7 This section contains a list of smaller janitorial tasks in the kernel DRM
8 graphics subsystem useful as newbie projects. Or for slow rainy days.
13 To make it easier task are categorized into different levels:
15 Starter: Good tasks to get started with the DRM subsystem.
17 Intermediate: Tasks which need some experience with working in the DRM
18 subsystem, or some specific GPU/display graphics knowledge. For debugging issue
19 it's good to have the relevant hardware (or a virtual driver set up) available
22 Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem
23 and graphics topics. Generally need the relevant hardware for development and
26 Expert: Only attempt these if you've successfully completed some tricky
27 refactorings already and are an expert in the specific area
29 Subsystem-wide refactorings
30 ===========================
32 Remove custom dumb_map_offset implementations
33 ---------------------------------------------
35 All GEM based drivers should be using drm_gem_create_mmap_offset() instead.
36 Audit each individual driver, make sure it'll work with the generic
37 implementation (there's lots of outdated locking leftovers in various
38 implementations), and then remove it.
40 Contact: Daniel Vetter, respective driver maintainers
44 Convert existing KMS drivers to atomic modesetting
45 --------------------------------------------------
47 3.19 has the atomic modeset interfaces and helpers, so drivers can now be
48 converted over. Modern compositors like Wayland or Surfaceflinger on Android
49 really want an atomic modeset interface, so this is all about the bright
52 There is a conversion guide for atomic [1]_ and all you need is a GPU for a
53 non-converted driver. The "Atomic mode setting design overview" series [2]_
54 [3]_ at LWN.net can also be helpful.
56 As part of this drivers also need to convert to universal plane (which means
57 exposing primary & cursor as proper plane objects). But that's much easier to
58 do by directly using the new atomic helper driver callbacks.
60 .. [1] https://blog.ffwll.ch/2014/11/atomic-modeset-support-for-kms-drivers.html
61 .. [2] https://lwn.net/Articles/653071/
62 .. [3] https://lwn.net/Articles/653466/
64 Contact: Daniel Vetter, respective driver maintainers
68 Clean up the clipped coordination confusion around planes
69 ---------------------------------------------------------
71 We have a helper to get this right with drm_plane_helper_check_update(), but
72 it's not consistently used. This should be fixed, preferably in the atomic
73 helpers (and drivers then moved over to clipped coordinates). Probably the
74 helper should also be moved from drm_plane_helper.c to the atomic helpers, to
75 avoid confusion - the other helpers in that file are all deprecated legacy
78 Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
82 Improve plane atomic_check helpers
83 ----------------------------------
85 Aside from the clipped coordinates right above there's a few suboptimal things
86 with the current helpers:
88 - drm_plane_helper_funcs->atomic_check gets called for enabled or disabled
89 planes. At best this seems to confuse drivers, worst it means they blow up
90 when the plane is disabled without the CRTC. The only special handling is
91 resetting values in the plane state structures, which instead should be moved
92 into the drm_plane_funcs->atomic_duplicate_state functions.
94 - Once that's done, helpers could stop calling ->atomic_check for disabled
97 - Then we could go through all the drivers and remove the more-or-less confused
98 checks for plane_state->fb and plane_state->crtc.
100 Contact: Daniel Vetter
104 Convert early atomic drivers to async commit helpers
105 ----------------------------------------------------
107 For the first year the atomic modeset helpers didn't support asynchronous /
108 nonblocking commits, and every driver had to hand-roll them. This is fixed
109 now, but there's still a pile of existing drivers that easily could be
110 converted over to the new infrastructure.
112 One issue with the helpers is that they require that drivers handle completion
113 events for atomic commits correctly. But fixing these bugs is good anyway.
115 Somewhat related is the legacy_cursor_update hack, which should be replaced with
116 the new atomic_async_check/commit functionality in the helpers in drivers that
117 still look at that flag.
