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 Subsystem-wide refactorings
27 ===========================
29 Remove custom dumb_map_offset implementations
30 ---------------------------------------------
32 All GEM based drivers should be using drm_gem_create_mmap_offset() instead.
33 Audit each individual driver, make sure it'll work with the generic
34 implementation (there's lots of outdated locking leftovers in various
35 implementations), and then remove it.
37 Contact: Daniel Vetter, respective driver maintainers
41 Convert existing KMS drivers to atomic modesetting
42 --------------------------------------------------
44 3.19 has the atomic modeset interfaces and helpers, so drivers can now be
45 converted over. Modern compositors like Wayland or Surfaceflinger on Android
46 really want an atomic modeset interface, so this is all about the bright
49 There is a conversion guide for atomic and all you need is a GPU for a
50 non-converted driver (again virtual HW drivers for KVM are still all
53 As part of this drivers also need to convert to universal plane (which means
54 exposing primary & cursor as proper plane objects). But that's much easier to
55 do by directly using the new atomic helper driver callbacks.
57 Contact: Daniel Vetter, respective driver maintainers
61 Clean up the clipped coordination confusion around planes
62 ---------------------------------------------------------
64 We have a helper to get this right with drm_plane_helper_check_update(), but
65 it's not consistently used. This should be fixed, preferrably in the atomic
66 helpers (and drivers then moved over to clipped coordinates). Probably the
67 helper should also be moved from drm_plane_helper.c to the atomic helpers, to
68 avoid confusion - the other helpers in that file are all deprecated legacy
71 Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
75 Improve plane atomic_check helpers
76 ----------------------------------
78 Aside from the clipped coordinates right above there's a few suboptimal things
79 with the current helpers:
81 - drm_plane_helper_funcs->atomic_check gets called for enabled or disabled
82 planes. At best this seems to confuse drivers, worst it means they blow up
83 when the plane is disabled without the CRTC. The only special handling is
84 resetting values in the plane state structures, which instead should be moved
85 into the drm_plane_funcs->atomic_duplicate_state functions.
87 - Once that's done, helpers could stop calling ->atomic_check for disabled
90 - Then we could go through all the drivers and remove the more-or-less confused
91 checks for plane_state->fb and plane_state->crtc.
93 Contact: Daniel Vetter
97 Convert early atomic drivers to async commit helpers
98 ----------------------------------------------------
100 For the first year the atomic modeset helpers didn't support asynchronous /
101 nonblocking commits, and every driver had to hand-roll them. This is fixed
102 now, but there's still a pile of existing drivers that easily could be
103 converted over to the new infrastructure.
105 One issue with the helpers is that they require that drivers handle completion
106 events for atomic commits correctly. But fixing these bugs is good anyway.
108 Contact: Daniel Vetter, respective driver maintainers
112 Fallout from atomic KMS
113 -----------------------
115 ``drm_atomic_helper.c`` provides a batch of functions which implement legacy
116 IOCTLs on top of the new atomic driver interface. Which is really nice for
117 gradual conversion of drivers, but unfortunately the semantic mismatches are
118 a bit too severe. So there's some follow-up work to adjust the function
119 interfaces to fix these issues:
121 * atomic needs the lock acquire context. At the moment that's passed around
122 implicitly with some horrible hacks, and it's also allocate with
123 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
124 the acquire context explicitly on stack and then also pass it down into
125 drivers explicitly so that the legacy-on-atomic functions can use them.
127 Except for some driver code this is done. This task should be finished by
128 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().
130 * A bunch of the vtable hooks are now in the wrong place: DRM has a split
131 between core vfunc tables (named ``drm_foo_funcs``), which are used to
132 implement the userspace ABI. And then there's the optional hooks for the
133 helper libraries (name ``drm_foo_helper_funcs``), which are purely for
134 internal use. Some of these hooks should be move from ``_funcs`` to
135 ``_helper_funcs`` since they are not part of the core ABI. There's a
136 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
138 Contact: Daniel Vetter
142 Get rid of dev->struct_mutex from GEM drivers
143 ---------------------------------------------
145 ``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
146 everything. Nowadays in modern drivers the only bit where it's mandatory is
147 serializing GEM buffer object destruction. Which unfortunately means drivers
148 have to keep track of that lock and either call ``unreference`` or
149 ``unreference_locked`` depending upon context.
151 Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
152 and there's a ``gem_free_object_unlocked`` callback for any drivers which are
153 entirely ``struct_mutex`` free.
155 For drivers that need ``struct_mutex`` it should be replaced with a driver-
156 private lock. The tricky part is the BO free functions, since those can't
157 reliably take that lock any more. Instead state needs to be protected with
158 suitable subordinate locks or some cleanup work pushed to a worker thread. For
159 performance-critical drivers it might also be better to go with a more
160 fine-grained per-buffer object and per-context lockings scheme. Currently only the
161 ``msm`` driver still use ``struct_mutex``.
