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_platform.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 Implement deferred fbdev setup in the helper
79 --------------------------------------------
81 Many (especially embedded drivers) want to delay fbdev setup until there's a
82 real screen plugged in. This is to avoid the dreaded fallback to the low-res
83 fbdev default. Many drivers have a hacked-up (and often broken) version of this,
84 better to do it once in the shared helpers. Thierry has a patch series, but that
85 one needs to be rebased and final polish applied.
87 Contact: Thierry Reding, Daniel Vetter, driver maintainers
89 Convert early atomic drivers to async commit helpers
90 ----------------------------------------------------
92 For the first year the atomic modeset helpers didn't support asynchronous /
93 nonblocking commits, and every driver had to hand-roll them. This is fixed
94 now, but there's still a pile of existing drivers that easily could be
95 converted over to the new infrastructure.
97 One issue with the helpers is that they require that drivers handle completion
98 events for atomic commits correctly. But fixing these bugs is good anyway.
100 Contact: Daniel Vetter, respective driver maintainers
102 Better manual-upload support for atomic
103 ---------------------------------------
105 This would be especially useful for tinydrm:
107 - Add a struct drm_rect dirty_clip to drm_crtc_state. When duplicating the
108 crtc state, clear that to the max values, x/y = 0 and w/h = MAX_INT, in
109 __drm_atomic_helper_crtc_duplicate_state().
111 - Move tinydrm_merge_clips into drm_framebuffer.c, dropping the tinydrm_
112 prefix ofc and using drm_fb_. drm_framebuffer.c makes sense since this
113 is a function useful to implement the fb->dirty function.
115 - Create a new drm_fb_dirty function which does essentially what e.g.
116 mipi_dbi_fb_dirty does. You can use e.g. drm_atomic_helper_update_plane as the
117 template. But instead of doing a simple full-screen plane update, this new
118 helper also sets crtc_state->dirty_clip to the right coordinates. And of
119 course it needs to check whether the fb is actually active (and maybe where),
120 so there's some book-keeping involved. There's also some good fun involved in
121 scaling things appropriately. For that case we might simply give up and
122 declare the entire area covered by the plane as dirty.
124 Contact: Noralf Trønnes, Daniel Vetter
126 Fallout from atomic KMS
127 -----------------------
129 ``drm_atomic_helper.c`` provides a batch of functions which implement legacy
130 IOCTLs on top of the new atomic driver interface. Which is really nice for
131 gradual conversion of drivers, but unfortunately the semantic mismatches are
132 a bit too severe. So there's some follow-up work to adjust the function
133 interfaces to fix these issues:
135 * atomic needs the lock acquire context. At the moment that's passed around
136 implicitly with some horrible hacks, and it's also allocate with
137 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
138 the acquire context explicitly on stack and then also pass it down into
139 drivers explicitly so that the legacy-on-atomic functions can use them.
141 * A bunch of the vtable hooks are now in the wrong place: DRM has a split
142 between core vfunc tables (named ``drm_foo_funcs``), which are used to
143 implement the userspace ABI. And then there's the optional hooks for the
144 helper libraries (name ``drm_foo_helper_funcs``), which are purely for
145 internal use. Some of these hooks should be move from ``_funcs`` to
146 ``_helper_funcs`` since they are not part of the core ABI. There's a
147 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
149 * There's a new helper ``drm_atomic_helper_best_encoder()`` which could be
150 used by all atomic drivers which don't select the encoder for a given
151 connector at runtime. That's almost all of them, and would allow us to get
152 rid of a lot of ``best_encoder`` boilerplate in drivers.
154 Contact: Daniel Vetter
156 Get rid of dev->struct_mutex from GEM drivers
157 ---------------------------------------------
159 ``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
160 everything. Nowadays in modern drivers the only bit where it's mandatory is
161 serializing GEM buffer object destruction. Which unfortunately means drivers
162 have to keep track of that lock and either call ``unreference`` or
163 ``unreference_locked`` depending upon context.
165 Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
166 and there's a ``gem_free_object_unlocked`` callback for any drivers which are
167 entirely ``struct_mutex`` free.
169 For drivers that need ``struct_mutex`` it should be replaced with a driver-
170 private lock. The tricky part is the BO free functions, since those can't
171 reliably take that lock any more. Instead state needs to be protected with
172 suitable subordinate locks or some cleanup work pushed to a worker thread. For
173 performance-critical drivers it might also be better to go with a more
174 fine-grained per-buffer object and per-context lockings scheme. Currently the
175 following drivers still use ``struct_mutex``: ``msm``, ``omapdrm`` and
178 Contact: Daniel Vetter
180 Switch to drm_connector_list_iter for any connector_list walking
181 ----------------------------------------------------------------
183 Connectors can be hotplugged, and we now have a special list of helpers to walk
184 the connector_list in a race-free fashion, without incurring deadlocks on
185 mutexes and other fun stuff.
