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instead. 36Audit each individual driver, make sure it'll work with the generic 37implementation (there's lots of outdated locking leftovers in various 38implementations), and then remove it. 39 40Contact: Daniel Vetter, respective driver maintainers 41 42Level: Intermediate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- Need to switch to drm_gem_fb_create(), as now drm_gem_fb_create() checks for 280 valid formats for atomic drivers. 281 282- Many drivers subclass drm_framebuffer, we'd need a embedding compatible 283 version of the varios drm_gem_fb_create functions. Maybe called 284 drm_gem_fb_create/_with_dirty/_with_funcs as needed. 285 286Contact: Daniel Vetter 287 288Level: Intermediate 289 290Generic fbdev defio support 291--------------------------- 292 293The defio support code in the fbdev core has some very specific requirements, 294which means drivers need to have a special framebuffer for fbdev. The main 295issue is that it uses some fields in struct page itself, which breaks shmem 296gem objects (and other things). To support defio, affected drivers require 297the use of a shadow buffer, which may add CPU and memory overhead. 298 299Possible solution would be to write our own defio mmap code in the drm fbdev 300emulation. It would need to fully wrap the existing mmap ops, forwarding 301everything after it has done the write-protect/mkwrite trickery: 302 303- In the drm_fbdev_fb_mmap helper, if we need defio, change the 304 default page prots to write-protected with something like this:: 305 306 vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot); 307 308- Set the mkwrite and fsync callbacks with similar implementions to the core 309 fbdev defio stuff. These should all work on plain ptes, they don't actually 310 require a struct page. uff. These should all work on plain ptes, they don't 311 actually require a struct page. 312 313- Track the dirty pages in a separate structure (bitfield with one bit per page 314 should work) to avoid clobbering struct page. 315 316Might be good to also have some igt testcases for this. 317 318Contact: Daniel Vetter, Noralf Tronnes 319 320Level: Advanced 321 322struct drm_gem_object_funcs 323--------------------------- 324 325GEM objects can now have a function table instead of having the callbacks on the 326DRM driver struct. This is now the preferred way. Callbacks in drivers have been 327converted, except for struct drm_driver.gem_prime_mmap. 328 329Level: Intermediate 330 331connector register/unregister fixes 332----------------------------------- 333 334- For most connectors it's a no-op to call drm_connector_register/unregister 335 directly from driver code, drm_dev_register/unregister take care of this 336 already. We can remove all of them. 337 338- For dp drivers it's a bit more a mess, since we need the connector to be 339 registered when calling drm_dp_aux_register. Fix this by instead calling 340 drm_dp_aux_init, and moving the actual registering into a late_register 341 callback as recommended in the kerneldoc. 342 343Level: Intermediate 344 345Remove load/unload callbacks from all non-DRIVER_LEGACY drivers 346--------------------------------------------------------------- 347 348The load/unload callbacks in struct &drm_driver are very much midlayers, plus 349for historical reasons they get the ordering wrong (and we can't fix that) 350between setting up the &drm_driver structure and calling drm_dev_register(). 351 352- Rework drivers to no longer use the load/unload callbacks, directly coding the 353 load/unload sequence into the driver's probe function. 354 355- Once all non-DRIVER_LEGACY drivers are converted, disallow the load/unload 356 callbacks for all modern drivers. 357 358Contact: Daniel Vetter 359 360Level: Intermediate 361 362Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi 363--------------------------------------------------------------- 364 365Once EDID is parsed, the monitor HDMI support information is available through 366drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor() to 367retrieve the same information, which is less efficient. 368 369Audit each individual driver calling drm_detect_hdmi_monitor() and switch to 370drm_display_info.is_hdmi if applicable. 371 372Contact: Laurent Pinchart, respective driver maintainers 373 374Level: Intermediate 375 376Consolidate custom driver modeset properties 377-------------------------------------------- 378 379Before atomic modeset took place, many drivers where creating their own 380properties. Among other things, atomic brought the requirement that custom, 381driver specific properties should not be used. 382 383For this task, we aim to introduce core helpers or reuse the existing ones 384if available: 385 386A quick, unconfirmed, examples list. 387 388Introduce core helpers: 389- audio (amdgpu, intel, gma500, radeon) 390- brightness, contrast, etc (armada, nouveau) - overlay only (?) 391- broadcast rgb (gma500, intel) 392- colorkey (armada, nouveau, rcar) - overlay only (?) 