781 lines
30 KiB
ReStructuredText
781 lines
30 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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Difficulty
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----------
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To make it easier task are categorized into different levels:
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Starter: Good tasks to get started with the DRM subsystem.
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Intermediate: Tasks which need some experience with working in the DRM
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subsystem, or some specific GPU/display graphics knowledge. For debugging issue
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it's good to have the relevant hardware (or a virtual driver set up) available
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for testing.
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Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem
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and graphics topics. Generally need the relevant hardware for development and
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testing.
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Expert: Only attempt these if you've successfully completed some tricky
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refactorings already and are an expert in the specific area
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Subsystem-wide refactorings
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===========================
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Remove custom dumb_map_offset implementations
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---------------------------------------------
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All GEM based drivers should be using drm_gem_create_mmap_offset() instead.
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Audit each individual driver, make sure it'll work with the generic
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implementation (there's lots of outdated locking leftovers in various
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implementations), and then remove it.
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Contact: Daniel Vetter, respective driver maintainers
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Level: Intermediate
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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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Level: Advanced
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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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Level: Advanced
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Improve plane atomic_check helpers
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----------------------------------
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Aside from the clipped coordinates right above there's a few suboptimal things
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with the current helpers:
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- drm_plane_helper_funcs->atomic_check gets called for enabled or disabled
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planes. At best this seems to confuse drivers, worst it means they blow up
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when the plane is disabled without the CRTC. The only special handling is
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resetting values in the plane state structures, which instead should be moved
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into the drm_plane_funcs->atomic_duplicate_state functions.
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- Once that's done, helpers could stop calling ->atomic_check for disabled
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planes.
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- Then we could go through all the drivers and remove the more-or-less confused
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checks for plane_state->fb and plane_state->crtc.
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Contact: Daniel Vetter
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Level: Advanced
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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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Somewhat related is the legacy_cursor_update hack, which should be replaced with
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the new atomic_async_check/commit functionality in the helpers in drivers that
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still look at that flag.
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Contact: Daniel Vetter, respective driver maintainers
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Level: Advanced
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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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Except for some driver code this is done. This task should be finished by
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adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().
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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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Contact: Daniel Vetter
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Level: Intermediate
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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 object ``free`` 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 only
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the ``msm`` and `i915` drivers use ``struct_mutex``.
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Contact: Daniel Vetter, respective driver maintainers
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Level: Advanced
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Move Buffer Object Locking to dma_resv_lock()
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---------------------------------------------
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Many drivers have their own per-object locking scheme, usually using
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mutex_lock(). This causes all kinds of trouble for buffer sharing, since
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depending which driver is the exporter and importer, the locking hierarchy is
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reversed.
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To solve this we need one standard per-object locking mechanism, which is
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dma_resv_lock(). This lock needs to be called as the outermost lock, with all
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other driver specific per-object locks removed. The problem is tha rolling out
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the actual change to the locking contract is a flag day, due to struct dma_buf
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buffer sharing.
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Level: Expert
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Convert logging to drm_* functions with drm_device paramater
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------------------------------------------------------------
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For drivers which could have multiple instances, it is necessary to
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differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR
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don't do this, drivers used dev_info/warn/err to make this differentiation. We
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now have drm_* variants of the drm print functions, so we can start to convert
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those drivers back to using drm-formatted specific log messages.
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Before you start this conversion please contact the relevant maintainers to make
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sure your work will be merged - not everyone agrees that the DRM dmesg macros
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are better.
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Contact: Sean Paul, Maintainer of the driver you plan to convert
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Level: Starter
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Convert drivers to use simple modeset suspend/resume
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----------------------------------------------------
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Most drivers (except i915 and nouveau) that use
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drm_atomic_helper_suspend/resume() can probably be converted to use
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drm_mode_config_helper_suspend/resume(). Also there's still open-coded version
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of the atomic suspend/resume code in older atomic modeset drivers.