119 Contact: Daniel Vetter, respective driver maintainers
123 Fallout from atomic KMS
124 -----------------------
126 ``drm_atomic_helper.c`` provides a batch of functions which implement legacy
127 IOCTLs on top of the new atomic driver interface. Which is really nice for
128 gradual conversion of drivers, but unfortunately the semantic mismatches are
129 a bit too severe. So there's some follow-up work to adjust the function
130 interfaces to fix these issues:
132 * atomic needs the lock acquire context. At the moment that's passed around
133 implicitly with some horrible hacks, and it's also allocate with
134 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
135 the acquire context explicitly on stack and then also pass it down into
136 drivers explicitly so that the legacy-on-atomic functions can use them.
138 Except for some driver code this is done. This task should be finished by
139 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().
141 * A bunch of the vtable hooks are now in the wrong place: DRM has a split
142 between core vfunc tables (named ``drm_foo_funcs``), which are used to
143 implement the userspace ABI. And then there's the optional hooks for the
144 helper libraries (name ``drm_foo_helper_funcs``), which are purely for
145 internal use. Some of these hooks should be move from ``_funcs`` to
146 ``_helper_funcs`` since they are not part of the core ABI. There's a
147 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
149 Contact: Daniel Vetter
153 Get rid of dev->struct_mutex from GEM drivers
154 ---------------------------------------------
156 ``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
157 everything. Nowadays in modern drivers the only bit where it's mandatory is
158 serializing GEM buffer object destruction. Which unfortunately means drivers
159 have to keep track of that lock and either call ``unreference`` or
160 ``unreference_locked`` depending upon context.
162 Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
163 and there's a GEM object ``free`` callback for any drivers which are
164 entirely ``struct_mutex`` free.
166 For drivers that need ``struct_mutex`` it should be replaced with a driver-
167 private lock. The tricky part is the BO free functions, since those can't
168 reliably take that lock any more. Instead state needs to be protected with
169 suitable subordinate locks or some cleanup work pushed to a worker thread. For
170 performance-critical drivers it might also be better to go with a more
171 fine-grained per-buffer object and per-context lockings scheme. Currently only
172 the ``msm`` and `i915` drivers use ``struct_mutex``.
174 Contact: Daniel Vetter, respective driver maintainers
178 Move Buffer Object Locking to dma_resv_lock()
179 ---------------------------------------------
181 Many drivers have their own per-object locking scheme, usually using
182 mutex_lock(). This causes all kinds of trouble for buffer sharing, since
183 depending which driver is the exporter and importer, the locking hierarchy is
186 To solve this we need one standard per-object locking mechanism, which is
187 dma_resv_lock(). This lock needs to be called as the outermost lock, with all
188 other driver specific per-object locks removed. The problem is that rolling out
189 the actual change to the locking contract is a flag day, due to struct dma_buf
194 Convert logging to drm_* functions with drm_device parameter
195 ------------------------------------------------------------
197 For drivers which could have multiple instances, it is necessary to
198 differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
199 don't do this, drivers used dev_info/warn/err to make this differentiation. We
200 now have drm_* variants of the drm print functions, so we can start to convert
201 those drivers back to using drm-formatted specific log messages.
203 Before you start this conversion please contact the relevant maintainers to make
204 sure your work will be merged - not everyone agrees that the DRM dmesg macros
207 Contact: Sean Paul, Maintainer of the driver you plan to convert
211 Convert drivers to use simple modeset suspend/resume
212 ----------------------------------------------------
214 Most drivers (except i915 and nouveau) that use
215 drm_atomic_helper_suspend/resume() can probably be converted to use
216 drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
217 of the atomic suspend/resume code in older atomic modeset drivers.
219 Contact: Maintainer of the driver you plan to convert
223 Convert drivers to use drm_fbdev_generic_setup()
224 ------------------------------------------------
226 Most drivers can use drm_fbdev_generic_setup(). Driver have to implement
227 atomic modesetting and GEM vmap support. Historically, generic fbdev emulation
228 expected the framebuffer in system memory or system-like memory. By employing
229 struct iosys_map, drivers with frambuffers in I/O memory can be supported
232 Contact: Maintainer of the driver you plan to convert
236 Reimplement functions in drm_fbdev_fb_ops without fbdev
237 -------------------------------------------------------
239 A number of callback functions in drm_fbdev_fb_ops could benefit from
240 being rewritten without dependencies on the fbdev module. Some of the
241 helpers could further benefit from using struct iosys_map instead of
244 Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
248 Benchmark and optimize blitting and format-conversion function
249 --------------------------------------------------------------
251 Drawing to display memory quickly is crucial for many applications'
254 On at least x86-64, sys_imageblit() is significantly slower than
255 cfb_imageblit(), even though both use the same blitting algorithm and
256 the latter is written for I/O memory. It turns out that cfb_imageblit()
257 uses movl instructions, while sys_imageblit apparently does not. This
258 seems to be a problem with gcc's optimizer. DRM's format-conversion
259 helpers might be subject to similar issues.
