TODO list¶
This section contains a list of smaller janitorial tasks in the kernel DRM graphics subsystem useful as newbie projects. Or for slow rainy days.
Difficulty¶
To make it easier task are categorized into different levels:
Starter: Good tasks to get started with the DRM subsystem.
Intermediate: Tasks which need some experience with working in the DRM subsystem, or some specific GPU/display graphics knowledge. For debugging issue it’s good to have the relevant hardware (or a virtual driver set up) available for testing.
Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem and graphics topics. Generally need the relevant hardware for development and testing.
Expert: Only attempt these if you’ve successfully completed some tricky refactorings already and are an expert in the specific area
Subsystem-wide refactorings¶
Remove custom dumb_map_offset implementations¶
All GEM based drivers should be using drm_gem_create_mmap_offset()
instead.
Audit each individual driver, make sure it’ll work with the generic
implementation (there’s lots of outdated locking leftovers in various
implementations), and then remove it.
Contact: Simona Vetter, respective driver maintainers
Level: Intermediate
Convert existing KMS drivers to atomic modesetting¶
3.19 has the atomic modeset interfaces and helpers, so drivers can now be converted over. Modern compositors like Wayland or Surfaceflinger on Android really want an atomic modeset interface, so this is all about the bright future.
There is a conversion guide for atomic [1] and all you need is a GPU for a non-converted driver. The “Atomic mode setting design overview” series [2] [3] at LWN.net can also be helpful.
As part of this drivers also need to convert to universal plane (which means exposing primary & cursor as proper plane objects). But that’s much easier to do by directly using the new atomic helper driver callbacks.
Contact: Simona Vetter, respective driver maintainers
Level: Advanced
Clean up the clipped coordination confusion around planes¶
We have a helper to get this right with drm_plane_helper_check_update(), but it’s not consistently used. This should be fixed, preferably in the atomic helpers (and drivers then moved over to clipped coordinates). Probably the helper should also be moved from drm_plane_helper.c to the atomic helpers, to avoid confusion - the other helpers in that file are all deprecated legacy helpers.
Contact: Ville Syrjälä, Simona Vetter, driver maintainers
Level: Advanced
Improve plane atomic_check helpers¶
Aside from the clipped coordinates right above there’s a few suboptimal things with the current helpers:
drm_plane_helper_funcs->atomic_check gets called for enabled or disabled planes. At best this seems to confuse drivers, worst it means they blow up when the plane is disabled without the CRTC. The only special handling is resetting values in the plane state structures, which instead should be moved into the drm_plane_funcs->atomic_duplicate_state functions.
Once that’s done, helpers could stop calling ->atomic_check for disabled planes.
Then we could go through all the drivers and remove the more-or-less confused checks for plane_state->fb and plane_state->crtc.
Contact: Simona Vetter
Level: Advanced
Convert early atomic drivers to async commit helpers¶
For the first year the atomic modeset helpers didn’t support asynchronous / nonblocking commits, and every driver had to hand-roll them. This is fixed now, but there’s still a pile of existing drivers that easily could be converted over to the new infrastructure.
One issue with the helpers is that they require that drivers handle completion events for atomic commits correctly. But fixing these bugs is good anyway.
Somewhat related is the legacy_cursor_update hack, which should be replaced with the new atomic_async_check/commit functionality in the helpers in drivers that still look at that flag.
Contact: Simona Vetter, respective driver maintainers
Level: Advanced
Rename drm_atomic_state¶
The KMS framework uses two slightly different definitions for the state
concept. For a given object (plane, CRTC, encoder, etc., so
drm_$OBJECT_state
), the state is the entire state of that object. However,
at the device level, drm_atomic_state
refers to a state update for a
limited number of objects.
The state isn’t the entire device state, but only the full state of some
objects in that device. This is confusing to newcomers, and
drm_atomic_state
should be renamed to something clearer like
drm_atomic_commit
.
In addition to renaming the structure itself, it would also imply renaming some
related functions (drm_atomic_state_alloc
, drm_atomic_state_get
,
drm_atomic_state_put
, drm_atomic_state_init
,
__drm_atomic_state_free
, etc.).
