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 13Remove custom dumb_map_offset implementations 14--------------------------------------------- 15 16All GEM based drivers should be using drm_gem_create_mmap_offset() instead. 17Audit each individual driver, make sure it'll work with the generic 18implementation (there's lots of outdated locking leftovers in various 19implementations), and then remove it. 20 21Contact: Daniel Vetter, respective driver maintainers 22 23Convert existing KMS drivers to atomic modesetting 24-------------------------------------------------- 25 263.19 has the atomic modeset interfaces and helpers, so drivers can now be 27converted over. Modern compositors like Wayland or Surfaceflinger on Android 28really want an atomic modeset interface, so this is all about the bright 29future. 30 31There is a conversion guide for atomic and all you need is a GPU for a 32non-converted driver (again virtual HW drivers for KVM are still all 33suitable). 34 35As part of this drivers also need to convert to universal plane (which means 36exposing primary & cursor as proper plane objects). But that's much easier to 37do by directly using the new atomic helper driver callbacks. 38 39Contact: Daniel Vetter, respective driver maintainers 40 41Clean up the clipped coordination confusion around planes 42--------------------------------------------------------- 43 44We have a helper to get this right with drm_plane_helper_check_update(), but 45it's not consistently used. This should be fixed, preferrably in the atomic 46helpers (and drivers then moved over to clipped coordinates). Probably the 47helper should also be moved from drm_plane_helper.c to the atomic helpers, to 48avoid confusion - the other helpers in that file are all deprecated legacy 49helpers. 50 51Contact: Ville Syrjälä, Daniel Vetter, driver maintainers 52 53Convert early atomic drivers to async commit helpers 54---------------------------------------------------- 55 56For the first year the atomic modeset helpers didn't support asynchronous / 57nonblocking commits, and every driver had to hand-roll them. This is fixed 58now, but there's still a pile of existing drivers that easily could be 59converted over to the new infrastructure. 60 61One issue with the helpers is that they require that drivers handle completion 62events for atomic commits correctly. But fixing these bugs is good anyway. 63 64Contact: Daniel Vetter, respective driver maintainers 65 66Fallout from atomic KMS 67----------------------- 68 69``drm_atomic_helper.c`` provides a batch of functions which implement legacy 70IOCTLs on top of the new atomic driver interface. Which is really nice for 71gradual conversion of drivers, but unfortunately the semantic mismatches are 72a bit too severe. So there's some follow-up work to adjust the function 73interfaces to fix these issues: 74 75* atomic needs the lock acquire context. At the moment that's passed around 76 implicitly with some horrible hacks, and it's also allocate with 77 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating 78 the acquire context explicitly on stack and then also pass it down into 79 drivers explicitly so that the legacy-on-atomic functions can use them. 80 81 Except for some driver code this is done. This task should be finished by 82 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all(). 83 84* A bunch of the vtable hooks are now in the wrong place: DRM has a split 85 between core vfunc tables (named ``drm_foo_funcs``), which are used to 86 implement the userspace ABI. And then there's the optional hooks for the 87 helper libraries (name ``drm_foo_helper_funcs``), which are purely for 88 internal use. Some of these hooks should be move from ``_funcs`` to 89 ``_helper_funcs`` since they are not part of the core ABI. There's a 90 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``. 91 92Contact: Daniel Vetter 93 94Get rid of dev->struct_mutex from GEM drivers 95--------------------------------------------- 96 97``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested 98everything. Nowadays in modern drivers the only bit where it's mandatory is 99serializing GEM buffer object destruction. Which unfortunately means drivers 100have to keep track of that lock and either call ``unreference`` or 101``unreference_locked`` depending upon context. 102 103Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8, 104and there's a ``gem_free_object_unlocked`` callback for any drivers which are 105entirely ``struct_mutex`` free. 106 107For drivers that need ``struct_mutex`` it should be replaced with a driver- 108private lock. The tricky part is the BO free functions, since those can't 109reliably take that lock any more. Instead state needs to be protected with 110suitable subordinate locks or some cleanup work pushed to a worker thread. For 111performance-critical drivers it might also be better to go with a more 112fine-grained per-buffer object and per-context lockings scheme. Currently only the 113``msm`` driver still use ``struct_mutex``. 