261 Benchmark and optimize fbdev's sys_() helpers and DRM's format-conversion
262 helpers. In cases that can be further optimized, maybe implement a different
263 algorithm. For micro-optimizations, use movl/movq instructions explicitly.
264 That might possibly require architecture-specific helpers (e.g., storel()
267 Contact: Thomas Zimmermann <tzimmermann@suse.de>
271 drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup
272 -----------------------------------------------------------------
274 A lot more drivers could be switched over to the drm_gem_framebuffer helpers.
277 - Need to switch over to the generic dirty tracking code using
278 drm_atomic_helper_dirtyfb first (e.g. qxl).
280 - Need to switch to drm_fbdev_generic_setup(), otherwise a lot of the custom fb
281 setup code can't be deleted.
283 - Need to switch to drm_gem_fb_create(), as now drm_gem_fb_create() checks for
284 valid formats for atomic drivers.
286 - Many drivers subclass drm_framebuffer, we'd need a embedding compatible
287 version of the varios drm_gem_fb_create functions. Maybe called
288 drm_gem_fb_create/_with_dirty/_with_funcs as needed.
290 Contact: Daniel Vetter
294 Generic fbdev defio support
295 ---------------------------
297 The defio support code in the fbdev core has some very specific requirements,
298 which means drivers need to have a special framebuffer for fbdev. The main
299 issue is that it uses some fields in struct page itself, which breaks shmem
300 gem objects (and other things). To support defio, affected drivers require
301 the use of a shadow buffer, which may add CPU and memory overhead.
303 Possible solution would be to write our own defio mmap code in the drm fbdev
304 emulation. It would need to fully wrap the existing mmap ops, forwarding
305 everything after it has done the write-protect/mkwrite trickery:
307 - In the drm_fbdev_fb_mmap helper, if we need defio, change the
308 default page prots to write-protected with something like this::
310 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
312 - Set the mkwrite and fsync callbacks with similar implementions to the core
313 fbdev defio stuff. These should all work on plain ptes, they don't actually
314 require a struct page. uff. These should all work on plain ptes, they don't
315 actually require a struct page.
317 - Track the dirty pages in a separate structure (bitfield with one bit per page
318 should work) to avoid clobbering struct page.
320 Might be good to also have some igt testcases for this.
322 Contact: Daniel Vetter, Noralf Tronnes
326 connector register/unregister fixes
327 -----------------------------------
329 - For most connectors it's a no-op to call drm_connector_register/unregister
330 directly from driver code, drm_dev_register/unregister take care of this
331 already. We can remove all of them.
333 - For dp drivers it's a bit more a mess, since we need the connector to be
334 registered when calling drm_dp_aux_register. Fix this by instead calling
335 drm_dp_aux_init, and moving the actual registering into a late_register
336 callback as recommended in the kerneldoc.
340 Remove load/unload callbacks
341 ----------------------------
343 The load/unload callbacks in struct &drm_driver are very much midlayers, plus
344 for historical reasons they get the ordering wrong (and we can't fix that)
345 between setting up the &drm_driver structure and calling drm_dev_register().
347 - Rework drivers to no longer use the load/unload callbacks, directly coding the
348 load/unload sequence into the driver's probe function.
350 - Once all drivers are converted, remove the load/unload callbacks.
352 Contact: Daniel Vetter
356 Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi
357 ---------------------------------------------------------------
359 Once EDID is parsed, the monitor HDMI support information is available through
360 drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor() to
361 retrieve the same information, which is less efficient.
363 Audit each individual driver calling drm_detect_hdmi_monitor() and switch to
364 drm_display_info.is_hdmi if applicable.
