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 26Expert: Only attempt these if you've successfully completed some tricky 27refactorings already and are an expert in the specific area 28 29Subsystem-wide refactorings 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instead. 36Audit each individual driver, make sure it'll work with the generic 37implementation (there's lots of outdated locking leftovers in various 38implementations), and then remove it. 39 40Contact: Daniel Vetter, respective driver maintainers 41 42Level: Intermediate 43 44Convert existing KMS drivers to atomic modesetting 45-------------------------------------------------- 46 473.19 has the atomic modeset interfaces and helpers, so drivers can now be 48converted over. Modern compositors like Wayland or Surfaceflinger on Android 49really want an atomic modeset interface, so this is all about the bright 50future. 51 52There is a conversion guide for atomic and all you need is a GPU for a 53non-converted driver (again virtual HW drivers for KVM are still all 54suitable). 55 56As part of this drivers also need to convert to universal plane (which means 57exposing primary & cursor as proper plane objects). But that's much easier to 58do by directly using the new atomic helper driver callbacks. 59 60Contact: Daniel Vetter, respective driver maintainers 61 62Level: Advanced 63 64Clean up the clipped coordination confusion around planes 65--------------------------------------------------------- 66 67We have a helper to get this right with drm_plane_helper_check_update(), but 68it's not consistently used. This should be fixed, preferrably in the atomic 69helpers (and drivers then moved over to clipped coordinates). Probably the 70helper should also be moved from drm_plane_helper.c to the atomic helpers, to 71avoid confusion - the other helpers in that file are all deprecated legacy 72helpers. 73 74Contact: Ville Syrjälä, Daniel Vetter, driver maintainers 75 76Level: Advanced 77 78Improve plane atomic_check helpers 79---------------------------------- 80 81Aside from the clipped coordinates right above there's a few suboptimal things 82with the current helpers: 83 84- drm_plane_helper_funcs->atomic_check gets called for enabled or disabled 85 planes. At best this seems to confuse drivers, worst it means they blow up 86 when the plane is disabled without the CRTC. The only special handling is 87 resetting values in the plane state structures, which instead should be moved 88 into the drm_plane_funcs->atomic_duplicate_state functions. 89 90- Once that's done, helpers could stop calling ->atomic_check for disabled 91 planes. 92 93- Then we could go through all the drivers and remove the more-or-less confused 94 checks for plane_state->fb and plane_state->crtc. 95 96Contact: Daniel Vetter 97 98Level: Advanced 99 100Convert early atomic drivers to async commit helpers 101---------------------------------------------------- 102 103For the first year the atomic modeset helpers didn't support asynchronous / 104nonblocking commits, and every driver had to hand-roll them. This is fixed 105now, but there's still a pile of existing drivers that easily could be 106converted over to the new infrastructure. 107 108One issue with the helpers is that they require that drivers handle completion 109events for atomic commits correctly. But fixing these bugs is good anyway. 110 111Somewhat related is the legacy_cursor_update hack, which should be replaced with 112the new atomic_async_check/commit functionality in the helpers in drivers that 113still look at that flag. 114 115Contact: Daniel Vetter, respective driver maintainers 116 117Level: Advanced 118 119Fallout from atomic KMS 120----------------------- 121 122``drm_atomic_helper.c`` provides a batch of functions which implement legacy 123IOCTLs on top of the new atomic driver interface. Which is really nice for 124gradual conversion of drivers, but unfortunately the semantic mismatches are 125a bit too severe. So there's some follow-up work to adjust the function 126interfaces to fix these issues: 127 128* atomic needs the lock acquire context. At the moment that's passed around 129 implicitly with some horrible hacks, and it's also allocate with 130 ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating 131 the acquire context explicitly on stack and then also pass it down into 132 drivers explicitly so that the legacy-on-atomic functions can use them. 133 134 Except for some driver code this is done. This task should be finished by 135 adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all(). 