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 10Difficulty 11---------- 12 13To make it easier task are categorized into different levels: 14 15Starter: Good tasks to get started with the DRM subsystem. 16 17Intermediate: Tasks which need some experience with working in the DRM 18subsystem, or some specific GPU/display graphics knowledge. For debugging issue 19it's good to have the relevant hardware (or a virtual driver set up) available 20for testing. 21 22Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem 23and graphics topics. Generally need the relevant hardware for development and 24testing. 25 26Subsystem-wide refactorings 27=========================== 28 29Remove custom dumb_map_offset implementations 30--------------------------------------------- 31 32All GEM based drivers should be using drm_gem_create_mmap_offset() instead. 33Audit each individual driver, make sure it'll work with the generic 34implementation (there's lots of outdated locking leftovers in various 35implementations), and then remove it. 36 37Contact: Daniel Vetter, respective driver maintainers 38 39Level: Intermediate 40 41Convert existing KMS drivers to atomic modesetting 42-------------------------------------------------- 43 443.19 has the atomic modeset interfaces and helpers, so drivers can now be 45converted over. Modern compositors like Wayland or Surfaceflinger on Android 46really want an atomic modeset interface, so this is all about the bright 47future. 48 49There is a conversion guide for atomic and all you need is a GPU for a 50non-converted driver (again virtual HW drivers for KVM are still all 51suitable). 52 53As part of this drivers also need to convert to universal plane (which means 54exposing primary & cursor as proper plane objects). But that's much easier to 55do by directly using the new atomic helper driver callbacks. 56 57Contact: Daniel Vetter, respective driver maintainers 58 59Level: Advanced 60 61Clean up the clipped coordination confusion around planes 62--------------------------------------------------------- 63 64We have a helper to get this right with drm_plane_helper_check_update(), but 65it's not consistently used. This should be fixed, preferrably in the atomic 66helpers (and drivers then moved over to clipped coordinates). Probably the 67helper should also be moved from drm_plane_helper.c to the atomic helpers, to 68avoid confusion - the other helpers in that file are all deprecated legacy 69helpers. 70 71Contact: Ville Syrjälä, Daniel Vetter, driver maintainers 72 73Level: Advanced 74 75Convert early atomic drivers to async commit helpers 76---------------------------------------------------- 77 78For the first year the atomic modeset helpers didn't support asynchronous / 79nonblocking commits, and every driver had to hand-roll them. This is fixed 80now, but there's still a pile of existing drivers that easily could be 81converted over to the new infrastructure. 82 83One issue with the helpers is that they require that drivers handle completion 84events for atomic commits correctly. But fixing these bugs is good anyway. 85 86Contact: Daniel Vetter, respective driver maintainers 87 88Level: Advanced 89 90Fallout from atomic KMS 91----------------------- 92 93``drm_atomic_helper.c`` provides a batch of functions which implement legacy 94IOCTLs on top of the new atomic driver interface. Which is really nice for 95gradual conversion of drivers, but unfortunately the semantic mismatches are 96a bit too severe. So there's some follow-up work to adjust the function 97interfaces to fix these issues: 98 99* atomic needs the lock acquire context. At the moment that's passed around 100 implicitly with some horrible hacks, and it's also allocate with 101 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating 102 the acquire context explicitly on stack and then also pass it down into 103 drivers explicitly so that the legacy-on-atomic functions can use them. 104 105 Except for some driver code this is done. This task should be finished by 106 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all(). 107 108* A bunch of the vtable hooks are now in the wrong place: DRM has a split 109 between core vfunc tables (named ``drm_foo_funcs``), which are used to 110 implement the userspace ABI. And then there's the optional hooks for the 111 helper libraries (name ``drm_foo_helper_funcs``), which are purely for 112 internal use. Some of these hooks should be move from ``_funcs`` to 113 ``_helper_funcs`` since they are not part of the core ABI. There's a 114 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``. 