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Contact: Maintainer of the driver you plan to convert
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Level: Intermediate
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Convert drivers to use drm_fbdev_generic_setup()
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------------------------------------------------
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Most drivers can use drm_fbdev_generic_setup(). Driver have to implement
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atomic modesetting and GEM vmap support. Historically, generic fbdev emulation
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expected the framebuffer in system memory or system-like memory. By employing
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struct iosys_map, drivers with frambuffers in I/O memory can be supported
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as well.
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Contact: Maintainer of the driver you plan to convert
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Level: Intermediate
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Reimplement functions in drm_fbdev_fb_ops without fbdev
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-------------------------------------------------------
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A number of callback functions in drm_fbdev_fb_ops could benefit from
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being rewritten without dependencies on the fbdev module. Some of the
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helpers could further benefit from using struct iosys_map instead of
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raw pointers.
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Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
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Level: Advanced
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Benchmark and optimize blitting and format-conversion function
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--------------------------------------------------------------
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Drawing to dispay memory quickly is crucial for many applications'
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performance.
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On at least x86-64, sys_imageblit() is significantly slower than
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cfb_imageblit(), even though both use the same blitting algorithm and
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the latter is written for I/O memory. It turns out that cfb_imageblit()
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uses movl instructions, while sys_imageblit apparently does not. This
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seems to be a problem with gcc's optimizer. DRM's format-conversion
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helpers might be subject to similar issues.
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Benchmark and optimize fbdev's sys_() helpers and DRM's format-conversion
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helpers. In cases that can be further optimized, maybe implement a different
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algorithm. For micro-optimizations, use movl/movq instructions explicitly.
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That might possibly require architecture-specific helpers (e.g., storel()
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storeq()).
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Contact: Thomas Zimmermann <tzimmermann@suse.de>
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Level: Intermediate
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drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup
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-----------------------------------------------------------------
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A lot more drivers could be switched over to the drm_gem_framebuffer helpers.
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Various hold-ups:
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- Need to switch over to the generic dirty tracking code using
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drm_atomic_helper_dirtyfb first (e.g. qxl).
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- Need to switch to drm_fbdev_generic_setup(), otherwise a lot of the custom fb
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setup code can't be deleted.
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- Many drivers wrap drm_gem_fb_create() only to check for valid formats. For
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atomic drivers we could check for valid formats by calling
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drm_plane_check_pixel_format() against all planes, and pass if any plane
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supports the format. For non-atomic that's not possible since like the format
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list for the primary plane is fake and we'd therefor reject valid formats.
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- Many drivers subclass drm_framebuffer, we'd need a embedding compatible
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version of the varios drm_gem_fb_create functions. Maybe called
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drm_gem_fb_create/_with_dirty/_with_funcs as needed.
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Contact: Daniel Vetter
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Level: Intermediate
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Generic fbdev defio support
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---------------------------
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The defio support code in the fbdev core has some very specific requirements,
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which means drivers need to have a special framebuffer for fbdev. The main
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issue is that it uses some fields in struct page itself, which breaks shmem
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gem objects (and other things). To support defio, affected drivers require
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the use of a shadow buffer, which may add CPU and memory overhead.
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Possible solution would be to write our own defio mmap code in the drm fbdev
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emulation. It would need to fully wrap the existing mmap ops, forwarding
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everything after it has done the write-protect/mkwrite trickery:
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- In the drm_fbdev_fb_mmap helper, if we need defio, change the
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default page prots to write-protected with something like this::
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vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
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- Set the mkwrite and fsync callbacks with similar implementions to the core
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fbdev defio stuff. These should all work on plain ptes, they don't actually
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require a struct page. uff. These should all work on plain ptes, they don't
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actually require a struct page.
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- Track the dirty pages in a separate structure (bitfield with one bit per page
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should work) to avoid clobbering struct page.
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Might be good to also have some igt testcases for this.
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Contact: Daniel Vetter, Noralf Tronnes
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Level: Advanced
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struct drm_gem_object_funcs
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---------------------------
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GEM objects can now have a function table instead of having the callbacks on the
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DRM driver struct. This is now the preferred way. Callbacks in drivers have been
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converted, except for struct drm_driver.gem_prime_mmap.
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Level: Intermediate
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connector register/unregister fixes
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-----------------------------------
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- For most connectors it's a no-op to call drm_connector_register/unregister
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directly from driver code, drm_dev_register/unregister take care of this
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already. We can remove all of them.
