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.
10 Subsystem-wide refactorings
11 ===========================
16 With the recent ``drm_bus`` cleanup patches for 3.17 it is no longer required
17 to have a ``drm_bus`` structure set up. Drivers can directly set up the
18 ``drm_device`` structure instead of relying on bus methods in ``drm_usb.c``
19 and ``drm_pci.c``. The goal is to get rid of the driver's ``->load`` /
20 ``->unload`` callbacks and open-code the load/unload sequence properly, using
21 the new two-stage ``drm_device`` setup/teardown.
23 Once all existing drivers are converted we can also remove those bus support
24 files for USB and platform devices.
26 All you need is a GPU for a non-converted driver (currently almost all of
27 them, but also all the virtual ones used by KVM, so everyone qualifies).
29 Contact: Daniel Vetter, Thierry Reding, respective driver maintainers
32 Remove custom dumb_map_offset implementations
33 ---------------------------------------------
35 All GEM based drivers should be using drm_gem_create_mmap_offset() instead.
36 Audit each individual driver, make sure it'll work with the generic
37 implementation (there's lots of outdated locking leftovers in various
38 implementations), and then remove it.
40 Contact: Daniel Vetter, respective driver maintainers
42 Convert existing KMS drivers to atomic modesetting
43 --------------------------------------------------
45 3.19 has the atomic modeset interfaces and helpers, so drivers can now be
46 converted over. Modern compositors like Wayland or Surfaceflinger on Android
47 really want an atomic modeset interface, so this is all about the bright
50 There is a conversion guide for atomic and all you need is a GPU for a
51 non-converted driver (again virtual HW drivers for KVM are still all
54 As part of this drivers also need to convert to universal plane (which means
55 exposing primary & cursor as proper plane objects). But that's much easier to
56 do by directly using the new atomic helper driver callbacks.
58 Contact: Daniel Vetter, respective driver maintainers
60 Clean up the clipped coordination confusion around planes
61 ---------------------------------------------------------
63 We have a helper to get this right with drm_plane_helper_check_update(), but
64 it's not consistently used. This should be fixed, preferrably in the atomic
65 helpers (and drivers then moved over to clipped coordinates). Probably the
66 helper should also be moved from drm_plane_helper.c to the atomic helpers, to
67 avoid confusion - the other helpers in that file are all deprecated legacy
70 Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
72 Convert early atomic drivers to async commit helpers
73 ----------------------------------------------------
75 For the first year the atomic modeset helpers didn't support asynchronous /
76 nonblocking commits, and every driver had to hand-roll them. This is fixed
77 now, but there's still a pile of existing drivers that easily could be
78 converted over to the new infrastructure.
80 One issue with the helpers is that they require that drivers handle completion
81 events for atomic commits correctly. But fixing these bugs is good anyway.
83 Contact: Daniel Vetter, respective driver maintainers
85 Better manual-upload support for atomic
86 ---------------------------------------
88 This would be especially useful for tinydrm:
90 - Add a struct drm_rect dirty_clip to drm_crtc_state. When duplicating the
91 crtc state, clear that to the max values, x/y = 0 and w/h = MAX_INT, in
92 __drm_atomic_helper_crtc_duplicate_state().
94 - Move tinydrm_merge_clips into drm_framebuffer.c, dropping the tinydrm\_
95 prefix ofc and using drm_fb\_. drm_framebuffer.c makes sense since this
96 is a function useful to implement the fb->dirty function.
98 - Create a new drm_fb_dirty function which does essentially what e.g.
99 mipi_dbi_fb_dirty does. You can use e.g. drm_atomic_helper_update_plane as the
100 template. But instead of doing a simple full-screen plane update, this new
101 helper also sets crtc_state->dirty_clip to the right coordinates. And of
102 course it needs to check whether the fb is actually active (and maybe where),
103 so there's some book-keeping involved. There's also some good fun involved in
104 scaling things appropriately. For that case we might simply give up and
105 declare the entire area covered by the plane as dirty.
107 Contact: Noralf Trønnes, Daniel Vetter
109 Fallout from atomic KMS
110 -----------------------
112 ``drm_atomic_helper.c`` provides a batch of functions which implement legacy
113 IOCTLs on top of the new atomic driver interface. Which is really nice for
114 gradual conversion of drivers, but unfortunately the semantic mismatches are
115 a bit too severe. So there's some follow-up work to adjust the function
116 interfaces to fix these issues:
118 * atomic needs the lock acquire context. At the moment that's passed around
119 implicitly with some horrible hacks, and it's also allocate with
120 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
121 the acquire context explicitly on stack and then also pass it down into
122 drivers explicitly so that the legacy-on-atomic functions can use them.