136 137* A bunch of the vtable hooks are now in the wrong place: DRM has a split 138 between core vfunc tables (named ``drm_foo_funcs``), which are used to 139 implement the userspace ABI. And then there's the optional hooks for the 140 helper libraries (name ``drm_foo_helper_funcs``), which are purely for 141 internal use. Some of these hooks should be move from ``_funcs`` to 142 ``_helper_funcs`` since they are not part of the core ABI. There's a 143 ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``. 144 145Contact: Daniel Vetter 146 147Level: Intermediate 148 149Get rid of dev->struct_mutex from GEM drivers 150--------------------------------------------- 151 152``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested 153everything. Nowadays in modern drivers the only bit where it's mandatory is 154serializing GEM buffer object destruction. Which unfortunately means drivers 155have to keep track of that lock and either call ``unreference`` or 156``unreference_locked`` depending upon context. 157 158Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8, 159and there's a GEM object ``free`` callback for any drivers which are 160entirely ``struct_mutex`` free. 161 162For drivers that need ``struct_mutex`` it should be replaced with a driver- 163private lock. The tricky part is the BO free functions, since those can't 164reliably take that lock any more. Instead state needs to be protected with 165suitable subordinate locks or some cleanup work pushed to a worker thread. For 166performance-critical drivers it might also be better to go with a more 167fine-grained per-buffer object and per-context lockings scheme. Currently only 168the ``msm`` and `i915` drivers use ``struct_mutex``. 169 170Contact: Daniel Vetter, respective driver maintainers 171 172Level: Advanced 173 174Move Buffer Object Locking to dma_resv_lock() 175--------------------------------------------- 176 177Many drivers have their own per-object locking scheme, usually using 178mutex_lock(). This causes all kinds of trouble for buffer sharing, since 179depending which driver is the exporter and importer, the locking hierarchy is 180reversed. 181 182To solve this we need one standard per-object locking mechanism, which is 183dma_resv_lock(). This lock needs to be called as the outermost lock, with all 184other driver specific per-object locks removed. The problem is tha rolling out 185the actual change to the locking contract is a flag day, due to struct dma_buf 186buffer sharing. 187 188Level: Expert 189 190Convert logging to drm_* functions with drm_device paramater 191------------------------------------------------------------ 192 193For drivers which could have multiple instances, it is necessary to 194differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR 195don't do this, drivers used dev_info/warn/err to make this differentiation. We 196now have drm_* variants of the drm print functions, so we can start to convert 197those drivers back to using drm-formatted specific log messages. 198 199Before you start this conversion please contact the relevant maintainers to make 200sure your work will be merged - not everyone agrees that the DRM dmesg macros 201are better. 202 203Contact: Sean Paul, Maintainer of the driver you plan to convert 204 205Level: Starter 206 207Convert drivers to use simple modeset suspend/resume 208---------------------------------------------------- 209 210Most drivers (except i915 and nouveau) that use 211drm_atomic_helper_suspend/resume() can probably be converted to use 212drm_mode_config_helper_suspend/resume(). Also there's still open-coded version 213of the atomic suspend/resume code in older atomic modeset drivers. 214 215Contact: Maintainer of the driver you plan to convert 216 217Level: Intermediate 218 219Convert drivers to use drm_fbdev_generic_setup() 220------------------------------------------------ 221 222Most drivers can use drm_fbdev_generic_setup(). Driver have to implement 223atomic modesetting and GEM vmap support. Historically, generic fbdev emulation 224expected the framebuffer in system memory or system-like memory. By employing 225struct iosys_map, drivers with frambuffers in I/O memory can be supported 226as well. 227 228Contact: Maintainer of the driver you plan to convert 229 230Level: Intermediate 231 232Reimplement functions in drm_fbdev_fb_ops without fbdev 233------------------------------------------------------- 234 235A number of callback functions in drm_fbdev_fb_ops could benefit from 236being rewritten without dependencies on the fbdev module. Some of the 237helpers could further benefit from using struct iosys_map instead of 238raw pointers. 