1.. _todo:
  2
  3=========
  4TODO list
  5=========
  6
  7This section contains a list of smaller janitorial tasks in the kernel DRM
  8graphics subsystem useful as newbie projects. Or for slow rainy days.
  9
 10Difficulty
 11----------
 12
 13To make it easier task are categorized into different levels:
 14
 15Starter: Good tasks to get started with the DRM subsystem.
 16
 17Intermediate: Tasks which need some experience with working in the DRM
 18subsystem, or some specific GPU/display graphics knowledge. For debugging issue
 19it's good to have the relevant hardware (or a virtual driver set up) available
 20for testing.
 21
 22Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem
 23and graphics topics. Generally need the relevant hardware for development and
 24testing.
 25
 26Expert: Only attempt these if you've successfully completed some tricky
 27refactorings already and are an expert in the specific area
 28
 29Subsystem-wide refactorings
 30===========================
 31
 32Remove custom dumb_map_offset implementations
 33---------------------------------------------
 34
 35All GEM based drivers should be using drm_gem_create_mmap_offset() instead.
 36Audit each individual driver, make sure it'll work with the generic
 37implementation (there's lots of outdated locking leftovers in various
 38implementations), and then remove it.
 39
 40Contact: Simona Vetter, respective driver maintainers
 41
 42Level: Intermediate
 43
 44Convert existing KMS drivers to atomic modesetting
 45--------------------------------------------------
 46
 473.19 has the atomic modeset interfaces and helpers, so drivers can now be
 48converted over. Modern compositors like Wayland or Surfaceflinger on Android
 49really want an atomic modeset interface, so this is all about the bright
 50future.
 51
 52There is a conversion guide for atomic [1]_ and all you need is a GPU for a
 53non-converted driver.  The "Atomic mode setting design overview" series [2]_
 54[3]_ at LWN.net can also be helpful.
 55
 56As part of this drivers also need to convert to universal plane (which means
 57exposing primary & cursor as proper plane objects). But that's much easier to
 58do by directly using the new atomic helper driver callbacks.
 59
 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/
 63
 64Contact: Simona Vetter, respective driver maintainers
 65
 66Level: Advanced
 67
 68Clean up the clipped coordination confusion around planes
 69---------------------------------------------------------
 70
 71We have a helper to get this right with drm_plane_helper_check_update(), but
 72it's not consistently used. This should be fixed, preferably in the atomic
 73helpers (and drivers then moved over to clipped coordinates). Probably the
 74helper should also be moved from drm_plane_helper.c to the atomic helpers, to
 75avoid confusion - the other helpers in that file are all deprecated legacy
 76helpers.
 77
 78Contact: Ville Syrjälä, Simona Vetter, driver maintainers
 79
 80Level: Advanced
 81
 82Improve plane atomic_check helpers
 83----------------------------------
 84
 85Aside from the clipped coordinates right above there's a few suboptimal things
 86with the current helpers:
 87
 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.
 93
 94- Once that's done, helpers could stop calling ->atomic_check for disabled
 95  planes.
 96
 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.
 99
100Contact: Simona Vetter
101
102Level: Advanced
103
104Convert early atomic drivers to async commit helpers
105----------------------------------------------------
106
107For the first year the atomic modeset helpers didn't support asynchronous /
108nonblocking commits, and every driver had to hand-roll them. This is fixed
109now, but there's still a pile of existing drivers that easily could be
110converted over to the new infrastructure.
111
112One issue with the helpers is that they require that drivers handle completion
113events for atomic commits correctly. But fixing these bugs is good anyway.
114
115Somewhat related is the legacy_cursor_update hack, which should be replaced with
116the new atomic_async_check/commit functionality in the helpers in drivers that
117still look at that flag.
118
119Contact: Simona Vetter, respective driver maintainers
120
121Level: Advanced
122
123Rename drm_atomic_state
124-----------------------
125
126The KMS framework uses two slightly different definitions for the ``state``
127concept. For a given object (plane, CRTC, encoder, etc., so
128``drm_$OBJECT_state``), the state is the entire state of that object. However,
129at the device level, ``drm_atomic_state`` refers to a state update for a
130limited number of objects.
131
132The state isn't the entire device state, but only the full state of some
133objects in that device. This is confusing to newcomers, and
134``drm_atomic_state`` should be renamed to something clearer like
135``drm_atomic_commit``.
136
137In addition to renaming the structure itself, it would also imply renaming some
138related functions (``drm_atomic_state_alloc``, ``drm_atomic_state_get``,
139``drm_atomic_state_put``, ``drm_atomic_state_init``,
140``__drm_atomic_state_free``, etc.).
141
142Contact: Maxime Ripard <mripard@kernel.org>
143
144Level: Advanced
145
146Fallout from atomic KMS
147-----------------------
148
149``drm_atomic_helper.c`` provides a batch of functions which implement legacy
150IOCTLs on top of the new atomic driver interface. Which is really nice for
151gradual conversion of drivers, but unfortunately the semantic mismatches are
152a bit too severe. So there's some follow-up work to adjust the function
153interfaces to fix these issues:
154
155* atomic needs the lock acquire context. At the moment that's passed around
156  implicitly with some horrible hacks, and it's also allocate with
157  ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
158  the acquire context explicitly on stack and then also pass it down into
159  drivers explicitly so that the legacy-on-atomic functions can use them.
160
161  Except for some driver code this is done. This task should be finished by
162  adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().
163
164* A bunch of the vtable hooks are now in the wrong place: DRM has a split
165  between core vfunc tables (named ``drm_foo_funcs``), which are used to
166  implement the userspace ABI. And then there's the optional hooks for the
167  helper libraries (name ``drm_foo_helper_funcs``), which are purely for
168  internal use. Some of these hooks should be move from ``_funcs`` to
169  ``_helper_funcs`` since they are not part of the core ABI. There's a
170  ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
171
172Contact: Simona Vetter
173
174Level: Intermediate
175
176Get rid of dev->struct_mutex from GEM drivers
177---------------------------------------------
178
179``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
180everything. Nowadays in modern drivers the only bit where it's mandatory is
181serializing GEM buffer object destruction. Which unfortunately means drivers
182have to keep track of that lock and either call ``unreference`` or
183``unreference_locked`` depending upon context.
184
185Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
186and there's a GEM object ``free`` callback for any drivers which are
187entirely ``struct_mutex`` free.
188
189For drivers that need ``struct_mutex`` it should be replaced with a driver-
190private lock. The tricky part is the BO free functions, since those can't
191reliably take that lock any more. Instead state needs to be protected with
192suitable subordinate locks or some cleanup work pushed to a worker thread. For
193performance-critical drivers it might also be better to go with a more
194fine-grained per-buffer object and per-context lockings scheme. Currently only
195the ``msm`` and `i915` drivers use ``struct_mutex``.
196
197Contact: Simona Vetter, respective driver maintainers
198
199Level: Advanced
200
201Move Buffer Object Locking to dma_resv_lock()
202---------------------------------------------
203
204Many drivers have their own per-object locking scheme, usually using
205mutex_lock(). This causes all kinds of trouble for buffer sharing, since
206depending which driver is the exporter and importer, the locking hierarchy is
207reversed.