Contact: Maxime Ripard <mripard@kernel.org>
Level: Advanced
Fallout from atomic KMS¶
drm_atomic_helper.c
provides a batch of functions which implement legacy
IOCTLs on top of the new atomic driver interface. Which is really nice for
gradual conversion of drivers, but unfortunately the semantic mismatches are
a bit too severe. So there’s some follow-up work to adjust the function
interfaces to fix these issues:
atomic needs the lock acquire context. At the moment that’s passed around implicitly with some horrible hacks, and it’s also allocate with
GFP_NOFAIL
behind the scenes. All legacy paths need to start allocating the acquire context explicitly on stack and then also pass it down into drivers explicitly so that the legacy-on-atomic functions can use them.Except for some driver code this is done. This task should be finished by adding WARN_ON(!drm_drv_uses_atomic_modeset) in
drm_modeset_lock_all()
.A bunch of the vtable hooks are now in the wrong place: DRM has a split between core vfunc tables (named
drm_foo_funcs
), which are used to implement the userspace ABI. And then there’s the optional hooks for the helper libraries (namedrm_foo_helper_funcs
), which are purely for internal use. Some of these hooks should be move from_funcs
to_helper_funcs
since they are not part of the core ABI. There’s aFIXME
comment in the kerneldoc for each such case indrm_crtc.h
.
Contact: Simona Vetter
Level: Intermediate
Get rid of dev->struct_mutex from GEM drivers¶
dev->struct_mutex
is the Big DRM Lock from legacy days and infested
everything. Nowadays in modern drivers the only bit where it’s mandatory is
serializing GEM buffer object destruction. Which unfortunately means drivers
have to keep track of that lock and either call unreference
or
unreference_locked
depending upon context.
Core GEM doesn’t have a need for struct_mutex
any more since kernel 4.8,
and there’s a GEM object free
callback for any drivers which are
entirely struct_mutex
free.
For drivers that need struct_mutex
it should be replaced with a driver-
private lock. The tricky part is the BO free functions, since those can’t
reliably take that lock any more. Instead state needs to be protected with
suitable subordinate locks or some cleanup work pushed to a worker thread. For
performance-critical drivers it might also be better to go with a more
fine-grained per-buffer object and per-context lockings scheme. Currently only
the msm
and i915 drivers use struct_mutex
.
Contact: Simona Vetter, respective driver maintainers
Level: Advanced
Move Buffer Object Locking to dma_resv_lock()¶
Many drivers have their own per-object locking scheme, usually using
mutex_lock()
. This causes all kinds of trouble for buffer sharing, since
depending which driver is the exporter and importer, the locking hierarchy is
reversed.
To solve this we need one standard per-object locking mechanism, which is
dma_resv_lock()
. This lock needs to be called as the outermost lock, with all
other driver specific per-object locks removed. The problem is that rolling out
the actual change to the locking contract is a flag day, due to struct dma_buf
buffer sharing.
Level: Expert
Convert logging to drm_* functions with drm_device parameter¶
For drivers which could have multiple instances, it is necessary to differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR don’t do this, drivers used dev_info/warn/err to make this differentiation. We now have drm_* variants of the drm print functions, so we can start to convert those drivers back to using drm-formatted specific log messages.
Before you start this conversion please contact the relevant maintainers to make sure your work will be merged - not everyone agrees that the DRM dmesg macros are better.
Contact: Sean Paul, Maintainer of the driver you plan to convert
Level: Starter
Convert drivers to use simple modeset suspend/resume¶
Most drivers (except i915 and nouveau) that use drm_atomic_helper_suspend/resume() can probably be converted to use drm_mode_config_helper_suspend/resume(). Also there’s still open-coded version of the atomic suspend/resume code in older atomic modeset drivers.
Contact: Maintainer of the driver you plan to convert
Level: Intermediate
Reimplement functions in drm_fbdev_fb_ops without fbdev¶
A number of callback functions in drm_fbdev_fb_ops could benefit from
being rewritten without dependencies on the fbdev module. Some of the
helpers could further benefit from using struct iosys_map
instead of
raw pointers.