114 115Contact: Daniel Vetter, respective driver maintainers 116 117Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent 118---------------------------------------------------------------------------- 119 120For drivers which could have multiple instances, it is necessary to 121differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR 122don't do this, drivers used dev_info/warn/err to make this differentiation. We 123now have DRM_DEV_* variants of the drm print macros, so we can start to convert 124those drivers back to using drm-formwatted specific log messages. 125 126Before you start this conversion please contact the relevant maintainers to make 127sure your work will be merged - not everyone agrees that the DRM dmesg macros 128are better. 129 130Contact: Sean Paul, Maintainer of the driver you plan to convert 131 132Convert drivers to use simple modeset suspend/resume 133---------------------------------------------------- 134 135Most drivers (except i915 and nouveau) that use 136drm_atomic_helper_suspend/resume() can probably be converted to use 137drm_mode_config_helper_suspend/resume(). Also there's still open-coded version 138of the atomic suspend/resume code in older atomic modeset drivers. 139 140Contact: Maintainer of the driver you plan to convert 141 142Convert drivers to use drm_fb_helper_fbdev_setup/teardown() 143----------------------------------------------------------- 144 145Most drivers can use drm_fb_helper_fbdev_setup() except maybe: 146 147- amdgpu which has special logic to decide whether to call 148 drm_helper_disable_unused_functions() 149 150- armada which isn't atomic and doesn't call 151 drm_helper_disable_unused_functions() 152 153- i915 which calls drm_fb_helper_initial_config() in a worker 154 155Drivers that use drm_framebuffer_remove() to clean up the fbdev framebuffer can 156probably use drm_fb_helper_fbdev_teardown(). 157 158Contact: Maintainer of the driver you plan to convert 159 160Clean up mmap forwarding 161------------------------ 162 163A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers. 164And also a lot of them forward dma-buf mmap to the gem mmap implementations. 165Would be great to refactor this all into a set of small common helpers. 166 167Contact: Daniel Vetter 168 169Generic fbdev defio support 170--------------------------- 171 172The defio support code in the fbdev core has some very specific requirements, 173which means drivers need to have a special framebuffer for fbdev. Which prevents 174us from using the generic fbdev emulation code everywhere. The main issue is 175that it uses some fields in struct page itself, which breaks shmem gem objects 176(and other things). 177 178Possible solution would be to write our own defio mmap code in the drm fbdev 179emulation. It would need to fully wrap the existing mmap ops, forwarding 180everything after it has done the write-protect/mkwrite trickery: 181 182- In the drm_fbdev_fb_mmap helper, if we need defio, change the 183 default page prots to write-protected with something like this:: 184 185 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot); 186 187- Set the mkwrite and fsync callbacks with similar implementions to the core 188 fbdev defio stuff. These should all work on plain ptes, they don't actually 189 require a struct page. uff. These should all work on plain ptes, they don't 190 actually require a struct page. 191 192- Track the dirty pages in a separate structure (bitfield with one bit per page 193 should work) to avoid clobbering struct page. 194 195Might be good to also have some igt testcases for this. 196 197Contact: Daniel Vetter, Noralf Tronnes 198 199Remove the ->gem_prime_res_obj callback 200-------------------------------------------- 201 202The ->gem_prime_res_obj callback can be removed from drivers by using the 203reservation_object in the drm_gem_object. It may also be possible to use the 204generic drm_gem_reservation_object_wait helper for waiting for a bo. 205 206Contact: Daniel Vetter 207 208idr_init_base() 209--------------- 210 211DRM core&drivers uses a lot of idr (integer lookup directories) for mapping 212userspace IDs to internal objects, and in most places ID=0 means NULL and hence 213is never used. Switching to idr_init_base() for these would make the idr more 214efficient. 215 216Contact: Daniel Vetter 217 218Defaults for .gem_prime_import and export 219----------------------------------------- 220 221Most drivers don't need to set drm_driver->gem_prime_import and 222->gem_prime_export now that drm_gem_prime_import() and drm_gem_prime_export() 223are the default. 224 225struct drm_gem_object_funcs 226--------------------------- 227 228GEM objects can now have a function table instead of having the callbacks on the 229DRM driver struct. This is now the preferred way and drivers can be moved over. 230 231Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate 232--------------------------------------------------------- 233 234For cases where drivers are attempting to grab the modeset locks with a local 235acquire context. Replace the boilerplate code surrounding 236drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and 237DRM_MODESET_LOCK_ALL_END() instead. 