115 116Contact: Daniel Vetter 117 118Level: Intermediate 119 120Get rid of dev->struct_mutex from GEM drivers 121--------------------------------------------- 122 123``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested 124everything. Nowadays in modern drivers the only bit where it's mandatory is 125serializing GEM buffer object destruction. Which unfortunately means drivers 126have to keep track of that lock and either call ``unreference`` or 127``unreference_locked`` depending upon context. 128 129Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8, 130and there's a ``gem_free_object_unlocked`` callback for any drivers which are 131entirely ``struct_mutex`` free. 132 133For drivers that need ``struct_mutex`` it should be replaced with a driver- 134private lock. The tricky part is the BO free functions, since those can't 135reliably take that lock any more. Instead state needs to be protected with 136suitable subordinate locks or some cleanup work pushed to a worker thread. For 137performance-critical drivers it might also be better to go with a more 138fine-grained per-buffer object and per-context lockings scheme. Currently only the 139``msm`` driver still use ``struct_mutex``. 140 141Contact: Daniel Vetter, respective driver maintainers 142 143Level: Advanced 144 145Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent 146---------------------------------------------------------------------------- 147 148For drivers which could have multiple instances, it is necessary to 149differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR 150don't do this, drivers used dev_info/warn/err to make this differentiation. We 151now have DRM_DEV_* variants of the drm print macros, so we can start to convert 152those drivers back to using drm-formwatted specific log messages. 153 154Before you start this conversion please contact the relevant maintainers to make 155sure your work will be merged - not everyone agrees that the DRM dmesg macros 156are better. 157 158Contact: Sean Paul, Maintainer of the driver you plan to convert 159 160Level: Starter 161 162Convert drivers to use simple modeset suspend/resume 163---------------------------------------------------- 164 165Most drivers (except i915 and nouveau) that use 166drm_atomic_helper_suspend/resume() can probably be converted to use 167drm_mode_config_helper_suspend/resume(). Also there's still open-coded version 168of the atomic suspend/resume code in older atomic modeset drivers. 169 170Contact: Maintainer of the driver you plan to convert 171 172Level: Intermediate 173 174Convert drivers to use drm_fb_helper_fbdev_setup/teardown() 175----------------------------------------------------------- 176 177Most drivers can use drm_fb_helper_fbdev_setup() except maybe: 178 179- amdgpu which has special logic to decide whether to call 180 drm_helper_disable_unused_functions() 181 182- armada which isn't atomic and doesn't call 183 drm_helper_disable_unused_functions() 184 185- i915 which calls drm_fb_helper_initial_config() in a worker 186 187Drivers that use drm_framebuffer_remove() to clean up the fbdev framebuffer can 188probably use drm_fb_helper_fbdev_teardown(). 189 190Contact: Maintainer of the driver you plan to convert 191 192Level: Intermediate 193 194Clean up mmap forwarding 195------------------------ 196 197A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers. 198And also a lot of them forward dma-buf mmap to the gem mmap implementations. 199There's drm_gem_prime_mmap() for this now, but still needs to be rolled out. 200 201Contact: Daniel Vetter 202 203Level: Intermediate 204 205Generic fbdev defio support 206--------------------------- 207 208The defio support code in the fbdev core has some very specific requirements, 209which means drivers need to have a special framebuffer for fbdev. The main 210issue is that it uses some fields in struct page itself, which breaks shmem 211gem objects (and other things). To support defio, affected drivers require 212the use of a shadow buffer, which may add CPU and memory overhead. 