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- For dp drivers it's a bit more a mess, since we need the connector to be
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registered when calling drm_dp_aux_register. Fix this by instead calling
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drm_dp_aux_init, and moving the actual registering into a late_register
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callback as recommended in the kerneldoc.
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Level: Intermediate
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Remove load/unload callbacks from all non-DRIVER_LEGACY drivers
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---------------------------------------------------------------
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The load/unload callbacks in struct &drm_driver are very much midlayers, plus
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for historical reasons they get the ordering wrong (and we can't fix that)
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between setting up the &drm_driver structure and calling drm_dev_register().
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- Rework drivers to no longer use the load/unload callbacks, directly coding the
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load/unload sequence into the driver's probe function.
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- Once all non-DRIVER_LEGACY drivers are converted, disallow the load/unload
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callbacks for all modern drivers.
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Contact: Daniel Vetter
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Level: Intermediate
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Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi
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---------------------------------------------------------------
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Once EDID is parsed, the monitor HDMI support information is available through
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drm_display_info.is_hdmi. Many drivers still call drm_detect_hdmi_monitor() to
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retrieve the same information, which is less efficient.
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Audit each individual driver calling drm_detect_hdmi_monitor() and switch to
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drm_display_info.is_hdmi if applicable.
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Contact: Laurent Pinchart, respective driver maintainers
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Level: Intermediate
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Consolidate custom driver modeset properties
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--------------------------------------------
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Before atomic modeset took place, many drivers where creating their own
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properties. Among other things, atomic brought the requirement that custom,
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driver specific properties should not be used.
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For this task, we aim to introduce core helpers or reuse the existing ones
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if available:
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A quick, unconfirmed, examples list.
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Introduce core helpers:
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- audio (amdgpu, intel, gma500, radeon)
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- brightness, contrast, etc (armada, nouveau) - overlay only (?)
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- broadcast rgb (gma500, intel)
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- colorkey (armada, nouveau, rcar) - overlay only (?)
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- dither (amdgpu, nouveau, radeon) - varies across drivers
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- underscan family (amdgpu, radeon, nouveau)
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Already in core:
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- colorspace (sti)
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- tv format names, enhancements (gma500, intel)
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- tv overscan, margins, etc. (gma500, intel)
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- zorder (omapdrm) - same as zpos (?)
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Contact: Emil Velikov, respective driver maintainers
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Level: Intermediate
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Use struct iosys_map throughout codebase
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----------------------------------------
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Pointers to shared device memory are stored in struct iosys_map. Each
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instance knows whether it refers to system or I/O memory. Most of the DRM-wide
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interface have been converted to use struct iosys_map, but implementations
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often still use raw pointers.
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The task is to use struct iosys_map where it makes sense.
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* Memory managers should use struct iosys_map for dma-buf-imported buffers.
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* TTM might benefit from using struct iosys_map internally.
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* Framebuffer copying and blitting helpers should operate on struct iosys_map.
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Contact: Thomas Zimmermann <tzimmermann@suse.de>, Christian König, Daniel Vetter
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Level: Intermediate
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Review all drivers for setting struct drm_mode_config.{max_width,max_height} correctly
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--------------------------------------------------------------------------------------
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The values in struct drm_mode_config.{max_width,max_height} describe the
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maximum supported framebuffer size. It's the virtual screen size, but many
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drivers treat it like limitations of the physical resolution.
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The maximum width depends on the hardware's maximum scanline pitch. The
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maximum height depends on the amount of addressable video memory. Review all
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drivers to initialize the fields to the correct values.
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Contact: Thomas Zimmermann <tzimmermann@suse.de>
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Level: Intermediate
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Request memory regions in all drivers
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-------------------------------------
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Go through all drivers and add code to request the memory regions that the
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driver uses. This requires adding calls to request_mem_region(),
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pci_request_region() or similar functions. Use helpers for managed cleanup
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where possible.
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Drivers are pretty bad at doing this and there used to be conflicts among
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DRM and fbdev drivers. Still, it's the correct thing to do.
