OpenCloudOS-Kernel/Documentation/gpu/todo.rst

.. _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implementation (there's lots of outdated locking leftovers in various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 [1]_ and all you need is a GPU for a
non-converted driver.  The "Atomic mode setting design overview" series [2]_
[3]_ at LWN.net can also be helpful.

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.

  .. [1] https://blog.ffwll.ch/2014/11/atomic-modeset-support-for-kms-drivers.html
  .. [2] https://lwn.net/Articles/653071/
  .. [3] https://lwn.net/Articles/653466/

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, preferably 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 that 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 parameter
------------------------------------------------------------

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 display 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.

- Need to switch to drm_gem_fb_create(), as now drm_gem_fb_create() checks for
  valid formats for atomic drivers.

- 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

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
----------------------------

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 drivers are converted, remove the load/unload callbacks.

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

Remove driver dependencies on FB_DEVICE
---------------------------------------

A number of fbdev drivers provide attributes via sysfs and therefore depend
on CONFIG_FB_DEVICE to be selected. Review each driver and attempt to make
any dependencies on CONFIG_FB_DEVICE optional. At the minimum, the respective
code in the driver could be conditionalized via ifdef CONFIG_FB_DEVICE. Not
all drivers might be able to drop CONFIG_FB_DEVICE.

Contact: Thomas Zimmermann <tzimmermann@suse.de>

Level: Starter

Clean up checks for already prepared/enabled in panels
------------------------------------------------------

In a whole pile of panel drivers, we have code to make the
prepare/unprepare/enable/disable callbacks behave as no-ops if they've already
been called. To get some idea of the duplicated code, try::

  git grep 'if.*>prepared' -- drivers/gpu/drm/panel
  git grep 'if.*>enabled' -- drivers/gpu/drm/panel

In the patch ("drm/panel: Check for already prepared/enabled in drm_panel")
we've moved this check to the core. Now we can most definitely remove the
check from the individual panels and save a pile of code.

In adition to removing the check from the individual panels, it is believed
that even the core shouldn't need this check and that should be considered
an error if other code ever relies on this check. The check in the core
currently prints a warning whenever something is relying on this check with
dev_warn(). After a little while, we likely want to promote this to a
WARN(1) to help encourage folks not to rely on this behavior.

Contact: Douglas Anderson <dianders@chromium.org>

Level: Starter/Intermediate


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:

- Convert drivers to support the drm_debugfs_add_files() function instead of
  the drm_debugfs_create_files() function.

- Improve late-register debugfs by rolling out the same debugfs pre-register
  infrastructure for connector and crtc too. That way, the drivers won't need to
  split their setup code into init and register anymore.

- 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_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.

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 [4]_. 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 in Thomas Zimmermann's fbconv tree
[4]_, as well as a tutorial of this process [5]_. The result is a primitive
DRM driver that can run X11 and Weston.

 .. [4] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv
 .. [5] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c

Contact: Thomas Zimmermann <tzimmermann@suse.de>

Level: Advanced