1.. _todo: 2 3========= 4TODO list 5========= 6 7This section contains a list of smaller janitorial tasks in the kernel DRM 8graphics subsystem useful as newbie projects. Or for slow rainy days. 9 10Subsystem-wide refactorings 11=========================== 12 13De-midlayer drivers 14------------------- 15 16With the recent ``drm_bus`` cleanup patches for 3.17 it is no longer required 17to 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`` 19and ``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 21the new two-stage ``drm_device`` setup/teardown. 22 23Once all existing drivers are converted we can also remove those bus support 24files for USB and platform devices. 25 26All you need is a GPU for a non-converted driver (currently almost all of 27them, but also all the virtual ones used by KVM, so everyone qualifies). 28 29Contact: Daniel Vetter, Thierry Reding, respective driver maintainers 30 31Switch from reference/unreference to get/put 32-------------------------------------------- 33 34For some reason DRM core uses ``reference``/``unreference`` suffixes for 35refcounting functions, but kernel uses ``get``/``put`` (e.g. 36``kref_get``/``put()``). It would be good to switch over for consistency, and 37it's shorter. Needs to be done in 3 steps for each pair of functions: 38 39* Create new ``get``/``put`` functions, define the old names as compatibility 40 wrappers 41* Switch over each file/driver using a cocci-generated spatch. 42* Once all users of the old names are gone, remove them. 43 44This way drivers/patches in the progress of getting merged won't break. 45 46Contact: Daniel Vetter 47 48Convert existing KMS drivers to atomic modesetting 49-------------------------------------------------- 50 513.19 has the atomic modeset interfaces and helpers, so drivers can now be 52converted over. Modern compositors like Wayland or Surfaceflinger on Android 53really want an atomic modeset interface, so this is all about the bright 54future. 55 56There is a conversion guide for atomic and all you need is a GPU for a 57non-converted driver (again virtual HW drivers for KVM are still all 58suitable). 59 60As part of this drivers also need to convert to universal plane (which means 61exposing primary & cursor as proper plane objects). But that's much easier to 62do by directly using the new atomic helper driver callbacks. 63 64Contact: Daniel Vetter, respective driver maintainers 65 66Clean up the clipped coordination confusion around planes 67--------------------------------------------------------- 68 69We have a helper to get this right with drm_plane_helper_check_update(), but 70it's not consistently used. This should be fixed, preferrably in the atomic 71helpers (and drivers then moved over to clipped coordinates). Probably the 72helper should also be moved from drm_plane_helper.c to the atomic helpers, to 73avoid confusion - the other helpers in that file are all deprecated legacy 74helpers. 75 76Contact: Ville Syrjälä, Daniel Vetter, driver maintainers 77 78Implement deferred fbdev setup in the helper 79-------------------------------------------- 80 81Many (especially embedded drivers) want to delay fbdev setup until there's a 82real screen plugged in. This is to avoid the dreaded fallback to the low-res 83fbdev default. Many drivers have a hacked-up (and often broken) version of this, 84better to do it once in the shared helpers. Thierry has a patch series, but that 85one needs to be rebased and final polish applied. 86 87Contact: Thierry Reding, Daniel Vetter, driver maintainers 88 89Convert early atomic drivers to async commit helpers 90---------------------------------------------------- 91 92For the first year the atomic modeset helpers didn't support asynchronous / 93nonblocking commits, and every driver had to hand-roll them. This is fixed 94now, but there's still a pile of existing drivers that easily could be 95converted over to the new infrastructure. 96 97One issue with the helpers is that they require that drivers handle completion 98events for atomic commits correctly. But fixing these bugs is good anyway. 99 100Contact: Daniel Vetter, respective driver maintainers 101 102Better manual-upload support for atomic 103--------------------------------------- 104 105This would be especially useful for tinydrm: 106 107- 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(). 110 111- 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. 114 115- 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. 123 124Contact: Noralf Trønnes, Daniel Vetter 125 126Fallout from atomic KMS 127----------------------- 128 129``drm_atomic_helper.c`` provides a batch of functions which implement legacy 130IOCTLs on top of the new atomic driver interface. Which is really nice for 131gradual conversion of drivers, but unfortunately the semantic mismatches are 132a bit too severe. So there's some follow-up work to adjust the function 133interfaces to fix these issues: 134 135* 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. 140 141* 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``. 148 149* 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. 153 154Contact: Daniel Vetter 155 156Get rid of dev->struct_mutex from GEM drivers 157--------------------------------------------- 158 159``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested 160everything. Nowadays in modern drivers the only bit where it's mandatory is 161serializing GEM buffer object destruction. Which unfortunately means drivers 162have to keep track of that lock and either call ``unreference`` or 163``unreference_locked`` depending upon context. 