124 Except for some driver code this is done. This task should be finished by
125 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().
127 * A bunch of the vtable hooks are now in the wrong place: DRM has a split
128 between core vfunc tables (named ``drm_foo_funcs``), which are used to
129 implement the userspace ABI. And then there's the optional hooks for the
130 helper libraries (name ``drm_foo_helper_funcs``), which are purely for
131 internal use. Some of these hooks should be move from ``_funcs`` to
132 ``_helper_funcs`` since they are not part of the core ABI. There's a
133 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
135 Contact: Daniel Vetter
137 Get rid of dev->struct_mutex from GEM drivers
138 ---------------------------------------------
140 ``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
141 everything. Nowadays in modern drivers the only bit where it's mandatory is
142 serializing GEM buffer object destruction. Which unfortunately means drivers
143 have to keep track of that lock and either call ``unreference`` or
144 ``unreference_locked`` depending upon context.
146 Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
147 and there's a ``gem_free_object_unlocked`` callback for any drivers which are
148 entirely ``struct_mutex`` free.
150 For drivers that need ``struct_mutex`` it should be replaced with a driver-
151 private lock. The tricky part is the BO free functions, since those can't
152 reliably take that lock any more. Instead state needs to be protected with
153 suitable subordinate locks or some cleanup work pushed to a worker thread. For
154 performance-critical drivers it might also be better to go with a more
155 fine-grained per-buffer object and per-context lockings scheme. Currently only the
156 ``msm`` driver still use ``struct_mutex``.
158 Contact: Daniel Vetter, respective driver maintainers
160 Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent
161 ----------------------------------------------------------------------------
163 For drivers which could have multiple instances, it is necessary to
164 differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
165 don't do this, drivers used dev_info/warn/err to make this differentiation. We
166 now have DRM_DEV_* variants of the drm print macros, so we can start to convert
167 those drivers back to using drm-formwatted specific log messages.
169 Before you start this conversion please contact the relevant maintainers to make
170 sure your work will be merged - not everyone agrees that the DRM dmesg macros
173 Contact: Sean Paul, Maintainer of the driver you plan to convert
175 Convert drivers to use simple modeset suspend/resume
176 ----------------------------------------------------
178 Most drivers (except i915 and nouveau) that use
179 drm_atomic_helper_suspend/resume() can probably be converted to use
180 drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
181 of the atomic suspend/resume code in older atomic modeset drivers.
183 Contact: Maintainer of the driver you plan to convert
185 Convert drivers to use drm_fb_helper_fbdev_setup/teardown()
186 -----------------------------------------------------------
188 Most drivers can use drm_fb_helper_fbdev_setup() except maybe:
190 - amdgpu which has special logic to decide whether to call
191 drm_helper_disable_unused_functions()
193 - armada which isn't atomic and doesn't call
194 drm_helper_disable_unused_functions()
196 - i915 which calls drm_fb_helper_initial_config() in a worker
198 Drivers that use drm_framebuffer_remove() to clean up the fbdev framebuffer can
199 probably use drm_fb_helper_fbdev_teardown().
201 Contact: Maintainer of the driver you plan to convert
203 Clean up mmap forwarding
204 ------------------------
206 A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers.
207 And also a lot of them forward dma-buf mmap to the gem mmap implementations.
208 Would be great to refactor this all into a set of small common helpers.
210 Contact: Daniel Vetter
212 Put a reservation_object into drm_gem_object
213 --------------------------------------------
215 This would remove the need for the ->gem_prime_res_obj callback. It would also
216 allow us to implement generic helpers for waiting for a bo, allowing for quite a
217 bit of refactoring in the various wait ioctl implementations.
