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34 implementation (there's lots of outdated locking leftovers in various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 Convert early atomic drivers to async commit helpers
76 ----------------------------------------------------
78 For the first year the atomic modeset helpers didn't support asynchronous /
79 nonblocking commits, and every driver had to hand-roll them. This is fixed
80 now, but there's still a pile of existing drivers that easily could be
81 converted over to the new infrastructure.
83 One issue with the helpers is that they require that drivers handle completion
84 events for atomic commits correctly. But fixing these bugs is good anyway.
86 Contact: Daniel Vetter, respective driver maintainers
90 Fallout from atomic KMS
91 -----------------------
93 ``drm_atomic_helper.c`` provides a batch of functions which implement legacy
94 IOCTLs on top of the new atomic driver interface. Which is really nice for
95 gradual conversion of drivers, but unfortunately the semantic mismatches are
96 a bit too severe. So there's some follow-up work to adjust the function
97 interfaces to fix these issues:
99 * atomic needs the lock acquire context. At the moment that's passed around
100 implicitly with some horrible hacks, and it's also allocate with
101 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
102 the acquire context explicitly on stack and then also pass it down into
103 drivers explicitly so that the legacy-on-atomic functions can use them.
105 Except for some driver code this is done. This task should be finished by
106 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().
108 * A bunch of the vtable hooks are now in the wrong place: DRM has a split
109 between core vfunc tables (named ``drm_foo_funcs``), which are used to
110 implement the userspace ABI. And then there's the optional hooks for the
111 helper libraries (name ``drm_foo_helper_funcs``), which are purely for
112 internal use. Some of these hooks should be move from ``_funcs`` to
113 ``_helper_funcs`` since they are not part of the core ABI. There's a
114 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
116 Contact: Daniel Vetter
120 Get rid of dev->struct_mutex from GEM drivers
121 ---------------------------------------------
123 ``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
124 everything. Nowadays in modern drivers the only bit where it's mandatory is
125 serializing GEM buffer object destruction. Which unfortunately means drivers
126 have to keep track of that lock and either call ``unreference`` or
127 ``unreference_locked`` depending upon context.
129 Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
130 and there's a ``gem_free_object_unlocked`` callback for any drivers which are
131 entirely ``struct_mutex`` free.
133 For drivers that need ``struct_mutex`` it should be replaced with a driver-
134 private lock. The tricky part is the BO free functions, since those can't
135 reliably take that lock any more. Instead state needs to be protected with
136 suitable subordinate locks or some cleanup work pushed to a worker thread. For
137 performance-critical drivers it might also be better to go with a more
138 fine-grained per-buffer object and per-context lockings scheme. Currently only the
139 ``msm`` driver still use ``struct_mutex``.
141 Contact: Daniel Vetter, respective driver maintainers
145 Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent
146 ----------------------------------------------------------------------------
148 For drivers which could have multiple instances, it is necessary to
149 differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
150 don't do this, drivers used dev_info/warn/err to make this differentiation. We
151 now have DRM_DEV_* variants of the drm print macros, so we can start to convert
152 those drivers back to using drm-formwatted specific log messages.
154 Before you start this conversion please contact the relevant maintainers to make
155 sure your work will be merged - not everyone agrees that the DRM dmesg macros
158 Contact: Sean Paul, Maintainer of the driver you plan to convert
162 Convert drivers to use simple modeset suspend/resume
163 ----------------------------------------------------
165 Most drivers (except i915 and nouveau) that use
166 drm_atomic_helper_suspend/resume() can probably be converted to use
167 drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
168 of the atomic suspend/resume code in older atomic modeset drivers.
170 Contact: Maintainer of the driver you plan to convert
174 Convert drivers to use drm_fb_helper_fbdev_setup/teardown()
175 -----------------------------------------------------------
177 Most drivers can use drm_fb_helper_fbdev_setup() except maybe:
179 - amdgpu which has special logic to decide whether to call
180 drm_helper_disable_unused_functions()
182 - armada which isn't atomic and doesn't call
183 drm_helper_disable_unused_functions()
185 - i915 which calls drm_fb_helper_initial_config() in a worker
187 Drivers that use drm_framebuffer_remove() to clean up the fbdev framebuffer can
188 probably use drm_fb_helper_fbdev_teardown().