239 240Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter 241 242Level: Advanced 243 244Benchmark and optimize blitting and format-conversion function 245-------------------------------------------------------------- 246 247Drawing to dispay memory quickly is crucial for many applications' 248performance. 249 250On at least x86-64, sys_imageblit() is significantly slower than 251cfb_imageblit(), even though both use the same blitting algorithm and 252the latter is written for I/O memory. It turns out that cfb_imageblit() 253uses movl instructions, while sys_imageblit apparently does not. This 254seems to be a problem with gcc's optimizer. DRM's format-conversion 255helpers might be subject to similar issues. 256 257Benchmark and optimize fbdev's sys_() helpers and DRM's format-conversion 258helpers. In cases that can be further optimized, maybe implement a different 259algorithm. For micro-optimizations, use movl/movq instructions explicitly. 260That might possibly require architecture-specific helpers (e.g., storel() 261storeq()). 262 263Contact: Thomas Zimmermann <tzimmermann@suse.de> 264 265Level: Intermediate 266 267drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup 268----------------------------------------------------------------- 269 270A lot more drivers could be switched over to the drm_gem_framebuffer helpers. 271Various hold-ups: 272 273- Need to switch over to the generic dirty tracking code using 274 drm_atomic_helper_dirtyfb first (e.g. qxl). 275 276- Need to switch to drm_fbdev_generic_setup(), otherwise a lot of the custom fb 277 setup code can't be deleted. 278 279- Many drivers wrap drm_gem_fb_create() only to check for valid formats. For 280 atomic drivers we could check for valid formats by calling 281 drm_plane_check_pixel_format() against all planes, and pass if any plane 282 supports the format. For non-atomic that's not possible since like the format 283 list for the primary plane is fake and we'd therefor reject valid formats. 284 285- Many drivers subclass drm_framebuffer, we'd need a embedding compatible 286 version of the varios drm_gem_fb_create functions. Maybe called 287 drm_gem_fb_create/_with_dirty/_with_funcs as needed. 288 289Contact: Daniel Vetter 290 291Level: Intermediate 292 293Generic fbdev defio support 294--------------------------- 295 296The defio support code in the fbdev core has some very specific requirements, 297which means drivers need to have a special framebuffer for fbdev. The main 298issue is that it uses some fields in struct page itself, which breaks shmem 299gem objects (and other things). To support defio, affected drivers require 300the use of a shadow buffer, which may add CPU and memory overhead. 301 302Possible solution would be to write our own defio mmap code in the drm fbdev 303emulation. It would need to fully wrap the existing mmap ops, forwarding 304everything after it has done the write-protect/mkwrite trickery: 305 306- In the drm_fbdev_fb_mmap helper, if we need defio, change the 307 default page prots to write-protected with something like this:: 308 309 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot); 310 311- Set the mkwrite and fsync callbacks with similar implementions to the core 312 fbdev defio stuff. These should all work on plain ptes, they don't actually 313 require a struct page. uff. These should all work on plain ptes, they don't 314 actually require a struct page. 315 316- Track the dirty pages in a separate structure (bitfield with one bit per page 317 should work) to avoid clobbering struct page. 318 319Might be good to also have some igt testcases for this. 320 321Contact: Daniel Vetter, Noralf Tronnes 322 323Level: Advanced 324 325struct drm_gem_object_funcs 326--------------------------- 327 328GEM objects can now have a function table instead of having the callbacks on the 329DRM driver struct. This is now the preferred way. Callbacks in drivers have been 330converted, except for struct drm_driver.gem_prime_mmap. 331 332Level: Intermediate 333 334connector register/unregister fixes 335----------------------------------- 336 337- For most connectors it's a no-op to call drm_connector_register/unregister 338 directly from driver code, drm_dev_register/unregister take care of this 339 already. We can remove all of them. 340 341- For dp drivers it's a bit more a mess, since we need the connector to be 342 registered when calling drm_dp_aux_register. Fix this by instead calling 343 drm_dp_aux_init, and moving the actual registering into a late_register 344 callback as recommended in the kerneldoc. 