238 239This should also be done for all places where drm_modest_lock_all() is still 240used. 241 242As a reference, take a look at the conversions already completed in drm core. 243 244Contact: Sean Paul, respective driver maintainers 245 246Rename CMA helpers to DMA helpers 247--------------------------------- 248 249CMA (standing for contiguous memory allocator) is really a bit an accident of 250what these were used for first, a much better name would be DMA helpers. In the 251text these should even be called coherent DMA memory helpers (so maybe CDM, but 252no one knows what that means) since underneath they just use dma_alloc_coherent. 253 254Contact: Laurent Pinchart, Daniel Vetter 255 256Convert direct mode.vrefresh accesses to use drm_mode_vrefresh() 257---------------------------------------------------------------- 258 259drm_display_mode.vrefresh isn't guaranteed to be populated. As such, using it 260is risky and has been known to cause div-by-zero bugs. Fortunately, drm core 261has helper which will use mode.vrefresh if it's !0 and will calculate it from 262the timings when it's 0. 263 264Use simple search/replace, or (more fun) cocci to replace instances of direct 265vrefresh access with a call to the helper. Check out 266https://lists.freedesktop.org/archives/dri-devel/2019-January/205186.html for 267inspiration. 268 269Once all instances of vrefresh have been converted, remove vrefresh from 270drm_display_mode to avoid future use. 271 272Contact: Sean Paul 273 274Remove drm_display_mode.hsync 275----------------------------- 276 277We have drm_mode_hsync() to calculate this from hsync_start/end, since drivers 278shouldn't/don't use this, remove this member to avoid any temptations to use it 279in the future. If there is any debug code using drm_display_mode.hsync, convert 280it to use drm_mode_hsync() instead. 281 282Contact: Sean Paul 283 284drm_fb_helper tasks 285------------------- 286 287- drm_fb_helper_restore_fbdev_mode_unlocked() should call restore_fbdev_mode() 288 not the _force variant so it can bail out if there is a master. But first 289 these igt tests need to be fixed: kms_fbcon_fbt@psr and 290 kms_fbcon_fbt@psr-suspend. 291 292Core refactorings 293================= 294 295Clean up the DRM header mess 296---------------------------- 297 298The DRM subsystem originally had only one huge global header, ``drmP.h``. This 299is now split up, but many source files still include it. The remaining part of 300the cleanup work here is to replace any ``#include <drm/drmP.h>`` by only the 301headers needed (and fixing up any missing pre-declarations in the headers). 302 303In the end no .c file should need to include ``drmP.h`` anymore. 304 305Contact: Daniel Vetter 306 307Add missing kerneldoc for exported functions 308-------------------------------------------- 309 310The DRM reference documentation is still lacking kerneldoc in a few areas. The 311task would be to clean up interfaces like moving functions around between 312files to better group them and improving the interfaces like dropping return 313values for functions that never fail. Then write kerneldoc for all exported 314functions and an overview section and integrate it all into the drm book. 315 316See https://dri.freedesktop.org/docs/drm/ for what's there already. 317 318Contact: Daniel Vetter 319 320Make panic handling work 321------------------------ 322 323This is a really varied tasks with lots of little bits and pieces: 324 325* The panic path can't be tested currently, leading to constant breaking. The 326 main issue here is that panics can be triggered from hardirq contexts and 327 hence all panic related callback can run in hardirq context. It would be 328 awesome if we could test at least the fbdev helper code and driver code by 329 e.g. trigger calls through drm debugfs files. hardirq context could be 330 achieved by using an IPI to the local processor. 331 332* There's a massive confusion of different panic handlers. DRM fbdev emulation 333 helpers have one, but on top of that the fbcon code itself also has one. We 334 need to make sure that they stop fighting over each another. 335 336* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and 337 isn't a full solution for panic paths. We need to make sure that it only 338 returns true if there's a panic going on for real, and fix up all the 339 fallout. 340 341* The panic handler must never sleep, which also means it can't ever 342 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not 343 even spinlocks (because NMI and hardirq can panic too). We need to either 344 make sure to not call such paths, or trylock everything. Really tricky. 345 346* For the above locking troubles reasons it's pretty much impossible to 347 attempt a synchronous modeset from panic handlers. The only thing we could 348 try to achive is an atomic ``set_base`` of the primary plane, and hope that 349 it shows up. Everything else probably needs to be delayed to some worker or 350 something else which happens later on. Otherwise it just kills the box 351 harder, prevent the panic from going out on e.g. netconsole. 352 353* There's also proposal for a simplied DRM console instead of the full-blown 354 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should 355 obviously work for both console, in case we ever get kmslog merged. 