213 214Possible solution would be to write our own defio mmap code in the drm fbdev 215emulation. It would need to fully wrap the existing mmap ops, forwarding 216everything after it has done the write-protect/mkwrite trickery: 217 218- In the drm_fbdev_fb_mmap helper, if we need defio, change the 219 default page prots to write-protected with something like this:: 220 221 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot); 222 223- Set the mkwrite and fsync callbacks with similar implementions to the core 224 fbdev defio stuff. These should all work on plain ptes, they don't actually 225 require a struct page. uff. These should all work on plain ptes, they don't 226 actually require a struct page. 227 228- Track the dirty pages in a separate structure (bitfield with one bit per page 229 should work) to avoid clobbering struct page. 230 231Might be good to also have some igt testcases for this. 232 233Contact: Daniel Vetter, Noralf Tronnes 234 235Level: Advanced 236 237idr_init_base() 238--------------- 239 240DRM core&drivers uses a lot of idr (integer lookup directories) for mapping 241userspace IDs to internal objects, and in most places ID=0 means NULL and hence 242is never used. Switching to idr_init_base() for these would make the idr more 243efficient. 244 245Contact: Daniel Vetter 246 247Level: Starter 248 249struct drm_gem_object_funcs 250--------------------------- 251 252GEM objects can now have a function table instead of having the callbacks on the 253DRM driver struct. This is now the preferred way and drivers can be moved over. 254 255We also need a 2nd version of the CMA define that doesn't require the 256vmapping to be present (different hook for prime importing). Plus this needs to 257be rolled out to all drivers using their own implementations, too. 258 259Level: Intermediate 260 261Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate 262--------------------------------------------------------- 263 264For cases where drivers are attempting to grab the modeset locks with a local 265acquire context. Replace the boilerplate code surrounding 266drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and 267DRM_MODESET_LOCK_ALL_END() instead. 268 269This should also be done for all places where drm_modest_lock_all() is still 270used. 271 272As a reference, take a look at the conversions already completed in drm core. 273 274Contact: Sean Paul, respective driver maintainers 275 276Level: Starter 277 278Rename CMA helpers to DMA helpers 279--------------------------------- 280 281CMA (standing for contiguous memory allocator) is really a bit an accident of 282what these were used for first, a much better name would be DMA helpers. In the 283text these should even be called coherent DMA memory helpers (so maybe CDM, but 284no one knows what that means) since underneath they just use dma_alloc_coherent. 285 286Contact: Laurent Pinchart, Daniel Vetter 287 288Level: Intermediate (mostly because it is a huge tasks without good partial 289milestones, not technically itself that challenging) 290 291Convert direct mode.vrefresh accesses to use drm_mode_vrefresh() 292---------------------------------------------------------------- 293 294drm_display_mode.vrefresh isn't guaranteed to be populated. As such, using it 295is risky and has been known to cause div-by-zero bugs. Fortunately, drm core 296has helper which will use mode.vrefresh if it's !0 and will calculate it from 297the timings when it's 0. 298 299Use simple search/replace, or (more fun) cocci to replace instances of direct 300vrefresh access with a call to the helper. Check out 301https://lists.freedesktop.org/archives/dri-devel/2019-January/205186.html for 302inspiration. 303 304Once all instances of vrefresh have been converted, remove vrefresh from 305drm_display_mode to avoid future use. 306 307Contact: Sean Paul 308 309Level: Starter 310 311Remove drm_display_mode.hsync 312----------------------------- 313 314We have drm_mode_hsync() to calculate this from hsync_start/end, since drivers 315shouldn't/don't use this, remove this member to avoid any temptations to use it 316in the future. If there is any debug code using drm_display_mode.hsync, convert 317it to use drm_mode_hsync() instead. 318 319Contact: Sean Paul 320 321Level: Starter 322 323drm_fb_helper tasks 324------------------- 325 326- drm_fb_helper_restore_fbdev_mode_unlocked() should call restore_fbdev_mode() 327 not the _force variant so it can bail out if there is a master. But first 328 these igt tests need to be fixed: kms_fbcon_fbt@psr and 329 kms_fbcon_fbt@psr-suspend. 330 331- The max connector argument for drm_fb_helper_init() and 332 drm_fb_helper_fbdev_setup() isn't used anymore and can be removed. 