345 346Level: Intermediate 347 348Remove load/unload callbacks from all non-DRIVER_LEGACY drivers 349--------------------------------------------------------------- 350 351The load/unload callbacks in struct &drm_driver are very much midlayers, plus 352for historical reasons they get the ordering wrong (and we can't fix that) 353between setting up the &drm_driver structure and calling drm_dev_register(). 354 355- Rework drivers to no longer use the load/unload callbacks, directly coding the 356 load/unload sequence into the driver's probe function. 357 358- Once all non-DRIVER_LEGACY drivers are converted, disallow the load/unload 359 callbacks for all modern drivers. 360 361Contact: Daniel Vetter 362 363Level: Intermediate 364 365Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi 366--------------------------------------------------------------- 367 368Once EDID is parsed, the monitor HDMI support information is available through 369drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor() to 370retrieve the same information, which is less efficient. 371 372Audit each individual driver calling drm_detect_hdmi_monitor() and switch to 373drm_display_info.is_hdmi if applicable. 374 375Contact: Laurent Pinchart, respective driver maintainers 376 377Level: Intermediate 378 379Consolidate custom driver modeset properties 380-------------------------------------------- 381 382Before atomic modeset took place, many drivers where creating their own 383properties. Among other things, atomic brought the requirement that custom, 384driver specific properties should not be used. 385 386For this task, we aim to introduce core helpers or reuse the existing ones 387if available: 388 389A quick, unconfirmed, examples list. 390 391Introduce core helpers: 392- audio (amdgpu, intel, gma500, radeon) 393- brightness, contrast, etc (armada, nouveau) - overlay only (?) 394- broadcast rgb (gma500, intel) 395- colorkey (armada, nouveau, rcar) - overlay only (?) 396- dither (amdgpu, nouveau, radeon) - varies across drivers 397- underscan family (amdgpu, radeon, nouveau) 398 399Already in core: 400- colorspace (sti) 401- tv format names, enhancements (gma500, intel) 402- tv overscan, margins, etc. (gma500, intel) 403- zorder (omapdrm) - same as zpos (?) 404 405 406Contact: Emil Velikov, respective driver maintainers 407 408Level: Intermediate 409 410Use struct iosys_map throughout codebase 411---------------------------------------- 412 413Pointers to shared device memory are stored in struct iosys_map. Each 414instance knows whether it refers to system or I/O memory. Most of the DRM-wide 415interface have been converted to use struct iosys_map, but implementations 416often still use raw pointers. 417 418The task is to use struct iosys_map where it makes sense. 419 420* Memory managers should use struct iosys_map for dma-buf-imported buffers. 421* TTM might benefit from using struct iosys_map internally. 422* Framebuffer copying and blitting helpers should operate on struct iosys_map. 423 424Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Daniel Vetter 425 426Level: Intermediate 427 428Review all drivers for setting struct drm_mode_config.{max_width,max_height} correctly 429-------------------------------------------------------------------------------------- 430 431The values in struct drm_mode_config.{max_width,max_height} describe the 432maximum supported framebuffer size. It's the virtual screen size, but many 433drivers treat it like limitations of the physical resolution. 434 435The maximum width depends on the hardware's maximum scanline pitch. The 436maximum height depends on the amount of addressable video memory. Review all 437drivers to initialize the fields to the correct values. 438 439Contact: Thomas Zimmermann <tzimmermann@suse.de> 440 441Level: Intermediate 442 443Request memory regions in all drivers 444------------------------------------- 445 446Go through all drivers and add code to request the memory regions that the 447driver uses. This requires adding calls to request_mem_region(), 448pci_request_region() or similar functions. Use helpers for managed cleanup 449where possible. 