Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. _todo:
+
+=========
+TODO list
+=========
+
+This section contains a list of smaller janitorial tasks in the kernel DRM
+graphics subsystem useful as newbie projects. Or for slow rainy days.
+
+Difficulty
+----------
+
+To make it easier task are categorized into different levels:
+
+Starter: Good tasks to get started with the DRM subsystem.
+
+Intermediate: Tasks which need some experience with working in the DRM
+subsystem, or some specific GPU/display graphics knowledge. For debugging issue
+it's good to have the relevant hardware (or a virtual driver set up) available
+for testing.
+
+Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem
+and graphics topics. Generally need the relevant hardware for development and
+testing.
+
+Expert: Only attempt these if you've successfully completed some tricky
+refactorings already and are an expert in the specific area
+
+Subsystem-wide refactorings
+===========================
+
+Remove custom dumb_map_offset implementations
+---------------------------------------------
+
+All GEM based drivers should be using drm_gem_create_mmap_offset() instead.
+Audit each individual driver, make sure it'll work with the generic
+implementation (there's lots of outdated locking leftovers in various
+implementations), and then remove it.
+
+Contact: Daniel Vetter, respective driver maintainers
+
+Level: Intermediate
+
+Convert existing KMS drivers to atomic modesetting
+--------------------------------------------------
+
+3.19 has the atomic modeset interfaces and helpers, so drivers can now be
+converted over. Modern compositors like Wayland or Surfaceflinger on Android
+really want an atomic modeset interface, so this is all about the bright
+future.
+
+There is a conversion guide for atomic and all you need is a GPU for a
+non-converted driver (again virtual HW drivers for KVM are still all
+suitable).
+
+As part of this drivers also need to convert to universal plane (which means
+exposing primary & cursor as proper plane objects). But that's much easier to
+do by directly using the new atomic helper driver callbacks.
+
+Contact: Daniel Vetter, respective driver maintainers
+
+Level: Advanced
+
+Clean up the clipped coordination confusion around planes
+---------------------------------------------------------
+
+We have a helper to get this right with drm_plane_helper_check_update(), but
+it's not consistently used. This should be fixed, preferrably in the atomic
+helpers (and drivers then moved over to clipped coordinates). Probably the
+helper should also be moved from drm_plane_helper.c to the atomic helpers, to
+avoid confusion - the other helpers in that file are all deprecated legacy
+helpers.
+
+Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
+
+Level: Advanced
+
+Improve plane atomic_check helpers
+----------------------------------
+
+Aside from the clipped coordinates right above there's a few suboptimal things
+with the current helpers:
+
+- drm_plane_helper_funcs->atomic_check gets called for enabled or disabled
+  planes. At best this seems to confuse drivers, worst it means they blow up
+  when the plane is disabled without the CRTC. The only special handling is
+  resetting values in the plane state structures, which instead should be moved
+  into the drm_plane_funcs->atomic_duplicate_state functions.
+
+- Once that's done, helpers could stop calling ->atomic_check for disabled
+  planes.
+
+- Then we could go through all the drivers and remove the more-or-less confused
+  checks for plane_state->fb and plane_state->crtc.
+
+Contact: Daniel Vetter
+
+Level: Advanced
+
+Convert early atomic drivers to async commit helpers
+----------------------------------------------------
+
+For the first year the atomic modeset helpers didn't support asynchronous /
+nonblocking commits, and every driver had to hand-roll them. This is fixed
+now, but there's still a pile of existing drivers that easily could be
+converted over to the new infrastructure.
+
+One issue with the helpers is that they require that drivers handle completion
+events for atomic commits correctly. But fixing these bugs is good anyway.
+
+Somewhat related is the legacy_cursor_update hack, which should be replaced with
+the new atomic_async_check/commit functionality in the helpers in drivers that
+still look at that flag.
