated changes by Randy Dunlap. Signed-off-by: Nicolas Kaiser <nikai@nikai.net> Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
192 lines
8.1 KiB
Text
192 lines
8.1 KiB
Text
|
||
VGA Arbiter
|
||
===========
|
||
|
||
Graphic devices are accessed through ranges in I/O or memory space. While most
|
||
modern devices allow relocation of such ranges, some "Legacy" VGA devices
|
||
implemented on PCI will typically have the same "hard-decoded" addresses as
|
||
they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
|
||
Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
|
||
Section 7, Legacy Devices.
|
||
|
||
The Resource Access Control (RAC) module inside the X server [0] existed for
|
||
the legacy VGA arbitration task (besides other bus management tasks) when more
|
||
than one legacy device co-exists on the same machine. But the problem happens
|
||
when these devices are trying to be accessed by different userspace clients
|
||
(e.g. two server in parallel). Their address assignments conflict. Moreover,
|
||
ideally, being a userspace application, it is not the role of the X server to
|
||
control bus resources. Therefore an arbitration scheme outside of the X server
|
||
is needed to control the sharing of these resources. This document introduces
|
||
the operation of the VGA arbiter implemented for the Linux kernel.
|
||
|
||
----------------------------------------------------------------------------
|
||
|
||
I. Details and Theory of Operation
|
||
I.1 vgaarb
|
||
I.2 libpciaccess
|
||
I.3 xf86VGAArbiter (X server implementation)
|
||
II. Credits
|
||
III.References
|
||
|
||
|
||
I. Details and Theory of Operation
|
||
==================================
|
||
|
||
I.1 vgaarb
|
||
----------
|
||
|
||
The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
|
||
scans all PCI devices and adds the VGA ones inside the arbitration. The
|
||
arbiter then enables/disables the decoding on different devices of the VGA
|
||
legacy instructions. Devices which do not want/need to use the arbiter may
|
||
explicitly tell it by calling vga_set_legacy_decoding().
|
||
|
||
The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
|
||
which has the following semantics:
|
||
|
||
open : open user instance of the arbiter. By default, it's attached to
|
||
the default VGA device of the system.
|
||
|
||
close : close user instance. Release locks made by the user
|
||
|
||
read : return a string indicating the status of the target like:
|
||
|
||
"<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"
|
||
|
||
An IO state string is of the form {io,mem,io+mem,none}, mc and
|
||
ic are respectively mem and io lock counts (for debugging/
|
||
diagnostic only). "decodes" indicate what the card currently
|
||
decodes, "owns" indicates what is currently enabled on it, and
|
||
"locks" indicates what is locked by this card. If the card is
|
||
unplugged, we get "invalid" then for card_ID and an -ENODEV
|
||
error is returned for any command until a new card is targeted.
|
||
|
||
|
||
write : write a command to the arbiter. List of commands:
|
||
|
||
target <card_ID> : switch target to card <card_ID> (see below)
|
||
lock <io_state> : acquires locks on target ("none" is an invalid io_state)
|
||
trylock <io_state> : non-blocking acquire locks on target (returns EBUSY if
|
||
unsuccessful)
|
||
unlock <io_state> : release locks on target
|
||
unlock all : release all locks on target held by this user (not
|
||
implemented yet)
|
||
decodes <io_state> : set the legacy decoding attributes for the card
|
||
|
||
poll : event if something changes on any card (not just the
|
||
target)
|
||
|
||
card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
|
||
to go back to the system default card (TODO: not implemented yet). Currently,
|
||
only PCI is supported as a prefix, but the userland API may support other bus
|
||
types in the future, even if the current kernel implementation doesn't.
|
||
|
||
Note about locks:
|
||
|
||
The driver keeps track of which user has which locks on which card. It
|
||
supports stacking, like the kernel one. This complexifies the implementation
|
||
a bit, but makes the arbiter more tolerant to user space problems and able
|
||
to properly cleanup in all cases when a process dies.
|
||
Currently, a max of 16 cards can have locks simultaneously issued from
|
||
user space for a given user (file descriptor instance) of the arbiter.
