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
31 Switch from reference/unreference to get/put
32 --------------------------------------------
34 For some reason DRM core uses ``reference``/``unreference`` suffixes for
35 refcounting functions, but kernel uses ``get``/``put`` (e.g.
36 ``kref_get``/``put()``). It would be good to switch over for consistency, and
37 it's shorter. Needs to be done in 3 steps for each pair of functions:
39 * Create new ``get``/``put`` functions, define the old names as compatibility
41 * Switch over each file/driver using a cocci-generated spatch.
42 * Once all users of the old names are gone, remove them.
44 This way drivers/patches in the progress of getting merged won't break.
46 Contact: Daniel Vetter
48 Convert existing KMS drivers to atomic modesetting
49 --------------------------------------------------
51 3.19 has the atomic modeset interfaces and helpers, so drivers can now be
52 converted over. Modern compositors like Wayland or Surfaceflinger on Android
53 really want an atomic modeset interface, so this is all about the bright
56 There is a conversion guide for atomic and all you need is a GPU for a
57 non-converted driver (again virtual HW drivers for KVM are still all
60 As part of this drivers also need to convert to universal plane (which means
61 exposing primary & cursor as proper plane objects). But that's much easier to
62 do by directly using the new atomic helper driver callbacks.
64 Contact: Daniel Vetter, respective driver maintainers
66 Clean up the clipped coordination confusion around planes
67 ---------------------------------------------------------
69 We have a helper to get this right with drm_plane_helper_check_update(), but
70 it's not consistently used. This should be fixed, preferrably in the atomic
71 helpers (and drivers then moved over to clipped coordinates). Probably the
72 helper should also be moved from drm_plane_helper.c to the atomic helpers, to
73 avoid confusion - the other helpers in that file are all deprecated legacy
76 Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
78 Convert early atomic drivers to async commit helpers
79 ----------------------------------------------------
81 For the first year the atomic modeset helpers didn't support asynchronous /
82 nonblocking commits, and every driver had to hand-roll them. This is fixed
83 now, but there's still a pile of existing drivers that easily could be
84 converted over to the new infrastructure.
86 One issue with the helpers is that they require that drivers handle completion
87 events for atomic commits correctly. But fixing these bugs is good anyway.
89 Contact: Daniel Vetter, respective driver maintainers
91 Better manual-upload support for atomic
92 ---------------------------------------
94 This would be especially useful for tinydrm:
96 - Add a struct drm_rect dirty_clip to drm_crtc_state. When duplicating the
97 crtc state, clear that to the max values, x/y = 0 and w/h = MAX_INT, in
98 __drm_atomic_helper_crtc_duplicate_state().
100 - Move tinydrm_merge_clips into drm_framebuffer.c, dropping the tinydrm\_
101 prefix ofc and using drm_fb\_. drm_framebuffer.c makes sense since this
102 is a function useful to implement the fb->dirty function.
104 - Create a new drm_fb_dirty function which does essentially what e.g.
105 mipi_dbi_fb_dirty does. You can use e.g. drm_atomic_helper_update_plane as the
106 template. But instead of doing a simple full-screen plane update, this new
107 helper also sets crtc_state->dirty_clip to the right coordinates. And of
108 course it needs to check whether the fb is actually active (and maybe where),
109 so there's some book-keeping involved. There's also some good fun involved in
110 scaling things appropriately. For that case we might simply give up and
111 declare the entire area covered by the plane as dirty.
113 Contact: Noralf Trønnes, Daniel Vetter
115 Fallout from atomic KMS
116 -----------------------
118 ``drm_atomic_helper.c`` provides a batch of functions which implement legacy
119 IOCTLs on top of the new atomic driver interface. Which is really nice for
120 gradual conversion of drivers, but unfortunately the semantic mismatches are
121 a bit too severe. So there's some follow-up work to adjust the function
122 interfaces to fix these issues:
124 * atomic needs the lock acquire context. At the moment that's passed around
125 implicitly with some horrible hacks, and it's also allocate with
126 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
127 the acquire context explicitly on stack and then also pass it down into
128 drivers explicitly so that the legacy-on-atomic functions can use them.