+
+Contact: Daniel Vetter, respective driver maintainers
+
+Level: Advanced
+
+Fallout from atomic KMS
+-----------------------
+
+``drm_atomic_helper.c`` provides a batch of functions which implement legacy
+IOCTLs on top of the new atomic driver interface. Which is really nice for
+gradual conversion of drivers, but unfortunately the semantic mismatches are
+a bit too severe. So there's some follow-up work to adjust the function
+interfaces to fix these issues:
+
+* atomic needs the lock acquire context. At the moment that's passed around
+  implicitly with some horrible hacks, and it's also allocate with
+  ``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
+  the acquire context explicitly on stack and then also pass it down into
+  drivers explicitly so that the legacy-on-atomic functions can use them.
+
+  Except for some driver code this is done. This task should be finished by
+  adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().
+
+* A bunch of the vtable hooks are now in the wrong place: DRM has a split
+  between core vfunc tables (named ``drm_foo_funcs``), which are used to
+  implement the userspace ABI. And then there's the optional hooks for the
+  helper libraries (name ``drm_foo_helper_funcs``), which are purely for
+  internal use. Some of these hooks should be move from ``_funcs`` to
+  ``_helper_funcs`` since they are not part of the core ABI. There's a
+  ``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
+
+Contact: Daniel Vetter
+
+Level: Intermediate
+
+Get rid of dev->struct_mutex from GEM drivers
+---------------------------------------------
+
+``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
+everything. Nowadays in modern drivers the only bit where it's mandatory is
+serializing GEM buffer object destruction. Which unfortunately means drivers
+have to keep track of that lock and either call ``unreference`` or
+``unreference_locked`` depending upon context.
+
+Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
+and there's a GEM object ``free`` callback for any drivers which are
+entirely ``struct_mutex`` free.
+
+For drivers that need ``struct_mutex`` it should be replaced with a driver-
+private lock. The tricky part is the BO free functions, since those can't
+reliably take that lock any more. Instead state needs to be protected with
+suitable subordinate locks or some cleanup work pushed to a worker thread. For
+performance-critical drivers it might also be better to go with a more
+fine-grained per-buffer object and per-context lockings scheme. Currently only
+the ``msm`` and `i915` drivers use ``struct_mutex``.
+
+Contact: Daniel Vetter, respective driver maintainers
+
+Level: Advanced
+
+Move Buffer Object Locking to dma_resv_lock()
+---------------------------------------------
+
+Many drivers have their own per-object locking scheme, usually using
+mutex_lock(). This causes all kinds of trouble for buffer sharing, since
+depending which driver is the exporter and importer, the locking hierarchy is
+reversed.
+
+To solve this we need one standard per-object locking mechanism, which is
+dma_resv_lock(). This lock needs to be called as the outermost lock, with all
+other driver specific per-object locks removed. The problem is tha rolling out
+the actual change to the locking contract is a flag day, due to struct dma_buf
+buffer sharing.
+
+Level: Expert
+
+Convert logging to drm_* functions with drm_device paramater
+------------------------------------------------------------
+
+For drivers which could have multiple instances, it is necessary to
+differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
+don't do this, drivers used dev_info/warn/err to make this differentiation. We
+now have drm_* variants of the drm print functions, so we can start to convert
+those drivers back to using drm-formatted specific log messages.
+
+Before you start this conversion please contact the relevant maintainers to make
+sure your work will be merged - not everyone agrees that the DRM dmesg macros
+are better.
+
+Contact: Sean Paul, Maintainer of the driver you plan to convert
+
+Level: Starter
+
+Convert drivers to use simple modeset suspend/resume
+----------------------------------------------------
+
+Most drivers (except i915 and nouveau) that use
+drm_atomic_helper_suspend/resume() can probably be converted to use
+drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
+of the atomic suspend/resume code in older atomic modeset drivers.
+
+Contact: Maintainer of the driver you plan to convert
+
+Level: Intermediate
+
+Convert drivers to use drm_fbdev_generic_setup()
+------------------------------------------------
+
+Most drivers can use drm_fbdev_generic_setup(). Driver have to implement
+atomic modesetting and GEM vmap support. Historically, generic fbdev emulation
+expected the framebuffer in system memory or system-like memory. By employing
+struct iosys_map, drivers with frambuffers in I/O memory can be supported
+as well.
