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37 implementation (there's lots of outdated locking leftovers in various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 Fallout from atomic KMS
86 -----------------------
88 ``drm_atomic_helper.c`` provides a batch of functions which implement legacy
89 IOCTLs on top of the new atomic driver interface. Which is really nice for
90 gradual conversion of drivers, but unfortunately the semantic mismatches are
91 a bit too severe. So there's some follow-up work to adjust the function
92 interfaces to fix these issues:
94 * atomic needs the lock acquire context. At the moment that's passed around
95 implicitly with some horrible hacks, and it's also allocate with
96 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
97 the acquire context explicitly on stack and then also pass it down into
98 drivers explicitly so that the legacy-on-atomic functions can use them.
100 Except for some driver code this is done. This task should be finished by
101 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().
103 * A bunch of the vtable hooks are now in the wrong place: DRM has a split
104 between core vfunc tables (named ``drm_foo_funcs``), which are used to
105 implement the userspace ABI. And then there's the optional hooks for the
106 helper libraries (name ``drm_foo_helper_funcs``), which are purely for
107 internal use. Some of these hooks should be move from ``_funcs`` to
108 ``_helper_funcs`` since they are not part of the core ABI. There's a
109 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
111 Contact: Daniel Vetter
113 Get rid of dev->struct_mutex from GEM drivers
114 ---------------------------------------------
116 ``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
117 everything. Nowadays in modern drivers the only bit where it's mandatory is
118 serializing GEM buffer object destruction. Which unfortunately means drivers
119 have to keep track of that lock and either call ``unreference`` or
120 ``unreference_locked`` depending upon context.
122 Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
123 and there's a ``gem_free_object_unlocked`` callback for any drivers which are
124 entirely ``struct_mutex`` free.
126 For drivers that need ``struct_mutex`` it should be replaced with a driver-
127 private lock. The tricky part is the BO free functions, since those can't
128 reliably take that lock any more. Instead state needs to be protected with
129 suitable subordinate locks or some cleanup work pushed to a worker thread. For
130 performance-critical drivers it might also be better to go with a more
131 fine-grained per-buffer object and per-context lockings scheme. Currently only the
132 ``msm`` driver still use ``struct_mutex``.
134 Contact: Daniel Vetter, respective driver maintainers
136 Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent
137 ----------------------------------------------------------------------------
139 For drivers which could have multiple instances, it is necessary to
140 differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
141 don't do this, drivers used dev_info/warn/err to make this differentiation. We
142 now have DRM_DEV_* variants of the drm print macros, so we can start to convert
143 those drivers back to using drm-formwatted specific log messages.
145 Before you start this conversion please contact the relevant maintainers to make
146 sure your work will be merged - not everyone agrees that the DRM dmesg macros
149 Contact: Sean Paul, Maintainer of the driver you plan to convert
151 Convert drivers to use simple modeset suspend/resume
152 ----------------------------------------------------
154 Most drivers (except i915 and nouveau) that use
155 drm_atomic_helper_suspend/resume() can probably be converted to use
156 drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
157 of the atomic suspend/resume code in older atomic modeset drivers.
159 Contact: Maintainer of the driver you plan to convert
161 Convert drivers to use drm_fb_helper_fbdev_setup/teardown()
162 -----------------------------------------------------------
164 Most drivers can use drm_fb_helper_fbdev_setup() except maybe:
166 - amdgpu which has special logic to decide whether to call
167 drm_helper_disable_unused_functions()
169 - armada which isn't atomic and doesn't call
170 drm_helper_disable_unused_functions()
172 - i915 which calls drm_fb_helper_initial_config() in a worker
174 Drivers that use drm_framebuffer_remove() to clean up the fbdev framebuffer can
175 probably use drm_fb_helper_fbdev_teardown().
177 Contact: Maintainer of the driver you plan to convert
179 Clean up mmap forwarding
180 ------------------------
182 A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers.
183 And also a lot of them forward dma-buf mmap to the gem mmap implementations.
184 Would be great to refactor this all into a set of small common helpers.
186 Contact: Daniel Vetter
188 Generic fbdev defio support
189 ---------------------------
191 The defio support code in the fbdev core has some very specific requirements,
192 which means drivers need to have a special framebuffer for fbdev. Which prevents
193 us from using the generic fbdev emulation code everywhere. The main issue is
194 that it uses some fields in struct page itself, which breaks shmem gem objects
197 Possible solution would be to write our own defio mmap code in the drm fbdev
198 emulation. It would need to fully wrap the existing mmap ops, forwarding
199 everything after it has done the write-protect/mkwrite trickery:
201 - In the drm_fbdev_fb_mmap helper, if we need defio, change the
202 default page prots to write-protected with something like this::
204 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
206 - Set the mkwrite and fsync callbacks with similar implementions to the core
207 fbdev defio stuff. These should all work on plain ptes, they don't actually
208 require a struct page. uff. These should all work on plain ptes, they don't
209 actually require a struct page.