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Contact: Thomas Zimmermann <tzimmermann@suse.de>
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Level: Starter
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Core refactorings
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=================
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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 had their own (long removed), but on top of that the fbcon code itself
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also has one. We need to make sure that they stop fighting over each other.
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This is worked around by checking ``oops_in_progress`` at various entry points
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into the DRM fbdev emulation helpers. A much cleaner approach here would be to
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switch fbcon to the `threaded printk support
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<https://lwn.net/Articles/800946/>`_.
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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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* A clean solution would be an entirely separate panic output support in KMS,
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bypassing the current fbcon support. See `[PATCH v2 0/3] drm: Add panic handling
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<https://lore.kernel.org/dri-devel/20190311174218.51899-1-noralf@tronnes.org/>`_.
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* Encoding the actual oops and preceding dmesg in a QR might help with the
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dread "important stuff scrolled away" problem. See `[RFC][PATCH] Oops messages
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transfer using QR codes
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<https://lore.kernel.org/lkml/1446217392-11981-1-git-send-email-alexandru.murtaza@intel.com/>`_
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for some example code that could be reused.
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Contact: Daniel Vetter
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Level: Advanced
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Clean up the debugfs support
|
|
----------------------------
|
|
|
|
There's a bunch of issues with it:
|
|
|
|
- The drm_info_list ->show() function doesn't even bother to cast to the drm
|
|
structure for you. This is lazy.
|
|
|
|
- We probably want to have some support for debugfs files on crtc/connectors and
|
|
maybe other kms objects directly in core. There's even drm_print support in
|
|
the funcs for these objects to dump kms state, so it's all there. And then the
|
|
->show() functions should obviously give you a pointer to the right object.
|
|
|
|
- The drm_info_list stuff is centered on drm_minor instead of drm_device. For
|
|
anything we want to print drm_device (or maybe drm_file) is the right thing.
|
|
|
|
- The drm_driver->debugfs_init hooks we have is just an artifact of the old
|
|
midlayered load sequence. DRM debugfs should work more like sysfs, where you
|
|
can create properties/files for an object anytime you want, and the core
|
|
takes care of publishing/unpuplishing all the files at register/unregister
|
|
time. Drivers shouldn't need to worry about these technicalities, and fixing
|
|
this (together with the drm_minor->drm_device move) would allow us to remove
|
|
debugfs_init.
|
|
|
|
Previous RFC that hasn't landed yet: https://lore.kernel.org/dri-devel/20200513114130.28641-2-wambui.karugax@gmail.com/
|
|
|
|
Contact: Daniel Vetter
|
|
|
|
Level: Intermediate
|
|
|
|
Object lifetime fixes
|
|
---------------------
|
|
|
|
There's two related issues here
|
|
|
|
- Cleanup up the various ->destroy callbacks, which often are all the same
|
|
simple code.
|
|
|
|
- Lots of drivers erroneously allocate DRM modeset objects using devm_kzalloc,
|
|
which results in use-after free issues on driver unload. This can be serious
|
|
trouble even for drivers for hardware integrated on the SoC due to
|
|
EPROBE_DEFERRED backoff.
|
|
|
|
Both these problems can be solved by switching over to drmm_kzalloc(), and the
|
|
various convenience wrappers provided, e.g. drmm_crtc_alloc_with_planes(),
|
|
drmm_universal_plane_alloc(), ... and so on.
|
|
|
|
Contact: Daniel Vetter
|
|
|
|
Level: Intermediate
|
|
|
|
Remove automatic page mapping from dma-buf importing
|
|
----------------------------------------------------
|
|
|
|
When importing dma-bufs, the dma-buf and PRIME frameworks automatically map
|
|
imported pages into the importer's DMA area. drm_gem_prime_fd_to_handle() and
|
|
drm_gem_prime_handle_to_fd() require that importers call dma_buf_attach()
|
|
even if they never do actual device DMA, but only CPU access through
|
|
dma_buf_vmap(). This is a problem for USB devices, which do not support DMA
|
|
operations.