+
+Contact: Maintainer of the driver you plan to convert
+
+Level: Intermediate
+
+Reimplement functions in drm_fbdev_fb_ops without fbdev
+-------------------------------------------------------
+
+A number of callback functions in drm_fbdev_fb_ops could benefit from
+being rewritten without dependencies on the fbdev module. Some of the
+helpers could further benefit from using struct iosys_map instead of
+raw pointers.
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
+
+Level: Advanced
+
+Benchmark and optimize blitting and format-conversion function
+--------------------------------------------------------------
+
+Drawing to dispay memory quickly is crucial for many applications'
+performance.
+
+On at least x86-64, sys_imageblit() is significantly slower than
+cfb_imageblit(), even though both use the same blitting algorithm and
+the latter is written for I/O memory. It turns out that cfb_imageblit()
+uses movl instructions, while sys_imageblit apparently does not. This
+seems to be a problem with gcc's optimizer. DRM's format-conversion
+helpers might be subject to similar issues.
+
+Benchmark and optimize fbdev's sys_() helpers and DRM's format-conversion
+helpers. In cases that can be further optimized, maybe implement a different
+algorithm. For micro-optimizations, use movl/movq instructions explicitly.
+That might possibly require architecture-specific helpers (e.g., storel()
+storeq()).
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>
+
+Level: Intermediate
+
+drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup
+-----------------------------------------------------------------
+
+A lot more drivers could be switched over to the drm_gem_framebuffer helpers.
+Various hold-ups:
+
+- Need to switch over to the generic dirty tracking code using
+  drm_atomic_helper_dirtyfb first (e.g. qxl).
+
+- Need to switch to drm_fbdev_generic_setup(), otherwise a lot of the custom fb
+  setup code can't be deleted.
+
+- Many drivers wrap drm_gem_fb_create() only to check for valid formats. For
+  atomic drivers we could check for valid formats by calling
+  drm_plane_check_pixel_format() against all planes, and pass if any plane
+  supports the format. For non-atomic that's not possible since like the format
+  list for the primary plane is fake and we'd therefor reject valid formats.
+
+- Many drivers subclass drm_framebuffer, we'd need a embedding compatible
+  version of the varios drm_gem_fb_create functions. Maybe called
+  drm_gem_fb_create/_with_dirty/_with_funcs as needed.
+
+Contact: Daniel Vetter
+
+Level: Intermediate
+
+Generic fbdev defio support
+---------------------------
+
+The defio support code in the fbdev core has some very specific requirements,
+which means drivers need to have a special framebuffer for fbdev. The main
+issue is that it uses some fields in struct page itself, which breaks shmem
+gem objects (and other things). To support defio, affected drivers require
+the use of a shadow buffer, which may add CPU and memory overhead.
+
+Possible solution would be to write our own defio mmap code in the drm fbdev
+emulation. It would need to fully wrap the existing mmap ops, forwarding
+everything after it has done the write-protect/mkwrite trickery:
+
+- In the drm_fbdev_fb_mmap helper, if we need defio, change the
+  default page prots to write-protected with something like this::
+
+      vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
+
+- Set the mkwrite and fsync callbacks with similar implementions to the core
+  fbdev defio stuff. These should all work on plain ptes, they don't actually
+  require a struct page.  uff. These should all work on plain ptes, they don't
+  actually require a struct page.
+
+- Track the dirty pages in a separate structure (bitfield with one bit per page
+  should work) to avoid clobbering struct page.
+
+Might be good to also have some igt testcases for this.
+
+Contact: Daniel Vetter, Noralf Tronnes
+
+Level: Advanced
+
+struct drm_gem_object_funcs
+---------------------------
+
+GEM objects can now have a function table instead of having the callbacks on the
+DRM driver struct. This is now the preferred way. Callbacks in drivers have been
+converted, except for struct drm_driver.gem_prime_mmap.