366 Contact: Laurent Pinchart, respective driver maintainers
370 Consolidate custom driver modeset properties
371 --------------------------------------------
373 Before atomic modeset took place, many drivers where creating their own
374 properties. Among other things, atomic brought the requirement that custom,
375 driver specific properties should not be used.
377 For this task, we aim to introduce core helpers or reuse the existing ones
380 A quick, unconfirmed, examples list.
382 Introduce core helpers:
383 - audio (amdgpu, intel, gma500, radeon)
384 - brightness, contrast, etc (armada, nouveau) - overlay only (?)
385 - broadcast rgb (gma500, intel)
386 - colorkey (armada, nouveau, rcar) - overlay only (?)
387 - dither (amdgpu, nouveau, radeon) - varies across drivers
388 - underscan family (amdgpu, radeon, nouveau)
392 - tv format names, enhancements (gma500, intel)
393 - tv overscan, margins, etc. (gma500, intel)
394 - zorder (omapdrm) - same as zpos (?)
397 Contact: Emil Velikov, respective driver maintainers
401 Use struct iosys_map throughout codebase
402 ----------------------------------------
404 Pointers to shared device memory are stored in struct iosys_map. Each
405 instance knows whether it refers to system or I/O memory. Most of the DRM-wide
406 interface have been converted to use struct iosys_map, but implementations
407 often still use raw pointers.
409 The task is to use struct iosys_map where it makes sense.
411 * Memory managers should use struct iosys_map for dma-buf-imported buffers.
412 * TTM might benefit from using struct iosys_map internally.
413 * Framebuffer copying and blitting helpers should operate on struct iosys_map.
415 Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Daniel Vetter
419 Review all drivers for setting struct drm_mode_config.{max_width,max_height} correctly
420 --------------------------------------------------------------------------------------
422 The values in struct drm_mode_config.{max_width,max_height} describe the
423 maximum supported framebuffer size. It's the virtual screen size, but many
424 drivers treat it like limitations of the physical resolution.
426 The maximum width depends on the hardware's maximum scanline pitch. The
427 maximum height depends on the amount of addressable video memory. Review all
428 drivers to initialize the fields to the correct values.
430 Contact: Thomas Zimmermann <tzimmermann@suse.de>
434 Request memory regions in all drivers
435 -------------------------------------
437 Go through all drivers and add code to request the memory regions that the
438 driver uses. This requires adding calls to request_mem_region(),
439 pci_request_region() or similar functions. Use helpers for managed cleanup
442 Drivers are pretty bad at doing this and there used to be conflicts among
443 DRM and fbdev drivers. Still, it's the correct thing to do.
445 Contact: Thomas Zimmermann <tzimmermann@suse.de>
449 Remove driver dependencies on FB_DEVICE
450 ---------------------------------------
452 A number of fbdev drivers provide attributes via sysfs and therefore depend
453 on CONFIG_FB_DEVICE to be selected. Review each driver and attempt to make
454 any dependencies on CONFIG_FB_DEVICE optional. At the minimum, the respective
455 code in the driver could be conditionalized via ifdef CONFIG_FB_DEVICE. Not
456 all drivers might be able to drop CONFIG_FB_DEVICE.
458 Contact: Thomas Zimmermann <tzimmermann@suse.de>
462 Clean up checks for already prepared/enabled in panels
463 ------------------------------------------------------
465 In a whole pile of panel drivers, we have code to make the
466 prepare/unprepare/enable/disable callbacks behave as no-ops if they've already
467 been called. To get some idea of the duplicated code, try::
469 git grep 'if.*>prepared' -- drivers/gpu/drm/panel
470 git grep 'if.*>enabled' -- drivers/gpu/drm/panel
472 In the patch ("drm/panel: Check for already prepared/enabled in drm_panel")
473 we've moved this check to the core. Now we can most definitely remove the
474 check from the individual panels and save a pile of code.