208
209To solve this we need one standard per-object locking mechanism, which is
210dma_resv_lock(). This lock needs to be called as the outermost lock, with all
211other driver specific per-object locks removed. The problem is that rolling out
212the actual change to the locking contract is a flag day, due to struct dma_buf
213buffer sharing.
214
215Level: Expert
216
217Convert logging to drm_* functions with drm_device parameter
218------------------------------------------------------------
219
220For drivers which could have multiple instances, it is necessary to
221differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
222don't do this, drivers used dev_info/warn/err to make this differentiation. We
223now have drm_* variants of the drm print functions, so we can start to convert
224those drivers back to using drm-formatted specific log messages.
225
226Before you start this conversion please contact the relevant maintainers to make
227sure your work will be merged - not everyone agrees that the DRM dmesg macros
228are better.
229
230Contact: Sean Paul, Maintainer of the driver you plan to convert
231
232Level: Starter
233
234Convert drivers to use simple modeset suspend/resume
235----------------------------------------------------
236
237Most drivers (except i915 and nouveau) that use
238drm_atomic_helper_suspend/resume() can probably be converted to use
239drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
240of the atomic suspend/resume code in older atomic modeset drivers.
241
242Contact: Maintainer of the driver you plan to convert
243
244Level: Intermediate
245
 
 
 
 
 
 
 
 
 
 
 
 
 
246Reimplement functions in drm_fbdev_fb_ops without fbdev
247-------------------------------------------------------
248
249A number of callback functions in drm_fbdev_fb_ops could benefit from
250being rewritten without dependencies on the fbdev module. Some of the
251helpers could further benefit from using struct iosys_map instead of
252raw pointers.
253
254Contact: Thomas Zimmermann <tzimmermann@suse.de>, Simona Vetter
255
256Level: Advanced
257
258Benchmark and optimize blitting and format-conversion function
259--------------------------------------------------------------
260
261Drawing to display memory quickly is crucial for many applications'
262performance.
263
264On at least x86-64, sys_imageblit() is significantly slower than
265cfb_imageblit(), even though both use the same blitting algorithm and
266the latter is written for I/O memory. It turns out that cfb_imageblit()
267uses movl instructions, while sys_imageblit apparently does not. This
268seems to be a problem with gcc's optimizer. DRM's format-conversion
269helpers might be subject to similar issues.
270
271Benchmark and optimize fbdev's sys_() helpers and DRM's format-conversion
272helpers. In cases that can be further optimized, maybe implement a different
273algorithm. For micro-optimizations, use movl/movq instructions explicitly.
274That might possibly require architecture-specific helpers (e.g., storel()
275storeq()).
276
277Contact: Thomas Zimmermann <tzimmermann@suse.de>
278
279Level: Intermediate
280
281drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup
282-----------------------------------------------------------------
283
284A lot more drivers could be switched over to the drm_gem_framebuffer helpers.
285Various hold-ups:
286
287- Need to switch over to the generic dirty tracking code using
288  drm_atomic_helper_dirtyfb first (e.g. qxl).
289
290- Need to switch to drm_fbdev_generic_setup(), otherwise a lot of the custom fb
291  setup code can't be deleted.
292
293- Need to switch to drm_gem_fb_create(), as now drm_gem_fb_create() checks for
294  valid formats for atomic drivers.
295
296- Many drivers subclass drm_framebuffer, we'd need a embedding compatible
297  version of the varios drm_gem_fb_create functions. Maybe called
298  drm_gem_fb_create/_with_dirty/_with_funcs as needed.
299
300Contact: Simona Vetter
301
302Level: Intermediate
303
304Generic fbdev defio support
305---------------------------
306
307The defio support code in the fbdev core has some very specific requirements,
308which means drivers need to have a special framebuffer for fbdev. The main
309issue is that it uses some fields in struct page itself, which breaks shmem
310gem objects (and other things). To support defio, affected drivers require
311the use of a shadow buffer, which may add CPU and memory overhead.
312
313Possible solution would be to write our own defio mmap code in the drm fbdev
314emulation. It would need to fully wrap the existing mmap ops, forwarding
315everything after it has done the write-protect/mkwrite trickery:
316
317- In the drm_fbdev_fb_mmap helper, if we need defio, change the
318  default page prots to write-protected with something like this::
319
320      vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
321
322- Set the mkwrite and fsync callbacks with similar implementions to the core
323  fbdev defio stuff. These should all work on plain ptes, they don't actually
324  require a struct page.  uff. These should all work on plain ptes, they don't
325  actually require a struct page.
326
327- Track the dirty pages in a separate structure (bitfield with one bit per page
328  should work) to avoid clobbering struct page.
329
330Might be good to also have some igt testcases for this.
331
332Contact: Simona Vetter, Noralf Tronnes
333
334Level: Advanced
335
336connector register/unregister fixes
337-----------------------------------
338
339- For most connectors it's a no-op to call drm_connector_register/unregister
340  directly from driver code, drm_dev_register/unregister take care of this
341  already. We can remove all of them.
342
343- For dp drivers it's a bit more a mess, since we need the connector to be
344  registered when calling drm_dp_aux_register. Fix this by instead calling
345  drm_dp_aux_init, and moving the actual registering into a late_register
346  callback as recommended in the kerneldoc.
347
348Level: Intermediate
349
350Remove load/unload callbacks
351----------------------------
352
353The load/unload callbacks in struct &drm_driver are very much midlayers, plus
354for historical reasons they get the ordering wrong (and we can't fix that)
355between setting up the &drm_driver structure and calling drm_dev_register().
356
357- Rework drivers to no longer use the load/unload callbacks, directly coding the
358  load/unload sequence into the driver's probe function.
359
360- Once all drivers are converted, remove the load/unload callbacks.
361
362Contact: Simona Vetter
363
364Level: Intermediate
365
366Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi
367---------------------------------------------------------------
368
369Once EDID is parsed, the monitor HDMI support information is available through
370drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor() to
371retrieve the same information, which is less efficient.
372
373Audit each individual driver calling drm_detect_hdmi_monitor() and switch to
374drm_display_info.is_hdmi if applicable.
375
376Contact: Laurent Pinchart, respective driver maintainers
377
378Level: Intermediate
379
380Consolidate custom driver modeset properties
381--------------------------------------------
382
383Before atomic modeset took place, many drivers where creating their own
384properties. Among other things, atomic brought the requirement that custom,
385driver specific properties should not be used.
386
387For this task, we aim to introduce core helpers or reuse the existing ones
388if available:
389
390A quick, unconfirmed, examples list.
391
392Introduce core helpers:
393- audio (amdgpu, intel, gma500, radeon)
394- brightness, contrast, etc (armada, nouveau) - overlay only (?)
395- broadcast rgb (gma500, intel)
396- colorkey (armada, nouveau, rcar) - overlay only (?)
397- dither (amdgpu, nouveau, radeon) - varies across drivers
398- underscan family (amdgpu, radeon, nouveau)
399
400Already in core:
401- colorspace (sti)
402- tv format names, enhancements (gma500, intel)
403- tv overscan, margins, etc. (gma500, intel)
404- zorder (omapdrm) - same as zpos (?)