+
+Level: Intermediate
+
+connector register/unregister fixes
+-----------------------------------
+
+- For most connectors it's a no-op to call drm_connector_register/unregister
+  directly from driver code, drm_dev_register/unregister take care of this
+  already. We can remove all of them.
+
+- For dp drivers it's a bit more a mess, since we need the connector to be
+  registered when calling drm_dp_aux_register. Fix this by instead calling
+  drm_dp_aux_init, and moving the actual registering into a late_register
+  callback as recommended in the kerneldoc.
+
+Level: Intermediate
+
+Remove load/unload callbacks from all non-DRIVER_LEGACY drivers
+---------------------------------------------------------------
+
+The load/unload callbacks in struct &drm_driver are very much midlayers, plus
+for historical reasons they get the ordering wrong (and we can't fix that)
+between setting up the &drm_driver structure and calling drm_dev_register().
+
+- Rework drivers to no longer use the load/unload callbacks, directly coding the
+  load/unload sequence into the driver's probe function.
+
+- Once all non-DRIVER_LEGACY drivers are converted, disallow the load/unload
+  callbacks for all modern drivers.
+
+Contact: Daniel Vetter
+
+Level: Intermediate
+
+Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi
+---------------------------------------------------------------
+
+Once EDID is parsed, the monitor HDMI support information is available through
+drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor() to
+retrieve the same information, which is less efficient.
+
+Audit each individual driver calling drm_detect_hdmi_monitor() and switch to
+drm_display_info.is_hdmi if applicable.
+
+Contact: Laurent Pinchart, respective driver maintainers
+
+Level: Intermediate
+
+Consolidate custom driver modeset properties
+--------------------------------------------
+
+Before atomic modeset took place, many drivers where creating their own
+properties. Among other things, atomic brought the requirement that custom,
+driver specific properties should not be used.
+
+For this task, we aim to introduce core helpers or reuse the existing ones
+if available:
+
+A quick, unconfirmed, examples list.
+
+Introduce core helpers:
+- audio (amdgpu, intel, gma500, radeon)
+- brightness, contrast, etc (armada, nouveau) - overlay only (?)
+- broadcast rgb (gma500, intel)
+- colorkey (armada, nouveau, rcar) - overlay only (?)
+- dither (amdgpu, nouveau, radeon) - varies across drivers
+- underscan family (amdgpu, radeon, nouveau)
+
+Already in core:
+- colorspace (sti)
+- tv format names, enhancements (gma500, intel)
+- tv overscan, margins, etc. (gma500, intel)
+- zorder (omapdrm) - same as zpos (?)
+
+
+Contact: Emil Velikov, respective driver maintainers
+
+Level: Intermediate
+
+Use struct iosys_map throughout codebase
+----------------------------------------
+
+Pointers to shared device memory are stored in struct iosys_map. Each
+instance knows whether it refers to system or I/O memory. Most of the DRM-wide
+interface have been converted to use struct iosys_map, but implementations
+often still use raw pointers.
+
+The task is to use struct iosys_map where it makes sense.
+
+* Memory managers should use struct iosys_map for dma-buf-imported buffers.
+* TTM might benefit from using struct iosys_map internally.
+* Framebuffer copying and blitting helpers should operate on struct iosys_map.
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Daniel Vetter
+
+Level: Intermediate
+
+Review all drivers for setting struct drm_mode_config.{max_width,max_height} correctly
+--------------------------------------------------------------------------------------
+
+The values in struct drm_mode_config.{max_width,max_height} describe the
+maximum supported framebuffer size. It's the virtual screen size, but many
+drivers treat it like limitations of the physical resolution.
+
+The maximum width depends on the hardware's maximum scanline pitch. The
+maximum height depends on the amount of addressable video memory. Review all
+drivers to initialize the fields to the correct values.