190 Contact: Maintainer of the driver you plan to convert
194 Clean up mmap forwarding
195 ------------------------
197 A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers.
198 And also a lot of them forward dma-buf mmap to the gem mmap implementations.
199 There's drm_gem_prime_mmap() for this now, but still needs to be rolled out.
201 Contact: Daniel Vetter
205 Generic fbdev defio support
206 ---------------------------
208 The defio support code in the fbdev core has some very specific requirements,
209 which means drivers need to have a special framebuffer for fbdev. The main
210 issue is that it uses some fields in struct page itself, which breaks shmem
211 gem objects (and other things). To support defio, affected drivers require
212 the use of a shadow buffer, which may add CPU and memory overhead.
214 Possible solution would be to write our own defio mmap code in the drm fbdev
215 emulation. It would need to fully wrap the existing mmap ops, forwarding
216 everything after it has done the write-protect/mkwrite trickery:
218 - In the drm_fbdev_fb_mmap helper, if we need defio, change the
219 default page prots to write-protected with something like this::
221 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
223 - Set the mkwrite and fsync callbacks with similar implementions to the core
224 fbdev defio stuff. These should all work on plain ptes, they don't actually
225 require a struct page. uff. These should all work on plain ptes, they don't
226 actually require a struct page.
228 - Track the dirty pages in a separate structure (bitfield with one bit per page
229 should work) to avoid clobbering struct page.
231 Might be good to also have some igt testcases for this.
233 Contact: Daniel Vetter, Noralf Tronnes
240 DRM core&drivers uses a lot of idr (integer lookup directories) for mapping
241 userspace IDs to internal objects, and in most places ID=0 means NULL and hence
242 is never used. Switching to idr_init_base() for these would make the idr more
245 Contact: Daniel Vetter
249 struct drm_gem_object_funcs
250 ---------------------------
252 GEM objects can now have a function table instead of having the callbacks on the
253 DRM driver struct. This is now the preferred way and drivers can be moved over.
255 We also need a 2nd version of the CMA define that doesn't require the
256 vmapping to be present (different hook for prime importing). Plus this needs to
257 be rolled out to all drivers using their own implementations, too.
261 Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate
262 ---------------------------------------------------------
264 For cases where drivers are attempting to grab the modeset locks with a local
265 acquire context. Replace the boilerplate code surrounding
266 drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and
267 DRM_MODESET_LOCK_ALL_END() instead.
269 This should also be done for all places where drm_modest_lock_all() is still
272 As a reference, take a look at the conversions already completed in drm core.
274 Contact: Sean Paul, respective driver maintainers
278 Rename CMA helpers to DMA helpers
279 ---------------------------------
281 CMA (standing for contiguous memory allocator) is really a bit an accident of
282 what these were used for first, a much better name would be DMA helpers. In the
283 text these should even be called coherent DMA memory helpers (so maybe CDM, but
284 no one knows what that means) since underneath they just use dma_alloc_coherent.
286 Contact: Laurent Pinchart, Daniel Vetter
288 Level: Intermediate (mostly because it is a huge tasks without good partial
289 milestones, not technically itself that challenging)
291 Convert direct mode.vrefresh accesses to use drm_mode_vrefresh()
292 ----------------------------------------------------------------
294 drm_display_mode.vrefresh isn't guaranteed to be populated. As such, using it
295 is risky and has been known to cause div-by-zero bugs. Fortunately, drm core
296 has helper which will use mode.vrefresh if it's !0 and will calculate it from
297 the timings when it's 0.