187 Unfortunately most drivers are not converted yet. At least all those supporting
188 DP MST hotplug should be converted, since for those drivers the difference
189 matters. See drm_for_each_connector_iter() vs. drm_for_each_connector().
191 Contact: Daniel Vetter
196 Use new IDR deletion interface to clean up drm_gem_handle_delete()
197 ------------------------------------------------------------------
199 See the "This is gross" comment -- apparently the IDR system now can return an
200 error code instead of oopsing.
202 Clean up the DRM header mess
203 ----------------------------
205 Currently the DRM subsystem has only one global header, ``drmP.h``. This is
206 used both for functions exported to helper libraries and drivers and functions
207 only used internally in the ``drm.ko`` module. The goal would be to move all
208 header declarations not needed outside of ``drm.ko`` into
209 ``drivers/gpu/drm/drm_*_internal.h`` header files. ``EXPORT_SYMBOL`` also
210 needs to be dropped for these functions.
212 This would nicely tie in with the below task to create kerneldoc after the API
213 is cleaned up. Or with the "hide legacy cruft better" task.
215 Note that this is well in progress, but ``drmP.h`` is still huge. The updated
216 plan is to switch to per-file driver API headers, which will also structure
217 the kerneldoc better. This should also allow more fine-grained ``#include``
220 Contact: Daniel Vetter
222 Add missing kerneldoc for exported functions
223 --------------------------------------------
225 The DRM reference documentation is still lacking kerneldoc in a few areas. The
226 task would be to clean up interfaces like moving functions around between
227 files to better group them and improving the interfaces like dropping return
228 values for functions that never fail. Then write kerneldoc for all exported
229 functions and an overview section and integrate it all into the drm DocBook.
231 See https://dri.freedesktop.org/docs/drm/ for what's there already.
233 Contact: Daniel Vetter
235 Hide legacy cruft better
236 ------------------------
238 Way back DRM supported only drivers which shadow-attached to PCI devices with
239 userspace or fbdev drivers setting up outputs. Modern DRM drivers take charge
240 of the entire device, you can spot them with the DRIVER_MODESET flag.
242 Unfortunately there's still large piles of legacy code around which needs to
243 be hidden so that driver writers don't accidentally end up using it. And to
244 prevent security issues in those legacy IOCTLs from being exploited on modern
245 drivers. This has multiple possible subtasks:
247 * Make sure legacy IOCTLs can't be used on modern drivers.
248 * Extract support code for legacy features into a ``drm-legacy.ko`` kernel
249 module and compile it only when one of the legacy drivers is enabled.
250 * Extract legacy functions into their own headers and remove it that from the
251 monolithic ``drmP.h`` header.
252 * Remove any lingering cruft from the OS abstraction layer from modern
255 This is mostly done, the only thing left is to split up ``drm_irq.c`` into
256 legacy cruft and the parts needed by modern KMS drivers.
258 Contact: Daniel Vetter
260 Make panic handling work
261 ------------------------
263 This is a really varied tasks with lots of little bits and pieces:
265 * The panic path can't be tested currently, leading to constant breaking. The
266 main issue here is that panics can be triggered from hardirq contexts and
267 hence all panic related callback can run in hardirq context. It would be
268 awesome if we could test at least the fbdev helper code and driver code by
269 e.g. trigger calls through drm debugfs files. hardirq context could be
270 achieved by using an IPI to the local processor.
272 * There's a massive confusion of different panic handlers. DRM fbdev emulation
273 helpers have one, but on top of that the fbcon code itself also has one. We
274 need to make sure that they stop fighting over each another.
276 * ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
277 isn't a full solution for panic paths. We need to make sure that it only
278 returns true if there's a panic going on for real, and fix up all the
281 * The panic handler must never sleep, which also means it can't ever
282 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
283 even spinlocks (because NMI and hardirq can panic too). We need to either
284 make sure to not call such paths, or trylock everything. Really tricky.
286 * For the above locking troubles reasons it's pretty much impossible to
287 attempt a synchronous modeset from panic handlers. The only thing we could
288 try to achive is an atomic ``set_base`` of the primary plane, and hope that
289 it shows up. Everything else probably needs to be delayed to some worker or
290 something else which happens later on. Otherwise it just kills the box
291 harder, prevent the panic from going out on e.g. netconsole.
293 * There's also proposal for a simplied DRM console instead of the full-blown
294 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
295 obviously work for both console, in case we ever get kmslog merged.
297 Contact: Daniel Vetter
299 Clean up the debugfs support
300 ----------------------------
302 There's a bunch of issues with it:
304 - The drm_info_list ->show() function doesn't even bother to cast to the drm
305 structure for you. This is lazy.