393- dither (amdgpu, nouveau, radeon) - varies across drivers 394- underscan family (amdgpu, radeon, nouveau) 395 396Already in core: 397- colorspace (sti) 398- tv format names, enhancements (gma500, intel) 399- tv overscan, margins, etc. (gma500, intel) 400- zorder (omapdrm) - same as zpos (?) 401 402 403Contact: Emil Velikov, respective driver maintainers 404 405Level: Intermediate 406 407Use struct iosys_map throughout codebase 408---------------------------------------- 409 410Pointers to shared device memory are stored in struct iosys_map. Each 411instance knows whether it refers to system or I/O memory. Most of the DRM-wide 412interface have been converted to use struct iosys_map, but implementations 413often still use raw pointers. 414 415The task is to use struct iosys_map where it makes sense. 416 417* Memory managers should use struct iosys_map for dma-buf-imported buffers. 418* TTM might benefit from using struct iosys_map internally. 419* Framebuffer copying and blitting helpers should operate on struct iosys_map. 420 421Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Daniel Vetter 422 423Level: Intermediate 424 425Review all drivers for setting struct drm_mode_config.{max_width,max_height} correctly 426-------------------------------------------------------------------------------------- 427 428The values in struct drm_mode_config.{max_width,max_height} describe the 429maximum supported framebuffer size. It's the virtual screen size, but many 430drivers treat it like limitations of the physical resolution. 431 432The maximum width depends on the hardware's maximum scanline pitch. The 433maximum height depends on the amount of addressable video memory. Review all 434drivers to initialize the fields to the correct values. 435 436Contact: Thomas Zimmermann <tzimmermann@suse.de> 437 438Level: Intermediate 439 440Request memory regions in all drivers 441------------------------------------- 442 443Go through all drivers and add code to request the memory regions that the 444driver uses. This requires adding calls to request_mem_region(), 445pci_request_region() or similar functions. Use helpers for managed cleanup 446where possible. 447 448Drivers are pretty bad at doing this and there used to be conflicts among 449DRM and fbdev drivers. Still, it's the correct thing to do. 450 451Contact: Thomas Zimmermann <tzimmermann@suse.de> 452 453Level: Starter 454 455 456Core refactorings 457================= 458 459Make panic handling work 460------------------------ 461 462This is a really varied tasks with lots of little bits and pieces: 463 464* The panic path can't be tested currently, leading to constant breaking. The 465 main issue here is that panics can be triggered from hardirq contexts and 466 hence all panic related callback can run in hardirq context. It would be 467 awesome if we could test at least the fbdev helper code and driver code by 468 e.g. trigger calls through drm debugfs files. hardirq context could be 469 achieved by using an IPI to the local processor. 470 471* There's a massive confusion of different panic handlers. DRM fbdev emulation 472 helpers had their own (long removed), but on top of that the fbcon code itself 473 also has one. We need to make sure that they stop fighting over each other. 474 This is worked around by checking ``oops_in_progress`` at various entry points 475 into the DRM fbdev emulation helpers. A much cleaner approach here would be to 476 switch fbcon to the `threaded printk support 477 <https://lwn.net/Articles/800946/>`_. 478 479* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and 480 isn't a full solution for panic paths. We need to make sure that it only 481 returns true if there's a panic going on for real, and fix up all the 482 fallout. 483 484* The panic handler must never sleep, which also means it can't ever 485 ``mutex_lock()``. Also it can't grab any other lock unconditionally, not 486 even spinlocks (because NMI and hardirq can panic too). We need to either 487 make sure to not call such paths, or trylock everything. Really tricky. 488 489* A clean solution would be an entirely separate panic output support in KMS, 490 bypassing the current fbcon support. See `[PATCH v2 0/3] drm: Add panic handling 491 <https://lore.kernel.org/dri-devel/20190311174218.51899-1-noralf@tronnes.org/>`_. 492 493* Encoding the actual oops and preceding dmesg in a QR might help with the 494 dread "important stuff scrolled away" problem. See `[RFC][PATCH] Oops messages 495 transfer using QR codes 496 <https://lore.kernel.org/lkml/1446217392-11981-1-git-send-email-alexandru.murtaza@intel.com/>`_ 497 for some example code that could be reused. 498 499Contact: Daniel Vetter 500 501Level: Advanced 502 503Clean up the debugfs support 504---------------------------- 505 506There's a bunch of issues with it: 507 508- Convert drivers to support the drm_debugfs_add_files() function instead of 509 the drm_debugfs_create_files() function. 510 511- Improve late-register debugfs by rolling out the same debugfs pre-register 512 infrastructure for connector and crtc too. That way, the drivers won't need to 513 split their setup code into init and register anymore. 