405
406
407Contact: Emil Velikov, respective driver maintainers
408
409Level: Intermediate
410
411Use struct iosys_map throughout codebase
412----------------------------------------
413
414Pointers to shared device memory are stored in struct iosys_map. Each
415instance knows whether it refers to system or I/O memory. Most of the DRM-wide
416interface have been converted to use struct iosys_map, but implementations
417often still use raw pointers.
418
419The task is to use struct iosys_map where it makes sense.
420
421* Memory managers should use struct iosys_map for dma-buf-imported buffers.
422* TTM might benefit from using struct iosys_map internally.
423* Framebuffer copying and blitting helpers should operate on struct iosys_map.
424
425Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Simona Vetter
426
427Level: Intermediate
428
429Review all drivers for setting struct drm_mode_config.{max_width,max_height} correctly
430--------------------------------------------------------------------------------------
431
432The values in struct drm_mode_config.{max_width,max_height} describe the
433maximum supported framebuffer size. It's the virtual screen size, but many
434drivers treat it like limitations of the physical resolution.
435
436The maximum width depends on the hardware's maximum scanline pitch. The
437maximum height depends on the amount of addressable video memory. Review all
438drivers to initialize the fields to the correct values.
439
440Contact: Thomas Zimmermann <tzimmermann@suse.de>
441
442Level: Intermediate
443
444Request memory regions in all drivers
445-------------------------------------
446
447Go through all drivers and add code to request the memory regions that the
448driver uses. This requires adding calls to request_mem_region(),
449pci_request_region() or similar functions. Use helpers for managed cleanup
450where possible.
451
452Drivers are pretty bad at doing this and there used to be conflicts among
453DRM and fbdev drivers. Still, it's the correct thing to do.
454
455Contact: Thomas Zimmermann <tzimmermann@suse.de>
456
457Level: Starter
458
459Remove driver dependencies on FB_DEVICE
460---------------------------------------
461
462A number of fbdev drivers provide attributes via sysfs and therefore depend
463on CONFIG_FB_DEVICE to be selected. Review each driver and attempt to make
464any dependencies on CONFIG_FB_DEVICE optional. At the minimum, the respective
465code in the driver could be conditionalized via ifdef CONFIG_FB_DEVICE. Not
466all drivers might be able to drop CONFIG_FB_DEVICE.
467
468Contact: Thomas Zimmermann <tzimmermann@suse.de>
469
470Level: Starter
471
472Remove disable/unprepare in remove/shutdown in panel-simple and panel-edp
473-------------------------------------------------------------------------
474
475As of commit d2aacaf07395 ("drm/panel: Check for already prepared/enabled in
476drm_panel"), we have a check in the drm_panel core to make sure nobody
477double-calls prepare/enable/disable/unprepare. Eventually that should probably
478be turned into a WARN_ON() or somehow made louder.
479
480At the moment, we expect that we may still encounter the warnings in the
481drm_panel core when using panel-simple and panel-edp. Since those panel
482drivers are used with a lot of different DRM modeset drivers they still
483make an extra effort to disable/unprepare the panel themsevles at shutdown
484time. Specifically we could still encounter those warnings if the panel
485driver gets shutdown() _before_ the DRM modeset driver and the DRM modeset
486driver properly calls drm_atomic_helper_shutdown() in its own shutdown()
487callback. Warnings could be avoided in such a case by using something like
488device links to ensure that the panel gets shutdown() after the DRM modeset
489driver.
490
491Once all DRM modeset drivers are known to shutdown properly, the extra
492calls to disable/unprepare in remove/shutdown in panel-simple and panel-edp
493should be removed and this TODO item marked complete.
494
495Contact: Douglas Anderson <dianders@chromium.org>
496
497Level: Intermediate
498
499Transition away from using mipi_dsi_*_write_seq()
500-------------------------------------------------
501
502The macros mipi_dsi_generic_write_seq() and mipi_dsi_dcs_write_seq() are
503non-intuitive because, if there are errors, they return out of the *caller's*
504function. We should move all callers to use mipi_dsi_generic_write_seq_multi()
505and mipi_dsi_dcs_write_seq_multi() macros instead.
506
507Once all callers are transitioned, the macros and the functions that they call,
508mipi_dsi_generic_write_chatty() and mipi_dsi_dcs_write_buffer_chatty(), can
509probably be removed. Alternatively, if people feel like the _multi() variants
510are overkill for some use cases, we could keep the mipi_dsi_*_write_seq()
511variants but change them not to return out of the caller.
512
513Contact: Douglas Anderson <dianders@chromium.org>
514
515Level: Starter
516
517
518Core refactorings
519=================
520
521Make panic handling work
522------------------------
523
524This is a really varied tasks with lots of little bits and pieces:
525
526* The panic path can't be tested currently, leading to constant breaking. The
527  main issue here is that panics can be triggered from hardirq contexts and
528  hence all panic related callback can run in hardirq context. It would be
529  awesome if we could test at least the fbdev helper code and driver code by
530  e.g. trigger calls through drm debugfs files. hardirq context could be
531  achieved by using an IPI to the local processor.
532
533* There's a massive confusion of different panic handlers. DRM fbdev emulation
534  helpers had their own (long removed), but on top of that the fbcon code itself
535  also has one. We need to make sure that they stop fighting over each other.
536  This is worked around by checking ``oops_in_progress`` at various entry points
537  into the DRM fbdev emulation helpers. A much cleaner approach here would be to
538  switch fbcon to the `threaded printk support
539  <https://lwn.net/Articles/800946/>`_.
540
541* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
542  isn't a full solution for panic paths. We need to make sure that it only
543  returns true if there's a panic going on for real, and fix up all the
544  fallout.
545
546* The panic handler must never sleep, which also means it can't ever
547  ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
548  even spinlocks (because NMI and hardirq can panic too). We need to either
549  make sure to not call such paths, or trylock everything. Really tricky.
550
551* A clean solution would be an entirely separate panic output support in KMS,
552  bypassing the current fbcon support. See `[PATCH v2 0/3] drm: Add panic handling
553  <https://lore.kernel.org/dri-devel/20190311174218.51899-1-noralf@tronnes.org/>`_.
554
555* Encoding the actual oops and preceding dmesg in a QR might help with the
556  dread "important stuff scrolled away" problem. See `[RFC][PATCH] Oops messages
557  transfer using QR codes
558  <https://lore.kernel.org/lkml/1446217392-11981-1-git-send-email-alexandru.murtaza@intel.com/>`_
559  for some example code that could be reused.
560
561Contact: Simona Vetter
562
563Level: Advanced
564
565Clean up the debugfs support
566----------------------------
567
568There's a bunch of issues with it:
569
570- Convert drivers to support the drm_debugfs_add_files() function instead of
571  the drm_debugfs_create_files() function.
572
573- Improve late-register debugfs by rolling out the same debugfs pre-register
574  infrastructure for connector and crtc too. That way, the drivers won't need to
575  split their setup code into init and register anymore.