Contact: Thomas Zimmermann <tzimmermann@suse.de>, Simona Vetter
Level: Advanced
Benchmark and optimize blitting and format-conversion function¶
Drawing to display memory quickly is crucial for many applications’ performance.
On at least x86-64, sys_imageblit() is significantly slower than cfb_imageblit(), even though both use the same blitting algorithm and the latter is written for I/O memory. It turns out that cfb_imageblit() uses movl instructions, while sys_imageblit apparently does not. This seems to be a problem with gcc’s optimizer. DRM’s format-conversion helpers might be subject to similar issues.
Benchmark and optimize fbdev’s sys_() helpers and DRM’s format-conversion helpers. In cases that can be further optimized, maybe implement a different algorithm. For micro-optimizations, use movl/movq instructions explicitly. That might possibly require architecture-specific helpers (e.g., storel() storeq()).
Contact: Thomas Zimmermann <tzimmermann@suse.de>
Level: Intermediate
drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup¶
A lot more drivers could be switched over to the drm_gem_framebuffer helpers. Various hold-ups:
Need to switch over to the generic dirty tracking code using drm_atomic_helper_dirtyfb first (e.g. qxl).
Need to switch to drm_fbdev_generic_setup(), otherwise a lot of the custom fb setup code can’t be deleted.
Need to switch to
drm_gem_fb_create()
, as nowdrm_gem_fb_create()
checks for valid formats for atomic drivers.Many drivers subclass drm_framebuffer, we’d need a embedding compatible version of the varios drm_gem_fb_create functions. Maybe called drm_gem_fb_create/_with_dirty/_with_funcs as needed.
Contact: Simona Vetter
Level: Intermediate
Generic fbdev defio support¶
The defio support code in the fbdev core has some very specific requirements, which means drivers need to have a special framebuffer for fbdev. The main issue is that it uses some fields in struct page itself, which breaks shmem gem objects (and other things). To support defio, affected drivers require the use of a shadow buffer, which may add CPU and memory overhead.
Possible solution would be to write our own defio mmap code in the drm fbdev emulation. It would need to fully wrap the existing mmap ops, forwarding everything after it has done the write-protect/mkwrite trickery:
In the drm_fbdev_fb_mmap helper, if we need defio, change the default page prots to write-protected with something like this:
vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
Set the mkwrite and fsync callbacks with similar implementions to the core fbdev defio stuff. These should all work on plain ptes, they don’t actually require a struct page. uff. These should all work on plain ptes, they don’t actually require a struct page.
Track the dirty pages in a separate structure (bitfield with one bit per page should work) to avoid clobbering struct page.
Might be good to also have some igt testcases for this.
Contact: Simona Vetter, Noralf Tronnes
Level: Advanced
connector register/unregister fixes¶
For most connectors it’s a no-op to call drm_connector_register/unregister directly from driver code, drm_dev_register/unregister take care of this already. We can remove all of them.
For dp drivers it’s a bit more a mess, since we need the connector to be registered when calling drm_dp_aux_register. Fix this by instead calling drm_dp_aux_init, and moving the actual registering into a late_register callback as recommended in the kerneldoc.
Level: Intermediate
Remove load/unload callbacks¶
The load/unload callbacks in struct &drm_driver are very much midlayers, plus
for historical reasons they get the ordering wrong (and we can’t fix that)
between setting up the &drm_driver structure and calling drm_dev_register()
.
Rework drivers to no longer use the load/unload callbacks, directly coding the load/unload sequence into the driver’s probe function.
Once all drivers are converted, remove the load/unload callbacks.
Contact: Simona Vetter
Level: Intermediate
Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi¶
Once EDID is parsed, the monitor HDMI support information is available through
drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor()
to
retrieve the same information, which is less efficient.
Audit each individual driver calling drm_detect_hdmi_monitor()
and switch to
drm_display_info.is_hdmi if applicable.