164 165Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8, 166and there's a ``gem_free_object_unlocked`` callback for any drivers which are 167entirely ``struct_mutex`` free. 168 169For drivers that need ``struct_mutex`` it should be replaced with a driver- 170private lock. The tricky part is the BO free functions, since those can't 171reliably take that lock any more. Instead state needs to be protected with 172suitable subordinate locks or some cleanup work pushed to a worker thread. For 173performance-critical drivers it might also be better to go with a more 174fine-grained per-buffer object and per-context lockings scheme. Currently the 175following drivers still use ``struct_mutex``: ``msm``, ``omapdrm`` and 176``udl``. 177 178Contact: Daniel Vetter, respective driver maintainers 179 180Switch to drm_connector_list_iter for any connector_list walking 181---------------------------------------------------------------- 182 183Connectors can be hotplugged, and we now have a special list of helpers to walk 184the connector_list in a race-free fashion, without incurring deadlocks on 185mutexes and other fun stuff. 186 187Unfortunately most drivers are not converted yet. At least all those supporting 188DP MST hotplug should be converted, since for those drivers the difference 189matters. See drm_for_each_connector_iter() vs. drm_for_each_connector(). 190 191Contact: Daniel Vetter 192 193Core refactorings 194================= 195 196Use new IDR deletion interface to clean up drm_gem_handle_delete() 197------------------------------------------------------------------ 198 199See the "This is gross" comment -- apparently the IDR system now can return an 200error code instead of oopsing. 201 202Clean up the DRM header mess 203---------------------------- 204 205Currently the DRM subsystem has only one global header, ``drmP.h``. This is 206used both for functions exported to helper libraries and drivers and functions 207only used internally in the ``drm.ko`` module. The goal would be to move all 208header declarations not needed outside of ``drm.ko`` into 209``drivers/gpu/drm/drm_*_internal.h`` header files. ``EXPORT_SYMBOL`` also 210needs to be dropped for these functions. 211 212This would nicely tie in with the below task to create kerneldoc after the API 213is cleaned up. Or with the "hide legacy cruft better" task. 214 215Note that this is well in progress, but ``drmP.h`` is still huge. The updated 216plan is to switch to per-file driver API headers, which will also structure 217the kerneldoc better. This should also allow more fine-grained ``#include`` 218directives. 219 220In the end no .c file should need to include ``drmP.h`` anymore. 221 222Contact: Daniel Vetter 223 224Add missing kerneldoc for exported functions 225-------------------------------------------- 226 227The DRM reference documentation is still lacking kerneldoc in a few areas. The 228task would be to clean up interfaces like moving functions around between 229files to better group them and improving the interfaces like dropping return 230values for functions that never fail. Then write kerneldoc for all exported 231functions and an overview section and integrate it all into the drm DocBook. 232 233See https://dri.freedesktop.org/docs/drm/ for what's there already. 234 235Contact: Daniel Vetter 236 237Hide legacy cruft better 238------------------------ 239 240Way back DRM supported only drivers which shadow-attached to PCI devices with 241userspace or fbdev drivers setting up outputs. Modern DRM drivers take charge 242of the entire device, you can spot them with the DRIVER_MODESET flag. 243 244Unfortunately there's still large piles of legacy code around which needs to 245be hidden so that driver writers don't accidentally end up using it. And to 246prevent security issues in those legacy IOCTLs from being exploited on modern 247drivers. This has multiple possible subtasks: 248 249* Extract support code for legacy features into a ``drm-legacy.ko`` kernel 250 module and compile it only when one of the legacy drivers is enabled. 251 252This is mostly done, the only thing left is to split up ``drm_irq.c`` into 253legacy cruft and the parts needed by modern KMS drivers. 254 255Contact: Daniel Vetter 256 257Make panic handling work 258------------------------ 259 260This is a really varied tasks with lots of little bits and pieces: 261 262* The panic path can't be tested currently, leading to constant breaking. The 263 main issue here is that panics can be triggered from hardirq contexts and 264 hence all panic related callback can run in hardirq context. It would be 265 awesome if we could test at least the fbdev helper code and driver code by 266 e.g. trigger calls through drm debugfs files. hardirq context could be 267 achieved by using an IPI to the local processor. 268 269* There's a massive confusion of different panic handlers. DRM fbdev emulation 270 helpers have one, but on top of that the fbcon code itself also has one. We 271 need to make sure that they stop fighting over each another. 272 273* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and 274 isn't a full solution for panic paths. We need to make sure that it only 275 returns true if there's a panic going on for real, and fix up all the 276 fallout. 277 278* The panic handler must never sleep, which also means it can't ever 279 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not 280 even spinlocks (because NMI and hardirq can panic too). We need to either 281 make sure to not call such paths, or trylock everything. Really tricky. 