322 lines
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ReStructuredText
322 lines
14 KiB
ReStructuredText
.. _todo:
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=========
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TODO list
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=========
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This section contains a list of smaller janitorial tasks in the kernel DRM
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graphics subsystem useful as newbie projects. Or for slow rainy days.
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Subsystem-wide refactorings
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===========================
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De-midlayer drivers
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-------------------
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With the recent ``drm_bus`` cleanup patches for 3.17 it is no longer required
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to have a ``drm_bus`` structure set up. Drivers can directly set up the
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``drm_device`` structure instead of relying on bus methods in ``drm_usb.c``
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and ``drm_platform.c``. The goal is to get rid of the driver's ``->load`` /
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``->unload`` callbacks and open-code the load/unload sequence properly, using
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the new two-stage ``drm_device`` setup/teardown.
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Once all existing drivers are converted we can also remove those bus support
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files for USB and platform devices.
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All you need is a GPU for a non-converted driver (currently almost all of
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them, but also all the virtual ones used by KVM, so everyone qualifies).
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Contact: Daniel Vetter, Thierry Reding, respective driver maintainers
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Switch from reference/unreference to get/put
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--------------------------------------------
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For some reason DRM core uses ``reference``/``unreference`` suffixes for
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refcounting functions, but kernel uses ``get``/``put`` (e.g.
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``kref_get``/``put()``). It would be good to switch over for consistency, and
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it's shorter. Needs to be done in 3 steps for each pair of functions:
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* Create new ``get``/``put`` functions, define the old names as compatibility
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wrappers
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* Switch over each file/driver using a cocci-generated spatch.
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* Once all users of the old names are gone, remove them.
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This way drivers/patches in the progress of getting merged won't break.
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Contact: Daniel Vetter
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Convert existing KMS drivers to atomic modesetting
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--------------------------------------------------
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3.19 has the atomic modeset interfaces and helpers, so drivers can now be
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converted over. Modern compositors like Wayland or Surfaceflinger on Android
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really want an atomic modeset interface, so this is all about the bright
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future.
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There is a conversion guide for atomic and all you need is a GPU for a
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non-converted driver (again virtual HW drivers for KVM are still all
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suitable).
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As part of this drivers also need to convert to universal plane (which means
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exposing primary & cursor as proper plane objects). But that's much easier to
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do by directly using the new atomic helper driver callbacks.
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Contact: Daniel Vetter, respective driver maintainers
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Clean up the clipped coordination confusion around planes
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---------------------------------------------------------
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We have a helper to get this right with drm_plane_helper_check_update(), but
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it's not consistently used. This should be fixed, preferrably in the atomic
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helpers (and drivers then moved over to clipped coordinates). Probably the
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helper should also be moved from drm_plane_helper.c to the atomic helpers, to
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avoid confusion - the other helpers in that file are all deprecated legacy
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helpers.
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Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
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Implement deferred fbdev setup in the helper
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--------------------------------------------
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Many (especially embedded drivers) want to delay fbdev setup until there's a
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real screen plugged in. This is to avoid the dreaded fallback to the low-res
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fbdev default. Many drivers have a hacked-up (and often broken) version of this,
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better to do it once in the shared helpers. Thierry has a patch series, but that
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one needs to be rebased and final polish applied.
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Contact: Thierry Reding, Daniel Vetter, driver maintainers
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Convert early atomic drivers to async commit helpers
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----------------------------------------------------
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For the first year the atomic modeset helpers didn't support asynchronous /
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nonblocking commits, and every driver had to hand-roll them. This is fixed
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now, but there's still a pile of existing drivers that easily could be
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converted over to the new infrastructure.
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One issue with the helpers is that they require that drivers handle completion
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events for atomic commits correctly. But fixing these bugs is good anyway.
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Contact: Daniel Vetter, respective driver maintainers
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Fallout from atomic KMS
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-----------------------
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``drm_atomic_helper.c`` provides a batch of functions which implement legacy
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IOCTLs on top of the new atomic driver interface. Which is really nice for
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gradual conversion of drivers, but unfortunately the semantic mismatches are
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a bit too severe. So there's some follow-up work to adjust the function
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interfaces to fix these issues:
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* atomic needs the lock acquire context. At the moment that's passed around
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implicitly with some horrible hacks, and it's also allocate with
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``GFP_NOFAIL`` behind the scenes. All legacy paths need to start allocating
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the acquire context explicitly on stack and then also pass it down into
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drivers explicitly so that the legacy-on-atomic functions can use them.
