2016-06-21 11:49:00 +00:00
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=============
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2016-06-21 11:48:58 +00:00
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DRM Internals
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=============
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This chapter documents DRM internals relevant to driver authors and
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developers working to add support for the latest features to existing
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drivers.
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First, we go over some typical driver initialization requirements, like
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setting up command buffers, creating an initial output configuration,
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and initializing core services. Subsequent sections cover core internals
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in more detail, providing implementation notes and examples.
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The DRM layer provides several services to graphics drivers, many of
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them driven by the application interfaces it provides through libdrm,
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the library that wraps most of the DRM ioctls. These include vblank
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event handling, memory management, output management, framebuffer
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management, command submission & fencing, suspend/resume support, and
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DMA services.
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Driver Initialization
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2016-06-21 11:49:00 +00:00
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=====================
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2016-06-21 11:48:58 +00:00
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At the core of every DRM driver is a :c:type:`struct drm_driver
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<drm_driver>` structure. Drivers typically statically initialize
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a drm_driver structure, and then pass it to
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2019-12-04 10:19:33 +00:00
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drm_dev_alloc() to allocate a device instance. After the
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2016-06-21 11:48:58 +00:00
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device instance is fully initialized it can be registered (which makes
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2019-12-04 10:19:33 +00:00
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it accessible from userspace) using drm_dev_register().
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2016-06-21 11:48:58 +00:00
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The :c:type:`struct drm_driver <drm_driver>` structure
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contains static information that describes the driver and features it
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supports, and pointers to methods that the DRM core will call to
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implement the DRM API. We will first go through the :c:type:`struct
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drm_driver <drm_driver>` static information fields, and will
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then describe individual operations in details as they get used in later
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sections.
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Driver Information
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2016-06-21 11:49:00 +00:00
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------------------
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2016-06-21 11:48:58 +00:00
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Major, Minor and Patchlevel
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2016-06-21 11:49:02 +00:00
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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2016-06-21 11:48:58 +00:00
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int major; int minor; int patchlevel;
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The DRM core identifies driver versions by a major, minor and patch
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level triplet. The information is printed to the kernel log at
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initialization time and passed to userspace through the
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DRM_IOCTL_VERSION ioctl.
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The major and minor numbers are also used to verify the requested driver
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API version passed to DRM_IOCTL_SET_VERSION. When the driver API
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changes between minor versions, applications can call
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DRM_IOCTL_SET_VERSION to select a specific version of the API. If the
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requested major isn't equal to the driver major, or the requested minor
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is larger than the driver minor, the DRM_IOCTL_SET_VERSION call will
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return an error. Otherwise the driver's set_version() method will be
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called with the requested version.
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Name, Description and Date
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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char \*name; char \*desc; char \*date;
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The driver name is printed to the kernel log at initialization time,
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used for IRQ registration and passed to userspace through
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DRM_IOCTL_VERSION.
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The driver description is a purely informative string passed to
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userspace through the DRM_IOCTL_VERSION ioctl and otherwise unused by
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the kernel.
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The driver date, formatted as YYYYMMDD, is meant to identify the date of
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the latest modification to the driver. However, as most drivers fail to
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update it, its value is mostly useless. The DRM core prints it to the
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kernel log at initialization time and passes it to userspace through the
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DRM_IOCTL_VERSION ioctl.
