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 31 32Remove custom dumb_map_offset implementations 33--------------------------------------------- 34 35All GEM based drivers should be using drm_gem_create_mmap_offset() instead. 36Audit each individual driver, make sure it'll work with the generic 37implementation (there's lots of outdated locking leftovers in various 38implementations), and then remove it. 39 40Contact: Daniel Vetter, respective driver maintainers 41 42Convert existing KMS drivers to atomic modesetting 43-------------------------------------------------- 44 453.19 has the atomic modeset interfaces and helpers, so drivers can now be 46converted over. Modern compositors like Wayland or Surfaceflinger on Android 47really want an atomic modeset interface, so this is all about the bright 48future. 49 50There is a conversion guide for atomic and all you need is a GPU for a 51non-converted driver (again virtual HW drivers for KVM are still all 52suitable). 53 54As part of this drivers also need to convert to universal plane (which means 55exposing primary & cursor as proper plane objects). But that's much easier to 56do by directly using the new atomic helper driver callbacks. 57 58Contact: Daniel Vetter, respective driver maintainers 59 60Clean up the clipped coordination confusion around planes 61--------------------------------------------------------- 62 63We have a helper to get this right with drm_plane_helper_check_update(), but 64it's not consistently used. This should be fixed, preferrably in the atomic 65helpers (and drivers then moved over to clipped coordinates). Probably the 66helper should also be moved from drm_plane_helper.c to the atomic helpers, to 67avoid confusion - the other helpers in that file are all deprecated legacy 68helpers. 69 70Contact: Ville Syrjälä, Daniel Vetter, driver maintainers 71 72Convert early atomic drivers to async commit helpers 73---------------------------------------------------- 74 75For the first year the atomic modeset helpers didn't support asynchronous / 76nonblocking commits, and every driver had to hand-roll them. This is fixed 77now, but there's still a pile of existing drivers that easily could be 78converted over to the new infrastructure. 79 80One issue with the helpers is that they require that drivers handle completion 81events for atomic commits correctly. But fixing these bugs is good anyway. 82 83Contact: Daniel Vetter, respective driver maintainers 84 85Fallout from atomic KMS 86----------------------- 87 88``drm_atomic_helper.c`` provides a batch of functions which implement legacy 89IOCTLs on top of the new atomic driver interface. Which is really nice for 90gradual conversion of drivers, but unfortunately the semantic mismatches are 91a bit too severe. So there's some follow-up work to adjust the function 92interfaces to fix these issues: 93 94* atomic needs the lock acquire context. At the moment that's passed around 95 implicitly with some horrible hacks, and it's also allocate with 96 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating 97 the acquire context explicitly on stack and then also pass it down into 98 drivers explicitly so that the legacy-on-atomic functions can use them. 99 100 Except for some driver code this is done. This task should be finished by 101 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all(). 102 103* A bunch of the vtable hooks are now in the wrong place: DRM has a split 104 between core vfunc tables (named ``drm_foo_funcs``), which are used to 105 implement the userspace ABI. And then there's the optional hooks for the 106 helper libraries (name ``drm_foo_helper_funcs``), which are purely for 107 internal use. Some of these hooks should be move from ``_funcs`` to 108 ``_helper_funcs`` since they are not part of the core ABI. There's a 109 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``. 110 111Contact: Daniel Vetter 112 113Get rid of dev->struct_mutex from GEM drivers 114--------------------------------------------- 115 116``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested 117everything. Nowadays in modern drivers the only bit where it's mandatory is 118serializing GEM buffer object destruction. Which unfortunately means drivers 119have to keep track of that lock and either call ``unreference`` or 120``unreference_locked`` depending upon context. 121 122Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8, 123and there's a ``gem_free_object_unlocked`` callback for any drivers which are 124entirely ``struct_mutex`` free. 125 126For drivers that need ``struct_mutex`` it should be replaced with a driver- 127private lock. The tricky part is the BO free functions, since those can't 128reliably take that lock any more. Instead state needs to be protected with 129suitable subordinate locks or some cleanup work pushed to a worker thread. For 130performance-critical drivers it might also be better to go with a more 131fine-grained per-buffer object and per-context lockings scheme. Currently only the 132``msm`` driver still use ``struct_mutex``. 