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* A bunch of the vtable hooks are now in the wrong place: DRM has a split
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between core vfunc tables (named ``drm_foo_funcs``), which are used to
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implement the userspace ABI. And then there's the optional hooks for the
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helper libraries (name ``drm_foo_helper_funcs``), which are purely for
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internal use. Some of these hooks should be move from ``_funcs`` to
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``_helper_funcs`` since they are not part of the core ABI. There's a
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``FIXME`` comment in the kerneldoc for each such case in ``drm_crtc.h``.
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* There's a new helper ``drm_atomic_helper_best_encoder()`` which could be
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used by all atomic drivers which don't select the encoder for a given
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connector at runtime. That's almost all of them, and would allow us to get
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rid of a lot of ``best_encoder`` boilerplate in drivers.
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Contact: Daniel Vetter
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Get rid of dev->struct_mutex from GEM drivers
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---------------------------------------------
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``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
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everything. Nowadays in modern drivers the only bit where it's mandatory is
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serializing GEM buffer object destruction. Which unfortunately means drivers
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have to keep track of that lock and either call ``unreference`` or
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``unreference_locked`` depending upon context.
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Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
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and there's a ``gem_free_object_unlocked`` callback for any drivers which are
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entirely ``struct_mutex`` free.
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For drivers that need ``struct_mutex`` it should be replaced with a driver-
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private lock. The tricky part is the BO free functions, since those can't
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reliably take that lock any more. Instead state needs to be protected with
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suitable subordinate locks or some cleanup work pushed to a worker thread. For
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performance-critical drivers it might also be better to go with a more
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fine-grained per-buffer object and per-context lockings scheme. Currently the
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following drivers still use ``struct_mutex``: ``msm``, ``omapdrm`` and
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``udl``.
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Contact: Daniel Vetter
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Core refactorings
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=================
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Use new IDR deletion interface to clean up drm_gem_handle_delete()
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------------------------------------------------------------------
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See the "This is gross" comment -- apparently the IDR system now can return an
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error code instead of oopsing.
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Clean up the DRM header mess
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----------------------------
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Currently the DRM subsystem has only one global header, ``drmP.h``. This is
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used both for functions exported to helper libraries and drivers and functions
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only used internally in the ``drm.ko`` module. The goal would be to move all
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header declarations not needed outside of ``drm.ko`` into
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``drivers/gpu/drm/drm_*_internal.h`` header files. ``EXPORT_SYMBOL`` also
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needs to be dropped for these functions.
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This would nicely tie in with the below task to create kerneldoc after the API
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is cleaned up. Or with the "hide legacy cruft better" task.
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Note that this is well in progress, but ``drmP.h`` is still huge. The updated
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plan is to switch to per-file driver API headers, which will also structure
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the kerneldoc better. This should also allow more fine-grained ``#include``
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directives.
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Contact: Daniel Vetter
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Add missing kerneldoc for exported functions
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--------------------------------------------
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The DRM reference documentation is still lacking kerneldoc in a few areas. The
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task would be to clean up interfaces like moving functions around between
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files to better group them and improving the interfaces like dropping return
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values for functions that never fail. Then write kerneldoc for all exported
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functions and an overview section and integrate it all into the drm DocBook.
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See https://dri.freedesktop.org/docs/drm/ for what's there already.
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Contact: Daniel Vetter
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Hide legacy cruft better
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------------------------
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Way back DRM supported only drivers which shadow-attached to PCI devices with
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userspace or fbdev drivers setting up outputs. Modern DRM drivers take charge
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of the entire device, you can spot them with the DRIVER_MODESET flag.
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Unfortunately there's still large piles of legacy code around which needs to
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be hidden so that driver writers don't accidentally end up using it. And to
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prevent security issues in those legacy IOCTLs from being exploited on modern
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drivers. This has multiple possible subtasks:
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* Make sure legacy IOCTLs can't be used on modern drivers.