|
||
|
||
In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
|
||
notify them being added/removed in the system and automatically added/removed
|
||
in the arbiter.
|
||
|
||
There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
|
||
drivers want to use it.
|
||
|
||
|
||
I.2 libpciaccess
|
||
----------------
|
||
|
||
To use the vga arbiter char device it was implemented an API inside the
|
||
libpciaccess library. One field was added to struct pci_device (each device
|
||
on the system):
|
||
|
||
/* the type of resource decoded by the device */
|
||
int vgaarb_rsrc;
|
||
|
||
Besides it, in pci_system were added:
|
||
|
||
int vgaarb_fd;
|
||
int vga_count;
|
||
struct pci_device *vga_target;
|
||
struct pci_device *vga_default_dev;
|
||
|
||
|
||
The vga_count is used to track how many cards are being arbitrated, so for
|
||
instance, if there is only one card, then it can completely escape arbitration.
|
||
|
||
|
||
These functions below acquire VGA resources for the given card and mark those
|
||
resources as locked. If the resources requested are "normal" (and not legacy)
|
||
resources, the arbiter will first check whether the card is doing legacy
|
||
decoding for that type of resource. If yes, the lock is "converted" into a
|
||
legacy resource lock. The arbiter will first look for all VGA cards that
|
||
might conflict and disable their IOs and/or Memory access, including VGA
|
||
forwarding on P2P bridges if necessary, so that the requested resources can
|
||
be used. Then, the card is marked as locking these resources and the IO and/or
|
||
Memory access is enabled on the card (including VGA forwarding on parent
|
||
P2P bridges if any). In the case of vga_arb_lock(), the function will block
|
||
if some conflicting card is already locking one of the required resources (or
|
||
any resource on a different bus segment, since P2P bridges don't differentiate
|
||
VGA memory and IO afaik). If the card already owns the resources, the function
|
||
succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
|
||
calls are supported (a per-resource counter is maintained).
|
||
|
||
|
||
Set the target device of this client.
|
||
int pci_device_vgaarb_set_target (struct pci_device *dev);
|
||
|
||
|
||
For instance, in x86 if two devices on the same bus want to lock different
|
||
resources, both will succeed (lock). If devices are in different buses and
|
||
trying to lock different resources, only the first who tried succeeds.
|
||
int pci_device_vgaarb_lock (void);
|
||
int pci_device_vgaarb_trylock (void);
|
||
|
||
Unlock resources of device.
|
||
int pci_device_vgaarb_unlock (void);
|
||
|
||
Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
|
||
Memory, both, or none. All cards default to both, the card driver (fbdev for
|
||
example) should tell the arbiter if it has disabled legacy decoding, so the
|
||
card can be left out of the arbitration process (and can be safe to take
|
||
interrupts at any time.
|
||
int pci_device_vgaarb_decodes (int new_vgaarb_rsrc);
|
||
|
||
Connects to the arbiter device, allocates the struct
|
||
int pci_device_vgaarb_init (void);
|
||
|
||
Close the connection
|
||
void pci_device_vgaarb_fini (void);
|
||
|
||
|
||
I.3 xf86VGAArbiter (X server implementation)
|
||
--------------------------------------------
|
||
|
||
(TODO)
|
||
|
||
X server basically wraps all the functions that touch VGA registers somehow.
|
||
|
||
|
||
II. Credits
|
||
===========
|
||
|
||
Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
|
||
with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
|
||
Tiago Vignatti (both of C3SL/Federal University of Paran<61>) proceeded his work
|
||
enhancing the kernel code to adapt as a kernel module and also did the
|
||
implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
|
||
Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.
|
||
|
||
|
||
III. References
|
||
==============
|
||
|
||
[0] http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
|
||
[1] http://lists.freedesktop.org/archives/xorg/2005-March/006663.html
|
||
[2] http://lists.freedesktop.org/archives/xorg/2005-March/006745.html
|
||
[3] http://lists.freedesktop.org/archives/xorg/2007-October/029507.html
|