211 - Track the dirty pages in a separate structure (bitfield with one bit per page
212 should work) to avoid clobbering struct page.
214 Might be good to also have some igt testcases for this.
216 Contact: Daniel Vetter, Noralf Tronnes
218 Remove the ->gem_prime_res_obj callback
219 --------------------------------------------
221 The ->gem_prime_res_obj callback can be removed from drivers by using the
222 reservation_object in the drm_gem_object. It may also be possible to use the
223 generic drm_gem_reservation_object_wait helper for waiting for a bo.
225 Contact: Daniel Vetter
230 DRM core&drivers uses a lot of idr (integer lookup directories) for mapping
231 userspace IDs to internal objects, and in most places ID=0 means NULL and hence
232 is never used. Switching to idr_init_base() for these would make the idr more
235 Contact: Daniel Vetter
237 Defaults for .gem_prime_import and export
238 -----------------------------------------
240 Most drivers don't need to set drm_driver->gem_prime_import and
241 ->gem_prime_export now that drm_gem_prime_import() and drm_gem_prime_export()
244 struct drm_gem_object_funcs
245 ---------------------------
247 GEM objects can now have a function table instead of having the callbacks on the
248 DRM driver struct. This is now the preferred way and drivers can be moved over.
250 Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate
251 ---------------------------------------------------------
253 For cases where drivers are attempting to grab the modeset locks with a local
254 acquire context. Replace the boilerplate code surrounding
255 drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and
256 DRM_MODESET_LOCK_ALL_END() instead.
258 This should also be done for all places where drm_modest_lock_all() is still
261 As a reference, take a look at the conversions already completed in drm core.
263 Contact: Sean Paul, respective driver maintainers
265 Rename CMA helpers to DMA helpers
266 ---------------------------------
268 CMA (standing for contiguous memory allocator) is really a bit an accident of
269 what these were used for first, a much better name would be DMA helpers. In the
270 text these should even be called coherent DMA memory helpers (so maybe CDM, but
271 no one knows what that means) since underneath they just use dma_alloc_coherent.
273 Contact: Laurent Pinchart, Daniel Vetter
275 Convert direct mode.vrefresh accesses to use drm_mode_vrefresh()
276 ----------------------------------------------------------------
278 drm_display_mode.vrefresh isn't guaranteed to be populated. As such, using it
279 is risky and has been known to cause div-by-zero bugs. Fortunately, drm core
280 has helper which will use mode.vrefresh if it's !0 and will calculate it from
281 the timings when it's 0.
283 Use simple search/replace, or (more fun) cocci to replace instances of direct
284 vrefresh access with a call to the helper. Check out
285 https://lists.freedesktop.org/archives/dri-devel/2019-January/205186.html for
288 Once all instances of vrefresh have been converted, remove vrefresh from
289 drm_display_mode to avoid future use.
293 Remove drm_display_mode.hsync
294 -----------------------------
296 We have drm_mode_hsync() to calculate this from hsync_start/end, since drivers
297 shouldn't/don't use this, remove this member to avoid any temptations to use it
298 in the future. If there is any debug code using drm_display_mode.hsync, convert
299 it to use drm_mode_hsync() instead.
306 Clean up the DRM header mess
307 ----------------------------
309 The DRM subsystem originally had only one huge global header, ``drmP.h``. This
310 is now split up, but many source files still include it. The remaining part of
311 the cleanup work here is to replace any ``#include <drm/drmP.h>`` by only the
312 headers needed (and fixing up any missing pre-declarations in the headers).
314 In the end no .c file should need to include ``drmP.h`` anymore.
316 Contact: Daniel Vetter
318 Add missing kerneldoc for exported functions
319 --------------------------------------------
321 The DRM reference documentation is still lacking kerneldoc in a few areas. The
322 task would be to clean up interfaces like moving functions around between
323 files to better group them and improving the interfaces like dropping return
324 values for functions that never fail. Then write kerneldoc for all exported
325 functions and an overview section and integrate it all into the drm book.
327 See https://dri.freedesktop.org/docs/drm/ for what's there already.
329 Contact: Daniel Vetter
331 Make panic handling work
332 ------------------------
334 This is a really varied tasks with lots of little bits and pieces:
336 * The panic path can't be tested currently, leading to constant breaking. The
337 main issue here is that panics can be triggered from hardirq contexts and
338 hence all panic related callback can run in hardirq context. It would be
339 awesome if we could test at least the fbdev helper code and driver code by
340 e.g. trigger calls through drm debugfs files. hardirq context could be
341 achieved by using an IPI to the local processor.
343 * There's a massive confusion of different panic handlers. DRM fbdev emulation
344 helpers have one, but on top of that the fbcon code itself also has one. We
345 need to make sure that they stop fighting over each another.
347 * ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
348 isn't a full solution for panic paths. We need to make sure that it only
349 returns true if there's a panic going on for real, and fix up all the
352 * The panic handler must never sleep, which also means it can't ever
353 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
354 even spinlocks (because NMI and hardirq can panic too). We need to either
355 make sure to not call such paths, or trylock everything. Really tricky.