576
577- We probably want to have some support for debugfs files on crtc/connectors and
578  maybe other kms objects directly in core. There's even drm_print support in
579  the funcs for these objects to dump kms state, so it's all there. And then the
580  ->show() functions should obviously give you a pointer to the right object.
581
582- The drm_driver->debugfs_init hooks we have is just an artifact of the old
583  midlayered load sequence. DRM debugfs should work more like sysfs, where you
584  can create properties/files for an object anytime you want, and the core
585  takes care of publishing/unpuplishing all the files at register/unregister
586  time. Drivers shouldn't need to worry about these technicalities, and fixing
587  this (together with the drm_minor->drm_device move) would allow us to remove
588  debugfs_init.
589
590Contact: Simona Vetter
591
592Level: Intermediate
593
594Object lifetime fixes
595---------------------
596
597There's two related issues here
598
599- Cleanup up the various ->destroy callbacks, which often are all the same
600  simple code.
601
602- Lots of drivers erroneously allocate DRM modeset objects using devm_kzalloc,
603  which results in use-after free issues on driver unload. This can be serious
604  trouble even for drivers for hardware integrated on the SoC due to
605  EPROBE_DEFERRED backoff.
606
607Both these problems can be solved by switching over to drmm_kzalloc(), and the
608various convenience wrappers provided, e.g. drmm_crtc_alloc_with_planes(),
609drmm_universal_plane_alloc(), ... and so on.
610
611Contact: Simona Vetter
612
613Level: Intermediate
614
615Remove automatic page mapping from dma-buf importing
616----------------------------------------------------
617
618When importing dma-bufs, the dma-buf and PRIME frameworks automatically map
619imported pages into the importer's DMA area. drm_gem_prime_fd_to_handle() and
620drm_gem_prime_handle_to_fd() require that importers call dma_buf_attach()
621even if they never do actual device DMA, but only CPU access through
622dma_buf_vmap(). This is a problem for USB devices, which do not support DMA
623operations.
624
625To fix the issue, automatic page mappings should be removed from the
626buffer-sharing code. Fixing this is a bit more involved, since the import/export
627cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over
628this problem for USB devices by fishing out the USB host controller device, as
629long as that supports DMA. Otherwise importing can still needlessly fail.
630
631Contact: Thomas Zimmermann <tzimmermann@suse.de>, Simona Vetter
632
633Level: Advanced
634
635
636Better Testing
637==============
638
639Add unit tests using the Kernel Unit Testing (KUnit) framework
640--------------------------------------------------------------
641
642The `KUnit <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
643provides a common framework for unit tests within the Linux kernel. Having a
644test suite would allow to identify regressions earlier.
645
646A good candidate for the first unit tests are the format-conversion helpers in
647``drm_format_helper.c``.
648
649Contact: Javier Martinez Canillas <javierm@redhat.com>
650
651Level: Intermediate
652
653Clean up and document former selftests suites
654---------------------------------------------
655
656Some KUnit test suites (drm_buddy, drm_cmdline_parser, drm_damage_helper,
657drm_format, drm_framebuffer, drm_dp_mst_helper, drm_mm, drm_plane_helper and
658drm_rect) are former selftests suites that have been converted over when KUnit
659was first introduced.
660
661These suites were fairly undocumented, and with different goals than what unit
662tests can be. Trying to identify what each test in these suites actually test
663for, whether that makes sense for a unit test, and either remove it if it
664doesn't or document it if it does would be of great help.
665
666Contact: Maxime Ripard <mripard@kernel.org>
667
668Level: Intermediate
669
670Enable trinity for DRM
671----------------------
672
673And fix up the fallout. Should be really interesting ...
674
675Level: Advanced
676
677Make KMS tests in i-g-t generic
678-------------------------------
679
680The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
681including tons of testcases for corner-cases in the modesetting API. It would
682be awesome if those tests (at least the ones not relying on Intel-specific GEM
683features) could be made to run on any KMS driver.
684
685Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
686converting things over. For modeset tests we also first need a bit of
687infrastructure to use dumb buffers for untiled buffers, to be able to run all
688the non-i915 specific modeset tests.
689
690Level: Advanced
691
692Extend virtual test driver (VKMS)
693---------------------------------
694
695See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
696internship task, since it only requires a virtual machine and can be sized to
697fit the available time.
698
699Level: See details
700
701Backlight Refactoring
702---------------------
703
704Backlight drivers have a triple enable/disable state, which is a bit overkill.
705Plan to fix this:
706
7071. Roll out backlight_enable() and backlight_disable() helpers everywhere. This
708   has started already.
7092. In all, only look at one of the three status bits set by the above helpers.
7103. Remove the other two status bits.
711
712Contact: Simona Vetter
713
714Level: Intermediate
715
716Driver Specific
717===============
718
719AMD DC Display Driver
720---------------------
721
722AMD DC is the display driver for AMD devices starting with Vega. There has been
723a bunch of progress cleaning it up but there's still plenty of work to be done.
724
725See drivers/gpu/drm/amd/display/TODO for tasks.
726
727Contact: Harry Wentland, Alex Deucher
728
729Bootsplash
730==========
731
732There is support in place now for writing internal DRM clients making it
733possible to pick up the bootsplash work that was rejected because it was written
734for fbdev.
735
736- [v6,8/8] drm/client: Hack: Add bootsplash example
737  https://patchwork.freedesktop.org/patch/306579/
738
739- [RFC PATCH v2 00/13] Kernel based bootsplash
740  https://lore.kernel.org/r/20171213194755.3409-1-mstaudt@suse.de
741
742Contact: Sam Ravnborg
743
744Level: Advanced
745
746Brightness handling on devices with multiple internal panels
747============================================================
748
749On x86/ACPI devices there can be multiple backlight firmware interfaces:
750(ACPI) video, vendor specific and others. As well as direct/native (PWM)
751register programming by the KMS driver.
752
753To deal with this backlight drivers used on x86/ACPI call
754acpi_video_get_backlight_type() which has heuristics (+quirks) to select
755which backlight interface to use; and backlight drivers which do not match
756the returned type will not register themselves, so that only one backlight
757device gets registered (in a single GPU setup, see below).
758
759At the moment this more or less assumes that there will only
760be 1 (internal) panel on a system.
761
762On systems with 2 panels this may be a problem, depending on
763what interface acpi_video_get_backlight_type() selects:
764
7651. native: in this case the KMS driver is expected to know which backlight
766   device belongs to which output so everything should just work.
7672. video: this does support controlling multiple backlights, but some work
768   will need to be done to get the output <-> backlight device mapping
769
770The above assumes both panels will require the same backlight interface type.
771Things will break on systems with multiple panels where the 2 panels need
772a different type of control. E.g. one panel needs ACPI video backlight control,
773where as the other is using native backlight control. Currently in this case
774only one of the 2 required backlight devices will get registered, based on
775the acpi_video_get_backlight_type() return value.
776
777If this (theoretical) case ever shows up, then supporting this will need some
778work. A possible solution here would be to pass a device and connector-name
779to acpi_video_get_backlight_type() so that it can deal with this.