133 134Contact: Daniel Vetter, respective driver maintainers 135 136Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent 137---------------------------------------------------------------------------- 138 139For drivers which could have multiple instances, it is necessary to 140differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR 141don't do this, drivers used dev_info/warn/err to make this differentiation. We 142now have DRM_DEV_* variants of the drm print macros, so we can start to convert 143those drivers back to using drm-formwatted specific log messages. 144 145Before you start this conversion please contact the relevant maintainers to make 146sure your work will be merged - not everyone agrees that the DRM dmesg macros 147are better. 148 149Contact: Sean Paul, Maintainer of the driver you plan to convert 150 151Convert drivers to use simple modeset suspend/resume 152---------------------------------------------------- 153 154Most drivers (except i915 and nouveau) that use 155drm_atomic_helper_suspend/resume() can probably be converted to use 156drm_mode_config_helper_suspend/resume(). Also there's still open-coded version 157of the atomic suspend/resume code in older atomic modeset drivers. 158 159Contact: Maintainer of the driver you plan to convert 160 161Convert drivers to use drm_fb_helper_fbdev_setup/teardown() 162----------------------------------------------------------- 163 164Most drivers can use drm_fb_helper_fbdev_setup() except maybe: 165 166- amdgpu which has special logic to decide whether to call 167 drm_helper_disable_unused_functions() 168 169- armada which isn't atomic and doesn't call 170 drm_helper_disable_unused_functions() 171 172- i915 which calls drm_fb_helper_initial_config() in a worker 173 174Drivers that use drm_framebuffer_remove() to clean up the fbdev framebuffer can 175probably use drm_fb_helper_fbdev_teardown(). 176 177Contact: Maintainer of the driver you plan to convert 178 179Clean up mmap forwarding 180------------------------ 181 182A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers. 183And also a lot of them forward dma-buf mmap to the gem mmap implementations. 184Would be great to refactor this all into a set of small common helpers. 185 186Contact: Daniel Vetter 187 188Generic fbdev defio support 189--------------------------- 190 191The defio support code in the fbdev core has some very specific requirements, 192which means drivers need to have a special framebuffer for fbdev. Which prevents 193us from using the generic fbdev emulation code everywhere. The main issue is 194that it uses some fields in struct page itself, which breaks shmem gem objects 195(and other things). 196 197Possible solution would be to write our own defio mmap code in the drm fbdev 198emulation. It would need to fully wrap the existing mmap ops, forwarding 199everything after it has done the write-protect/mkwrite trickery: 200 201- In the drm_fbdev_fb_mmap helper, if we need defio, change the 202 default page prots to write-protected with something like this:: 203 204 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot); 205 206- Set the mkwrite and fsync callbacks with similar implementions to the core 207 fbdev defio stuff. These should all work on plain ptes, they don't actually 208 require a struct page. uff. These should all work on plain ptes, they don't 209 actually require a struct page. 210 211- Track the dirty pages in a separate structure (bitfield with one bit per page 212 should work) to avoid clobbering struct page. 213 214Might be good to also have some igt testcases for this. 215 216Contact: Daniel Vetter, Noralf Tronnes 217 218Put a reservation_object into drm_gem_object 219-------------------------------------------- 220 221This would remove the need for the ->gem_prime_res_obj callback. It would also 222allow us to implement generic helpers for waiting for a bo, allowing for quite a 223bit of refactoring in the various wait ioctl implementations. 224 225Contact: Daniel Vetter 226 227idr_init_base() 228--------------- 229 230DRM core&drivers uses a lot of idr (integer lookup directories) for mapping 231userspace IDs to internal objects, and in most places ID=0 means NULL and hence 232is never used. Switching to idr_init_base() for these would make the idr more 233efficient. 234 235Contact: Daniel Vetter 236 237Defaults for .gem_prime_import and export 238----------------------------------------- 239 240Most drivers don't need to set drm_driver->gem_prime_import and 241->gem_prime_export now that drm_gem_prime_import() and drm_gem_prime_export() 242are the default. 243 244struct drm_gem_object_funcs 245--------------------------- 246 247GEM objects can now have a function table instead of having the callbacks on the 248DRM driver struct. This is now the preferred way and drivers can be moved over. 249 250Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate 251--------------------------------------------------------- 252 253For cases where drivers are attempting to grab the modeset locks with a local 254acquire context. Replace the boilerplate code surrounding 255drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and 256DRM_MODESET_LOCK_ALL_END() instead. 