514 515- We probably want to have some support for debugfs files on crtc/connectors and 516 maybe other kms objects directly in core. There's even drm_print support in 517 the funcs for these objects to dump kms state, so it's all there. And then the 518 ->show() functions should obviously give you a pointer to the right object. 519 520- The drm_driver->debugfs_init hooks we have is just an artifact of the old 521 midlayered load sequence. DRM debugfs should work more like sysfs, where you 522 can create properties/files for an object anytime you want, and the core 523 takes care of publishing/unpuplishing all the files at register/unregister 524 time. Drivers shouldn't need to worry about these technicalities, and fixing 525 this (together with the drm_minor->drm_device move) would allow us to remove 526 debugfs_init. 527 528Contact: Daniel Vetter 529 530Level: Intermediate 531 532Object lifetime fixes 533--------------------- 534 535There's two related issues here 536 537- Cleanup up the various ->destroy callbacks, which often are all the same 538 simple code. 539 540- Lots of drivers erroneously allocate DRM modeset objects using devm_kzalloc, 541 which results in use-after free issues on driver unload. This can be serious 542 trouble even for drivers for hardware integrated on the SoC due to 543 EPROBE_DEFERRED backoff. 544 545Both these problems can be solved by switching over to drmm_kzalloc(), and the 546various convenience wrappers provided, e.g. drmm_crtc_alloc_with_planes(), 547drmm_universal_plane_alloc(), ... and so on. 548 549Contact: Daniel Vetter 550 551Level: Intermediate 552 553Remove automatic page mapping from dma-buf importing 554---------------------------------------------------- 555 556When importing dma-bufs, the dma-buf and PRIME frameworks automatically map 557imported pages into the importer's DMA area. drm_gem_prime_fd_to_handle() and 558drm_gem_prime_handle_to_fd() require that importers call dma_buf_attach() 559even if they never do actual device DMA, but only CPU access through 560dma_buf_vmap(). This is a problem for USB devices, which do not support DMA 561operations. 562 563To fix the issue, automatic page mappings should be removed from the 564buffer-sharing code. Fixing this is a bit more involved, since the import/export 565cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over 566this problem for USB devices by fishing out the USB host controller device, as 567long as that supports DMA. Otherwise importing can still needlessly fail. 568 569Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter 570 571Level: Advanced 572 573 574Better Testing 575============== 576 577Add unit tests using the Kernel Unit Testing (KUnit) framework 578-------------------------------------------------------------- 579 580The `KUnit <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_ 581provides a common framework for unit tests within the Linux kernel. Having a 582test suite would allow to identify regressions earlier. 583 584A good candidate for the first unit tests are the format-conversion helpers in 585``drm_format_helper.c``. 586 587Contact: Javier Martinez Canillas <javierm@redhat.com> 588 589Level: Intermediate 590 591Enable trinity for DRM 592---------------------- 593 594And fix up the fallout. Should be really interesting ... 595 596Level: Advanced 597 598Make KMS tests in i-g-t generic 599------------------------------- 600 601The i915 driver team maintains an extensive testsuite for the i915 DRM driver, 602including tons of testcases for corner-cases in the modesetting API. It would 603be awesome if those tests (at least the ones not relying on Intel-specific GEM 604features) could be made to run on any KMS driver. 605 606Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass- 607converting things over. For modeset tests we also first need a bit of 608infrastructure to use dumb buffers for untiled buffers, to be able to run all 609the non-i915 specific modeset tests. 610 611Level: Advanced 612 613Extend virtual test driver (VKMS) 614--------------------------------- 615 616See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal 617internship task, since it only requires a virtual machine and can be sized to 618fit the available time. 619 620Level: See details 621 622Backlight Refactoring 623--------------------- 624 625Backlight drivers have a triple enable/disable state, which is a bit overkill. 626Plan to fix this: 627 6281. Roll out backlight_enable() and backlight_disable() helpers everywhere. This 629 has started already. 6302. In all, only look at one of the three status bits set by the above helpers. 6313. Remove the other two status bits. 632 633Contact: Daniel Vetter 634 635Level: Intermediate 636 637Driver Specific 638=============== 639 640AMD DC Display Driver 641--------------------- 642 643AMD DC is the display driver for AMD devices starting with Vega. There has been 644a bunch of progress cleaning it up but there's still plenty of work to be done. 645 646See drivers/gpu/drm/amd/display/TODO for tasks. 647 648Contact: Harry Wentland, Alex Deucher 649 650Bootsplash 651========== 652 653There is support in place now for writing internal DRM clients making it 654possible to pick up the bootsplash work that was rejected because it was written 655for fbdev. 