780
781Note in a way we already have a case where userspace sees 2 panels,
782in dual GPU laptop setups with a mux. On those systems we may see
783either 2 native backlight devices; or 2 native backlight devices.
784
785Userspace already has code to deal with this by detecting if the related
786panel is active (iow which way the mux between the GPU and the panels
787points) and then uses that backlight device. Userspace here very much
788assumes a single panel though. It picks only 1 of the 2 backlight devices
789and then only uses that one.
790
791Note that all userspace code (that I know off) is currently hardcoded
792to assume a single panel.
793
794Before the recent changes to not register multiple (e.g. video + native)
795/sys/class/backlight devices for a single panel (on a single GPU laptop),
796userspace would see multiple backlight devices all controlling the same
797backlight.
798
799To deal with this userspace had to always picks one preferred device under
800/sys/class/backlight and will ignore the others. So to support brightness
801control on multiple panels userspace will need to be updated too.
802
803There are plans to allow brightness control through the KMS API by adding
804a "display brightness" property to drm_connector objects for panels. This
805solves a number of issues with the /sys/class/backlight API, including not
806being able to map a sysfs backlight device to a specific connector. Any
807userspace changes to add support for brightness control on devices with
808multiple panels really should build on top of this new KMS property.
809
810Contact: Hans de Goede
811
812Level: Advanced
813
814Buffer age or other damage accumulation algorithm for buffer damage
815===================================================================
816
817Drivers that do per-buffer uploads, need a buffer damage handling (rather than
818frame damage like drivers that do per-plane or per-CRTC uploads), but there is
819no support to get the buffer age or any other damage accumulation algorithm.
820
821For this reason, the damage helpers just fallback to a full plane update if the
822framebuffer attached to a plane has changed since the last page-flip. Drivers
823set &drm_plane_state.ignore_damage_clips to true as indication to
824drm_atomic_helper_damage_iter_init() and drm_atomic_helper_damage_iter_next()
825helpers that the damage clips should be ignored.
826
827This should be improved to get damage tracking properly working on drivers that
828do per-buffer uploads.
829
830More information about damage tracking and references to learning materials can
831be found in :ref:`damage_tracking_properties`.
832
833Contact: Javier Martinez Canillas <javierm@redhat.com>
834
835Level: Advanced
836
837Querying errors from drm_syncobj
838================================
839
840The drm_syncobj container can be used by driver independent code to signal
841complection of submission.
842
843One minor feature still missing is a generic DRM IOCTL to query the error
844status of binary and timeline drm_syncobj.
845
846This should probably be improved by implementing the necessary kernel interface
847and adding support for that in the userspace stack.
848
849Contact: Christian König
850
851Level: Starter
852
853Outside DRM
854===========
855
856Convert fbdev drivers to DRM
857----------------------------
858
859There are plenty of fbdev drivers for older hardware. Some hardware has
860become obsolete, but some still provides good(-enough) framebuffers. The
861drivers that are still useful should be converted to DRM and afterwards
862removed from fbdev.
863
864Very simple fbdev drivers can best be converted by starting with a new
865DRM driver. Simple KMS helpers and SHMEM should be able to handle any
866existing hardware. The new driver's call-back functions are filled from
867existing fbdev code.
868
869More complex fbdev drivers can be refactored step-by-step into a DRM
870driver with the help of the DRM fbconv helpers [4]_. These helpers provide
871the transition layer between the DRM core infrastructure and the fbdev
872driver interface. Create a new DRM driver on top of the fbconv helpers,
873copy over the fbdev driver, and hook it up to the DRM code. Examples for
874several fbdev drivers are available in Thomas Zimmermann's fbconv tree
875[4]_, as well as a tutorial of this process [5]_. The result is a primitive
876DRM driver that can run X11 and Weston.
877
878 .. [4] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv
879 .. [5] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c
880
881Contact: Thomas Zimmermann <tzimmermann@suse.de>
882
883Level: Advanced

  1.. _todo:
  2
  3=========
  4TODO list
  5=========
  6
  7This section contains a list of smaller janitorial tasks in the kernel DRM
  8graphics subsystem useful as newbie projects. Or for slow rainy days.
  9
 10Difficulty
 11----------
 12
 13To make it easier task are categorized into different levels:
 14
 15Starter: Good tasks to get started with the DRM subsystem.
 16
 17Intermediate: Tasks which need some experience with working in the DRM
 18subsystem, or some specific GPU/display graphics knowledge. For debugging issue
 19it's good to have the relevant hardware (or a virtual driver set up) available
 20for testing.
 21
 22Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem
 23and graphics topics. Generally need the relevant hardware for development and
 24testing.
 25
 26Expert: Only attempt these if you've successfully completed some tricky
 27refactorings already and are an expert in the specific area
 28
 29Subsystem-wide refactorings
 30===========================
 31
 32Remove custom dumb_map_offset implementations
 33---------------------------------------------
 34
 35All GEM based drivers should be using drm_gem_create_mmap_offset() instead.
 36Audit each individual driver, make sure it'll work with the generic
 37implementation (there's lots of outdated locking leftovers in various
 38implementations), and then remove it.
 39
 40Contact: Daniel Vetter, respective driver maintainers
 41
 42Level: Intermediate
 43
 44Convert existing KMS drivers to atomic modesetting
 45--------------------------------------------------
 46
 473.19 has the atomic modeset interfaces and helpers, so drivers can now be
 48converted over. Modern compositors like Wayland or Surfaceflinger on Android
 49really want an atomic modeset interface, so this is all about the bright
 50future.
 51
 52There is a conversion guide for atomic [1]_ and all you need is a GPU for a
 53non-converted driver.  The "Atomic mode setting design overview" series [2]_
 54[3]_ at LWN.net can also be helpful.
 55
 56As part of this drivers also need to convert to universal plane (which means
 57exposing primary & cursor as proper plane objects). But that's much easier to
 58do by directly using the new atomic helper driver callbacks.
 59
 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/
 63
 64Contact: Daniel Vetter, respective driver maintainers
 65
 66Level: Advanced
 67
 68Clean up the clipped coordination confusion around planes
 69---------------------------------------------------------
 70
 71We have a helper to get this right with drm_plane_helper_check_update(), but
 72it's not consistently used. This should be fixed, preferably in the atomic
 73helpers (and drivers then moved over to clipped coordinates). Probably the
 74helper should also be moved from drm_plane_helper.c to the atomic helpers, to
 75avoid confusion - the other helpers in that file are all deprecated legacy
 76helpers.
 77
 78Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
 79
 80Level: Advanced
 81
 82Improve plane atomic_check helpers
 83----------------------------------
 84
 85Aside from the clipped coordinates right above there's a few suboptimal things
 86with the current helpers:
 87
 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.
 93
 94- Once that's done, helpers could stop calling ->atomic_check for disabled
 95  planes.
 96
 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.
 99
100Contact: Daniel Vetter
101
102Level: Advanced
103
104Convert early atomic drivers to async commit helpers
105----------------------------------------------------
106
107For the first year the atomic modeset helpers didn't support asynchronous /
108nonblocking commits, and every driver had to hand-roll them. This is fixed
109now, but there's still a pile of existing drivers that easily could be
110converted over to the new infrastructure.
