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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.
Subsystem-wide refactorings
De-midlayer drivers
With the recent ``drm_bus`` cleanup patches for 3.17 it is no longer required
to have a ``drm_bus`` structure set up. Drivers can directly set up the
``drm_device`` structure instead of relying on bus methods in ``drm_usb.c``
and ``drm_pci.c``. The goal is to get rid of the driver's ``->load`` /
``->unload`` callbacks and open-code the load/unload sequence properly, using
the new two-stage ``drm_device`` setup/teardown.
Once all existing drivers are converted we can also remove those bus support
files for USB and platform devices.
All you need is a GPU for a non-converted driver (currently almost all of
them, but also all the virtual ones used by KVM, so everyone qualifies).
Contact: Daniel Vetter, Thierry Reding, respective driver maintainers
Switch from reference/unreference to get/put
For some reason DRM core uses ``reference``/``unreference`` suffixes for
refcounting functions, but kernel uses ``get``/``put`` (e.g.
``kref_get``/``put()``). It would be good to switch over for consistency, and
it's shorter. Needs to be done in 3 steps for each pair of functions:
* Create new ``get``/``put`` functions, define the old names as compatibility
* Switch over each file/driver using a cocci-generated spatch.
* Once all users of the old names are gone, remove them.
This way drivers/patches in the progress of getting merged won't break.
Contact: Daniel Vetter
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
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
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
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
Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
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.
Contact: Daniel Vetter, respective driver maintainers
Better manual-upload support for atomic
This would be especially useful for tinydrm:
- Add a struct drm_rect dirty_clip to drm_crtc_state. When duplicating the
crtc state, clear that to the max values, x/y = 0 and w/h = MAX_INT, in
- Move tinydrm_merge_clips into drm_framebuffer.c, dropping the tinydrm\_
prefix ofc and using drm_fb\_. drm_framebuffer.c makes sense since this
is a function useful to implement the fb->dirty function.
- Create a new drm_fb_dirty function which does essentially what e.g.
mipi_dbi_fb_dirty does. You can use e.g. drm_atomic_helper_update_plane as the
template. But instead of doing a simple full-screen plane update, this new
helper also sets crtc_state->dirty_clip to the right coordinates. And of
course it needs to check whether the fb is actually active (and maybe where),
so there's some book-keeping involved. There's also some good fun involved in
scaling things appropriately. For that case we might simply give up and
declare the entire area covered by the plane as dirty.
Contact: Noralf Trønnes, Daniel Vetter
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.
* 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``.
* There's a new helper ``drm_atomic_helper_best_encoder()`` which could be
used by all atomic drivers which don't select the encoder for a given
connector at runtime. That's almost all of them, and would allow us to get
rid of a lot of ``best_encoder`` boilerplate in drivers.
This was almost done, but new drivers added a few more cases again.
Contact: Daniel Vetter
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_free_object_unlocked`` 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 the
following drivers still use ``struct_mutex``: ``msm``, ``omapdrm`` and
Contact: Daniel Vetter, respective driver maintainers
Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent
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_DEV_* variants of the drm print macros, so we can start to convert
those drivers back to using drm-formwatted 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
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
Contact: Maintainer of the driver you plan to convert
Convert drivers to use drm_fb_helper_fbdev_setup/teardown()
Most drivers can use drm_fb_helper_fbdev_setup() except maybe:
- amdgpu which has special logic to decide whether to call
- armada which isn't atomic and doesn't call
- i915 which calls drm_fb_helper_initial_config() in a worker
Drivers that use drm_framebuffer_remove() to clean up the fbdev framebuffer can
probably use drm_fb_helper_fbdev_teardown().
Contact: Maintainer of the driver you plan to convert
Clean up mmap forwarding
A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers.
And also a lot of them forward dma-buf mmap to the gem mmap implementations.
Would be great to refactor this all into a set of small common helpers.
Contact: Daniel Vetter
Put a reservation_object into drm_gem_object
This would remove the need for the ->gem_prime_res_obj callback. It would also
allow us to implement generic helpers for waiting for a bo, allowing for quite a
bit of refactoring in the various wait ioctl implementations.
Contact: Daniel Vetter
DRM core&drivers uses a lot of idr (integer lookup directories) for mapping
userspace IDs to internal objects, and in most places ID=0 means NULL and hence
is never used. Switching to idr_init_base() for these would make the idr more
Contact: Daniel Vetter
Core refactorings
Clean up the DRM header mess
Currently the DRM subsystem has only one global header, ``drmP.h``. This is
used both for functions exported to helper libraries and drivers and functions
only used internally in the ``drm.ko`` module. The goal would be to move all
header declarations not needed outside of ``drm.ko`` into
``drivers/gpu/drm/drm_*_internal.h`` header files. ``EXPORT_SYMBOL`` also
needs to be dropped for these functions.
This would nicely tie in with the below task to create kerneldoc after the API
is cleaned up. Or with the "hide legacy cruft better" task.