656 657- [v6,8/8] drm/client: Hack: Add bootsplash example 658 https://patchwork.freedesktop.org/patch/306579/ 659 660- [RFC PATCH v2 00/13] Kernel based bootsplash 661 https://lore.kernel.org/r/20171213194755.3409-1-mstaudt@suse.de 662 663Contact: Sam Ravnborg 664 665Level: Advanced 666 667Brightness handling on devices with multiple internal panels 668============================================================ 669 670On x86/ACPI devices there can be multiple backlight firmware interfaces: 671(ACPI) video, vendor specific and others. As well as direct/native (PWM) 672register programming by the KMS driver. 673 674To deal with this backlight drivers used on x86/ACPI call 675acpi_video_get_backlight_type() which has heuristics (+quirks) to select 676which backlight interface to use; and backlight drivers which do not match 677the returned type will not register themselves, so that only one backlight 678device gets registered (in a single GPU setup, see below). 679 680At the moment this more or less assumes that there will only 681be 1 (internal) panel on a system. 682 683On systems with 2 panels this may be a problem, depending on 684what interface acpi_video_get_backlight_type() selects: 685 6861. native: in this case the KMS driver is expected to know which backlight 687 device belongs to which output so everything should just work. 6882. video: this does support controlling multiple backlights, but some work 689 will need to be done to get the output <-> backlight device mapping 690 691The above assumes both panels will require the same backlight interface type. 692Things will break on systems with multiple panels where the 2 panels need 693a different type of control. E.g. one panel needs ACPI video backlight control, 694where as the other is using native backlight control. Currently in this case 695only one of the 2 required backlight devices will get registered, based on 696the acpi_video_get_backlight_type() return value. 697 698If this (theoretical) case ever shows up, then supporting this will need some 699work. A possible solution here would be to pass a device and connector-name 700to acpi_video_get_backlight_type() so that it can deal with this. 701 702Note in a way we already have a case where userspace sees 2 panels, 703in dual GPU laptop setups with a mux. On those systems we may see 704either 2 native backlight devices; or 2 native backlight devices. 705 706Userspace already has code to deal with this by detecting if the related 707panel is active (iow which way the mux between the GPU and the panels 708points) and then uses that backlight device. Userspace here very much 709assumes a single panel though. It picks only 1 of the 2 backlight devices 710and then only uses that one. 711 712Note that all userspace code (that I know off) is currently hardcoded 713to assume a single panel. 714 715Before the recent changes to not register multiple (e.g. video + native) 716/sys/class/backlight devices for a single panel (on a single GPU laptop), 717userspace would see multiple backlight devices all controlling the same 718backlight. 719 720To deal with this userspace had to always picks one preferred device under 721/sys/class/backlight and will ignore the others. So to support brightness 722control on multiple panels userspace will need to be updated too. 723 724There are plans to allow brightness control through the KMS API by adding 725a "display brightness" property to drm_connector objects for panels. This 726solves a number of issues with the /sys/class/backlight API, including not 727being able to map a sysfs backlight device to a specific connector. Any 728userspace changes to add support for brightness control on devices with 729multiple panels really should build on top of this new KMS property. 730 731Contact: Hans de Goede 732 733Level: Advanced 734 735Outside DRM 736=========== 737 738Convert fbdev drivers to DRM 739---------------------------- 740 741There are plenty of fbdev drivers for older hardware. Some hardware has 742become obsolete, but some still provides good(-enough) framebuffers. The 743drivers that are still useful should be converted to DRM and afterwards 744removed from fbdev. 745 746Very simple fbdev drivers can best be converted by starting with a new 747DRM driver. Simple KMS helpers and SHMEM should be able to handle any 748existing hardware. The new driver's call-back functions are filled from 749existing fbdev code. 750 751More complex fbdev drivers can be refactored step-by-step into a DRM 752driver with the help of the DRM fbconv helpers. [1] These helpers provide 753the transition layer between the DRM core infrastructure and the fbdev 754driver interface. Create a new DRM driver on top of the fbconv helpers, 755copy over the fbdev driver, and hook it up to the DRM code. Examples for 756several fbdev drivers are available at [1] and a tutorial of this process 757available at [2]. The result is a primitive DRM driver that can run X11 758and Weston. 759 760 - [1] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv 761 - [2] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c 762 763Contact: Thomas Zimmermann <tzimmermann@suse.de> 764 765Level: Advanced 766