299 Use simple search/replace, or (more fun) cocci to replace instances of direct
300 vrefresh access with a call to the helper. Check out
301 https://lists.freedesktop.org/archives/dri-devel/2019-January/205186.html for
304 Once all instances of vrefresh have been converted, remove vrefresh from
305 drm_display_mode to avoid future use.
311 Remove drm_display_mode.hsync
312 -----------------------------
314 We have drm_mode_hsync() to calculate this from hsync_start/end, since drivers
315 shouldn't/don't use this, remove this member to avoid any temptations to use it
316 in the future. If there is any debug code using drm_display_mode.hsync, convert
317 it to use drm_mode_hsync() instead.
326 - drm_fb_helper_restore_fbdev_mode_unlocked() should call restore_fbdev_mode()
327 not the _force variant so it can bail out if there is a master. But first
328 these igt tests need to be fixed: kms_fbcon_fbt@psr and
329 kms_fbcon_fbt@psr-suspend.
331 - The max connector argument for drm_fb_helper_init() and
332 drm_fb_helper_fbdev_setup() isn't used anymore and can be removed.
334 - The helper doesn't keep an array of connectors anymore so these can be
335 removed: drm_fb_helper_single_add_all_connectors(),
336 drm_fb_helper_add_one_connector() and drm_fb_helper_remove_one_connector().
340 connector register/unregister fixes
341 -----------------------------------
343 - For most connectors it's a no-op to call drm_connector_register/unregister
344 directly from driver code, drm_dev_register/unregister take care of this
345 already. We can remove all of them.
347 - For dp drivers it's a bit more a mess, since we need the connector to be
348 registered when calling drm_dp_aux_register. Fix this by instead calling
349 drm_dp_aux_init, and moving the actual registering into a late_register
350 callback as recommended in the kerneldoc.
357 Make panic handling work
358 ------------------------
360 This is a really varied tasks with lots of little bits and pieces:
362 * The panic path can't be tested currently, leading to constant breaking. The
363 main issue here is that panics can be triggered from hardirq contexts and
364 hence all panic related callback can run in hardirq context. It would be
365 awesome if we could test at least the fbdev helper code and driver code by
366 e.g. trigger calls through drm debugfs files. hardirq context could be
367 achieved by using an IPI to the local processor.
369 * There's a massive confusion of different panic handlers. DRM fbdev emulation
370 helpers have one, but on top of that the fbcon code itself also has one. We
371 need to make sure that they stop fighting over each another.
373 * ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
374 isn't a full solution for panic paths. We need to make sure that it only
375 returns true if there's a panic going on for real, and fix up all the
378 * The panic handler must never sleep, which also means it can't ever
379 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
380 even spinlocks (because NMI and hardirq can panic too). We need to either
381 make sure to not call such paths, or trylock everything. Really tricky.
383 * For the above locking troubles reasons it's pretty much impossible to
384 attempt a synchronous modeset from panic handlers. The only thing we could
385 try to achive is an atomic ``set_base`` of the primary plane, and hope that
386 it shows up. Everything else probably needs to be delayed to some worker or
387 something else which happens later on. Otherwise it just kills the box
388 harder, prevent the panic from going out on e.g. netconsole.
390 * There's also proposal for a simplied DRM console instead of the full-blown
391 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
392 obviously work for both console, in case we ever get kmslog merged.
394 Contact: Daniel Vetter
398 Clean up the debugfs support
399 ----------------------------
401 There's a bunch of issues with it:
403 - The drm_info_list ->show() function doesn't even bother to cast to the drm
404 structure for you. This is lazy.
406 - We probably want to have some support for debugfs files on crtc/connectors and
407 maybe other kms objects directly in core. There's even drm_print support in
408 the funcs for these objects to dump kms state, so it's all there. And then the
409 ->show() functions should obviously give you a pointer to the right object.
411 - The drm_info_list stuff is centered on drm_minor instead of drm_device. For
412 anything we want to print drm_device (or maybe drm_file) is the right thing.