163 Contact: Daniel Vetter, respective driver maintainers
167 Convert logging to drm_* functions with drm_device paramater
168 ------------------------------------------------------------
170 For drivers which could have multiple instances, it is necessary to
171 differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
172 don't do this, drivers used dev_info/warn/err to make this differentiation. We
173 now have drm_* variants of the drm print functions, so we can start to convert
174 those drivers back to using drm-formatted specific log messages.
176 Before you start this conversion please contact the relevant maintainers to make
177 sure your work will be merged - not everyone agrees that the DRM dmesg macros
180 Contact: Sean Paul, Maintainer of the driver you plan to convert
184 Convert drivers to use simple modeset suspend/resume
185 ----------------------------------------------------
187 Most drivers (except i915 and nouveau) that use
188 drm_atomic_helper_suspend/resume() can probably be converted to use
189 drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
190 of the atomic suspend/resume code in older atomic modeset drivers.
192 Contact: Maintainer of the driver you plan to convert
196 Convert drivers to use drm_fbdev_generic_setup()
197 ------------------------------------------------
199 Most drivers can use drm_fbdev_generic_setup(). Driver have to implement
200 atomic modesetting and GEM vmap support. Current generic fbdev emulation
201 expects the framebuffer in system memory (or system-like memory).
203 Contact: Maintainer of the driver you plan to convert
207 drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup
208 -----------------------------------------------------------------
210 A lot more drivers could be switched over to the drm_gem_framebuffer helpers.
213 - Need to switch over to the generic dirty tracking code using
214 drm_atomic_helper_dirtyfb first (e.g. qxl).
216 - Need to switch to drm_fbdev_generic_setup(), otherwise a lot of the custom fb
217 setup code can't be deleted.
219 - Many drivers wrap drm_gem_fb_create() only to check for valid formats. For
220 atomic drivers we could check for valid formats by calling
221 drm_plane_check_pixel_format() against all planes, and pass if any plane
222 supports the format. For non-atomic that's not possible since like the format
223 list for the primary plane is fake and we'd therefor reject valid formats.
225 - Many drivers subclass drm_framebuffer, we'd need a embedding compatible
226 version of the varios drm_gem_fb_create functions. Maybe called
227 drm_gem_fb_create/_with_dirty/_with_funcs as needed.
229 Contact: Daniel Vetter
233 Clean up mmap forwarding
234 ------------------------
236 A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers.
237 And also a lot of them forward dma-buf mmap to the gem mmap implementations.
238 There's drm_gem_prime_mmap() for this now, but still needs to be rolled out.
240 Contact: Daniel Vetter
244 Generic fbdev defio support
245 ---------------------------
247 The defio support code in the fbdev core has some very specific requirements,
248 which means drivers need to have a special framebuffer for fbdev. The main
249 issue is that it uses some fields in struct page itself, which breaks shmem
250 gem objects (and other things). To support defio, affected drivers require
251 the use of a shadow buffer, which may add CPU and memory overhead.
253 Possible solution would be to write our own defio mmap code in the drm fbdev
254 emulation. It would need to fully wrap the existing mmap ops, forwarding
255 everything after it has done the write-protect/mkwrite trickery:
257 - In the drm_fbdev_fb_mmap helper, if we need defio, change the
258 default page prots to write-protected with something like this::
260 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
262 - Set the mkwrite and fsync callbacks with similar implementions to the core
263 fbdev defio stuff. These should all work on plain ptes, they don't actually
264 require a struct page. uff. These should all work on plain ptes, they don't
265 actually require a struct page.
267 - Track the dirty pages in a separate structure (bitfield with one bit per page
268 should work) to avoid clobbering struct page.
270 Might be good to also have some igt testcases for this.
272 Contact: Daniel Vetter, Noralf Tronnes
279 DRM core&drivers uses a lot of idr (integer lookup directories) for mapping
280 userspace IDs to internal objects, and in most places ID=0 means NULL and hence
281 is never used. Switching to idr_init_base() for these would make the idr more
284 Contact: Daniel Vetter
288 struct drm_gem_object_funcs
289 ---------------------------
291 GEM objects can now have a function table instead of having the callbacks on the
292 DRM driver struct. This is now the preferred way and drivers can be moved over.
294 We also need a 2nd version of the CMA define that doesn't require the
295 vmapping to be present (different hook for prime importing). Plus this needs to
296 be rolled out to all drivers using their own implementations, too.