Contact: Laurent Pinchart, respective driver maintainers
Level: Intermediate
Consolidate custom driver modeset properties¶
Before atomic modeset took place, many drivers where creating their own properties. Among other things, atomic brought the requirement that custom, driver specific properties should not be used.
For this task, we aim to introduce core helpers or reuse the existing ones if available:
A quick, unconfirmed, examples list.
Introduce core helpers: - audio (amdgpu, intel, gma500, radeon) - brightness, contrast, etc (armada, nouveau) - overlay only (?) - broadcast rgb (gma500, intel) - colorkey (armada, nouveau, rcar) - overlay only (?) - dither (amdgpu, nouveau, radeon) - varies across drivers - underscan family (amdgpu, radeon, nouveau)
Already in core: - colorspace (sti) - tv format names, enhancements (gma500, intel) - tv overscan, margins, etc. (gma500, intel) - zorder (omapdrm) - same as zpos (?)
Contact: Emil Velikov, respective driver maintainers
Level: Intermediate
Use struct iosys_map throughout codebase¶
Pointers to shared device memory are stored in struct iosys_map
. Each
instance knows whether it refers to system or I/O memory. Most of the DRM-wide
interface have been converted to use struct iosys_map
, but implementations
often still use raw pointers.
The task is to use struct iosys_map
where it makes sense.
Memory managers should use
struct iosys_map
for dma-buf-imported buffers.TTM might benefit from using
struct iosys_map
internally.Framebuffer copying and blitting helpers should operate on
struct iosys_map
.
Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Simona Vetter
Level: Intermediate
Review all drivers for setting struct drm_mode_config.{max_width,max_height} correctly¶
The values in struct drm_mode_config
.{max_width,max_height} describe the
maximum supported framebuffer size. It’s the virtual screen size, but many
drivers treat it like limitations of the physical resolution.
The maximum width depends on the hardware’s maximum scanline pitch. The maximum height depends on the amount of addressable video memory. Review all drivers to initialize the fields to the correct values.
Contact: Thomas Zimmermann <tzimmermann@suse.de>
Level: Intermediate
Request memory regions in all drivers¶
Go through all drivers and add code to request the memory regions that the
driver uses. This requires adding calls to request_mem_region(),
pci_request_region()
or similar functions. Use helpers for managed cleanup
where possible.
Drivers are pretty bad at doing this and there used to be conflicts among DRM and fbdev drivers. Still, it’s the correct thing to do.
Contact: Thomas Zimmermann <tzimmermann@suse.de>
Level: Starter
Remove driver dependencies on FB_DEVICE¶
A number of fbdev drivers provide attributes via sysfs and therefore depend on CONFIG_FB_DEVICE to be selected. Review each driver and attempt to make any dependencies on CONFIG_FB_DEVICE optional. At the minimum, the respective code in the driver could be conditionalized via ifdef CONFIG_FB_DEVICE. Not all drivers might be able to drop CONFIG_FB_DEVICE.
Contact: Thomas Zimmermann <tzimmermann@suse.de>
Level: Starter
Remove disable/unprepare in remove/shutdown in panel-simple and panel-edp¶
As of commit d2aacaf07395 (“drm/panel: Check for already prepared/enabled in drm_panel”), we have a check in the drm_panel core to make sure nobody double-calls prepare/enable/disable/unprepare. Eventually that should probably be turned into a WARN_ON() or somehow made louder.
At the moment, we expect that we may still encounter the warnings in the
drm_panel core when using panel-simple and panel-edp. Since those panel
drivers are used with a lot of different DRM modeset drivers they still
make an extra effort to disable/unprepare the panel themsevles at shutdown
time. Specifically we could still encounter those warnings if the panel
driver gets shutdown() _before_ the DRM modeset driver and the DRM modeset
driver properly calls drm_atomic_helper_shutdown()
in its own shutdown()
callback. Warnings could be avoided in such a case by using something like
device links to ensure that the panel gets shutdown() after the DRM modeset
driver.
Once all DRM modeset drivers are known to shutdown properly, the extra calls to disable/unprepare in remove/shutdown in panel-simple and panel-edp should be removed and this TODO item marked complete.