257 258This should also be done for all places where drm_modest_lock_all() is still 259used. 260 261As a reference, take a look at the conversions already completed in drm core. 262 263Contact: Sean Paul, respective driver maintainers 264 265Rename CMA helpers to DMA helpers 266--------------------------------- 267 268CMA (standing for contiguous memory allocator) is really a bit an accident of 269what these were used for first, a much better name would be DMA helpers. In the 270text these should even be called coherent DMA memory helpers (so maybe CDM, but 271no one knows what that means) since underneath they just use dma_alloc_coherent. 272 273Contact: Laurent Pinchart, Daniel Vetter 274 275Convert direct mode.vrefresh accesses to use drm_mode_vrefresh() 276---------------------------------------------------------------- 277 278drm_display_mode.vrefresh isn't guaranteed to be populated. As such, using it 279is risky and has been known to cause div-by-zero bugs. Fortunately, drm core 280has helper which will use mode.vrefresh if it's !0 and will calculate it from 281the timings when it's 0. 282 283Use simple search/replace, or (more fun) cocci to replace instances of direct 284vrefresh access with a call to the helper. Check out 285https://lists.freedesktop.org/archives/dri-devel/2019-January/205186.html for 286inspiration. 287 288Once all instances of vrefresh have been converted, remove vrefresh from 289drm_display_mode to avoid future use. 290 291Contact: Sean Paul 292 293Remove drm_display_mode.hsync 294----------------------------- 295 296We have drm_mode_hsync() to calculate this from hsync_start/end, since drivers 297shouldn't/don't use this, remove this member to avoid any temptations to use it 298in the future. If there is any debug code using drm_display_mode.hsync, convert 299it to use drm_mode_hsync() instead. 300 301Contact: Sean Paul 302 303Core refactorings 304================= 305 306Clean up the DRM header mess 307---------------------------- 308 309The DRM subsystem originally had only one huge global header, ``drmP.h``. This 310is now split up, but many source files still include it. The remaining part of 311the cleanup work here is to replace any ``#include <drm/drmP.h>`` by only the 312headers needed (and fixing up any missing pre-declarations in the headers). 313 314In the end no .c file should need to include ``drmP.h`` anymore. 315 316Contact: Daniel Vetter 317 318Add missing kerneldoc for exported functions 319-------------------------------------------- 320 321The DRM reference documentation is still lacking kerneldoc in a few areas. The 322task would be to clean up interfaces like moving functions around between 323files to better group them and improving the interfaces like dropping return 324values for functions that never fail. Then write kerneldoc for all exported 325functions and an overview section and integrate it all into the drm book. 326 327See https://dri.freedesktop.org/docs/drm/ for what's there already. 328 329Contact: Daniel Vetter 330 331Make panic handling work 332------------------------ 333 334This is a really varied tasks with lots of little bits and pieces: 335 336* The panic path can't be tested currently, leading to constant breaking. The 337 main issue here is that panics can be triggered from hardirq contexts and 338 hence all panic related callback can run in hardirq context. It would be 339 awesome if we could test at least the fbdev helper code and driver code by 340 e.g. trigger calls through drm debugfs files. hardirq context could be 341 achieved by using an IPI to the local processor. 342 343* There's a massive confusion of different panic handlers. DRM fbdev emulation 344 helpers have one, but on top of that the fbcon code itself also has one. We 345 need to make sure that they stop fighting over each another. 346 347* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and 348 isn't a full solution for panic paths. We need to make sure that it only 349 returns true if there's a panic going on for real, and fix up all the 350 fallout. 351 352* The panic handler must never sleep, which also means it can't ever 353 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not 354 even spinlocks (because NMI and hardirq can panic too). We need to either 355 make sure to not call such paths, or trylock everything. Really tricky. 356 357* For the above locking troubles reasons it's pretty much impossible to 358 attempt a synchronous modeset from panic handlers. The only thing we could 359 try to achive is an atomic ``set_base`` of the primary plane, and hope that 360 it shows up. Everything else probably needs to be delayed to some worker or 361 something else which happens later on. Otherwise it just kills the box 362 harder, prevent the panic from going out on e.g. netconsole. 363 364* There's also proposal for a simplied DRM console instead of the full-blown 365 fbcon and DRM fbdev emulation. Any kind of panic handling tricks should 366 obviously work for both console, in case we ever get kmslog merged. 