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>
+
+Level: Intermediate
+
+Request memory regions in all drivers
+-------------------------------------
+
+Go through all drivers and add code to request the memory regions that the
+driver uses. This requires adding calls to request_mem_region(),
+pci_request_region() or similar functions. Use helpers for managed cleanup
+where possible.
+
+Drivers are pretty bad at doing this and there used to be conflicts among
+DRM and fbdev drivers. Still, it's the correct thing to do.
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>
+
+Level: Starter
+
+
+Core refactorings
+=================
+
+Make panic handling work
+------------------------
+
+This is a really varied tasks with lots of little bits and pieces:
+
+* The panic path can't be tested currently, leading to constant breaking. The
+  main issue here is that panics can be triggered from hardirq contexts and
+  hence all panic related callback can run in hardirq context. It would be
+  awesome if we could test at least the fbdev helper code and driver code by
+  e.g. trigger calls through drm debugfs files. hardirq context could be
+  achieved by using an IPI to the local processor.
+
+* There's a massive confusion of different panic handlers. DRM fbdev emulation
+  helpers had their own (long removed), but on top of that the fbcon code itself
+  also has one. We need to make sure that they stop fighting over each other.
+  This is worked around by checking ``oops_in_progress`` at various entry points
+  into the DRM fbdev emulation helpers. A much cleaner approach here would be to
+  switch fbcon to the `threaded printk support
+  <https://lwn.net/Articles/800946/>`_.
+
+* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
+  isn't a full solution for panic paths. We need to make sure that it only
+  returns true if there's a panic going on for real, and fix up all the
+  fallout.
+
+* The panic handler must never sleep, which also means it can't ever
+  ``mutex_lock()``. Also it can't grab any other lock unconditionally, not
+  even spinlocks (because NMI and hardirq can panic too). We need to either
+  make sure to not call such paths, or trylock everything. Really tricky.
+
+* A clean solution would be an entirely separate panic output support in KMS,
+  bypassing the current fbcon support. See `[PATCH v2 0/3] drm: Add panic handling
+  <https://lore.kernel.org/dri-devel/20190311174218.51899-1-noralf@tronnes.org/>`_.
+
+* Encoding the actual oops and preceding dmesg in a QR might help with the
+  dread "important stuff scrolled away" problem. See `[RFC][PATCH] Oops messages
+  transfer using QR codes
+  <https://lore.kernel.org/lkml/1446217392-11981-1-git-send-email-alexandru.murtaza@intel.com/>`_
+  for some example code that could be reused.
+
+Contact: Daniel Vetter
+
+Level: Advanced
+
+Clean up the debugfs support
+----------------------------
+
+There's a bunch of issues with it:
+
+- The drm_info_list ->show() function doesn't even bother to cast to the drm
+  structure for you. This is lazy.
+
+- We probably want to have some support for debugfs files on crtc/connectors and
+  maybe other kms objects directly in core. There's even drm_print support in
+  the funcs for these objects to dump kms state, so it's all there. And then the
+  ->show() functions should obviously give you a pointer to the right object.
+
+- The drm_info_list stuff is centered on drm_minor instead of drm_device. For
+  anything we want to print drm_device (or maybe drm_file) is the right thing.
+
+- The drm_driver->debugfs_init hooks we have is just an artifact of the old
+  midlayered load sequence. DRM debugfs should work more like sysfs, where you
+  can create properties/files for an object anytime you want, and the core
+  takes care of publishing/unpuplishing all the files at register/unregister
+  time. Drivers shouldn't need to worry about these technicalities, and fixing
+  this (together with the drm_minor->drm_device move) would allow us to remove
+  debugfs_init.
+
+Previous RFC that hasn't landed yet: https://lore.kernel.org/dri-devel/20200513114130.28641-2-wambui.karugax@gmail.com/
+
+Contact: Daniel Vetter
+
+Level: Intermediate
+
+Object lifetime fixes
+---------------------
+
+There's two related issues here
+
+- Cleanup up the various ->destroy callbacks, which often are all the same
+  simple code.