476 In adition to removing the check from the individual panels, it is believed
477 that even the core shouldn't need this check and that should be considered
478 an error if other code ever relies on this check. The check in the core
479 currently prints a warning whenever something is relying on this check with
480 dev_warn(). After a little while, we likely want to promote this to a
481 WARN(1) to help encourage folks not to rely on this behavior.
483 Contact: Douglas Anderson <dianders@chromium.org>
485 Level: Starter/Intermediate
491 Make panic handling work
492 ------------------------
494 This is a really varied tasks with lots of little bits and pieces:
496 * The panic path can't be tested currently, leading to constant breaking. The
497 main issue here is that panics can be triggered from hardirq contexts and
498 hence all panic related callback can run in hardirq context. It would be
499 awesome if we could test at least the fbdev helper code and driver code by
500 e.g. trigger calls through drm debugfs files. hardirq context could be
501 achieved by using an IPI to the local processor.
503 * There's a massive confusion of different panic handlers. DRM fbdev emulation
504 helpers had their own (long removed), but on top of that the fbcon code itself
505 also has one. We need to make sure that they stop fighting over each other.
506 This is worked around by checking ``oops_in_progress`` at various entry points
507 into the DRM fbdev emulation helpers. A much cleaner approach here would be to
508 switch fbcon to the `threaded printk support
509 <https://lwn.net/Articles/800946/>`_.
511 * ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
512 isn't a full solution for panic paths. We need to make sure that it only
513 returns true if there's a panic going on for real, and fix up all the
516 * The panic handler must never sleep, which also means it can't ever
517 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
518 even spinlocks (because NMI and hardirq can panic too). We need to either
519 make sure to not call such paths, or trylock everything. Really tricky.
521 * A clean solution would be an entirely separate panic output support in KMS,
522 bypassing the current fbcon support. See `[PATCH v2 0/3] drm: Add panic handling
523 <https://lore.kernel.org/dri-devel/20190311174218.51899-1-noralf@tronnes.org/>`_.
525 * Encoding the actual oops and preceding dmesg in a QR might help with the
526 dread "important stuff scrolled away" problem. See `[RFC][PATCH] Oops messages
527 transfer using QR codes
528 <https://lore.kernel.org/lkml/1446217392-11981-1-git-send-email-alexandru.murtaza@intel.com/>`_
529 for some example code that could be reused.
531 Contact: Daniel Vetter
535 Clean up the debugfs support
536 ----------------------------
538 There's a bunch of issues with it:
540 - Convert drivers to support the drm_debugfs_add_files() function instead of
541 the drm_debugfs_create_files() function.
543 - Improve late-register debugfs by rolling out the same debugfs pre-register
544 infrastructure for connector and crtc too. That way, the drivers won't need to
545 split their setup code into init and register anymore.
547 - We probably want to have some support for debugfs files on crtc/connectors and
548 maybe other kms objects directly in core. There's even drm_print support in
549 the funcs for these objects to dump kms state, so it's all there. And then the
550 ->show() functions should obviously give you a pointer to the right object.
552 - The drm_driver->debugfs_init hooks we have is just an artifact of the old
553 midlayered load sequence. DRM debugfs should work more like sysfs, where you
554 can create properties/files for an object anytime you want, and the core
555 takes care of publishing/unpuplishing all the files at register/unregister
556 time. Drivers shouldn't need to worry about these technicalities, and fixing
557 this (together with the drm_minor->drm_device move) would allow us to remove
560 Contact: Daniel Vetter
564 Object lifetime fixes
565 ---------------------
567 There's two related issues here
569 - Cleanup up the various ->destroy callbacks, which often are all the same
572 - Lots of drivers erroneously allocate DRM modeset objects using devm_kzalloc,
573 which results in use-after free issues on driver unload. This can be serious
574 trouble even for drivers for hardware integrated on the SoC due to
575 EPROBE_DEFERRED backoff.
577 Both these problems can be solved by switching over to drmm_kzalloc(), and the
578 various convenience wrappers provided, e.g. drmm_crtc_alloc_with_planes(),
579 drmm_universal_plane_alloc(), ... and so on.