307 - We probably want to have some support for debugfs files on crtc/connectors and
308 maybe other kms objects directly in core. There's even drm_print support in
309 the funcs for these objects to dump kms state, so it's all there. And then the
310 ->show() functions should obviously give you a pointer to the right object.
312 - The drm_info_list stuff is centered on drm_minor instead of drm_device. For
313 anything we want to print drm_device (or maybe drm_file) is the right thing.
315 - The drm_driver->debugfs_init hooks we have is just an artifact of the old
316 midlayered load sequence. DRM debugfs should work more like sysfs, where you
317 can create properties/files for an object anytime you want, and the core
318 takes care of publishing/unpuplishing all the files at register/unregister
319 time. Drivers shouldn't need to worry about these technicalities, and fixing
320 this (together with the drm_minor->drm_device move) would allow us to remove
323 Contact: Daniel Vetter
328 Enable trinity for DRM
329 ----------------------
331 And fix up the fallout. Should be really interesting ...
333 Make KMS tests in i-g-t generic
334 -------------------------------
336 The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
337 including tons of testcases for corner-cases in the modesetting API. It would
338 be awesome if those tests (at least the ones not relying on Intel-specific GEM
339 features) could be made to run on any KMS driver.
341 Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
342 converting things over. For modeset tests we also first need a bit of
343 infrastructure to use dumb buffers for untiled buffers, to be able to run all
344 the non-i915 specific modeset tests.
346 Contact: Daniel Vetter
348 Create a virtual KMS driver for testing (vkms)
349 ----------------------------------------------
351 With all the latest helpers it should be fairly simple to create a virtual KMS
352 driver useful for testing, or for running X or similar on headless machines
353 (to be able to still use the GPU). This would be similar to vgem, but aimed at
356 Once the basics are there there's tons of possibilities to extend it.
358 Contact: Daniel Vetter
366 Tinydrm is the helper driver for really simple fb drivers. The goal is to make
367 those drivers as simple as possible, so lots of room for refactoring:
369 - backlight helpers, probably best to put them into a new drm_backlight.c.
370 This is because drivers/video is de-facto unmaintained. We could also
371 move drivers/video/backlight to drivers/gpu/backlight and take it all
372 over within drm-misc, but that's more work.
374 - spi helpers, probably best put into spi core/helper code. Thierry said
375 the spi maintainer is fast&reactive, so shouldn't be a big issue.
377 - extract the mipi-dbi helper (well, the non-tinydrm specific parts at
378 least) into a separate helper, like we have for mipi-dsi already. Or follow
379 one of the ideas for having a shared dsi/dbi helper, abstracting away the
380 transport details more.
382 - tinydrm_lastclose could be drm_fb_helper_lastclose. Only thing we need
383 for that is to store the drm_fb_helper pointer somewhere in
384 drm_device->mode_config. And then we could roll that out to all the
387 - tinydrm_gem_cma_prime_import_sg_table should probably go into the cma
388 helpers, as a _vmapped variant (since not every driver needs the vmap).
389 And tinydrm_gem_cma_free_object could the be merged into
390 drm_gem_cma_free_object().
392 - tinydrm_fb_create we could move into drm_simple_pipe, only need to add
393 the fb_create hook to drm_simple_pipe_funcs, which would again simplify a
394 bunch of things (since it gives you a one-stop vfunc for simple drivers).
396 - Quick aside: The unregister devm stuff is kinda getting the lifetimes of
397 a drm_device wrong. Doesn't matter, since everyone else gets it wrong
400 - With the fbdev pointer in dev->mode_config we could also make
401 suspend/resume helpers entirely generic, at least if we add a
402 dev->mode_config.suspend_state. We could even provide a generic pm_ops
403 structure with those.
405 - also rework the drm_framebuffer_funcs->dirty hook wire-up, see above.
407 Contact: Noralf Trønnes, Daniel Vetter
415 This is pretty much done, but there's some advanced topics:
417 Come up with a way to hyperlink to struct members. Currently you can hyperlink
418 to the struct using ``#struct_name``, but not to a member within. Would need
419 buy-in from kerneldoc maintainers, and the big question is how to make it work
420 without totally unsightly
421 ``drm_foo_bar_really_long_structure->even_longer_memeber`` all over the text
422 which breaks text flow.
424 Figure out how to integrate the asciidoc support for ascii-diagrams. We have a
425 few of those (e.g. to describe mode timings), and asciidoc supports converting
426 some ascii-art dialect into pngs. Would be really pretty to make that work.
428 Contact: Daniel Vetter, Jani Nikula
430 Jani is working on this already, hopefully lands in 4.8.
projects / karo-tx-linux.git / blob