Contact: Douglas Anderson <dianders@chromium.org>
Level: Intermediate
Transition away from using mipi_dsi_*_write_seq()¶
The macros mipi_dsi_generic_write_seq()
and mipi_dsi_dcs_write_seq()
are
non-intuitive because, if there are errors, they return out of the caller’s
function. We should move all callers to use mipi_dsi_generic_write_seq_multi()
and mipi_dsi_dcs_write_seq_multi()
macros instead.
Once all callers are transitioned, the macros and the functions that they call,
mipi_dsi_generic_write_chatty()
and mipi_dsi_dcs_write_buffer_chatty()
, can
probably be removed. Alternatively, if people feel like the _multi() variants
are overkill for some use cases, we could keep the mipi_dsi_*_write_seq()
variants but change them not to return out of the caller.
Contact: Douglas Anderson <dianders@chromium.org>
Level: Starter
Core refactorings¶
Make panic handling work¶
This is a really varied tasks with lots of little bits and pieces:
The panic path can’t be tested currently, leading to constant breaking. The main issue here is that panics can be triggered from hardirq contexts and hence all panic related callback can run in hardirq context. It would be awesome if we could test at least the fbdev helper code and driver code by e.g. trigger calls through drm debugfs files. hardirq context could be achieved by using an IPI to the local processor.
There’s a massive confusion of different panic handlers. DRM fbdev emulation helpers had their own (long removed), but on top of that the fbcon code itself also has one. We need to make sure that they stop fighting over each other. This is worked around by checking
oops_in_progress
at various entry points into the DRM fbdev emulation helpers. A much cleaner approach here would be to switch fbcon to the threaded printk support.drm_can_sleep()
is a mess. It hides real bugs in normal operations and isn’t a full solution for panic paths. We need to make sure that it only returns true if there’s a panic going on for real, and fix up all the fallout.The panic handler must never sleep, which also means it can’t ever
mutex_lock()
. Also it can’t grab any other lock unconditionally, not even spinlocks (because NMI and hardirq can panic too). We need to either make sure to not call such paths, or trylock everything. Really tricky.A clean solution would be an entirely separate panic output support in KMS, bypassing the current fbcon support. See [PATCH v2 0/3] drm: Add panic handling.
Encoding the actual oops and preceding dmesg in a QR might help with the dread “important stuff scrolled away” problem. See [RFC][PATCH] Oops messages transfer using QR codes for some example code that could be reused.
Contact: Simona Vetter
Level: Advanced
Clean up the debugfs support¶
There’s a bunch of issues with it:
Convert drivers to support the
drm_debugfs_add_files()
function instead of thedrm_debugfs_create_files()
function.Improve late-register debugfs by rolling out the same debugfs pre-register infrastructure for connector and crtc too. That way, the drivers won’t need to split their setup code into init and register anymore.
We probably want to have some support for debugfs files on crtc/connectors and maybe other kms objects directly in core. There’s even drm_print support in the funcs for these objects to dump kms state, so it’s all there. And then the ->show() functions should obviously give you a pointer to the right object.
The drm_driver->debugfs_init hooks we have is just an artifact of the old midlayered load sequence. DRM debugfs should work more like sysfs, where you can create properties/files for an object anytime you want, and the core takes care of publishing/unpuplishing all the files at register/unregister time. Drivers shouldn’t need to worry about these technicalities, and fixing this (together with the drm_minor->drm_device move) would allow us to remove debugfs_init.
Contact: Simona Vetter
Level: Intermediate
Object lifetime fixes¶
There’s two related issues here
Cleanup up the various ->destroy callbacks, which often are all the same simple code.
Lots of drivers erroneously allocate DRM modeset objects using devm_kzalloc, which results in use-after free issues on driver unload. This can be serious trouble even for drivers for hardware integrated on the SoC due to EPROBE_DEFERRED backoff.
Both these problems can be solved by switching over to drmm_kzalloc()
, and the
various convenience wrappers provided, e.g. drmm_crtc_alloc_with_planes()
,
drmm_universal_plane_alloc()
, ... and so on.