|
|
|
|
To fix the issue, automatic page mappings should be removed from the
|
|
buffer-sharing code. Fixing this is a bit more involved, since the import/export
|
|
cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over
|
|
this problem for USB devices by fishing out the USB host controller device, as
|
|
long as that supports DMA. Otherwise importing can still needlessly fail.
|
|
|
|
Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
|
|
|
|
Level: Advanced
|
|
|
|
|
|
Better Testing
|
|
==============
|
|
|
|
Add unit tests using the Kernel Unit Testing (KUnit) framework
|
|
--------------------------------------------------------------
|
|
|
|
The `KUnit <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
|
|
provides a common framework for unit tests within the Linux kernel. Having a
|
|
test suite would allow to identify regressions earlier.
|
|
|
|
A good candidate for the first unit tests are the format-conversion helpers in
|
|
``drm_format_helper.c``.
|
|
|
|
Contact: Javier Martinez Canillas <javierm@redhat.com>
|
|
|
|
Level: Intermediate
|
|
|
|
Enable trinity for DRM
|
|
----------------------
|
|
|
|
And fix up the fallout. Should be really interesting ...
|
|
|
|
Level: Advanced
|
|
|
|
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.
|
|
|
|
Level: Advanced
|
|
|
|
Extend virtual test driver (VKMS)
|
|
---------------------------------
|
|
|
|
See the documentation of :ref:`VKMS <vkms>` for more details. This is an ideal
|
|
internship task, since it only requires a virtual machine and can be sized to
|
|
fit the available time.
|
|
|
|
Level: See details
|
|
|
|
Backlight Refactoring
|
|
---------------------
|
|
|
|
Backlight drivers have a triple enable/disable state, which is a bit overkill.
|
|
Plan to fix this:
|
|
|
|
1. Roll out backlight_enable() and backlight_disable() helpers everywhere. This
|
|
has started already.
|
|
2. In all, only look at one of the three status bits set by the above helpers.
|
|
3. Remove the other two status bits.
|
|
|
|
Contact: Daniel Vetter
|
|
|
|
Level: Intermediate
|
|
|
|
Driver Specific
|
|
===============
|
|
|
|
AMD DC Display Driver
|
|
---------------------
|
|
|
|
AMD DC is the display driver for AMD devices starting with Vega. There has been
|
|
a bunch of progress cleaning it up but there's still plenty of work to be done.
|
|
|
|
See drivers/gpu/drm/amd/display/TODO for tasks.
|
|
|
|
Contact: Harry Wentland, Alex Deucher
|
|
|
|
vmwgfx: Replace hashtable with Linux' implementation
|
|
----------------------------------------------------
|
|
|
|
The vmwgfx driver uses its own hashtable implementation. Replace the
|
|
code with Linux' implementation and update the callers. It's mostly a
|
|
refactoring task, but the interfaces are different.
|
|
|
|
Contact: Zack Rusin, Thomas Zimmermann <tzimmermann@suse.de>
|
|
|
|
Level: Intermediate
|
|
|
|
Bootsplash
|
|
==========
|
|
|
|
There is support in place now for writing internal DRM clients making it
|
|
possible to pick up the bootsplash work that was rejected because it was written
|
|
for fbdev.
|
|
|
|
- [v6,8/8] drm/client: Hack: Add bootsplash example
|
|
https://patchwork.freedesktop.org/patch/306579/
|
|
|
|
- [RFC PATCH v2 00/13] Kernel based bootsplash
|
|
https://lore.kernel.org/r/20171213194755.3409-1-mstaudt@suse.de
|
|
|
|
Contact: Sam Ravnborg
|
|
|
|
Level: Advanced
|
|
|
|
Brightness handling on devices with multiple internal panels
|
|
============================================================
|
|
|
|
On x86/ACPI devices there can be multiple backlight firmware interfaces:
|
|
(ACPI) video, vendor specific and others. As well as direct/native (PWM)
|
|
register programming by the KMS driver.
|
|
|
|
To deal with this backlight drivers used on x86/ACPI call
|
|
acpi_video_get_backlight_type() which has heuristics (+quirks) to select
|
|
which backlight interface to use; and backlight drivers which do not match
|
|
the returned type will not register themselves, so that only one backlight
|
|
device gets registered (in a single GPU setup, see below).