Note that this is well in progress, but ``drmP.h`` is still huge. The updated
plan is to switch to per-file driver API headers, which will also structure
the kerneldoc better. This should also allow more fine-grained ``#include``
In the end no .c file should need to include ``drmP.h`` anymore.
Contact: Daniel Vetter
Add missing kerneldoc for exported functions
The DRM reference documentation is still lacking kerneldoc in a few areas. The
task would be to clean up interfaces like moving functions around between
files to better group them and improving the interfaces like dropping return
values for functions that never fail. Then write kerneldoc for all exported
functions and an overview section and integrate it all into the drm book.
See for what's there already.
Contact: Daniel Vetter
Hide legacy cruft better
Way back DRM supported only drivers which shadow-attached to PCI devices with
userspace or fbdev drivers setting up outputs. Modern DRM drivers take charge
of the entire device, you can spot them with the DRIVER_MODESET flag.
Unfortunately there's still large piles of legacy code around which needs to
be hidden so that driver writers don't accidentally end up using it. And to
prevent security issues in those legacy IOCTLs from being exploited on modern
drivers. This has multiple possible subtasks:
* Extract support code for legacy features into a ``drm-legacy.ko`` kernel
module and compile it only when one of the legacy drivers is enabled.
This is mostly done, the only thing left is to split up ``drm_irq.c`` into
legacy cruft and the parts needed by modern KMS drivers.
Contact: Daniel Vetter
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 have one, but on top of that the fbcon code itself also has one. We
need to make sure that they stop fighting over each another.
* ``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
* 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.
* For the above locking troubles reasons it's pretty much impossible to
attempt a synchronous modeset from panic handlers. The only thing we could
try to achive is an atomic ``set_base`` of the primary plane, and hope that
it shows up. Everything else probably needs to be delayed to some worker or
something else which happens later on. Otherwise it just kills the box
harder, prevent the panic from going out on e.g. netconsole.
* There's also proposal for a simplied DRM console instead of the full-blown
fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
obviously work for both console, in case we ever get kmslog merged.
Contact: Daniel Vetter
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
Contact: Daniel Vetter
KMS cleanups
Some of these date from the very introduction of KMS in 2008 ...
- drm_mode_config.crtc_idr is misnamed, since it contains all KMS object. Should
be renamed to drm_mode_config.object_idr.
- drm_display_mode doesn't need to be derived from drm_mode_object. That's
leftovers from older (never merged into upstream) KMS designs where modes
where set using their ID, including support to add/remove modes.
Better Testing
Enable trinity for DRM
And fix up the fallout. Should be really interesting ...
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.
Contact: Daniel Vetter
Create a virtual KMS driver for testing (vkms)
With all the latest helpers it should be fairly simple to create a virtual KMS
driver useful for testing, or for running X or similar on headless machines
(to be able to still use the GPU). This would be similar to vgem, but aimed at
the modeset side.
Once the basics are there there's tons of possibilities to extend it.
Contact: Daniel Vetter
Driver Specific
Tinydrm is the helper driver for really simple fb drivers. The goal is to make
those drivers as simple as possible, so lots of room for refactoring:
- backlight helpers, probably best to put them into a new drm_backlight.c.
This is because drivers/video is de-facto unmaintained. We could also
move drivers/video/backlight to drivers/gpu/backlight and take it all
over within drm-misc, but that's more work. Backlight helpers require a fair
bit of reworking and refactoring. A simple example is the enabling of a backlight.
Tinydrm has helpers for this. It would be good if other drivers can also use the
helper. However, there are various cases we need to consider i.e different
drivers seem to have different ways of enabling/disabling a backlight.
We also need to consider the backlight drivers (like gpio_backlight). The situation
is further complicated by the fact that the backlight is tied to fbdev
via fb_notifier_callback() which has complicated logic. For further details, refer
to the following discussion thread:!topic/outreachy-kernel/8rBe30lwtdA
- spi helpers, probably best put into spi core/helper code. Thierry said
the spi maintainer is fast&reactive, so shouldn't be a big issue.
- extract the mipi-dbi helper (well, the non-tinydrm specific parts at
least) into a separate helper, like we have for mipi-dsi already. Or follow
one of the ideas for having a shared dsi/dbi helper, abstracting away the
transport details more.
- tinydrm_gem_cma_prime_import_sg_table should probably go into the cma
helpers, as a _vmapped variant (since not every driver needs the vmap).
And tinydrm_gem_cma_free_object could the be merged into
- tinydrm_fb_create we could move into drm_simple_pipe, only need to add
the fb_create hook to drm_simple_pipe_funcs, which would again simplify a
bunch of things (since it gives you a one-stop vfunc for simple drivers).
- Quick aside: The unregister devm stuff is kinda getting the lifetimes of
a drm_device wrong. Doesn't matter, since everyone else gets it wrong
too :-)
- also rework the drm_framebuffer_funcs->dirty hook wire-up, see above.
Contact: Noralf Trønnes, Daniel Vetter
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
- Our early/late pm callbacks could be removed in favour of using
device_link_add to model the dependency between i915 and snd_had. See
Outside DRM