450 451Drivers are pretty bad at doing this and there used to be conflicts among 452DRM and fbdev drivers. Still, it's the correct thing to do. 453 454Contact: Thomas Zimmermann <tzimmermann@suse.de> 455 456Level: Starter 457 458 459Core refactorings 460================= 461 462Make panic handling work 463------------------------ 464 465This is a really varied tasks with lots of little bits and pieces: 466 467* The panic path can't be tested currently, leading to constant breaking. The 468 main issue here is that panics can be triggered from hardirq contexts and 469 hence all panic related callback can run in hardirq context. It would be 470 awesome if we could test at least the fbdev helper code and driver code by 471 e.g. trigger calls through drm debugfs files. hardirq context could be 472 achieved by using an IPI to the local processor. 473 474* There's a massive confusion of different panic handlers. DRM fbdev emulation 475 helpers had their own (long removed), but on top of that the fbcon code itself 476 also has one. We need to make sure that they stop fighting over each other. 477 This is worked around by checking ``oops_in_progress`` at various entry points 478 into the DRM fbdev emulation helpers. A much cleaner approach here would be to 479 switch fbcon to the `threaded printk support 480 <https://lwn.net/Articles/800946/>`_. 481 482* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and 483 isn't a full solution for panic paths. We need to make sure that it only 484 returns true if there's a panic going on for real, and fix up all the 485 fallout. 486 487* The panic handler must never sleep, which also means it can't ever 488 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not 489 even spinlocks (because NMI and hardirq can panic too). We need to either 490 make sure to not call such paths, or trylock everything. Really tricky. 491 492* A clean solution would be an entirely separate panic output support in KMS, 493 bypassing the current fbcon support. See `[PATCH v2 0/3] drm: Add panic handling 494 <https://lore.kernel.org/dri-devel/20190311174218.51899-1-noralf@tronnes.org/>`_. 495 496* Encoding the actual oops and preceding dmesg in a QR might help with the 497 dread "important stuff scrolled away" problem. See `[RFC][PATCH] Oops messages 498 transfer using QR codes 499 <https://lore.kernel.org/lkml/1446217392-11981-1-git-send-email-alexandru.murtaza@intel.com/>`_ 500 for some example code that could be reused. 501 502Contact: Daniel Vetter 503 504Level: Advanced 505 506Clean up the debugfs support 507---------------------------- 508 509There's a bunch of issues with it: 510 511- Convert drivers to support the drm_debugfs_add_files() function instead of 512 the drm_debugfs_create_files() function. 513 514- Improve late-register debugfs by rolling out the same debugfs pre-register 515 infrastructure for connector and crtc too. That way, the drivers won't need to 516 split their setup code into init and register anymore. 517 518- We probably want to have some support for debugfs files on crtc/connectors and 519 maybe other kms objects directly in core. There's even drm_print support in 520 the funcs for these objects to dump kms state, so it's all there. And then the 521 ->show() functions should obviously give you a pointer to the right object. 522 523- The drm_driver->debugfs_init hooks we have is just an artifact of the old 524 midlayered load sequence. DRM debugfs should work more like sysfs, where you 525 can create properties/files for an object anytime you want, and the core 526 takes care of publishing/unpuplishing all the files at register/unregister 527 time. Drivers shouldn't need to worry about these technicalities, and fixing 528 this (together with the drm_minor->drm_device move) would allow us to remove 529 debugfs_init. 530 531Contact: Daniel Vetter 532 533Level: Intermediate 534 535Object lifetime fixes 536--------------------- 537 538There's two related issues here 539 540- Cleanup up the various ->destroy callbacks, which often are all the same 541 simple code. 542 543- Lots of drivers erroneously allocate DRM modeset objects using devm_kzalloc, 544 which results in use-after free issues on driver unload. This can be serious 545 trouble even for drivers for hardware integrated on the SoC due to 546 EPROBE_DEFERRED backoff. 