581 Contact: Daniel Vetter
585 Remove automatic page mapping from dma-buf importing
586 ----------------------------------------------------
588 When importing dma-bufs, the dma-buf and PRIME frameworks automatically map
589 imported pages into the importer's DMA area. drm_gem_prime_fd_to_handle() and
590 drm_gem_prime_handle_to_fd() require that importers call dma_buf_attach()
591 even if they never do actual device DMA, but only CPU access through
592 dma_buf_vmap(). This is a problem for USB devices, which do not support DMA
595 To fix the issue, automatic page mappings should be removed from the
596 buffer-sharing code. Fixing this is a bit more involved, since the import/export
597 cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over
598 this problem for USB devices by fishing out the USB host controller device, as
599 long as that supports DMA. Otherwise importing can still needlessly fail.
601 Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
609 Add unit tests using the Kernel Unit Testing (KUnit) framework
610 --------------------------------------------------------------
612 The `KUnit <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
613 provides a common framework for unit tests within the Linux kernel. Having a
614 test suite would allow to identify regressions earlier.
616 A good candidate for the first unit tests are the format-conversion helpers in
617 ``drm_format_helper.c``.
619 Contact: Javier Martinez Canillas <javierm@redhat.com>
623 Clean up and document former selftests suites
624 ---------------------------------------------
626 Some KUnit test suites (drm_buddy, drm_cmdline_parser, drm_damage_helper,
627 drm_format, drm_framebuffer, drm_dp_mst_helper, drm_mm, drm_plane_helper and
628 drm_rect) are former selftests suites that have been converted over when KUnit
629 was first introduced.
631 These suites were fairly undocumented, and with different goals than what unit
632 tests can be. Trying to identify what each test in these suites actually test
633 for, whether that makes sense for a unit test, and either remove it if it
634 doesn't or document it if it does would be of great help.
636 Contact: Maxime Ripard <mripard@kernel.org>
640 Enable trinity for DRM
641 ----------------------
643 And fix up the fallout. Should be really interesting ...
647 Make KMS tests in i-g-t generic
648 -------------------------------
650 The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
651 including tons of testcases for corner-cases in the modesetting API. It would
652 be awesome if those tests (at least the ones not relying on Intel-specific GEM
653 features) could be made to run on any KMS driver.
655 Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
656 converting things over. For modeset tests we also first need a bit of
657 infrastructure to use dumb buffers for untiled buffers, to be able to run all
658 the non-i915 specific modeset tests.
662 Extend virtual test driver (VKMS)
663 ---------------------------------
665 See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
666 internship task, since it only requires a virtual machine and can be sized to
667 fit the available time.
671 Backlight Refactoring
672 ---------------------
674 Backlight drivers have a triple enable/disable state, which is a bit overkill.
677 1. Roll out backlight_enable() and backlight_disable() helpers everywhere. This
679 2. In all, only look at one of the three status bits set by the above helpers.
680 3. Remove the other two status bits.
682 Contact: Daniel Vetter
689 AMD DC Display Driver
690 ---------------------
692 AMD DC is the display driver for AMD devices starting with Vega. There has been
693 a bunch of progress cleaning it up but there's still plenty of work to be done.
695 See drivers/gpu/drm/amd/display/TODO for tasks.
697 Contact: Harry Wentland, Alex Deucher
702 There is support in place now for writing internal DRM clients making it
703 possible to pick up the bootsplash work that was rejected because it was written
706 - [v6,8/8] drm/client: Hack: Add bootsplash example
707 https://patchwork.freedesktop.org/patch/306579/
709 - [RFC PATCH v2 00/13] Kernel based bootsplash
710 https://lore.kernel.org/r/20171213194755.3409-1-mstaudt@suse.de
712 Contact: Sam Ravnborg
716 Brightness handling on devices with multiple internal panels
717 ============================================================
719 On x86/ACPI devices there can be multiple backlight firmware interfaces:
720 (ACPI) video, vendor specific and others. As well as direct/native (PWM)
721 register programming by the KMS driver.
723 To deal with this backlight drivers used on x86/ACPI call
724 acpi_video_get_backlight_type() which has heuristics (+quirks) to select
725 which backlight interface to use; and backlight drivers which do not match
726 the returned type will not register themselves, so that only one backlight
727 device gets registered (in a single GPU setup, see below).
729 At the moment this more or less assumes that there will only
730 be 1 (internal) panel on a system.