356 357Contact: Daniel Vetter 358 359Clean up the debugfs support 360---------------------------- 361 362There's a bunch of issues with it: 363 364- The drm_info_list ->show() function doesn't even bother to cast to the drm 365 structure for you. This is lazy. 366 367- We probably want to have some support for debugfs files on crtc/connectors and 368 maybe other kms objects directly in core. There's even drm_print support in 369 the funcs for these objects to dump kms state, so it's all there. And then the 370 ->show() functions should obviously give you a pointer to the right object. 371 372- The drm_info_list stuff is centered on drm_minor instead of drm_device. For 373 anything we want to print drm_device (or maybe drm_file) is the right thing. 374 375- The drm_driver->debugfs_init hooks we have is just an artifact of the old 376 midlayered load sequence. DRM debugfs should work more like sysfs, where you 377 can create properties/files for an object anytime you want, and the core 378 takes care of publishing/unpuplishing all the files at register/unregister 379 time. Drivers shouldn't need to worry about these technicalities, and fixing 380 this (together with the drm_minor->drm_device move) would allow us to remove 381 debugfs_init. 382 383Contact: Daniel Vetter 384 385KMS cleanups 386------------ 387 388Some of these date from the very introduction of KMS in 2008 ... 389 390- Make ->funcs and ->helper_private vtables optional. There's a bunch of empty 391 function tables in drivers, but before we can remove them we need to make sure 392 that all the users in helpers and drivers do correctly check for a NULL 393 vtable. 394 395- Cleanup up the various ->destroy callbacks. A lot of them just wrapt the 396 drm_*_cleanup implementations and can be removed. Some tack a kfree() at the 397 end, for which we could add drm_*_cleanup_kfree(). And then there's the (for 398 historical reasons) misnamed drm_primary_helper_destroy() function. 399 400Better Testing 401============== 402 403Enable trinity for DRM 404---------------------- 405 406And fix up the fallout. Should be really interesting ... 407 408Make KMS tests in i-g-t generic 409------------------------------- 410 411The i915 driver team maintains an extensive testsuite for the i915 DRM driver, 412including tons of testcases for corner-cases in the modesetting API. It would 413be awesome if those tests (at least the ones not relying on Intel-specific GEM 414features) could be made to run on any KMS driver. 415 416Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass- 417converting things over. For modeset tests we also first need a bit of 418infrastructure to use dumb buffers for untiled buffers, to be able to run all 419the non-i915 specific modeset tests. 420 421Extend virtual test driver (VKMS) 422--------------------------------- 423 424See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal 425internship task, since it only requires a virtual machine and can be sized to 426fit the available time. 427 428Contact: Daniel Vetter 429 430Driver Specific 431=============== 432 433tinydrm 434------- 435 436Tinydrm is the helper driver for really simple fb drivers. The goal is to make 437those drivers as simple as possible, so lots of room for refactoring: 438 439- backlight helpers, probably best to put them into a new drm_backlight.c. 440 This is because drivers/video is de-facto unmaintained. We could also 441 move drivers/video/backlight to drivers/gpu/backlight and take it all 442 over within drm-misc, but that's more work. Backlight helpers require a fair 443 bit of reworking and refactoring. A simple example is the enabling of a backlight. 444 Tinydrm has helpers for this. It would be good if other drivers can also use the 445 helper. However, there are various cases we need to consider i.e different 446 drivers seem to have different ways of enabling/disabling a backlight. 447 We also need to consider the backlight drivers (like gpio_backlight). The situation 448 is further complicated by the fact that the backlight is tied to fbdev 449 via fb_notifier_callback() which has complicated logic. For further details, refer 450 to the following discussion thread: 451 https://groups.google.com/forum/#!topic/outreachy-kernel/8rBe30lwtdA 452 453- spi helpers, probably best put into spi core/helper code. Thierry said 454 the spi maintainer is fast&reactive, so shouldn't be a big issue. 455 456- extract the mipi-dbi helper (well, the non-tinydrm specific parts at 457 least) into a separate helper, like we have for mipi-dsi already. Or follow 458 one of the ideas for having a shared dsi/dbi helper, abstracting away the 459 transport details more. 460 461Contact: Noralf Trønnes, Daniel Vetter 462 463AMD DC Display Driver 464--------------------- 465 466AMD DC is the display driver for AMD devices starting with Vega. There has been 467a bunch of progress cleaning it up but there's still plenty of work to be done. 468 469See drivers/gpu/drm/amd/display/TODO for tasks. 470 471Contact: Harry Wentland, Alex Deucher 472 473i915 474---- 475 476- Our early/late pm callbacks could be removed in favour of using 477 device_link_add to model the dependency between i915 and snd_had. See 478 https://dri.freedesktop.org/docs/drm/driver-api/device_link.html 479 480Outside DRM 481=========== 482