111
112One issue with the helpers is that they require that drivers handle completion
113events for atomic commits correctly. But fixing these bugs is good anyway.
114
115Somewhat related is the legacy_cursor_update hack, which should be replaced with
116the new atomic_async_check/commit functionality in the helpers in drivers that
117still look at that flag.
118
119Contact: Daniel Vetter, respective driver maintainers
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
120
121Level: Advanced
122
123Fallout from atomic KMS
124-----------------------
125
126``drm_atomic_helper.c`` provides a batch of functions which implement legacy
127IOCTLs on top of the new atomic driver interface. Which is really nice for
128gradual conversion of drivers, but unfortunately the semantic mismatches are
129a bit too severe. So there's some follow-up work to adjust the function
130interfaces to fix these issues:
131
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.
137
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().
140
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``.
148
149Contact: Daniel Vetter
150
151Level: Intermediate
152
153Get rid of dev->struct_mutex from GEM drivers
154---------------------------------------------
155
156``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
157everything. Nowadays in modern drivers the only bit where it's mandatory is
158serializing GEM buffer object destruction. Which unfortunately means drivers
159have to keep track of that lock and either call ``unreference`` or
160``unreference_locked`` depending upon context.
161
162Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
163and there's a GEM object ``free`` callback for any drivers which are
164entirely ``struct_mutex`` free.
165
166For drivers that need ``struct_mutex`` it should be replaced with a driver-
167private lock. The tricky part is the BO free functions, since those can't
168reliably take that lock any more. Instead state needs to be protected with
169suitable subordinate locks or some cleanup work pushed to a worker thread. For
170performance-critical drivers it might also be better to go with a more
171fine-grained per-buffer object and per-context lockings scheme. Currently only
172the ``msm`` and `i915` drivers use ``struct_mutex``.
173
174Contact: Daniel Vetter, respective driver maintainers
175
176Level: Advanced
177
178Move Buffer Object Locking to dma_resv_lock()
179---------------------------------------------
180
181Many drivers have their own per-object locking scheme, usually using
182mutex_lock(). This causes all kinds of trouble for buffer sharing, since
183depending which driver is the exporter and importer, the locking hierarchy is
184reversed.
185
186To solve this we need one standard per-object locking mechanism, which is
187dma_resv_lock(). This lock needs to be called as the outermost lock, with all
188other driver specific per-object locks removed. The problem is that rolling out
189the actual change to the locking contract is a flag day, due to struct dma_buf
190buffer sharing.
191
192Level: Expert
193
194Convert logging to drm_* functions with drm_device parameter
195------------------------------------------------------------
196
197For drivers which could have multiple instances, it is necessary to
198differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
199don't do this, drivers used dev_info/warn/err to make this differentiation. We
200now have drm_* variants of the drm print functions, so we can start to convert
201those drivers back to using drm-formatted specific log messages.
202
203Before you start this conversion please contact the relevant maintainers to make
204sure your work will be merged - not everyone agrees that the DRM dmesg macros
205are better.
206
207Contact: Sean Paul, Maintainer of the driver you plan to convert
208
209Level: Starter
210
211Convert drivers to use simple modeset suspend/resume
212----------------------------------------------------
213
214Most drivers (except i915 and nouveau) that use
215drm_atomic_helper_suspend/resume() can probably be converted to use
216drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
217of the atomic suspend/resume code in older atomic modeset drivers.
218
219Contact: Maintainer of the driver you plan to convert
220
221Level: Intermediate
222
223Convert drivers to use drm_fbdev_generic_setup()
224------------------------------------------------
225
226Most drivers can use drm_fbdev_generic_setup(). Driver have to implement
227atomic modesetting and GEM vmap support. Historically, generic fbdev emulation
228expected the framebuffer in system memory or system-like memory. By employing
229struct iosys_map, drivers with frambuffers in I/O memory can be supported
230as well.
231
232Contact: Maintainer of the driver you plan to convert
233
234Level: Intermediate
235
236Reimplement functions in drm_fbdev_fb_ops without fbdev
237-------------------------------------------------------
238
239A number of callback functions in drm_fbdev_fb_ops could benefit from
240being rewritten without dependencies on the fbdev module. Some of the
241helpers could further benefit from using struct iosys_map instead of
242raw pointers.
243
244Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
245
246Level: Advanced
247
248Benchmark and optimize blitting and format-conversion function
249--------------------------------------------------------------
250
251Drawing to display memory quickly is crucial for many applications'
252performance.
253
254On at least x86-64, sys_imageblit() is significantly slower than
255cfb_imageblit(), even though both use the same blitting algorithm and
256the latter is written for I/O memory. It turns out that cfb_imageblit()
257uses movl instructions, while sys_imageblit apparently does not. This
258seems to be a problem with gcc's optimizer. DRM's format-conversion
259helpers might be subject to similar issues.
260
261Benchmark and optimize fbdev's sys_() helpers and DRM's format-conversion
262helpers. In cases that can be further optimized, maybe implement a different
263algorithm. For micro-optimizations, use movl/movq instructions explicitly.
264That might possibly require architecture-specific helpers (e.g., storel()
265storeq()).
266
267Contact: Thomas Zimmermann <tzimmermann@suse.de>
268
269Level: Intermediate
270
271drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup
272-----------------------------------------------------------------
273
274A lot more drivers could be switched over to the drm_gem_framebuffer helpers.
275Various hold-ups:
276
277- Need to switch over to the generic dirty tracking code using
278  drm_atomic_helper_dirtyfb first (e.g. qxl).
279
280- Need to switch to drm_fbdev_generic_setup(), otherwise a lot of the custom fb
281  setup code can't be deleted.
282
283- Need to switch to drm_gem_fb_create(), as now drm_gem_fb_create() checks for
284  valid formats for atomic drivers.
285
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.
289
290Contact: Daniel Vetter
291
292Level: Intermediate
293
294Generic fbdev defio support
295---------------------------
296
297The defio support code in the fbdev core has some very specific requirements,
298which means drivers need to have a special framebuffer for fbdev. The main
299issue is that it uses some fields in struct page itself, which breaks shmem
300gem objects (and other things). To support defio, affected drivers require
301the use of a shadow buffer, which may add CPU and memory overhead.
302
303Possible solution would be to write our own defio mmap code in the drm fbdev
304emulation. It would need to fully wrap the existing mmap ops, forwarding
305everything after it has done the write-protect/mkwrite trickery:
306
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::
309
310      vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
311
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.
316
317- Track the dirty pages in a separate structure (bitfield with one bit per page
318  should work) to avoid clobbering struct page.
319
320Might be good to also have some igt testcases for this.
321
322Contact: Daniel Vetter, Noralf Tronnes
323
324Level: Advanced
325
326connector register/unregister fixes
327-----------------------------------
328
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.
332
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.
337
338Level: Intermediate
339
340Remove load/unload callbacks
341----------------------------
342
343The load/unload callbacks in struct &drm_driver are very much midlayers, plus
344for historical reasons they get the ordering wrong (and we can't fix that)
345between setting up the &drm_driver structure and calling drm_dev_register().