+
+- Lots of drivers erroneously allocate DRM modeset objects using devm_kzalloc,
+  which results in use-after free issues on driver unload. This can be serious
+  trouble even for drivers for hardware integrated on the SoC due to
+  EPROBE_DEFERRED backoff.
+
+Both these problems can be solved by switching over to drmm_kzalloc(), and the
+various convenience wrappers provided, e.g. drmm_crtc_alloc_with_planes(),
+drmm_universal_plane_alloc(), ... and so on.
+
+Contact: Daniel Vetter
+
+Level: Intermediate
+
+Remove automatic page mapping from dma-buf importing
+----------------------------------------------------
+
+When importing dma-bufs, the dma-buf and PRIME frameworks automatically map
+imported pages into the importer's DMA area. drm_gem_prime_fd_to_handle() and
+drm_gem_prime_handle_to_fd() require that importers call dma_buf_attach()
+even if they never do actual device DMA, but only CPU access through
+dma_buf_vmap(). This is a problem for USB devices, which do not support DMA
+operations.
+
+To fix the issue, automatic page mappings should be removed from the
+buffer-sharing code. Fixing this is a bit more involved, since the import/export
+cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over
+this problem for USB devices by fishing out the USB host controller device, as
+long as that supports DMA. Otherwise importing can still needlessly fail.
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
+
+Level: Advanced
+
+
+Better Testing
+==============
+
+Add unit tests using the Kernel Unit Testing (KUnit) framework
+--------------------------------------------------------------
+
+The `KUnit <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
+provides a common framework for unit tests within the Linux kernel. Having a
+test suite would allow to identify regressions earlier.
+
+A good candidate for the first unit tests are the format-conversion helpers in
+``drm_format_helper.c``.
+
+Contact: Javier Martinez Canillas <javierm@redhat.com>
+
+Level: Intermediate
+
+Enable trinity for DRM
+----------------------
+
+And fix up the fallout. Should be really interesting ...
+
+Level: Advanced
+
+Make KMS tests in i-g-t generic
+-------------------------------
+
+The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
+including tons of testcases for corner-cases in the modesetting API. It would
+be awesome if those tests (at least the ones not relying on Intel-specific GEM
+features) could be made to run on any KMS driver.
+
+Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
+converting things over. For modeset tests we also first need a bit of
+infrastructure to use dumb buffers for untiled buffers, to be able to run all
+the non-i915 specific modeset tests.
+
+Level: Advanced
+
+Extend virtual test driver (VKMS)
+---------------------------------
+
+See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
+internship task, since it only requires a virtual machine and can be sized to
+fit the available time.
+
+Level: See details
+
+Backlight Refactoring
+---------------------
+
+Backlight drivers have a triple enable/disable state, which is a bit overkill.
+Plan to fix this:
+
+1. Roll out backlight_enable() and backlight_disable() helpers everywhere. This
+   has started already.
+2. In all, only look at one of the three status bits set by the above helpers.
+3. Remove the other two status bits.
+
+Contact: Daniel Vetter
+
+Level: Intermediate
+
+Driver Specific
+===============
+
+AMD DC Display Driver
+---------------------
+
+AMD DC is the display driver for AMD devices starting with Vega. There has been
+a bunch of progress cleaning it up but there's still plenty of work to be done.
+
+See drivers/gpu/drm/amd/display/TODO for tasks.
+
+Contact: Harry Wentland, Alex Deucher
+
+Bootsplash
+==========
+
+There is support in place now for writing internal DRM clients making it
+possible to pick up the bootsplash work that was rejected because it was written
+for fbdev.
+
+- [v6,8/8] drm/client: Hack: Add bootsplash example
+  https://patchwork.freedesktop.org/patch/306579/
+
+- [RFC PATCH v2 00/13] Kernel based bootsplash
+  https://lore.kernel.org/r/20171213194755.3409-1-mstaudt@suse.de
+
+Contact: Sam Ravnborg
+
+Level: Advanced
+
+Brightness handling on devices with multiple internal panels
+============================================================
+
+On x86/ACPI devices there can be multiple backlight firmware interfaces:
+(ACPI) video, vendor specific and others. As well as direct/native (PWM)
+register programming by the KMS driver.