547 548Both these problems can be solved by switching over to drmm_kzalloc(), and the 549various convenience wrappers provided, e.g. drmm_crtc_alloc_with_planes(), 550drmm_universal_plane_alloc(), ... and so on. 551 552Contact: Daniel Vetter 553 554Level: Intermediate 555 556Remove automatic page mapping from dma-buf importing 557---------------------------------------------------- 558 559When importing dma-bufs, the dma-buf and PRIME frameworks automatically map 560imported pages into the importer's DMA area. drm_gem_prime_fd_to_handle() and 561drm_gem_prime_handle_to_fd() require that importers call dma_buf_attach() 562even if they never do actual device DMA, but only CPU access through 563dma_buf_vmap(). This is a problem for USB devices, which do not support DMA 564operations. 565 566To fix the issue, automatic page mappings should be removed from the 567buffer-sharing code. Fixing this is a bit more involved, since the import/export 568cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over 569this problem for USB devices by fishing out the USB host controller device, as 570long as that supports DMA. Otherwise importing can still needlessly fail. 571 572Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter 573 574Level: Advanced 575 576 577Better Testing 578============== 579 580Add unit tests using the Kernel Unit Testing (KUnit) framework 581-------------------------------------------------------------- 582 583The `KUnit <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_ 584provides a common framework for unit tests within the Linux kernel. Having a 585test suite would allow to identify regressions earlier. 586 587A good candidate for the first unit tests are the format-conversion helpers in 588``drm_format_helper.c``. 589 590Contact: Javier Martinez Canillas <javierm@redhat.com> 591 592Level: Intermediate 593 594Enable trinity for DRM 595---------------------- 596 597And fix up the fallout. Should be really interesting ... 598 599Level: Advanced 600 601Make KMS tests in i-g-t generic 602------------------------------- 603 604The i915 driver team maintains an extensive testsuite for the i915 DRM driver, 605including tons of testcases for corner-cases in the modesetting API. It would 606be awesome if those tests (at least the ones not relying on Intel-specific GEM 607features) could be made to run on any KMS driver. 608 609Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass- 610converting things over. For modeset tests we also first need a bit of 611infrastructure to use dumb buffers for untiled buffers, to be able to run all 612the non-i915 specific modeset tests. 613 614Level: Advanced 615 616Extend virtual test driver (VKMS) 617--------------------------------- 618 619See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal 620internship task, since it only requires a virtual machine and can be sized to 621fit the available time. 622 623Level: See details 624 625Backlight Refactoring 626--------------------- 627 628Backlight drivers have a triple enable/disable state, which is a bit overkill. 629Plan to fix this: 630 6311. Roll out backlight_enable() and backlight_disable() helpers everywhere. This 632 has started already. 6332. In all, only look at one of the three status bits set by the above helpers. 6343. Remove the other two status bits. 635 636Contact: Daniel Vetter 637 638Level: Intermediate 639 640Driver Specific 641=============== 642 643AMD DC Display Driver 644--------------------- 645 646AMD DC is the display driver for AMD devices starting with Vega. There has been 647a bunch of progress cleaning it up but there's still plenty of work to be done. 648 649See drivers/gpu/drm/amd/display/TODO for tasks. 650 651Contact: Harry Wentland, Alex Deucher 652 653Bootsplash 654========== 655 656There is support in place now for writing internal DRM clients making it 657possible to pick up the bootsplash work that was rejected because it was written 658for fbdev. 659 660- [v6,8/8] drm/client: Hack: Add bootsplash example 661 https://patchwork.freedesktop.org/patch/306579/ 662 663- [RFC PATCH v2 00/13] Kernel based bootsplash 664 https://lore.kernel.org/r/20171213194755.3409-1-mstaudt@suse.de 665 666Contact: Sam Ravnborg 667 668Level: Advanced 669 670Brightness handling on devices with multiple internal panels 671============================================================ 672 673On x86/ACPI devices there can be multiple backlight firmware interfaces: 674(ACPI) video, vendor specific and others. As well as direct/native (PWM) 675register programming by the KMS driver. 