130 Except for some driver code this is done.
132 * A bunch of the vtable hooks are now in the wrong place: DRM has a split
133 between core vfunc tables (named ``drm_foo_funcs``), which are used to
134 implement the userspace ABI. And then there's the optional hooks for the
135 helper libraries (name ``drm_foo_helper_funcs``), which are purely for
136 internal use. Some of these hooks should be move from ``_funcs`` to
137 ``_helper_funcs`` since they are not part of the core ABI. There's a
138 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
140 * There's a new helper ``drm_atomic_helper_best_encoder()`` which could be
141 used by all atomic drivers which don't select the encoder for a given
142 connector at runtime. That's almost all of them, and would allow us to get
143 rid of a lot of ``best_encoder`` boilerplate in drivers.
145 This was almost done, but new drivers added a few more cases again.
147 Contact: Daniel Vetter
149 Get rid of dev->struct_mutex from GEM drivers
150 ---------------------------------------------
152 ``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
153 everything. Nowadays in modern drivers the only bit where it's mandatory is
154 serializing GEM buffer object destruction. Which unfortunately means drivers
155 have to keep track of that lock and either call ``unreference`` or
156 ``unreference_locked`` depending upon context.
158 Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
159 and there's a ``gem_free_object_unlocked`` callback for any drivers which are
160 entirely ``struct_mutex`` free.
162 For drivers that need ``struct_mutex`` it should be replaced with a driver-
163 private lock. The tricky part is the BO free functions, since those can't
164 reliably take that lock any more. Instead state needs to be protected with
165 suitable subordinate locks or some cleanup work pushed to a worker thread. For
166 performance-critical drivers it might also be better to go with a more
167 fine-grained per-buffer object and per-context lockings scheme. Currently the
168 following drivers still use ``struct_mutex``: ``msm``, ``omapdrm`` and
171 Contact: Daniel Vetter, respective driver maintainers
173 Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent
174 ----------------------------------------------------------------------------
176 For drivers which could have multiple instances, it is necessary to
177 differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
178 don't do this, drivers used dev_info/warn/err to make this differentiation. We
179 now have DRM_DEV_* variants of the drm print macros, so we can start to convert
180 those drivers back to using drm-formwatted specific log messages.
182 Before you start this conversion please contact the relevant maintainers to make
183 sure your work will be merged - not everyone agrees that the DRM dmesg macros
186 Contact: Sean Paul, Maintainer of the driver you plan to convert
191 Clean up the DRM header mess
192 ----------------------------
194 Currently the DRM subsystem has only one global header, ``drmP.h``. This is
195 used both for functions exported to helper libraries and drivers and functions
196 only used internally in the ``drm.ko`` module. The goal would be to move all
197 header declarations not needed outside of ``drm.ko`` into
198 ``drivers/gpu/drm/drm_*_internal.h`` header files. ``EXPORT_SYMBOL`` also
199 needs to be dropped for these functions.
201 This would nicely tie in with the below task to create kerneldoc after the API
202 is cleaned up. Or with the "hide legacy cruft better" task.
204 Note that this is well in progress, but ``drmP.h`` is still huge. The updated
205 plan is to switch to per-file driver API headers, which will also structure
206 the kerneldoc better. This should also allow more fine-grained ``#include``
209 In the end no .c file should need to include ``drmP.h`` anymore.
211 Contact: Daniel Vetter
213 Add missing kerneldoc for exported functions
214 --------------------------------------------
216 The DRM reference documentation is still lacking kerneldoc in a few areas. The
217 task would be to clean up interfaces like moving functions around between
218 files to better group them and improving the interfaces like dropping return
219 values for functions that never fail. Then write kerneldoc for all exported
220 functions and an overview section and integrate it all into the drm book.