732 On systems with 2 panels this may be a problem, depending on
733 what interface acpi_video_get_backlight_type() selects:
735 1. native: in this case the KMS driver is expected to know which backlight
736 device belongs to which output so everything should just work.
737 2. video: this does support controlling multiple backlights, but some work
738 will need to be done to get the output <-> backlight device mapping
740 The above assumes both panels will require the same backlight interface type.
741 Things will break on systems with multiple panels where the 2 panels need
742 a different type of control. E.g. one panel needs ACPI video backlight control,
743 where as the other is using native backlight control. Currently in this case
744 only one of the 2 required backlight devices will get registered, based on
745 the acpi_video_get_backlight_type() return value.
747 If this (theoretical) case ever shows up, then supporting this will need some
748 work. A possible solution here would be to pass a device and connector-name
749 to acpi_video_get_backlight_type() so that it can deal with this.
751 Note in a way we already have a case where userspace sees 2 panels,
752 in dual GPU laptop setups with a mux. On those systems we may see
753 either 2 native backlight devices; or 2 native backlight devices.
755 Userspace already has code to deal with this by detecting if the related
756 panel is active (iow which way the mux between the GPU and the panels
757 points) and then uses that backlight device. Userspace here very much
758 assumes a single panel though. It picks only 1 of the 2 backlight devices
759 and then only uses that one.
761 Note that all userspace code (that I know off) is currently hardcoded
762 to assume a single panel.
764 Before the recent changes to not register multiple (e.g. video + native)
765 /sys/class/backlight devices for a single panel (on a single GPU laptop),
766 userspace would see multiple backlight devices all controlling the same
769 To deal with this userspace had to always picks one preferred device under
770 /sys/class/backlight and will ignore the others. So to support brightness
771 control on multiple panels userspace will need to be updated too.
773 There are plans to allow brightness control through the KMS API by adding
774 a "display brightness" property to drm_connector objects for panels. This
775 solves a number of issues with the /sys/class/backlight API, including not
776 being able to map a sysfs backlight device to a specific connector. Any
777 userspace changes to add support for brightness control on devices with
778 multiple panels really should build on top of this new KMS property.
780 Contact: Hans de Goede
784 Buffer age or other damage accumulation algorithm for buffer damage
785 ===================================================================
787 Drivers that do per-buffer uploads, need a buffer damage handling (rather than
788 frame damage like drivers that do per-plane or per-CRTC uploads), but there is
789 no support to get the buffer age or any other damage accumulation algorithm.
791 For this reason, the damage helpers just fallback to a full plane update if the
792 framebuffer attached to a plane has changed since the last page-flip. Drivers
793 set &drm_plane_state.ignore_damage_clips to true as indication to
794 drm_atomic_helper_damage_iter_init() and drm_atomic_helper_damage_iter_next()
795 helpers that the damage clips should be ignored.
797 This should be improved to get damage tracking properly working on drivers that
798 do per-buffer uploads.
800 More information about damage tracking and references to learning materials can
801 be found in :ref:`damage_tracking_properties`.
803 Contact: Javier Martinez Canillas <javierm@redhat.com>
810 Convert fbdev drivers to DRM
811 ----------------------------
813 There are plenty of fbdev drivers for older hardware. Some hardware has
814 become obsolete, but some still provides good(-enough) framebuffers. The
815 drivers that are still useful should be converted to DRM and afterwards
818 Very simple fbdev drivers can best be converted by starting with a new
819 DRM driver. Simple KMS helpers and SHMEM should be able to handle any
820 existing hardware. The new driver's call-back functions are filled from
823 More complex fbdev drivers can be refactored step-by-step into a DRM
824 driver with the help of the DRM fbconv helpers [4]_. These helpers provide
825 the transition layer between the DRM core infrastructure and the fbdev
826 driver interface. Create a new DRM driver on top of the fbconv helpers,
827 copy over the fbdev driver, and hook it up to the DRM code. Examples for
828 several fbdev drivers are available in Thomas Zimmermann's fbconv tree
829 [4]_, as well as a tutorial of this process [5]_. The result is a primitive
830 DRM driver that can run X11 and Weston.
832 .. [4] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv
833 .. [5] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c
835 Contact: Thomas Zimmermann <tzimmermann@suse.de>