+
+To deal with this backlight drivers used on x86/ACPI call
+acpi_video_get_backlight_type() which has heuristics (+quirks) to select
+which backlight interface to use; and backlight drivers which do not match
+the returned type will not register themselves, so that only one backlight
+device gets registered (in a single GPU setup, see below).
+
+At the moment this more or less assumes that there will only
+be 1 (internal) panel on a system.
+
+On systems with 2 panels this may be a problem, depending on
+what interface acpi_video_get_backlight_type() selects:
+
+1. native: in this case the KMS driver is expected to know which backlight
+   device belongs to which output so everything should just work.
+2. video: this does support controlling multiple backlights, but some work
+   will need to be done to get the output <-> backlight device mapping
+
+The above assumes both panels will require the same backlight interface type.
+Things will break on systems with multiple panels where the 2 panels need
+a different type of control. E.g. one panel needs ACPI video backlight control,
+where as the other is using native backlight control. Currently in this case
+only one of the 2 required backlight devices will get registered, based on
+the acpi_video_get_backlight_type() return value.
+
+If this (theoretical) case ever shows up, then supporting this will need some
+work. A possible solution here would be to pass a device and connector-name
+to acpi_video_get_backlight_type() so that it can deal with this.
+
+Note in a way we already have a case where userspace sees 2 panels,
+in dual GPU laptop setups with a mux. On those systems we may see
+either 2 native backlight devices; or 2 native backlight devices.
+
+Userspace already has code to deal with this by detecting if the related
+panel is active (iow which way the mux between the GPU and the panels
+points) and then uses that backlight device. Userspace here very much
+assumes a single panel though. It picks only 1 of the 2 backlight devices
+and then only uses that one.
+
+Note that all userspace code (that I know off) is currently hardcoded
+to assume a single panel.
+
+Before the recent changes to not register multiple (e.g. video + native)
+/sys/class/backlight devices for a single panel (on a single GPU laptop),
+userspace would see multiple backlight devices all controlling the same
+backlight.
+
+To deal with this userspace had to always picks one preferred device under
+/sys/class/backlight and will ignore the others. So to support brightness
+control on multiple panels userspace will need to be updated too.
+
+There are plans to allow brightness control through the KMS API by adding
+a "display brightness" property to drm_connector objects for panels. This
+solves a number of issues with the /sys/class/backlight API, including not
+being able to map a sysfs backlight device to a specific connector. Any
+userspace changes to add support for brightness control on devices with
+multiple panels really should build on top of this new KMS property.
+
+Contact: Hans de Goede
+
+Level: Advanced
+
+Outside DRM
+===========
+
+Convert fbdev drivers to DRM
+----------------------------
+
+There are plenty of fbdev drivers for older hardware. Some hardware has
+become obsolete, but some still provides good(-enough) framebuffers. The
+drivers that are still useful should be converted to DRM and afterwards
+removed from fbdev.
+
+Very simple fbdev drivers can best be converted by starting with a new
+DRM driver. Simple KMS helpers and SHMEM should be able to handle any
+existing hardware. The new driver's call-back functions are filled from
+existing fbdev code.
+
+More complex fbdev drivers can be refactored step-by-step into a DRM
+driver with the help of the DRM fbconv helpers. [1] These helpers provide
+the transition layer between the DRM core infrastructure and the fbdev
+driver interface. Create a new DRM driver on top of the fbconv helpers,
+copy over the fbdev driver, and hook it up to the DRM code. Examples for
+several fbdev drivers are available at [1] and a tutorial of this process
+available at [2]. The result is a primitive DRM driver that can run X11
+and Weston.
+
+ - [1] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv
+ - [2] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>
+
+Level: Advanced