282 283* For the above locking troubles reasons it's pretty much impossible to 284 attempt a synchronous modeset from panic handlers. The only thing we could 285 try to achive is an atomic ``set_base`` of the primary plane, and hope that 286 it shows up. Everything else probably needs to be delayed to some worker or 287 something else which happens later on. Otherwise it just kills the box 288 harder, prevent the panic from going out on e.g. netconsole. 289 290* There's also proposal for a simplied DRM console instead of the full-blown 291 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should 292 obviously work for both console, in case we ever get kmslog merged. 293 294Contact: Daniel Vetter 295 296Clean up the debugfs support 297---------------------------- 298 299There's a bunch of issues with it: 300 301- The drm_info_list ->show() function doesn't even bother to cast to the drm 302 structure for you. This is lazy. 303 304- We probably want to have some support for debugfs files on crtc/connectors and 305 maybe other kms objects directly in core. There's even drm_print support in 306 the funcs for these objects to dump kms state, so it's all there. And then the 307 ->show() functions should obviously give you a pointer to the right object. 308 309- The drm_info_list stuff is centered on drm_minor instead of drm_device. For 310 anything we want to print drm_device (or maybe drm_file) is the right thing. 311 312- The drm_driver->debugfs_init hooks we have is just an artifact of the old 313 midlayered load sequence. DRM debugfs should work more like sysfs, where you 314 can create properties/files for an object anytime you want, and the core 315 takes care of publishing/unpuplishing all the files at register/unregister 316 time. Drivers shouldn't need to worry about these technicalities, and fixing 317 this (together with the drm_minor->drm_device move) would allow us to remove 318 debugfs_init. 319 320Contact: Daniel Vetter 321 322Better Testing 323============== 324 325Enable trinity for DRM 326---------------------- 327 328And fix up the fallout. Should be really interesting ... 329 330Make KMS tests in i-g-t generic 331------------------------------- 332 333The i915 driver team maintains an extensive testsuite for the i915 DRM driver, 334including tons of testcases for corner-cases in the modesetting API. It would 335be awesome if those tests (at least the ones not relying on Intel-specific GEM 336features) could be made to run on any KMS driver. 337 338Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass- 339converting things over. For modeset tests we also first need a bit of 340infrastructure to use dumb buffers for untiled buffers, to be able to run all 341the non-i915 specific modeset tests. 342 343Contact: Daniel Vetter 344 345Create a virtual KMS driver for testing (vkms) 346---------------------------------------------- 347 348With all the latest helpers it should be fairly simple to create a virtual KMS 349driver useful for testing, or for running X or similar on headless machines 350(to be able to still use the GPU). This would be similar to vgem, but aimed at 351the modeset side. 352 353Once the basics are there there's tons of possibilities to extend it. 354 355Contact: Daniel Vetter 356 357Driver Specific 358=============== 359 360tinydrm 361------- 362 363Tinydrm is the helper driver for really simple fb drivers. The goal is to make 364those drivers as simple as possible, so lots of room for refactoring: 365 366- backlight helpers, probably best to put them into a new drm_backlight.c. 367 This is because drivers/video is de-facto unmaintained. We could also 368 move drivers/video/backlight to drivers/gpu/backlight and take it all 369 over within drm-misc, but that's more work. 370 371- spi helpers, probably best put into spi core/helper code. Thierry said 372 the spi maintainer is fast&reactive, so shouldn't be a big issue. 373 374- extract the mipi-dbi helper (well, the non-tinydrm specific parts at 375 least) into a separate helper, like we have for mipi-dsi already. Or follow 376 one of the ideas for having a shared dsi/dbi helper, abstracting away the 377 transport details more. 378 379- tinydrm_lastclose could be drm_fb_helper_lastclose. Only thing we need 380 for that is to store the drm_fb_helper pointer somewhere in 381 drm_device->mode_config. And then we could roll that out to all the 382 drivers. 383 384- tinydrm_gem_cma_prime_import_sg_table should probably go into the cma 385 helpers, as a _vmapped variant (since not every driver needs the vmap). 386 And tinydrm_gem_cma_free_object could the be merged into 387 drm_gem_cma_free_object(). 388 389- tinydrm_fb_create we could move into drm_simple_pipe, only need to add 390 the fb_create hook to drm_simple_pipe_funcs, which would again simplify a 391 bunch of things (since it gives you a one-stop vfunc for simple drivers). 392 393- Quick aside: The unregister devm stuff is kinda getting the lifetimes of 394 a drm_device wrong. Doesn't matter, since everyone else gets it wrong 395 too :-) 396 397- With the fbdev pointer in dev->mode_config we could also make 398 suspend/resume helpers entirely generic, at least if we add a 399 dev->mode_config.suspend_state. We could even provide a generic pm_ops 400 structure with those. 401 402- also rework the drm_framebuffer_funcs->dirty hook wire-up, see above. 403 404Contact: Noralf Trønnes, Daniel Vetter 405 406Outside DRM 407=========== 408