219 Contact: Daniel Vetter
224 DRM core&drivers uses a lot of idr (integer lookup directories) for mapping
225 userspace IDs to internal objects, and in most places ID=0 means NULL and hence
226 is never used. Switching to idr_init_base() for these would make the idr more
229 Contact: Daniel Vetter
231 Defaults for .gem_prime_import and export
232 -----------------------------------------
234 Most drivers don't need to set drm_driver->gem_prime_import and
235 ->gem_prime_export now that drm_gem_prime_import() and drm_gem_prime_export()
238 struct drm_gem_object_funcs
239 ---------------------------
241 GEM objects can now have a function table instead of having the callbacks on the
242 DRM driver struct. This is now the preferred way and drivers can be moved over.
244 Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate
245 ---------------------------------------------------------
247 For cases where drivers are attempting to grab the modeset locks with a local
248 acquire context. Replace the boilerplate code surrounding
249 drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and
250 DRM_MODESET_LOCK_ALL_END() instead.
252 This should also be done for all places where drm_modest_lock_all() is still
255 As a reference, take a look at the conversions already completed in drm core.
257 Contact: Sean Paul, respective driver maintainers
262 Clean up the DRM header mess
263 ----------------------------
265 The DRM subsystem originally had only one huge global header, ``drmP.h``. This
266 is now split up, but many source files still include it. The remaining part of
267 the cleanup work here is to replace any ``#include <drm/drmP.h>`` by only the
268 headers needed (and fixing up any missing pre-declarations in the headers).
270 In the end no .c file should need to include ``drmP.h`` anymore.
272 Contact: Daniel Vetter
274 Add missing kerneldoc for exported functions
275 --------------------------------------------
277 The DRM reference documentation is still lacking kerneldoc in a few areas. The
278 task would be to clean up interfaces like moving functions around between
279 files to better group them and improving the interfaces like dropping return
280 values for functions that never fail. Then write kerneldoc for all exported
281 functions and an overview section and integrate it all into the drm book.
283 See https://dri.freedesktop.org/docs/drm/ for what's there already.
285 Contact: Daniel Vetter
287 Make panic handling work
288 ------------------------
290 This is a really varied tasks with lots of little bits and pieces:
292 * The panic path can't be tested currently, leading to constant breaking. The
293 main issue here is that panics can be triggered from hardirq contexts and
294 hence all panic related callback can run in hardirq context. It would be
295 awesome if we could test at least the fbdev helper code and driver code by
296 e.g. trigger calls through drm debugfs files. hardirq context could be
297 achieved by using an IPI to the local processor.
299 * There's a massive confusion of different panic handlers. DRM fbdev emulation
300 helpers have one, but on top of that the fbcon code itself also has one. We
301 need to make sure that they stop fighting over each another.
303 * ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
304 isn't a full solution for panic paths. We need to make sure that it only
305 returns true if there's a panic going on for real, and fix up all the
308 * The panic handler must never sleep, which also means it can't ever
309 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
310 even spinlocks (because NMI and hardirq can panic too). We need to either
311 make sure to not call such paths, or trylock everything. Really tricky.
313 * For the above locking troubles reasons it's pretty much impossible to
314 attempt a synchronous modeset from panic handlers. The only thing we could
315 try to achive is an atomic ``set_base`` of the primary plane, and hope that
316 it shows up. Everything else probably needs to be delayed to some worker or
317 something else which happens later on. Otherwise it just kills the box
318 harder, prevent the panic from going out on e.g. netconsole.
320 * There's also proposal for a simplied DRM console instead of the full-blown
321 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
322 obviously work for both console, in case we ever get kmslog merged.
324 Contact: Daniel Vetter
326 Clean up the debugfs support
327 ----------------------------
329 There's a bunch of issues with it:
331 - The drm_info_list ->show() function doesn't even bother to cast to the drm
332 structure for you. This is lazy.
334 - We probably want to have some support for debugfs files on crtc/connectors and
335 maybe other kms objects directly in core. There's even drm_print support in
336 the funcs for these objects to dump kms state, so it's all there. And then the
337 ->show() functions should obviously give you a pointer to the right object.
339 - The drm_info_list stuff is centered on drm_minor instead of drm_device. For
340 anything we want to print drm_device (or maybe drm_file) is the right thing.
342 - The drm_driver->debugfs_init hooks we have is just an artifact of the old
343 midlayered load sequence. DRM debugfs should work more like sysfs, where you
344 can create properties/files for an object anytime you want, and the core
345 takes care of publishing/unpuplishing all the files at register/unregister
346 time. Drivers shouldn't need to worry about these technicalities, and fixing
347 this (together with the drm_minor->drm_device move) would allow us to remove
350 Contact: Daniel Vetter
355 Some of these date from the very introduction of KMS in 2008 ...
357 - drm_mode_config.crtc_idr is misnamed, since it contains all KMS object. Should
358 be renamed to drm_mode_config.object_idr.