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2021-12-22 08:28:22 +00:00
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Module Initialization
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---------------------
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.. kernel-doc:: include/drm/drm_module.h
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:doc: overview
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2021-04-12 13:10:41 +00:00
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Managing Ownership of the Framebuffer Aperture
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----------------------------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_aperture.c
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:doc: overview
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.. kernel-doc:: include/drm/drm_aperture.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_aperture.c
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:export:
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2016-06-21 11:48:58 +00:00
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Device Instance and Driver Handling
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2016-06-21 11:49:00 +00:00
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-----------------------------------
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2016-06-21 11:48:58 +00:00
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.. kernel-doc:: drivers/gpu/drm/drm_drv.c
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:doc: driver instance overview
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2019-01-11 16:40:48 +00:00
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.. kernel-doc:: include/drm/drm_device.h
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:internal:
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2016-11-14 11:58:20 +00:00
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.. kernel-doc:: include/drm/drm_drv.h
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:internal:
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2017-03-02 15:16:36 +00:00
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.. kernel-doc:: drivers/gpu/drm/drm_drv.c
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:export:
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2016-06-21 11:48:58 +00:00
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Driver Load
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2016-06-21 11:49:00 +00:00
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-----------
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2016-06-21 11:48:58 +00:00
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2019-02-12 16:46:15 +00:00
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Component Helper Usage
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~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/gpu/drm/drm_drv.c
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:doc: component helper usage recommendations
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2016-06-21 11:48:58 +00:00
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Memory Manager Initialization
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2016-06-21 11:49:02 +00:00
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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2016-06-21 11:48:58 +00:00
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Every DRM driver requires a memory manager which must be initialized at
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load time. DRM currently contains two memory managers, the Translation
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Table Manager (TTM) and the Graphics Execution Manager (GEM). This
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document describes the use of the GEM memory manager only. See ? for
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details.
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Miscellaneous Device Configuration
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2016-06-21 11:49:02 +00:00
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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2016-06-21 11:48:58 +00:00
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Another task that may be necessary for PCI devices during configuration
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is mapping the video BIOS. On many devices, the VBIOS describes device
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configuration, LCD panel timings (if any), and contains flags indicating
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device state. Mapping the BIOS can be done using the pci_map_rom()
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call, a convenience function that takes care of mapping the actual ROM,
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whether it has been shadowed into memory (typically at address 0xc0000)
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or exists on the PCI device in the ROM BAR. Note that after the ROM has
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been mapped and any necessary information has been extracted, it should
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be unmapped; on many devices, the ROM address decoder is shared with
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other BARs, so leaving it mapped could cause undesired behaviour like
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hangs or memory corruption.
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drm: add managed resources tied to drm_device
We have lots of these. And the cleanup code tends to be of dubious
quality. The biggest wrong pattern is that developers use devm_, which
ties the release action to the underlying struct device, whereas
all the userspace visible stuff attached to a drm_device can long
outlive that one (e.g. after a hotunplug while userspace has open
files and mmap'ed buffers). Give people what they want, but with more
correctness.
Mostly copied from devres.c, with types adjusted to fit drm_device and
a few simplifications - I didn't (yet) copy over everything. Since
the types don't match code sharing looked like a hopeless endeavour.
For now it's only super simplified, no groups, you can't remove
actions (but kfree exists, we'll need that soon). Plus all specific to
drm_device ofc, including the logging. Which I didn't bother to make
compile-time optional, since none of the other drm logging is compile
time optional either.
One tricky bit here is the chicken&egg between allocating your
drm_device structure and initiliazing it with drm_dev_init. For
perfect onion unwinding we'd need to have the action to kfree the
allocation registered before drm_dev_init registers any of its own
release handlers. But drm_dev_init doesn't know where exactly the
drm_device is emebedded into the overall structure, and by the time it
returns it'll all be too late. And forcing drivers to be able clean up
everything except the one kzalloc is silly.
Work around this by having a very special final_kfree pointer. This
also avoids troubles with the list head possibly disappearing from
underneath us when we release all resources attached to the
drm_device.
v2: Do all the kerneldoc at the end, to avoid lots of fairly pointless
shuffling while getting everything into shape.
v3: Add static to add/del_dr (Neil)
Move typo fix to the right patch (Neil)
v4: Enforce contract for drmm_add_final_kfree:
Use ksize() to check that the drm_device is indeed contained somewhere
in the final kfree(). Because we need that or the entire managed
release logic blows up in a pile of use-after-frees. Motivated by a
discussion with Laurent.
v5: Review from Laurent:
- %zu instead of casting size_t
- header guards
- sorting of includes
- guarding of data assignment if we didn't allocate it for a NULL
pointer
- delete spurious newline
- cast void* data parameter correctly in ->release call, no idea how
this even worked before
v6: Review from Sam
- Add the kerneldoc for the managed sub-struct back in, even if it
doesn't show up in the generated html somehow.