300 Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate
301 ---------------------------------------------------------
303 For cases where drivers are attempting to grab the modeset locks with a local
304 acquire context. Replace the boilerplate code surrounding
305 drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and
306 DRM_MODESET_LOCK_ALL_END() instead.
308 This should also be done for all places where drm_modest_lock_all() is still
311 As a reference, take a look at the conversions already completed in drm core.
313 Contact: Sean Paul, respective driver maintainers
317 Rename CMA helpers to DMA helpers
318 ---------------------------------
320 CMA (standing for contiguous memory allocator) is really a bit an accident of
321 what these were used for first, a much better name would be DMA helpers. In the
322 text these should even be called coherent DMA memory helpers (so maybe CDM, but
323 no one knows what that means) since underneath they just use dma_alloc_coherent.
325 Contact: Laurent Pinchart, Daniel Vetter
327 Level: Intermediate (mostly because it is a huge tasks without good partial
328 milestones, not technically itself that challenging)
330 Convert direct mode.vrefresh accesses to use drm_mode_vrefresh()
331 ----------------------------------------------------------------
333 drm_display_mode.vrefresh isn't guaranteed to be populated. As such, using it
334 is risky and has been known to cause div-by-zero bugs. Fortunately, drm core
335 has helper which will use mode.vrefresh if it's !0 and will calculate it from
336 the timings when it's 0.
338 Use simple search/replace, or (more fun) cocci to replace instances of direct
339 vrefresh access with a call to the helper. Check out
340 https://lists.freedesktop.org/archives/dri-devel/2019-January/205186.html for
343 Once all instances of vrefresh have been converted, remove vrefresh from
344 drm_display_mode to avoid future use.
350 Remove drm_display_mode.hsync
351 -----------------------------
353 We have drm_mode_hsync() to calculate this from hsync_start/end, since drivers
354 shouldn't/don't use this, remove this member to avoid any temptations to use it
355 in the future. If there is any debug code using drm_display_mode.hsync, convert
356 it to use drm_mode_hsync() instead.
362 connector register/unregister fixes
363 -----------------------------------
365 - For most connectors it's a no-op to call drm_connector_register/unregister
366 directly from driver code, drm_dev_register/unregister take care of this
367 already. We can remove all of them.
369 - For dp drivers it's a bit more a mess, since we need the connector to be
370 registered when calling drm_dp_aux_register. Fix this by instead calling
371 drm_dp_aux_init, and moving the actual registering into a late_register
372 callback as recommended in the kerneldoc.
376 Remove load/unload callbacks from all non-DRIVER_LEGACY drivers
377 ---------------------------------------------------------------
379 The load/unload callbacks in struct &drm_driver are very much midlayers, plus
380 for historical reasons they get the ordering wrong (and we can't fix that)
381 between setting up the &drm_driver structure and calling drm_dev_register().
383 - Rework drivers to no longer use the load/unload callbacks, directly coding the
384 load/unload sequence into the driver's probe function.
386 - Once all non-DRIVER_LEGACY drivers are converted, disallow the load/unload
387 callbacks for all modern drivers.
389 Contact: Daniel Vetter
393 Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi
394 ---------------------------------------------------------------
396 Once EDID is parsed, the monitor HDMI support information is available through
397 drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor() to
398 retrieve the same information, which is less efficient.
400 Audit each individual driver calling drm_detect_hdmi_monitor() and switch to
401 drm_display_info.is_hdmi if applicable.
403 Contact: Laurent Pinchart, respective driver maintainers
410 Make panic handling work
411 ------------------------
413 This is a really varied tasks with lots of little bits and pieces:
415 * The panic path can't be tested currently, leading to constant breaking. The
416 main issue here is that panics can be triggered from hardirq contexts and
417 hence all panic related callback can run in hardirq context. It would be
418 awesome if we could test at least the fbdev helper code and driver code by
419 e.g. trigger calls through drm debugfs files. hardirq context could be
420 achieved by using an IPI to the local processor.
422 * There's a massive confusion of different panic handlers. DRM fbdev emulation
423 helpers have one, but on top of that the fbcon code itself also has one. We
424 need to make sure that they stop fighting over each another.
426 * ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
427 isn't a full solution for panic paths. We need to make sure that it only
428 returns true if there's a panic going on for real, and fix up all the
431 * The panic handler must never sleep, which also means it can't ever
432 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
433 even spinlocks (because NMI and hardirq can panic too). We need to either
434 make sure to not call such paths, or trylock everything. Really tricky.
436 * For the above locking troubles reasons it's pretty much impossible to
437 attempt a synchronous modeset from panic handlers. The only thing we could
438 try to achive is an atomic ``set_base`` of the primary plane, and hope that
439 it shows up. Everything else probably needs to be delayed to some worker or
440 something else which happens later on. Otherwise it just kills the box
441 harder, prevent the panic from going out on e.g. netconsole.
443 * There's also proposal for a simplied DRM console instead of the full-blown
444 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
445 obviously work for both console, in case we ever get kmslog merged.