360 - drm_display_mode doesn't need to be derived from drm_mode_object. That's
361 leftovers from older (never merged into upstream) KMS designs where modes
362 where set using their ID, including support to add/remove modes.
364 - Make ->funcs and ->helper_private vtables optional. There's a bunch of empty
365 function tables in drivers, but before we can remove them we need to make sure
366 that all the users in helpers and drivers do correctly check for a NULL
369 - Cleanup up the various ->destroy callbacks. A lot of them just wrapt the
370 drm_*_cleanup implementations and can be removed. Some tack a kfree() at the
371 end, for which we could add drm_*_cleanup_kfree(). And then there's the (for
372 historical reasons) misnamed drm_primary_helper_destroy() function.
377 Enable trinity for DRM
378 ----------------------
380 And fix up the fallout. Should be really interesting ...
382 Make KMS tests in i-g-t generic
383 -------------------------------
385 The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
386 including tons of testcases for corner-cases in the modesetting API. It would
387 be awesome if those tests (at least the ones not relying on Intel-specific GEM
388 features) could be made to run on any KMS driver.
390 Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
391 converting things over. For modeset tests we also first need a bit of
392 infrastructure to use dumb buffers for untiled buffers, to be able to run all
393 the non-i915 specific modeset tests.
395 Extend virtual test driver (VKMS)
396 ---------------------------------
398 See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
399 internship task, since it only requires a virtual machine and can be sized to
400 fit the available time.
402 Contact: Daniel Vetter
410 Tinydrm is the helper driver for really simple fb drivers. The goal is to make
411 those drivers as simple as possible, so lots of room for refactoring:
413 - backlight helpers, probably best to put them into a new drm_backlight.c.
414 This is because drivers/video is de-facto unmaintained. We could also
415 move drivers/video/backlight to drivers/gpu/backlight and take it all
416 over within drm-misc, but that's more work. Backlight helpers require a fair
417 bit of reworking and refactoring. A simple example is the enabling of a backlight.
418 Tinydrm has helpers for this. It would be good if other drivers can also use the
419 helper. However, there are various cases we need to consider i.e different
420 drivers seem to have different ways of enabling/disabling a backlight.
421 We also need to consider the backlight drivers (like gpio_backlight). The situation
422 is further complicated by the fact that the backlight is tied to fbdev
423 via fb_notifier_callback() which has complicated logic. For further details, refer
424 to the following discussion thread:
425 https://groups.google.com/forum/#!topic/outreachy-kernel/8rBe30lwtdA
427 - spi helpers, probably best put into spi core/helper code. Thierry said
428 the spi maintainer is fast&reactive, so shouldn't be a big issue.
430 - extract the mipi-dbi helper (well, the non-tinydrm specific parts at
431 least) into a separate helper, like we have for mipi-dsi already. Or follow
432 one of the ideas for having a shared dsi/dbi helper, abstracting away the
433 transport details more.
435 - tinydrm_gem_cma_prime_import_sg_table should probably go into the cma
436 helpers, as a _vmapped variant (since not every driver needs the vmap).
437 And tinydrm_gem_cma_free_object could the be merged into
438 drm_gem_cma_free_object().
440 - tinydrm_fb_create we could move into drm_simple_pipe, only need to add
441 the fb_create hook to drm_simple_pipe_funcs, which would again simplify a
442 bunch of things (since it gives you a one-stop vfunc for simple drivers).
444 - Quick aside: The unregister devm stuff is kinda getting the lifetimes of
445 a drm_device wrong. Doesn't matter, since everyone else gets it wrong
448 - also rework the drm_framebuffer_funcs->dirty hook wire-up, see above.
450 Contact: Noralf Trønnes, Daniel Vetter
452 AMD DC Display Driver
453 ---------------------
455 AMD DC is the display driver for AMD devices starting with Vega. There has been
456 a bunch of progress cleaning it up but there's still plenty of work to be done.
458 See drivers/gpu/drm/amd/display/TODO for tasks.
460 Contact: Harry Wentland, Alex Deucher
465 - Our early/late pm callbacks could be removed in favour of using
466 device_link_add to model the dependency between i915 and snd_had. See
467 https://dri.freedesktop.org/docs/drm/driver-api/device_link.html