346
347- Rework drivers to no longer use the load/unload callbacks, directly coding the
348  load/unload sequence into the driver's probe function.
349
350- Once all drivers are converted, remove the load/unload callbacks.
351
352Contact: Daniel Vetter
353
354Level: Intermediate
355
356Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi
357---------------------------------------------------------------
358
359Once EDID is parsed, the monitor HDMI support information is available through
360drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor() to
361retrieve the same information, which is less efficient.
362
363Audit each individual driver calling drm_detect_hdmi_monitor() and switch to
364drm_display_info.is_hdmi if applicable.
365
366Contact: Laurent Pinchart, respective driver maintainers
367
368Level: Intermediate
369
370Consolidate custom driver modeset properties
371--------------------------------------------
372
373Before atomic modeset took place, many drivers where creating their own
374properties. Among other things, atomic brought the requirement that custom,
375driver specific properties should not be used.
376
377For this task, we aim to introduce core helpers or reuse the existing ones
378if available:
379
380A quick, unconfirmed, examples list.
381
382Introduce 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)
389
390Already in core:
391- colorspace (sti)
392- tv format names, enhancements (gma500, intel)
393- tv overscan, margins, etc. (gma500, intel)
394- zorder (omapdrm) - same as zpos (?)
395
396
397Contact: Emil Velikov, respective driver maintainers
398
399Level: Intermediate
400
401Use struct iosys_map throughout codebase
402----------------------------------------
403
404Pointers to shared device memory are stored in struct iosys_map. Each
405instance knows whether it refers to system or I/O memory. Most of the DRM-wide
406interface have been converted to use struct iosys_map, but implementations
407often still use raw pointers.
408
409The task is to use struct iosys_map where it makes sense.
410
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.
414
415Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Daniel Vetter
416
417Level: Intermediate
418
419Review all drivers for setting struct drm_mode_config.{max_width,max_height} correctly
420--------------------------------------------------------------------------------------
421
422The values in struct drm_mode_config.{max_width,max_height} describe the
423maximum supported framebuffer size. It's the virtual screen size, but many
424drivers treat it like limitations of the physical resolution.
425
426The maximum width depends on the hardware's maximum scanline pitch. The
427maximum height depends on the amount of addressable video memory. Review all
428drivers to initialize the fields to the correct values.
429
430Contact: Thomas Zimmermann <tzimmermann@suse.de>
431
432Level: Intermediate
433
434Request memory regions in all drivers
435-------------------------------------
436
437Go through all drivers and add code to request the memory regions that the
438driver uses. This requires adding calls to request_mem_region(),
439pci_request_region() or similar functions. Use helpers for managed cleanup
440where possible.
441
442Drivers are pretty bad at doing this and there used to be conflicts among
443DRM and fbdev drivers. Still, it's the correct thing to do.
444
445Contact: Thomas Zimmermann <tzimmermann@suse.de>
446
447Level: Starter
448
449Remove driver dependencies on FB_DEVICE
450---------------------------------------
451
452A number of fbdev drivers provide attributes via sysfs and therefore depend
453on CONFIG_FB_DEVICE to be selected. Review each driver and attempt to make
454any dependencies on CONFIG_FB_DEVICE optional. At the minimum, the respective
455code in the driver could be conditionalized via ifdef CONFIG_FB_DEVICE. Not
456all drivers might be able to drop CONFIG_FB_DEVICE.
457
458Contact: Thomas Zimmermann <tzimmermann@suse.de>
459
460Level: Starter
461
462Clean up checks for already prepared/enabled in panels
463------------------------------------------------------
464
465In a whole pile of panel drivers, we have code to make the
466prepare/unprepare/enable/disable callbacks behave as no-ops if they've already
467been called. To get some idea of the duplicated code, try::
468
469  git grep 'if.*>prepared' -- drivers/gpu/drm/panel
470  git grep 'if.*>enabled' -- drivers/gpu/drm/panel
471
472In the patch ("drm/panel: Check for already prepared/enabled in drm_panel")
473we've moved this check to the core. Now we can most definitely remove the
474check from the individual panels and save a pile of code.
475
476In adition to removing the check from the individual panels, it is believed
477that even the core shouldn't need this check and that should be considered
478an error if other code ever relies on this check. The check in the core
479currently prints a warning whenever something is relying on this check with
480dev_warn(). After a little while, we likely want to promote this to a
481WARN(1) to help encourage folks not to rely on this behavior.
 
 
482
483Contact: Douglas Anderson <dianders@chromium.org>
484
485Level: Starter/Intermediate
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
486
487
488Core refactorings
489=================
490
491Make panic handling work
492------------------------
493
494This is a really varied tasks with lots of little bits and pieces:
495
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.
502
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/>`_.
510
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
514  fallout.
515
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.
520
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/>`_.
524
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.
530
531Contact: Daniel Vetter
532
533Level: Advanced
534
535Clean up the debugfs support
536----------------------------
537
538There's a bunch of issues with it:
539
540- Convert drivers to support the drm_debugfs_add_files() function instead of
541  the drm_debugfs_create_files() function.
542
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.
546
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.
551
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
558  debugfs_init.
559
560Contact: Daniel Vetter
561
562Level: Intermediate
563
564Object lifetime fixes
565---------------------
566
567There's two related issues here
568
569- Cleanup up the various ->destroy callbacks, which often are all the same
570  simple code.
571
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.
576
577Both these problems can be solved by switching over to drmm_kzalloc(), and the
578various convenience wrappers provided, e.g. drmm_crtc_alloc_with_planes(),
579drmm_universal_plane_alloc(), ... and so on.
580
581Contact: Daniel Vetter
582
583Level: Intermediate
584
585Remove automatic page mapping from dma-buf importing
586----------------------------------------------------
587
588When importing dma-bufs, the dma-buf and PRIME frameworks automatically map
589imported pages into the importer's DMA area. drm_gem_prime_fd_to_handle() and
590drm_gem_prime_handle_to_fd() require that importers call dma_buf_attach()
591even if they never do actual device DMA, but only CPU access through
592dma_buf_vmap(). This is a problem for USB devices, which do not support DMA
593operations.
594
595To fix the issue, automatic page mappings should be removed from the
596buffer-sharing code. Fixing this is a bit more involved, since the import/export
597cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over
598this problem for USB devices by fishing out the USB host controller device, as
599long as that supports DMA. Otherwise importing can still needlessly fail.
600
601Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
602
603Level: Advanced
604
605
606Better Testing
607==============
608
609Add unit tests using the Kernel Unit Testing (KUnit) framework
610--------------------------------------------------------------
611
612The `KUnit <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
613provides a common framework for unit tests within the Linux kernel. Having a
614test suite would allow to identify regressions earlier.
615
616A good candidate for the first unit tests are the format-conversion helpers in
617``drm_format_helper.c``.
618
619Contact: Javier Martinez Canillas <javierm@redhat.com>
620
621Level: Intermediate
622
623Clean up and document former selftests suites
624---------------------------------------------
625
626Some KUnit test suites (drm_buddy, drm_cmdline_parser, drm_damage_helper,
627drm_format, drm_framebuffer, drm_dp_mst_helper, drm_mm, drm_plane_helper and
628drm_rect) are former selftests suites that have been converted over when KUnit
629was first introduced.