333 334- The helper doesn't keep an array of connectors anymore so these can be 335 removed: drm_fb_helper_single_add_all_connectors(), 336 drm_fb_helper_add_one_connector() and drm_fb_helper_remove_one_connector(). 337 338Level: Intermediate 339 340connector register/unregister fixes 341----------------------------------- 342 343- For most connectors it's a no-op to call drm_connector_register/unregister 344 directly from driver code, drm_dev_register/unregister take care of this 345 already. We can remove all of them. 346 347- For dp drivers it's a bit more a mess, since we need the connector to be 348 registered when calling drm_dp_aux_register. Fix this by instead calling 349 drm_dp_aux_init, and moving the actual registering into a late_register 350 callback as recommended in the kerneldoc. 351 352Level: Intermediate 353 354Core refactorings 355================= 356 357Make panic handling work 358------------------------ 359 360This is a really varied tasks with lots of little bits and pieces: 361 362* The panic path can't be tested currently, leading to constant breaking. The 363 main issue here is that panics can be triggered from hardirq contexts and 364 hence all panic related callback can run in hardirq context. It would be 365 awesome if we could test at least the fbdev helper code and driver code by 366 e.g. trigger calls through drm debugfs files. hardirq context could be 367 achieved by using an IPI to the local processor. 368 369* There's a massive confusion of different panic handlers. DRM fbdev emulation 370 helpers have one, but on top of that the fbcon code itself also has one. We 371 need to make sure that they stop fighting over each another. 372 373* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and 374 isn't a full solution for panic paths. We need to make sure that it only 375 returns true if there's a panic going on for real, and fix up all the 376 fallout. 377 378* The panic handler must never sleep, which also means it can't ever 379 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not 380 even spinlocks (because NMI and hardirq can panic too). We need to either 381 make sure to not call such paths, or trylock everything. Really tricky. 382 383* For the above locking troubles reasons it's pretty much impossible to 384 attempt a synchronous modeset from panic handlers. The only thing we could 385 try to achive is an atomic ``set_base`` of the primary plane, and hope that 386 it shows up. Everything else probably needs to be delayed to some worker or 387 something else which happens later on. Otherwise it just kills the box 388 harder, prevent the panic from going out on e.g. netconsole. 389 390* There's also proposal for a simplied DRM console instead of the full-blown 391 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should 392 obviously work for both console, in case we ever get kmslog merged. 393 394Contact: Daniel Vetter 395 396Level: Advanced 397 398Clean up the debugfs support 399---------------------------- 400 401There's a bunch of issues with it: 402 403- The drm_info_list ->show() function doesn't even bother to cast to the drm 404 structure for you. This is lazy. 405 406- We probably want to have some support for debugfs files on crtc/connectors and 407 maybe other kms objects directly in core. There's even drm_print support in 408 the funcs for these objects to dump kms state, so it's all there. And then the 409 ->show() functions should obviously give you a pointer to the right object. 410 411- The drm_info_list stuff is centered on drm_minor instead of drm_device. For 412 anything we want to print drm_device (or maybe drm_file) is the right thing. 413 414- The drm_driver->debugfs_init hooks we have is just an artifact of the old 415 midlayered load sequence. DRM debugfs should work more like sysfs, where you 416 can create properties/files for an object anytime you want, and the core 417 takes care of publishing/unpuplishing all the files at register/unregister 418 time. Drivers shouldn't need to worry about these technicalities, and fixing 419 this (together with the drm_minor->drm_device move) would allow us to remove 420 debugfs_init. 421 422- Drop the return code and error checking from all debugfs functions. Greg KH is 423 working on this already. 424 425Contact: Daniel Vetter 426 427Level: Intermediate 428 429KMS cleanups 430------------ 431 432Some of these date from the very introduction of KMS in 2008 ... 