447 Contact: Daniel Vetter
451 Clean up the debugfs support
452 ----------------------------
454 There's a bunch of issues with it:
456 - The drm_info_list ->show() function doesn't even bother to cast to the drm
457 structure for you. This is lazy.
459 - We probably want to have some support for debugfs files on crtc/connectors and
460 maybe other kms objects directly in core. There's even drm_print support in
461 the funcs for these objects to dump kms state, so it's all there. And then the
462 ->show() functions should obviously give you a pointer to the right object.
464 - The drm_info_list stuff is centered on drm_minor instead of drm_device. For
465 anything we want to print drm_device (or maybe drm_file) is the right thing.
467 - The drm_driver->debugfs_init hooks we have is just an artifact of the old
468 midlayered load sequence. DRM debugfs should work more like sysfs, where you
469 can create properties/files for an object anytime you want, and the core
470 takes care of publishing/unpuplishing all the files at register/unregister
471 time. Drivers shouldn't need to worry about these technicalities, and fixing
472 this (together with the drm_minor->drm_device move) would allow us to remove
475 - Drop the return code and error checking from all debugfs functions. Greg KH is
476 working on this already.
478 Contact: Daniel Vetter
485 Some of these date from the very introduction of KMS in 2008 ...
487 - Make ->funcs and ->helper_private vtables optional. There's a bunch of empty
488 function tables in drivers, but before we can remove them we need to make sure
489 that all the users in helpers and drivers do correctly check for a NULL
492 - Cleanup up the various ->destroy callbacks. A lot of them just wrapt the
493 drm_*_cleanup implementations and can be removed. Some tack a kfree() at the
494 end, for which we could add drm_*_cleanup_kfree(). And then there's the (for
495 historical reasons) misnamed drm_primary_helper_destroy() function.
502 Enable trinity for DRM
503 ----------------------
505 And fix up the fallout. Should be really interesting ...
509 Make KMS tests in i-g-t generic
510 -------------------------------
512 The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
513 including tons of testcases for corner-cases in the modesetting API. It would
514 be awesome if those tests (at least the ones not relying on Intel-specific GEM
515 features) could be made to run on any KMS driver.
517 Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
518 converting things over. For modeset tests we also first need a bit of
519 infrastructure to use dumb buffers for untiled buffers, to be able to run all
520 the non-i915 specific modeset tests.
524 Extend virtual test driver (VKMS)
525 ---------------------------------
527 See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
528 internship task, since it only requires a virtual machine and can be sized to
529 fit the available time.
531 Contact: Daniel Vetter
535 Backlight Refactoring
536 ---------------------
538 Backlight drivers have a triple enable/disable state, which is a bit overkill.
541 1. Roll out backlight_enable() and backlight_disable() helpers everywhere. This
543 2. In all, only look at one of the three status bits set by the above helpers.
544 3. Remove the other two status bits.
546 Contact: Daniel Vetter
553 AMD DC Display Driver
554 ---------------------
556 AMD DC is the display driver for AMD devices starting with Vega. There has been
557 a bunch of progress cleaning it up but there's still plenty of work to be done.
559 See drivers/gpu/drm/amd/display/TODO for tasks.
561 Contact: Harry Wentland, Alex Deucher
566 There is support in place now for writing internal DRM clients making it
567 possible to pick up the bootsplash work that was rejected because it was written
570 - [v6,8/8] drm/client: Hack: Add bootsplash example
571 https://patchwork.freedesktop.org/patch/306579/
573 - [RFC PATCH v2 00/13] Kernel based bootsplash
574 https://lkml.org/lkml/2017/12/13/764
576 Contact: Sam Ravnborg
583 Convert fbdev drivers to DRM
584 ----------------------------
586 There are plenty of fbdev drivers for older hardware. Some hwardware has
587 become obsolete, but some still provides good(-enough) framebuffers. The
588 drivers that are still useful should be converted to DRM and afterwards
591 Very simple fbdev drivers can best be converted by starting with a new
592 DRM driver. Simple KMS helpers and SHMEM should be able to handle any
593 existing hardware. The new driver's call-back functions are filled from
596 More complex fbdev drivers can be refactored step-by-step into a DRM
597 driver with the help of the DRM fbconv helpers. [1] These helpers provide
598 the transition layer between the DRM core infrastructure and the fbdev
599 driver interface. Create a new DRM driver on top of the fbconv helpers,
600 copy over the fbdev driver, and hook it up to the DRM code. Examples for
601 several fbdev drivers are available at [1] and a tutorial of this process
602 available at [2]. The result is a primitive DRM driver that can run X11
605 - [1] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv
606 - [2] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c
608 Contact: Thomas Zimmermann <tzimmermann@suse.de>
projects / linux-2.6-microblaze.git / blob