Contact: Simona Vetter
Level: Intermediate
Remove automatic page mapping from dma-buf importing¶
When importing dma-bufs, the dma-buf and PRIME frameworks automatically map
imported pages into the importer’s DMA area. drm_gem_prime_fd_to_handle()
and
drm_gem_prime_handle_to_fd()
require that importers call dma_buf_attach()
even if they never do actual device DMA, but only CPU access through
dma_buf_vmap()
. This is a problem for USB devices, which do not support DMA
operations.
To fix the issue, automatic page mappings should be removed from the buffer-sharing code. Fixing this is a bit more involved, since the import/export cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over this problem for USB devices by fishing out the USB host controller device, as long as that supports DMA. Otherwise importing can still needlessly fail.
Contact: Thomas Zimmermann <tzimmermann@suse.de>, Simona Vetter
Level: Advanced
Better Testing¶
Add unit tests using the Kernel Unit Testing (KUnit) framework¶
The KUnit provides a common framework for unit tests within the Linux kernel. Having a test suite would allow to identify regressions earlier.
A good candidate for the first unit tests are the format-conversion helpers in
drm_format_helper.c
.
Contact: Javier Martinez Canillas <javierm@redhat.com>
Level: Intermediate
Clean up and document former selftests suites¶
Some KUnit test suites (drm_buddy, drm_cmdline_parser, drm_damage_helper, drm_format, drm_framebuffer, drm_dp_mst_helper, drm_mm, drm_plane_helper and drm_rect) are former selftests suites that have been converted over when KUnit was first introduced.
These suites were fairly undocumented, and with different goals than what unit tests can be. Trying to identify what each test in these suites actually test for, whether that makes sense for a unit test, and either remove it if it doesn’t or document it if it does would be of great help.
Contact: Maxime Ripard <mripard@kernel.org>
Level: Intermediate
Enable trinity for DRM¶
And fix up the fallout. Should be really interesting ...
Level: Advanced
Make KMS tests in i-g-t generic¶
The i915 driver team maintains an extensive testsuite for the i915 DRM driver, including tons of testcases for corner-cases in the modesetting API. It would be awesome if those tests (at least the ones not relying on Intel-specific GEM features) could be made to run on any KMS driver.
Basic work to run i-g-t tests on non-i915 is done, what’s now missing is mass- converting things over. For modeset tests we also first need a bit of infrastructure to use dumb buffers for untiled buffers, to be able to run all the non-i915 specific modeset tests.
Level: Advanced
Extend virtual test driver (VKMS)¶
See the documentation of VKMS for more details. This is an ideal internship task, since it only requires a virtual machine and can be sized to fit the available time.
Level: See details
Backlight Refactoring¶
Backlight drivers have a triple enable/disable state, which is a bit overkill. Plan to fix this:
Roll out
backlight_enable()
andbacklight_disable()
helpers everywhere. This has started already.In all, only look at one of the three status bits set by the above helpers.
Remove the other two status bits.
Contact: Simona Vetter
Level: Intermediate
Driver Specific¶
AMD DC Display Driver¶
AMD DC is the display driver for AMD devices starting with Vega. There has been a bunch of progress cleaning it up but there’s still plenty of work to be done.
See drivers/gpu/drm/amd/display/TODO for tasks.
Contact: Harry Wentland, Alex Deucher
Bootsplash¶
There is support in place now for writing internal DRM clients making it possible to pick up the bootsplash work that was rejected because it was written for fbdev.
[v6,8/8] drm/client: Hack: Add bootsplash example https://patchwork.freedesktop.org/patch/306579/
[RFC PATCH v2 00/13] Kernel based bootsplash https://lore.kernel.org/r/20171213194755.3409-1-mstaudt@suse.de
Contact: Sam Ravnborg
Level: Advanced
Brightness handling on devices with multiple internal panels¶
On x86/ACPI devices there can be multiple backlight firmware interfaces: (ACPI) video, vendor specific and others. As well as direct/native (PWM) register programming by the KMS driver.