222 See https://dri.freedesktop.org/docs/drm/ for what's there already.
224 Contact: Daniel Vetter
226 Hide legacy cruft better
227 ------------------------
229 Way back DRM supported only drivers which shadow-attached to PCI devices with
230 userspace or fbdev drivers setting up outputs. Modern DRM drivers take charge
231 of the entire device, you can spot them with the DRIVER_MODESET flag.
233 Unfortunately there's still large piles of legacy code around which needs to
234 be hidden so that driver writers don't accidentally end up using it. And to
235 prevent security issues in those legacy IOCTLs from being exploited on modern
236 drivers. This has multiple possible subtasks:
238 * Extract support code for legacy features into a ``drm-legacy.ko`` kernel
239 module and compile it only when one of the legacy drivers is enabled.
241 This is mostly done, the only thing left is to split up ``drm_irq.c`` into
242 legacy cruft and the parts needed by modern KMS drivers.
244 Contact: Daniel Vetter
246 Make panic handling work
247 ------------------------
249 This is a really varied tasks with lots of little bits and pieces:
251 * The panic path can't be tested currently, leading to constant breaking. The
252 main issue here is that panics can be triggered from hardirq contexts and
253 hence all panic related callback can run in hardirq context. It would be
254 awesome if we could test at least the fbdev helper code and driver code by
255 e.g. trigger calls through drm debugfs files. hardirq context could be
256 achieved by using an IPI to the local processor.
258 * There's a massive confusion of different panic handlers. DRM fbdev emulation
259 helpers have one, but on top of that the fbcon code itself also has one. We
260 need to make sure that they stop fighting over each another.
262 * ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
263 isn't a full solution for panic paths. We need to make sure that it only
264 returns true if there's a panic going on for real, and fix up all the
267 * The panic handler must never sleep, which also means it can't ever
268 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
269 even spinlocks (because NMI and hardirq can panic too). We need to either
270 make sure to not call such paths, or trylock everything. Really tricky.
272 * For the above locking troubles reasons it's pretty much impossible to
273 attempt a synchronous modeset from panic handlers. The only thing we could
274 try to achive is an atomic ``set_base`` of the primary plane, and hope that
275 it shows up. Everything else probably needs to be delayed to some worker or
276 something else which happens later on. Otherwise it just kills the box
277 harder, prevent the panic from going out on e.g. netconsole.
279 * There's also proposal for a simplied DRM console instead of the full-blown
280 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
281 obviously work for both console, in case we ever get kmslog merged.
283 Contact: Daniel Vetter
285 Clean up the debugfs support
286 ----------------------------
288 There's a bunch of issues with it:
290 - The drm_info_list ->show() function doesn't even bother to cast to the drm
291 structure for you. This is lazy.
293 - We probably want to have some support for debugfs files on crtc/connectors and
294 maybe other kms objects directly in core. There's even drm_print support in
295 the funcs for these objects to dump kms state, so it's all there. And then the
296 ->show() functions should obviously give you a pointer to the right object.
298 - The drm_info_list stuff is centered on drm_minor instead of drm_device. For
299 anything we want to print drm_device (or maybe drm_file) is the right thing.
301 - The drm_driver->debugfs_init hooks we have is just an artifact of the old
302 midlayered load sequence. DRM debugfs should work more like sysfs, where you
303 can create properties/files for an object anytime you want, and the core
304 takes care of publishing/unpuplishing all the files at register/unregister
305 time. Drivers shouldn't need to worry about these technicalities, and fixing
306 this (together with the drm_minor->drm_device move) would allow us to remove
309 Contact: Daniel Vetter
314 Some of these date from the very introduction of KMS in 2008 ...
316 - drm_mode_config.crtc_idr is misnamed, since it contains all KMS object. Should
317 be renamed to drm_mode_config.object_idr.
319 - drm_display_mode doesn't need to be derived from drm_mode_object. That's
320 leftovers from older (never merged into upstream) KMS designs where modes
321 where set using their ID, including support to add/remove modes.