|
|
|
|
At the moment this more or less assumes that there will only
|
|
be 1 (internal) panel on a system.
|
|
|
|
On systems with 2 panels this may be a problem, depending on
|
|
what interface acpi_video_get_backlight_type() selects:
|
|
|
|
1. native: in this case the KMS driver is expected to know which backlight
|
|
device belongs to which output so everything should just work.
|
|
2. video: this does support controlling multiple backlights, but some work
|
|
will need to be done to get the output <-> backlight device mapping
|
|
|
|
The above assumes both panels will require the same backlight interface type.
|
|
Things will break on systems with multiple panels where the 2 panels need
|
|
a different type of control. E.g. one panel needs ACPI video backlight control,
|
|
where as the other is using native backlight control. Currently in this case
|
|
only one of the 2 required backlight devices will get registered, based on
|
|
the acpi_video_get_backlight_type() return value.
|
|
|
|
If this (theoretical) case ever shows up, then supporting this will need some
|
|
work. A possible solution here would be to pass a device and connector-name
|
|
to acpi_video_get_backlight_type() so that it can deal with this.
|
|
|
|
Note in a way we already have a case where userspace sees 2 panels,
|
|
in dual GPU laptop setups with a mux. On those systems we may see
|
|
either 2 native backlight devices; or 2 native backlight devices.
|
|
|
|
Userspace already has code to deal with this by detecting if the related
|
|
panel is active (iow which way the mux between the GPU and the panels
|
|
points) and then uses that backlight device. Userspace here very much
|
|
assumes a single panel though. It picks only 1 of the 2 backlight devices
|
|
and then only uses that one.
|
|
|
|
Note that all userspace code (that I know off) is currently hardcoded
|
|
to assume a single panel.
|
|
|
|
Before the recent changes to not register multiple (e.g. video + native)
|
|
/sys/class/backlight devices for a single panel (on a single GPU laptop),
|
|
userspace would see multiple backlight devices all controlling the same
|
|
backlight.
|
|
|
|
To deal with this userspace had to always picks one preferred device under
|
|
/sys/class/backlight and will ignore the others. So to support brightness
|
|
control on multiple panels userspace will need to be updated too.
|
|
|
|
There are plans to allow brightness control through the KMS API by adding
|
|
a "display brightness" property to drm_connector objects for panels. This
|
|
solves a number of issues with the /sys/class/backlight API, including not
|
|
being able to map a sysfs backlight device to a specific connector. Any
|
|
userspace changes to add support for brightness control on devices with
|
|
multiple panels really should build on top of this new KMS property.
|
|
|
|
Contact: Hans de Goede
|
|
|
|
Level: Advanced
|
|
|
|
Outside DRM
|
|
===========
|
|
|
|
Convert fbdev drivers to DRM
|
|
----------------------------
|
|
|
|
There are plenty of fbdev drivers for older hardware. Some hardware has
|
|
become obsolete, but some still provides good(-enough) framebuffers. The
|
|
drivers that are still useful should be converted to DRM and afterwards
|
|
removed from fbdev.
|
|
|
|
Very simple fbdev drivers can best be converted by starting with a new
|
|
DRM driver. Simple KMS helpers and SHMEM should be able to handle any
|
|
existing hardware. The new driver's call-back functions are filled from
|
|
existing fbdev code.
|
|
|
|
More complex fbdev drivers can be refactored step-by-step into a DRM
|
|
driver with the help of the DRM fbconv helpers. [1] These helpers provide
|
|
the transition layer between the DRM core infrastructure and the fbdev
|
|
driver interface. Create a new DRM driver on top of the fbconv helpers,
|
|
copy over the fbdev driver, and hook it up to the DRM code. Examples for
|
|
several fbdev drivers are available at [1] and a tutorial of this process
|
|
available at [2]. The result is a primitive DRM driver that can run X11
|
|
and Weston.
|
|
|
|
- [1] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv
|
|
- [2] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c
|
|
|
|
Contact: Thomas Zimmermann <tzimmermann@suse.de>
|
|
|
|
Level: Advanced
|