To deal with this backlight drivers used on x86/ACPI call acpi_video_get_backlight_type() which has heuristics (+quirks) to select which backlight interface to use; and backlight drivers which do not match the returned type will not register themselves, so that only one backlight device gets registered (in a single GPU setup, see below).
At the moment this more or less assumes that there will only be 1 (internal) panel on a system.
On systems with 2 panels this may be a problem, depending on what interface acpi_video_get_backlight_type() selects:
native: in this case the KMS driver is expected to know which backlight device belongs to which output so everything should just work.
video: this does support controlling multiple backlights, but some work will need to be done to get the output <-> backlight device mapping
The above assumes both panels will require the same backlight interface type. Things will break on systems with multiple panels where the 2 panels need a different type of control. E.g. one panel needs ACPI video backlight control, where as the other is using native backlight control. Currently in this case only one of the 2 required backlight devices will get registered, based on the acpi_video_get_backlight_type() return value.
If this (theoretical) case ever shows up, then supporting this will need some work. A possible solution here would be to pass a device and connector-name to acpi_video_get_backlight_type() so that it can deal with this.
Note in a way we already have a case where userspace sees 2 panels, in dual GPU laptop setups with a mux. On those systems we may see either 2 native backlight devices; or 2 native backlight devices.
Userspace already has code to deal with this by detecting if the related panel is active (iow which way the mux between the GPU and the panels points) and then uses that backlight device. Userspace here very much assumes a single panel though. It picks only 1 of the 2 backlight devices and then only uses that one.
Note that all userspace code (that I know off) is currently hardcoded to assume a single panel.
Before the recent changes to not register multiple (e.g. video + native) /sys/class/backlight devices for a single panel (on a single GPU laptop), userspace would see multiple backlight devices all controlling the same backlight.
To deal with this userspace had to always picks one preferred device under /sys/class/backlight and will ignore the others. So to support brightness control on multiple panels userspace will need to be updated too.
There are plans to allow brightness control through the KMS API by adding a “display brightness” property to drm_connector objects for panels. This solves a number of issues with the /sys/class/backlight API, including not being able to map a sysfs backlight device to a specific connector. Any userspace changes to add support for brightness control on devices with multiple panels really should build on top of this new KMS property.
Contact: Hans de Goede
Level: Advanced
Buffer age or other damage accumulation algorithm for buffer damage¶
Drivers that do per-buffer uploads, need a buffer damage handling (rather than frame damage like drivers that do per-plane or per-CRTC uploads), but there is no support to get the buffer age or any other damage accumulation algorithm.
For this reason, the damage helpers just fallback to a full plane update if the
framebuffer attached to a plane has changed since the last page-flip. Drivers
set &drm_plane_state.ignore_damage_clips to true as indication to
drm_atomic_helper_damage_iter_init()
and drm_atomic_helper_damage_iter_next()
helpers that the damage clips should be ignored.
This should be improved to get damage tracking properly working on drivers that do per-buffer uploads.
More information about damage tracking and references to learning materials can be found in Damage Tracking Properties.
Contact: Javier Martinez Canillas <javierm@redhat.com>
Level: Advanced
Outside DRM¶
Convert fbdev drivers to DRM¶
There are plenty of fbdev drivers for older hardware. Some hardware has become obsolete, but some still provides good(-enough) framebuffers. The drivers that are still useful should be converted to DRM and afterwards removed from fbdev.
Very simple fbdev drivers can best be converted by starting with a new DRM driver. Simple KMS helpers and SHMEM should be able to handle any existing hardware. The new driver’s call-back functions are filled from existing fbdev code.
More complex fbdev drivers can be refactored step-by-step into a DRM driver with the help of the DRM fbconv helpers [4]. These helpers provide the transition layer between the DRM core infrastructure and the fbdev driver interface. Create a new DRM driver on top of the fbconv helpers, copy over the fbdev driver, and hook it up to the DRM code. Examples for several fbdev drivers are available in Thomas Zimmermann’s fbconv tree [4], as well as a tutorial of this process [5]. The result is a primitive DRM driver that can run X11 and Weston.
Contact: Thomas Zimmermann <tzimmermann@suse.de>
Level: Advanced