357 * For the above locking troubles reasons it's pretty much impossible to
358 attempt a synchronous modeset from panic handlers. The only thing we could
359 try to achive is an atomic ``set_base`` of the primary plane, and hope that
360 it shows up. Everything else probably needs to be delayed to some worker or
361 something else which happens later on. Otherwise it just kills the box
362 harder, prevent the panic from going out on e.g. netconsole.
364 * There's also proposal for a simplied DRM console instead of the full-blown
365 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
366 obviously work for both console, in case we ever get kmslog merged.
368 Contact: Daniel Vetter
370 Clean up the debugfs support
371 ----------------------------
373 There's a bunch of issues with it:
375 - The drm_info_list ->show() function doesn't even bother to cast to the drm
376 structure for you. This is lazy.
378 - We probably want to have some support for debugfs files on crtc/connectors and
379 maybe other kms objects directly in core. There's even drm_print support in
380 the funcs for these objects to dump kms state, so it's all there. And then the
381 ->show() functions should obviously give you a pointer to the right object.
383 - The drm_info_list stuff is centered on drm_minor instead of drm_device. For
384 anything we want to print drm_device (or maybe drm_file) is the right thing.
386 - The drm_driver->debugfs_init hooks we have is just an artifact of the old
387 midlayered load sequence. DRM debugfs should work more like sysfs, where you
388 can create properties/files for an object anytime you want, and the core
389 takes care of publishing/unpuplishing all the files at register/unregister
390 time. Drivers shouldn't need to worry about these technicalities, and fixing
391 this (together with the drm_minor->drm_device move) would allow us to remove
394 Contact: Daniel Vetter
399 Some of these date from the very introduction of KMS in 2008 ...
401 - Make ->funcs and ->helper_private vtables optional. There's a bunch of empty
402 function tables in drivers, but before we can remove them we need to make sure
403 that all the users in helpers and drivers do correctly check for a NULL
406 - Cleanup up the various ->destroy callbacks. A lot of them just wrapt the
407 drm_*_cleanup implementations and can be removed. Some tack a kfree() at the
408 end, for which we could add drm_*_cleanup_kfree(). And then there's the (for
409 historical reasons) misnamed drm_primary_helper_destroy() function.
414 Enable trinity for DRM
415 ----------------------
417 And fix up the fallout. Should be really interesting ...
419 Make KMS tests in i-g-t generic
420 -------------------------------
422 The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
423 including tons of testcases for corner-cases in the modesetting API. It would
424 be awesome if those tests (at least the ones not relying on Intel-specific GEM
425 features) could be made to run on any KMS driver.
427 Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
428 converting things over. For modeset tests we also first need a bit of
429 infrastructure to use dumb buffers for untiled buffers, to be able to run all
430 the non-i915 specific modeset tests.
432 Extend virtual test driver (VKMS)
433 ---------------------------------
435 See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
436 internship task, since it only requires a virtual machine and can be sized to
437 fit the available time.
439 Contact: Daniel Vetter
447 Tinydrm is the helper driver for really simple fb drivers. The goal is to make
448 those drivers as simple as possible, so lots of room for refactoring:
450 - backlight helpers, probably best to put them into a new drm_backlight.c.
451 This is because drivers/video is de-facto unmaintained. We could also
452 move drivers/video/backlight to drivers/gpu/backlight and take it all
453 over within drm-misc, but that's more work. Backlight helpers require a fair
454 bit of reworking and refactoring. A simple example is the enabling of a backlight.
455 Tinydrm has helpers for this. It would be good if other drivers can also use the
456 helper. However, there are various cases we need to consider i.e different
457 drivers seem to have different ways of enabling/disabling a backlight.
458 We also need to consider the backlight drivers (like gpio_backlight). The situation
459 is further complicated by the fact that the backlight is tied to fbdev
460 via fb_notifier_callback() which has complicated logic. For further details, refer
461 to the following discussion thread:
462 https://groups.google.com/forum/#!topic/outreachy-kernel/8rBe30lwtdA
464 - spi helpers, probably best put into spi core/helper code. Thierry said
465 the spi maintainer is fast&reactive, so shouldn't be a big issue.
467 - extract the mipi-dbi helper (well, the non-tinydrm specific parts at
468 least) into a separate helper, like we have for mipi-dsi already. Or follow
469 one of the ideas for having a shared dsi/dbi helper, abstracting away the
470 transport details more.
472 Contact: Noralf Trønnes, Daniel Vetter
474 AMD DC Display Driver
475 ---------------------
477 AMD DC is the display driver for AMD devices starting with Vega. There has been
478 a bunch of progress cleaning it up but there's still plenty of work to be done.
480 See drivers/gpu/drm/amd/display/TODO for tasks.
482 Contact: Harry Wentland, Alex Deucher
487 - Our early/late pm callbacks could be removed in favour of using
488 device_link_add to model the dependency between i915 and snd_had. See
489 https://dri.freedesktop.org/docs/drm/driver-api/device_link.html