- Explain why __always_inline.
- Fix bisectability around the final kfree() in drm_dev_relase(). This
is just interim code which will disappear again.
- Some whitespace polish.
- Add debug output when drmm_add_action or drmm_kmalloc fail.
v7: My bisectability fix wasn't up to par as noticed by smatch.
v8: Remove unecessary {} around if else
v9: Use kstrdup_const, which requires kfree_const and introducing a free_dr()
helper (Thomas).
v10: kfree_const goes boom on the plain "kmalloc" assignment, somehow
we need to wrap that in kstrdup_const() too!! Also renumber revision
log, I somehow reset it midway thruh.
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Neil Armstrong <narmstrong@baylibre.com
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200324124540.3227396-1-daniel.vetter@ffwll.ch
2020-03-24 12:45:40 +00:00
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Managed Resources
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-----------------
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.. kernel-doc:: drivers/gpu/drm/drm_managed.c
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:doc: managed resources
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drm: Add docs for managed resources
All collected together to provide a consistent story in one patch,
instead of the somewhat bumpy refactor-evolution leading to this.
Also some thoughts on what the next steps could be:
- Create a macro called devm_drm_dev_alloc() which essentially wraps
the kzalloc(); devm_drm_dev_init(); drmm_add_final_kfree() combo.
Needs to be a macro since we'll have to do some typeof trickery and
casting to make this fully generic for all drivers that embed struct
drm_device into their own thing.
- A lot of the simple drivers now have essentially just
drm_dev_unplug(); drm_atomic_helper_shutdown(); as their
$bus_driver->remove hook. We could create a devm_mode_config_reset
which sets drm_atomic_helper_shutdown as it's cleanup action, and a
devm_drm_dev_register with drm_dev_unplug as it's cleanup action,
and simple drivers wouldn't have a need for a ->remove function at
all, and we could delete them.
- For more complicated drivers we need drmm_ versions of a _lot_ more
things. All the userspace visible objects (crtc, plane, encoder,
crtc), anything else hanging of those (maybe a drmm_get_edid, at
least for panels and other built-in stuff).
Also some more thoughts on why we're not reusing devm_ with maybe a
fake struct device embedded into the drm_device (we can't use the
kdev, since that's in each drm_minor).
- Code review gets extremely tricky, since every time you see a devm_
you need to carefully check whether the fake device (with the
drm_device lifetim) or the real device (with the lifetim of the
underlying physical device and driver binding) are used. That's not
going to help at all, and we have enormous amounts of drivers who
use devm_ where they really shouldn't. Having different types makes
sure the compiler type checks this for us and ensures correctness.
- The set of functions are very much non-overlapping. E.g.
devm_ioremap makes total sense, drmm_ioremap has the wrong lifetime,
since hw resources need to be cleaned out at driver unbind and wont
outlive that like a drm_device. Similar, but other way round for
drmm_connector_init (which is the only correct version, devm_ for
drm_connector is just buggy). Simply not having the wrong version
again prevents bugs.
Finally I guess this opens a huge todo for all the drivers. I'm
semi-tempted to do a tree-wide s/devm_kzalloc/drmm_kzalloc/ since most
likely that'll fix an enormous amount of bugs and most likely not
cause any issues at all (aside from maybe holding onto memory slightly
too long).
v2:
- Doc improvements from Laurent.
- Also add kerneldoc for the new drmm_add_action_or_reset.
v3:
- Remove kerneldoc for drmm_remove_action.