630
631These suites were fairly undocumented, and with different goals than what unit
632tests can be. Trying to identify what each test in these suites actually test
633for, whether that makes sense for a unit test, and either remove it if it
634doesn't or document it if it does would be of great help.
635
636Contact: Maxime Ripard <mripard@kernel.org>
637
638Level: Intermediate
639
640Enable trinity for DRM
641----------------------
642
643And fix up the fallout. Should be really interesting ...
644
645Level: Advanced
646
647Make KMS tests in i-g-t generic
648-------------------------------
649
650The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
651including tons of testcases for corner-cases in the modesetting API. It would
652be awesome if those tests (at least the ones not relying on Intel-specific GEM
653features) could be made to run on any KMS driver.
654
655Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
656converting things over. For modeset tests we also first need a bit of
657infrastructure to use dumb buffers for untiled buffers, to be able to run all
658the non-i915 specific modeset tests.
659
660Level: Advanced
661
662Extend virtual test driver (VKMS)
663---------------------------------
664
665See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
666internship task, since it only requires a virtual machine and can be sized to
667fit the available time.
668
669Level: See details
670
671Backlight Refactoring
672---------------------
673
674Backlight drivers have a triple enable/disable state, which is a bit overkill.
675Plan to fix this:
676
6771. Roll out backlight_enable() and backlight_disable() helpers everywhere. This
678   has started already.
6792. In all, only look at one of the three status bits set by the above helpers.
6803. Remove the other two status bits.
681
682Contact: Daniel Vetter
683
684Level: Intermediate
685
686Driver Specific
687===============
688
689AMD DC Display Driver
690---------------------
691
692AMD DC is the display driver for AMD devices starting with Vega. There has been
693a bunch of progress cleaning it up but there's still plenty of work to be done.
694
695See drivers/gpu/drm/amd/display/TODO for tasks.
696
697Contact: Harry Wentland, Alex Deucher
698
699Bootsplash
700==========
701
702There is support in place now for writing internal DRM clients making it
703possible to pick up the bootsplash work that was rejected because it was written
704for fbdev.
705
706- [v6,8/8] drm/client: Hack: Add bootsplash example
707  https://patchwork.freedesktop.org/patch/306579/
708
709- [RFC PATCH v2 00/13] Kernel based bootsplash
710  https://lore.kernel.org/r/20171213194755.3409-1-mstaudt@suse.de
711
712Contact: Sam Ravnborg
713
714Level: Advanced
715
716Brightness handling on devices with multiple internal panels
717============================================================
718
719On x86/ACPI devices there can be multiple backlight firmware interfaces:
720(ACPI) video, vendor specific and others. As well as direct/native (PWM)
721register programming by the KMS driver.
722
723To deal with this backlight drivers used on x86/ACPI call
724acpi_video_get_backlight_type() which has heuristics (+quirks) to select
725which backlight interface to use; and backlight drivers which do not match
726the returned type will not register themselves, so that only one backlight
727device gets registered (in a single GPU setup, see below).
728
729At the moment this more or less assumes that there will only
730be 1 (internal) panel on a system.
731
732On systems with 2 panels this may be a problem, depending on
733what interface acpi_video_get_backlight_type() selects:
734
7351. native: in this case the KMS driver is expected to know which backlight
736   device belongs to which output so everything should just work.
7372. video: this does support controlling multiple backlights, but some work
738   will need to be done to get the output <-> backlight device mapping
739
740The above assumes both panels will require the same backlight interface type.
741Things will break on systems with multiple panels where the 2 panels need
742a different type of control. E.g. one panel needs ACPI video backlight control,
743where as the other is using native backlight control. Currently in this case
744only one of the 2 required backlight devices will get registered, based on
745the acpi_video_get_backlight_type() return value.
746
747If this (theoretical) case ever shows up, then supporting this will need some
748work. A possible solution here would be to pass a device and connector-name
749to acpi_video_get_backlight_type() so that it can deal with this.
750
751Note in a way we already have a case where userspace sees 2 panels,
752in dual GPU laptop setups with a mux. On those systems we may see
753either 2 native backlight devices; or 2 native backlight devices.
754
755Userspace already has code to deal with this by detecting if the related
756panel is active (iow which way the mux between the GPU and the panels
757points) and then uses that backlight device. Userspace here very much
758assumes a single panel though. It picks only 1 of the 2 backlight devices
759and then only uses that one.
760
761Note that all userspace code (that I know off) is currently hardcoded
762to assume a single panel.
763
764Before 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),
766userspace would see multiple backlight devices all controlling the same
767backlight.
768
769To 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
771control on multiple panels userspace will need to be updated too.
772
773There are plans to allow brightness control through the KMS API by adding
774a "display brightness" property to drm_connector objects for panels. This
775solves a number of issues with the /sys/class/backlight API, including not
776being able to map a sysfs backlight device to a specific connector. Any
777userspace changes to add support for brightness control on devices with
778multiple panels really should build on top of this new KMS property.
779
780Contact: Hans de Goede
781
782Level: Advanced
783
784Buffer age or other damage accumulation algorithm for buffer damage
785===================================================================
786
787Drivers that do per-buffer uploads, need a buffer damage handling (rather than
788frame damage like drivers that do per-plane or per-CRTC uploads), but there is
789no support to get the buffer age or any other damage accumulation algorithm.
790
791For this reason, the damage helpers just fallback to a full plane update if the
792framebuffer attached to a plane has changed since the last page-flip. Drivers
793set &drm_plane_state.ignore_damage_clips to true as indication to
794drm_atomic_helper_damage_iter_init() and drm_atomic_helper_damage_iter_next()
795helpers that the damage clips should be ignored.
796
797This should be improved to get damage tracking properly working on drivers that
798do per-buffer uploads.
799
800More information about damage tracking and references to learning materials can
801be found in :ref:`damage_tracking_properties`.
802
803Contact: Javier Martinez Canillas <javierm@redhat.com>
804
805Level: Advanced
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
806
807Outside DRM
808===========
809
810Convert fbdev drivers to DRM
811----------------------------
812
813There are plenty of fbdev drivers for older hardware. Some hardware has
814become obsolete, but some still provides good(-enough) framebuffers. The
815drivers that are still useful should be converted to DRM and afterwards
816removed from fbdev.
817
818Very simple fbdev drivers can best be converted by starting with a new
819DRM driver. Simple KMS helpers and SHMEM should be able to handle any
820existing hardware. The new driver's call-back functions are filled from
821existing fbdev code.
822
823More complex fbdev drivers can be refactored step-by-step into a DRM
824driver with the help of the DRM fbconv helpers [4]_. These helpers provide
825the transition layer between the DRM core infrastructure and the fbdev
826driver interface. Create a new DRM driver on top of the fbconv helpers,
827copy over the fbdev driver, and hook it up to the DRM code. Examples for
828several 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
830DRM driver that can run X11 and Weston.
831
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
834
835Contact: Thomas Zimmermann <tzimmermann@suse.de>
836
837Level: Advanced