367 368Contact: Daniel Vetter 369 370Clean up the debugfs support 371---------------------------- 372 373There's a bunch of issues with it: 374 375- The drm_info_list ->show() function doesn't even bother to cast to the drm 376 structure for you. This is lazy. 377 378- We probably want to have some support for debugfs files on crtc/connectors and 379 maybe other kms objects directly in core. There's even drm_print support in 380 the funcs for these objects to dump kms state, so it's all there. And then the 381 ->show() functions should obviously give you a pointer to the right object. 382 383- The drm_info_list stuff is centered on drm_minor instead of drm_device. For 384 anything we want to print drm_device (or maybe drm_file) is the right thing. 385 386- The drm_driver->debugfs_init hooks we have is just an artifact of the old 387 midlayered load sequence. DRM debugfs should work more like sysfs, where you 388 can create properties/files for an object anytime you want, and the core 389 takes care of publishing/unpuplishing all the files at register/unregister 390 time. Drivers shouldn't need to worry about these technicalities, and fixing 391 this (together with the drm_minor->drm_device move) would allow us to remove 392 debugfs_init. 393 394Contact: Daniel Vetter 395 396KMS cleanups 397------------ 398 399Some of these date from the very introduction of KMS in 2008 ... 400 401- Make ->funcs and ->helper_private vtables optional. There's a bunch of empty 402 function tables in drivers, but before we can remove them we need to make sure 403 that all the users in helpers and drivers do correctly check for a NULL 404 vtable. 405 406- Cleanup up the various ->destroy callbacks. A lot of them just wrapt the 407 drm_*_cleanup implementations and can be removed. Some tack a kfree() at the 408 end, for which we could add drm_*_cleanup_kfree(). And then there's the (for 409 historical reasons) misnamed drm_primary_helper_destroy() function. 410 411Better Testing 412============== 413 414Enable trinity for DRM 415---------------------- 416 417And fix up the fallout. Should be really interesting ... 418 419Make KMS tests in i-g-t generic 420------------------------------- 421 422The i915 driver team maintains an extensive testsuite for the i915 DRM driver, 423including tons of testcases for corner-cases in the modesetting API. It would 424be awesome if those tests (at least the ones not relying on Intel-specific GEM 425features) could be made to run on any KMS driver. 426 427Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass- 428converting things over. For modeset tests we also first need a bit of 429infrastructure to use dumb buffers for untiled buffers, to be able to run all 430the non-i915 specific modeset tests. 431 432Extend virtual test driver (VKMS) 433--------------------------------- 434 435See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal 436internship task, since it only requires a virtual machine and can be sized to 437fit the available time. 438 439Contact: Daniel Vetter 440 441Driver Specific 442=============== 443 444tinydrm 445------- 446 447Tinydrm is the helper driver for really simple fb drivers. The goal is to make 448those drivers as simple as possible, so lots of room for refactoring: 449 450- backlight helpers, probably best to put them into a new drm_backlight.c. 451 This is because drivers/video is de-facto unmaintained. We could also 452 move drivers/video/backlight to drivers/gpu/backlight and take it all 453 over within drm-misc, but that's more work. Backlight helpers require a fair 454 bit of reworking and refactoring. A simple example is the enabling of a backlight. 455 Tinydrm has helpers for this. It would be good if other drivers can also use the 456 helper. However, there are various cases we need to consider i.e different 457 drivers seem to have different ways of enabling/disabling a backlight. 458 We also need to consider the backlight drivers (like gpio_backlight). The situation 459 is further complicated by the fact that the backlight is tied to fbdev 460 via fb_notifier_callback() which has complicated logic. For further details, refer 461 to the following discussion thread: 462 https://groups.google.com/forum/#!topic/outreachy-kernel/8rBe30lwtdA 463 464- spi helpers, probably best put into spi core/helper code. Thierry said 465 the spi maintainer is fast&reactive, so shouldn't be a big issue. 466 467- extract the mipi-dbi helper (well, the non-tinydrm specific parts at 468 least) into a separate helper, like we have for mipi-dsi already. Or follow 469 one of the ideas for having a shared dsi/dbi helper, abstracting away the 470 transport details more. 471 472- Quick aside: The unregister devm stuff is kinda getting the lifetimes of 473 a drm_device wrong. Doesn't matter, since everyone else gets it wrong 474 too :-) 475 476Contact: Noralf Trønnes, Daniel Vetter 477 478AMD DC Display Driver 479--------------------- 480 481AMD DC is the display driver for AMD devices starting with Vega. There has been 482a bunch of progress cleaning it up but there's still plenty of work to be done. 483 484See drivers/gpu/drm/amd/display/TODO for tasks. 485 486Contact: Harry Wentland, Alex Deucher 487 488i915 489---- 490 491- Our early/late pm callbacks could be removed in favour of using 492 device_link_add to model the dependency between i915 and snd_had. See 493 https://dri.freedesktop.org/docs/drm/driver-api/device_link.html 494 495Outside DRM 496=========== 497