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* Extract support code for legacy features into a ``drm-legacy.ko`` kernel
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module and compile it only when one of the legacy drivers is enabled.
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* Extract legacy functions into their own headers and remove it that from the
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monolithic ``drmP.h`` header.
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* Remove any lingering cruft from the OS abstraction layer from modern
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drivers.
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This is mostly done, the only thing left is to split up ``drm_irq.c`` into
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legacy cruft and the parts needed by modern KMS drivers.
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Contact: Daniel Vetter
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Make panic handling work
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------------------------
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This is a really varied tasks with lots of little bits and pieces:
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* The panic path can't be tested currently, leading to constant breaking. The
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main issue here is that panics can be triggered from hardirq contexts and
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hence all panic related callback can run in hardirq context. It would be
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awesome if we could test at least the fbdev helper code and driver code by
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e.g. trigger calls through drm debugfs files. hardirq context could be
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achieved by using an IPI to the local processor.
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* There's a massive confusion of different panic handlers. DRM fbdev emulation
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helpers have one, but on top of that the fbcon code itself also has one. We
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need to make sure that they stop fighting over each another.
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* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
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isn't a full solution for panic paths. We need to make sure that it only
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returns true if there's a panic going on for real, and fix up all the
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fallout.
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* The panic handler must never sleep, which also means it can't ever
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``mutex_lock()``. Also it can't grab any other lock unconditionally, not
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even spinlocks (because NMI and hardirq can panic too). We need to either
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make sure to not call such paths, or trylock everything. Really tricky.
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* For the above locking troubles reasons it's pretty much impossible to
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attempt a synchronous modeset from panic handlers. The only thing we could
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try to achive is an atomic ``set_base`` of the primary plane, and hope that
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it shows up. Everything else probably needs to be delayed to some worker or
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something else which happens later on. Otherwise it just kills the box
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harder, prevent the panic from going out on e.g. netconsole.
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* There's also proposal for a simplied DRM console instead of the full-blown
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fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
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obviously work for both console, in case we ever get kmslog merged.
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Contact: Daniel Vetter
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Better Testing
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==============
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Enable trinity for DRM
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----------------------
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And fix up the fallout. Should be really interesting ...
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Make KMS tests in i-g-t generic
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-------------------------------
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The i915 driver team maintains an extensive testsuite for the i915 DRM driver,
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including tons of testcases for corner-cases in the modesetting API. It would
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be awesome if those tests (at least the ones not relying on Intel-specific GEM
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features) could be made to run on any KMS driver.
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Basic work to run i-g-t tests on non-i915 is done, what's now missing is mass-
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converting things over. For modeset tests we also first need a bit of
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infrastructure to use dumb buffers for untiled buffers, to be able to run all
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the non-i915 specific modeset tests.
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Contact: Daniel Vetter
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Create a virtual KMS driver for testing (vkms)
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----------------------------------------------
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With all the latest helpers it should be fairly simple to create a virtual KMS
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driver useful for testing, or for running X or similar on headless machines
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(to be able to still use the GPU). This would be similar to vgem, but aimed at
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the modeset side.
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Once the basics are there there's tons of possibilities to extend it.
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Contact: Daniel Vetter
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Driver Specific
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===============
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Outside DRM
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===========
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Better kerneldoc
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----------------
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This is pretty much done, but there's some advanced topics:
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Come up with a way to hyperlink to struct members. Currently you can hyperlink
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to the struct using ``#struct_name``, but not to a member within. Would need
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buy-in from kerneldoc maintainers, and the big question is how to make it work
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without totally unsightly
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``drm_foo_bar_really_long_structure->even_longer_memeber`` all over the text
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which breaks text flow.
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Figure out how to integrate the asciidoc support for ascii-diagrams. We have a
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few of those (e.g. to describe mode timings), and asciidoc supports converting
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some ascii-art dialect into pngs. Would be really pretty to make that work.
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Contact: Daniel Vetter, Jani Nikula
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Jani is working on this already, hopefully lands in 4.8.
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