todo.rst



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

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implementation (there's lots of outdated locking leftovers in various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

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

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

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
__drm_atomic_helper_crtc_duplicate_state().
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. 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

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 only the
msm driver still use struct_mutex.
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
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

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
drm_helper_disable_unused_functions()
armada which isn't atomic and doesn't call
drm_helper_disable_unused_functions()
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