--- /dev/null
+.. _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.
projects / GitHub / moto-9609 / android_kernel_motorola_exynos9610.git / commitdiff