433 434- Make ->funcs and ->helper_private vtables optional. There's a bunch of empty 435 function tables in drivers, but before we can remove them we need to make sure 436 that all the users in helpers and drivers do correctly check for a NULL 437 vtable. 438 439- Cleanup up the various ->destroy callbacks. A lot of them just wrapt the 440 drm_*_cleanup implementations and can be removed. Some tack a kfree() at the 441 end, for which we could add drm_*_cleanup_kfree(). And then there's the (for 442 historical reasons) misnamed drm_primary_helper_destroy() function. 443 444Level: Intermediate 445 446Better Testing 447============== 448 449Enable trinity for DRM 450---------------------- 451 452And fix up the fallout. Should be really interesting ... 453 454Level: Advanced 455 456Make KMS tests in i-g-t generic 457------------------------------- 458 459The i915 driver team maintains an extensive testsuite for the i915 DRM driver, 460including tons of testcases for corner-cases in the modesetting API. It would 461be awesome if those tests (at least the ones not relying on Intel-specific GEM 462features) could be made to run on any KMS driver. 463 464Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass- 465converting things over. For modeset tests we also first need a bit of 466infrastructure to use dumb buffers for untiled buffers, to be able to run all 467the non-i915 specific modeset tests. 468 469Level: Advanced 470 471Extend virtual test driver (VKMS) 472--------------------------------- 473 474See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal 475internship task, since it only requires a virtual machine and can be sized to 476fit the available time. 477 478Contact: Daniel Vetter 479 480Level: See details 481 482Backlight Refactoring 483--------------------- 484 485Backlight drivers have a triple enable/disable state, which is a bit overkill. 486Plan to fix this: 487 4881. Roll out backlight_enable() and backlight_disable() helpers everywhere. This 489 has started already. 4902. In all, only look at one of the three status bits set by the above helpers. 4913. Remove the other two status bits. 492 493Contact: Daniel Vetter 494 495Level: Intermediate 496 497Driver Specific 498=============== 499 500AMD DC Display Driver 501--------------------- 502 503AMD DC is the display driver for AMD devices starting with Vega. There has been 504a bunch of progress cleaning it up but there's still plenty of work to be done. 505 506See drivers/gpu/drm/amd/display/TODO for tasks. 507 508Contact: Harry Wentland, Alex Deucher 509 510Bootsplash 511========== 512 513There is support in place now for writing internal DRM clients making it 514possible to pick up the bootsplash work that was rejected because it was written 515for fbdev. 516 517- [v6,8/8] drm/client: Hack: Add bootsplash example 518 https://patchwork.freedesktop.org/patch/306579/ 519 520- [RFC PATCH v2 00/13] Kernel based bootsplash 521 https://lkml.org/lkml/2017/12/13/764 522 523Contact: Sam Ravnborg 524 525Level: Advanced 526 527Outside DRM 528=========== 529 530Convert fbdev drivers to DRM 531---------------------------- 532 533There are plenty of fbdev drivers for older hardware. Some hwardware has 534become obsolete, but some still provides good(-enough) framebuffers. The 535drivers that are still useful should be converted to DRM and afterwards 536removed from fbdev. 537 538Very simple fbdev drivers can best be converted by starting with a new 539DRM driver. Simple KMS helpers and SHMEM should be able to handle any 540existing hardware. The new driver's call-back functions are filled from 541existing fbdev code. 542 543More complex fbdev drivers can be refactored step-by-step into a DRM 544driver with the help of the DRM fbconv helpers. [1] These helpers provide 545the transition layer between the DRM core infrastructure and the fbdev 546driver interface. Create a new DRM driver on top of the fbconv helpers, 547copy over the fbdev driver, and hook it up to the DRM code. Examples for 548several fbdev drivers are available at [1] and a tutorial of this process 549available at [2]. The result is a primitive DRM driver that can run X11 550and Weston. 551 552 - [1] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv 553 - [2] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c 554 555Contact: Thomas Zimmermann <tzimmermann@suse.de> 556 557Level: Advanced 558