326 Enable trinity for DRM
327 ----------------------
329 And fix up the fallout. Should be really interesting ...
331 Make KMS tests in i-g-t generic
332 -------------------------------
334 The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
335 including tons of testcases for corner-cases in the modesetting API. It would
336 be awesome if those tests (at least the ones not relying on Intel-specific GEM
337 features) could be made to run on any KMS driver.
339 Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
340 converting things over. For modeset tests we also first need a bit of
341 infrastructure to use dumb buffers for untiled buffers, to be able to run all
342 the non-i915 specific modeset tests.
344 Contact: Daniel Vetter
346 Create a virtual KMS driver for testing (vkms)
347 ----------------------------------------------
349 With all the latest helpers it should be fairly simple to create a virtual KMS
350 driver useful for testing, or for running X or similar on headless machines
351 (to be able to still use the GPU). This would be similar to vgem, but aimed at
354 Once the basics are there there's tons of possibilities to extend it.
356 Contact: Daniel Vetter
364 Tinydrm is the helper driver for really simple fb drivers. The goal is to make
365 those drivers as simple as possible, so lots of room for refactoring:
367 - backlight helpers, probably best to put them into a new drm_backlight.c.
368 This is because drivers/video is de-facto unmaintained. We could also
369 move drivers/video/backlight to drivers/gpu/backlight and take it all
370 over within drm-misc, but that's more work. Backlight helpers require a fair
371 bit of reworking and refactoring. A simple example is the enabling of a backlight.
372 Tinydrm has helpers for this. It would be good if other drivers can also use the
373 helper. However, there are various cases we need to consider i.e different
374 drivers seem to have different ways of enabling/disabling a backlight.
375 We also need to consider the backlight drivers (like gpio_backlight). The situation
376 is further complicated by the fact that the backlight is tied to fbdev
377 via fb_notifier_callback() which has complicated logic. For further details, refer
378 to the following discussion thread:
379 https://groups.google.com/forum/#!topic/outreachy-kernel/8rBe30lwtdA
381 - spi helpers, probably best put into spi core/helper code. Thierry said
382 the spi maintainer is fast&reactive, so shouldn't be a big issue.
384 - extract the mipi-dbi helper (well, the non-tinydrm specific parts at
385 least) into a separate helper, like we have for mipi-dsi already. Or follow
386 one of the ideas for having a shared dsi/dbi helper, abstracting away the
387 transport details more.
389 - tinydrm_lastclose could be drm_fb_helper_lastclose. Only thing we need
390 for that is to store the drm_fb_helper pointer somewhere in
391 drm_device->mode_config. And then we could roll that out to all the
394 - tinydrm_gem_cma_prime_import_sg_table should probably go into the cma
395 helpers, as a _vmapped variant (since not every driver needs the vmap).
396 And tinydrm_gem_cma_free_object could the be merged into
397 drm_gem_cma_free_object().
399 - tinydrm_fb_create we could move into drm_simple_pipe, only need to add
400 the fb_create hook to drm_simple_pipe_funcs, which would again simplify a
401 bunch of things (since it gives you a one-stop vfunc for simple drivers).
403 - Quick aside: The unregister devm stuff is kinda getting the lifetimes of
404 a drm_device wrong. Doesn't matter, since everyone else gets it wrong
407 - With the fbdev pointer in dev->mode_config we could also make
408 suspend/resume helpers entirely generic, at least if we add a
409 dev->mode_config.suspend_state. We could even provide a generic pm_ops
410 structure with those.
412 - also rework the drm_framebuffer_funcs->dirty hook wire-up, see above.
414 Contact: Noralf Trønnes, Daniel Vetter
416 AMD DC Display Driver
417 ---------------------
419 AMD DC is the display driver for AMD devices starting with Vega. There has been
420 a bunch of progress cleaning it up but there's still plenty of work to be done.
422 See drivers/gpu/drm/amd/display/TODO for tasks.
424 Contact: Harry Wentland, Alex Deucher
projects / linux-2.6-microblaze.git / blob