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
fixup docs
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-52-daniel.vetter@ffwll.ch
2020-03-23 14:49:50 +00:00
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.. kernel-doc:: drivers/gpu/drm/drm_managed.c
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:export:
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.. kernel-doc:: include/drm/drm_managed.h
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:internal:
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2016-06-21 11:48:58 +00:00
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Open/Close, File Operations and IOCTLs
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2016-06-21 11:49:00 +00:00
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======================================
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2016-06-21 11:48:58 +00:00
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2017-05-31 09:20:45 +00:00
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.. _drm_driver_fops:
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2016-06-21 11:48:58 +00:00
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File Operations
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2016-06-21 11:49:00 +00:00
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---------------
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2017-03-08 14:12:40 +00:00
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.. kernel-doc:: drivers/gpu/drm/drm_file.c
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2016-06-21 11:48:58 +00:00
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:doc: file operations
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2017-03-08 14:12:44 +00:00
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.. kernel-doc:: include/drm/drm_file.h
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:internal:
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2017-03-08 14:12:40 +00:00
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.. kernel-doc:: drivers/gpu/drm/drm_file.c
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:export:
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2016-11-05 15:08:07 +00:00
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Misc Utilities
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==============
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Printer
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-------
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.. kernel-doc:: include/drm/drm_print.h
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:doc: print
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.. kernel-doc:: include/drm/drm_print.h
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:internal:
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2016-11-14 11:58:22 +00:00
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.. kernel-doc:: drivers/gpu/drm/drm_print.c
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2016-11-05 15:08:07 +00:00
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:export:
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2019-01-12 19:32:44 +00:00
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Utilities
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---------
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.. kernel-doc:: include/drm/drm_util.h
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:doc: drm utils
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.. kernel-doc:: include/drm/drm_util.h
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:internal:
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2016-11-05 15:08:07 +00:00
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2022-06-20 16:06:40 +00:00
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Unit testing
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============
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KUnit
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-----
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KUnit (Kernel unit testing framework) provides a common framework for unit tests
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within the Linux kernel.
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This section covers the specifics for the DRM subsystem. For general information
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about KUnit, please refer to Documentation/dev-tools/kunit/start.rst.
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How to run the tests?
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~~~~~~~~~~~~~~~~~~~~~
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In order to facilitate running the test suite, a configuration file is present
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in ``drivers/gpu/drm/tests/.kunitconfig``. It can be used by ``kunit.py`` as
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follows:
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.. code-block:: bash
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$ ./tools/testing/kunit/kunit.py run --kunitconfig=drivers/gpu/drm/tests \
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--kconfig_add CONFIG_VIRTIO_UML=y \
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--kconfig_add CONFIG_UML_PCI_OVER_VIRTIO=y
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.. note::
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The configuration included in ``.kunitconfig`` should be as generic as
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possible.
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``CONFIG_VIRTIO_UML`` and ``CONFIG_UML_PCI_OVER_VIRTIO`` are not
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included in it because they are only required for User Mode Linux.
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2016-06-21 11:48:58 +00:00
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Legacy Support Code
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2016-06-21 11:49:00 +00:00
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===================
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2016-06-21 11:48:58 +00:00
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The section very briefly covers some of the old legacy support code
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which is only used by old DRM drivers which have done a so-called
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shadow-attach to the underlying device instead of registering as a real
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driver. This also includes some of the old generic buffer management and
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command submission code. Do not use any of this in new and modern
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drivers.
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Legacy Suspend/Resume
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2016-06-21 11:49:00 +00:00
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---------------------
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2016-06-21 11:48:58 +00:00
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The DRM core provides some suspend/resume code, but drivers wanting full
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suspend/resume support should provide save() and restore() functions.
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These are called at suspend, hibernate, or resume time, and should
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perform any state save or restore required by your device across suspend
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or hibernate states.
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int (\*suspend) (struct drm_device \*, pm_message_t state); int
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(\*resume) (struct drm_device \*);
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Those are legacy suspend and resume methods which *only* work with the
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legacy shadow-attach driver registration functions. New driver should
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use the power management interface provided by their bus type (usually
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through the :c:type:`struct device_driver <device_driver>`
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dev_pm_ops) and set these methods to NULL.
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Legacy DMA Services
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2016-06-21 11:49:00 +00:00
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-------------------
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2016-06-21 11:48:58 +00:00
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This should cover how DMA mapping etc. is supported by the core. These
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functions are deprecated and should not be used.
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