395 lines
17 KiB
ReStructuredText
395 lines
17 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_pci.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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Better manual-upload support for atomic
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---------------------------------------
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This would be especially useful for tinydrm:
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- Add a struct drm_rect dirty_clip to drm_crtc_state. When duplicating the
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crtc state, clear that to the max values, x/y = 0 and w/h = MAX_INT, in
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__drm_atomic_helper_crtc_duplicate_state().
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- Move tinydrm_merge_clips into drm_framebuffer.c, dropping the tinydrm\_
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prefix ofc and using drm_fb\_. drm_framebuffer.c makes sense since this
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is a function useful to implement the fb->dirty function.
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- Create a new drm_fb_dirty function which does essentially what e.g.
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mipi_dbi_fb_dirty does. You can use e.g. drm_atomic_helper_update_plane as the
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template. But instead of doing a simple full-screen plane update, this new
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helper also sets crtc_state->dirty_clip to the right coordinates. And of
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course it needs to check whether the fb is actually active (and maybe where),
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so there's some book-keeping involved. There's also some good fun involved in
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scaling things appropriately. For that case we might simply give up and
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declare the entire area covered by the plane as dirty.
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Contact: Noralf Trønnes, Daniel Vetter
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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, respective driver maintainers
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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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In the end no .c file should need to include ``drmP.h`` anymore.
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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 book.
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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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* 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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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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Clean up the debugfs support
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----------------------------
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There's a bunch of issues with it:
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- The drm_info_list ->show() function doesn't even bother to cast to the drm
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structure for you. This is lazy.
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- We probably want to have some support for debugfs files on crtc/connectors and
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maybe other kms objects directly in core. There's even drm_print support in
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the funcs for these objects to dump kms state, so it's all there. And then the
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->show() functions should obviously give you a pointer to the right object.
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- The drm_info_list stuff is centered on drm_minor instead of drm_device. For
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anything we want to print drm_device (or maybe drm_file) is the right thing.
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- The drm_driver->debugfs_init hooks we have is just an artifact of the old
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midlayered load sequence. DRM debugfs should work more like sysfs, where you
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can create properties/files for an object anytime you want, and the core
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takes care of publishing/unpuplishing all the files at register/unregister
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time. Drivers shouldn't need to worry about these technicalities, and fixing
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this (together with the drm_minor->drm_device move) would allow us to remove
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debugfs_init.
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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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tinydrm
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-------
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Tinydrm is the helper driver for really simple fb drivers. The goal is to make
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those drivers as simple as possible, so lots of room for refactoring:
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- backlight helpers, probably best to put them into a new drm_backlight.c.
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This is because drivers/video is de-facto unmaintained. We could also
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move drivers/video/backlight to drivers/gpu/backlight and take it all
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over within drm-misc, but that's more work.
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- spi helpers, probably best put into spi core/helper code. Thierry said
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the spi maintainer is fast&reactive, so shouldn't be a big issue.
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- extract the mipi-dbi helper (well, the non-tinydrm specific parts at
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least) into a separate helper, like we have for mipi-dsi already. Or follow
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one of the ideas for having a shared dsi/dbi helper, abstracting away the
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transport details more.
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- tinydrm_lastclose could be drm_fb_helper_lastclose. Only thing we need
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for that is to store the drm_fb_helper pointer somewhere in
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drm_device->mode_config. And then we could roll that out to all the
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drivers.
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- tinydrm_gem_cma_prime_import_sg_table should probably go into the cma
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helpers, as a _vmapped variant (since not every driver needs the vmap).
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And tinydrm_gem_cma_free_object could the be merged into
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drm_gem_cma_free_object().
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- tinydrm_fb_create we could move into drm_simple_pipe, only need to add
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the fb_create hook to drm_simple_pipe_funcs, which would again simplify a
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bunch of things (since it gives you a one-stop vfunc for simple drivers).
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- Quick aside: The unregister devm stuff is kinda getting the lifetimes of
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a drm_device wrong. Doesn't matter, since everyone else gets it wrong
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too :-)
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- With the fbdev pointer in dev->mode_config we could also make
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suspend/resume helpers entirely generic, at least if we add a
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dev->mode_config.suspend_state. We could even provide a generic pm_ops
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structure with those.
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- also rework the drm_framebuffer_funcs->dirty hook wire-up, see above.
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Contact: Noralf Trønnes, Daniel Vetter
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Outside DRM
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===========
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