414 - The drm_driver->debugfs_init hooks we have is just an artifact of the old
415 midlayered load sequence. DRM debugfs should work more like sysfs, where you
416 can create properties/files for an object anytime you want, and the core
417 takes care of publishing/unpuplishing all the files at register/unregister
418 time. Drivers shouldn't need to worry about these technicalities, and fixing
419 this (together with the drm_minor->drm_device move) would allow us to remove
422 - Drop the return code and error checking from all debugfs functions. Greg KH is
423 working on this already.
425 Contact: Daniel Vetter
432 Some of these date from the very introduction of KMS in 2008 ...
434 - Make ->funcs and ->helper_private vtables optional. There's a bunch of empty
435 function tables in drivers, but before we can remove them we need to make sure
436 that all the users in helpers and drivers do correctly check for a NULL
439 - Cleanup up the various ->destroy callbacks. A lot of them just wrapt the
440 drm_*_cleanup implementations and can be removed. Some tack a kfree() at the
441 end, for which we could add drm_*_cleanup_kfree(). And then there's the (for
442 historical reasons) misnamed drm_primary_helper_destroy() function.
449 Enable trinity for DRM
450 ----------------------
452 And fix up the fallout. Should be really interesting ...
456 Make KMS tests in i-g-t generic
457 -------------------------------
459 The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
460 including tons of testcases for corner-cases in the modesetting API. It would
461 be awesome if those tests (at least the ones not relying on Intel-specific GEM
462 features) could be made to run on any KMS driver.
464 Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
465 converting things over. For modeset tests we also first need a bit of
466 infrastructure to use dumb buffers for untiled buffers, to be able to run all
467 the non-i915 specific modeset tests.
471 Extend virtual test driver (VKMS)
472 ---------------------------------
474 See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
475 internship task, since it only requires a virtual machine and can be sized to
476 fit the available time.
478 Contact: Daniel Vetter
482 Backlight Refactoring
483 ---------------------
485 Backlight drivers have a triple enable/disable state, which is a bit overkill.
488 1. Roll out backlight_enable() and backlight_disable() helpers everywhere. This
490 2. In all, only look at one of the three status bits set by the above helpers.
491 3. Remove the other two status bits.
493 Contact: Daniel Vetter
500 AMD DC Display Driver
501 ---------------------
503 AMD DC is the display driver for AMD devices starting with Vega. There has been
504 a bunch of progress cleaning it up but there's still plenty of work to be done.
506 See drivers/gpu/drm/amd/display/TODO for tasks.
508 Contact: Harry Wentland, Alex Deucher
513 There is support in place now for writing internal DRM clients making it
514 possible to pick up the bootsplash work that was rejected because it was written
517 - [v6,8/8] drm/client: Hack: Add bootsplash example
518 https://patchwork.freedesktop.org/patch/306579/
520 - [RFC PATCH v2 00/13] Kernel based bootsplash
521 https://lkml.org/lkml/2017/12/13/764
523 Contact: Sam Ravnborg
530 Convert fbdev drivers to DRM
531 ----------------------------
533 There are plenty of fbdev drivers for older hardware. Some hwardware has
534 become obsolete, but some still provides good(-enough) framebuffers. The
535 drivers that are still useful should be converted to DRM and afterwards
538 Very simple fbdev drivers can best be converted by starting with a new
539 DRM driver. Simple KMS helpers and SHMEM should be able to handle any
540 existing hardware. The new driver's call-back functions are filled from
543 More complex fbdev drivers can be refactored step-by-step into a DRM
544 driver with the help of the DRM fbconv helpers. [1] These helpers provide
545 the transition layer between the DRM core infrastructure and the fbdev
546 driver interface. Create a new DRM driver on top of the fbconv helpers,
547 copy over the fbdev driver, and hook it up to the DRM code. Examples for
548 several fbdev drivers are available at [1] and a tutorial of this process
549 available at [2]. The result is a primitive DRM driver that can run X11
552 - [1] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv
553 - [2] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c
555 Contact: Thomas Zimmermann <tzimmermann@suse.de>