drm/doc: Add TODO list

This commit adds a TODO list to the GPU driver developer's guide. The
content was taken from the DRMJanitors page on the X.Org wiki:

	https://www.x.org/wiki/DRMJanitors/

The goal is to track a list of refactorings that would be nice to see
merged eventually. Sometimes these would be encountered during patch
review on the mailing list, and at other times one can come across
these while working in a specific area of code.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170207175113.2793-1-thierry.reding@gmail.com

1 files changed, 298 insertions, 0 deletions

diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
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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_platform.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
+  wrappers
+* 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
+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
+
+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
+
+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.
+
+* 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.
+
+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
+``udl``.
+
+Contact: Daniel Vetter
+
+Core refactorings
+=================
+
+Use new IDR deletion interface to clean up drm_gem_handle_delete()
+------------------------------------------------------------------
+
+See the "This is gross" comment -- apparently the IDR system now can return an
+error code instead of oopsing.
+
+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``
+directives.
+
+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 DocBook.
+
+See https://dri.freedesktop.org/docs/drm/ 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:
+
+* Make sure legacy IOCTLs can't be used on modern drivers.
+* 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.
+* Extract legacy functions into their own headers and remove it that from the
+  monolithic ``drmP.h`` header.
+* Remove any lingering cruft from the OS abstraction layer from modern
+  drivers.
+
+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
+  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.
+
+* 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
+
+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
+===============
+
+Outside DRM
+===========
+
+Better kerneldoc
+----------------
+
+This is pretty much done, but there's some advanced topics:
+
+Come up with a way to hyperlink to struct members. Currently you can hyperlink
+to the struct using ``#struct_name``, but not to a member within. Would need
+buy-in from kerneldoc maintainers, and the big question is how to make it work
+without totally unsightly
+``drm_foo_bar_really_long_structure->even_longer_memeber`` all over the text
+which breaks text flow.
+
+Figure out how to integrate the asciidoc support for ascii-diagrams. We have a
+few of those (e.g. to describe mode timings), and asciidoc supports converting
+some ascii-art dialect into pngs. Would be really pretty to make that work.
+
+Contact: Daniel Vetter, Jani Nikula
+
+Jani is working on this already, hopefully lands in 4.8.