676 677To deal with this backlight drivers used on x86/ACPI call 678acpi_video_get_backlight_type() which has heuristics (+quirks) to select 679which backlight interface to use; and backlight drivers which do not match 680the returned type will not register themselves, so that only one backlight 681device gets registered (in a single GPU setup, see below). 682 683At the moment this more or less assumes that there will only 684be 1 (internal) panel on a system. 685 686On systems with 2 panels this may be a problem, depending on 687what interface acpi_video_get_backlight_type() selects: 688 6891. native: in this case the KMS driver is expected to know which backlight 690 device belongs to which output so everything should just work. 6912. video: this does support controlling multiple backlights, but some work 692 will need to be done to get the output <-> backlight device mapping 693 694The above assumes both panels will require the same backlight interface type. 695Things will break on systems with multiple panels where the 2 panels need 696a different type of control. E.g. one panel needs ACPI video backlight control, 697where as the other is using native backlight control. Currently in this case 698only one of the 2 required backlight devices will get registered, based on 699the acpi_video_get_backlight_type() return value. 700 701If this (theoretical) case ever shows up, then supporting this will need some 702work. A possible solution here would be to pass a device and connector-name 703to acpi_video_get_backlight_type() so that it can deal with this. 704 705Note in a way we already have a case where userspace sees 2 panels, 706in dual GPU laptop setups with a mux. On those systems we may see 707either 2 native backlight devices; or 2 native backlight devices. 708 709Userspace already has code to deal with this by detecting if the related 710panel is active (iow which way the mux between the GPU and the panels 711points) and then uses that backlight device. Userspace here very much 712assumes a single panel though. It picks only 1 of the 2 backlight devices 713and then only uses that one. 714 715Note that all userspace code (that I know off) is currently hardcoded 716to assume a single panel. 717 718Before the recent changes to not register multiple (e.g. video + native) 719/sys/class/backlight devices for a single panel (on a single GPU laptop), 720userspace would see multiple backlight devices all controlling the same 721backlight. 722 723To deal with this userspace had to always picks one preferred device under 724/sys/class/backlight and will ignore the others. So to support brightness 725control on multiple panels userspace will need to be updated too. 726 727There are plans to allow brightness control through the KMS API by adding 728a "display brightness" property to drm_connector objects for panels. This 729solves a number of issues with the /sys/class/backlight API, including not 730being able to map a sysfs backlight device to a specific connector. Any 731userspace changes to add support for brightness control on devices with 732multiple panels really should build on top of this new KMS property. 733 734Contact: Hans de Goede 735 736Level: Advanced 737 738Outside DRM 739=========== 740 741Convert fbdev drivers to DRM 742---------------------------- 743 744There are plenty of fbdev drivers for older hardware. Some hardware has 745become obsolete, but some still provides good(-enough) framebuffers. The 746drivers that are still useful should be converted to DRM and afterwards 747removed from fbdev. 748 749Very simple fbdev drivers can best be converted by starting with a new 750DRM driver. Simple KMS helpers and SHMEM should be able to handle any 751existing hardware. The new driver's call-back functions are filled from 752existing fbdev code. 753 754More complex fbdev drivers can be refactored step-by-step into a DRM 755driver with the help of the DRM fbconv helpers. [1] These helpers provide 756the transition layer between the DRM core infrastructure and the fbdev 757driver interface. Create a new DRM driver on top of the fbconv helpers, 758copy over the fbdev driver, and hook it up to the DRM code. Examples for 759several fbdev drivers are available at [1] and a tutorial of this process 760available at [2]. The result is a primitive DRM driver that can run X11 761and Weston. 762 763 - [1] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv 764 - [2] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c 765 766Contact: Thomas Zimmermann <tzimmermann@suse.de> 767 768Level: Advanced 769