.. _todo:
|
|
=========
|
TODO list
|
=========
|
|
This section contains a list of smaller janitorial tasks in the kernel DRM
|
graphics subsystem useful as newbie projects. Or for slow rainy days.
|
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Difficulty
|
----------
|
|
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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Subsystem-wide refactorings
|
===========================
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Remove custom dumb_map_offset implementations
|
---------------------------------------------
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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implementation (there's lots of outdated locking leftovers in various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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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
|
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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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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Contact: Daniel Vetter, respective driver maintainers
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Level: Advanced
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Fallout from atomic KMS
|
-----------------------
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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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``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
|
everything. Nowadays in modern drivers the only bit where it's mandatory is
|
serializing GEM buffer object destruction. Which unfortunately means drivers
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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 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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Convert logging to drm_* functions with drm_device paramater
|
------------------------------------------------------------
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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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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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Most drivers can use drm_fbdev_generic_setup(). Driver have to implement
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atomic modesetting and GEM vmap support. Current generic fbdev emulation
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expects the framebuffer in system memory (or system-like memory).
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Contact: Maintainer of the driver you plan to convert
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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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Clean up mmap forwarding
|
------------------------
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A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers.
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And also a lot of them forward dma-buf mmap to the gem mmap implementations.
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There's drm_gem_prime_mmap() for this now, but still needs to be rolled out.
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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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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
|
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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idr_init_base()
|
---------------
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DRM core&drivers uses a lot of idr (integer lookup directories) for mapping
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userspace IDs to internal objects, and in most places ID=0 means NULL and hence
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is never used. Switching to idr_init_base() for these would make the idr more
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efficient.
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Contact: Daniel Vetter
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Level: Starter
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struct drm_gem_object_funcs
|
---------------------------
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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 and drivers can be moved over.
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We also need a 2nd version of the CMA define that doesn't require the
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vmapping to be present (different hook for prime importing). Plus this needs to
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be rolled out to all drivers using their own implementations, too.
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Level: Intermediate
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Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate
|
---------------------------------------------------------
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For cases where drivers are attempting to grab the modeset locks with a local
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acquire context. Replace the boilerplate code surrounding
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drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and
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DRM_MODESET_LOCK_ALL_END() instead.
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|
This should also be done for all places where drm_modeset_lock_all() is still
|
used.
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As a reference, take a look at the conversions already completed in drm core.
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Contact: Sean Paul, respective driver maintainers
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Level: Starter
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Rename CMA helpers to DMA helpers
|
---------------------------------
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|
CMA (standing for contiguous memory allocator) is really a bit an accident of
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what these were used for first, a much better name would be DMA helpers. In the
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text these should even be called coherent DMA memory helpers (so maybe CDM, but
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no one knows what that means) since underneath they just use dma_alloc_coherent.
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Contact: Laurent Pinchart, Daniel Vetter
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Level: Intermediate (mostly because it is a huge tasks without good partial
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milestones, not technically itself that challenging)
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connector register/unregister fixes
|
-----------------------------------
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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
|
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
|
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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|
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)
|
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.
|
|
- 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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|
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
|
retrieve the same information, which is less efficient.
|
|
Audit each individual driver calling drm_detect_hdmi_monitor() and switch to
|
drm_display_info.is_hdmi if applicable.
|
|
Contact: Laurent Pinchart, respective driver maintainers
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Level: Intermediate
|
|
Consolidate custom driver modeset properties
|
--------------------------------------------
|
|
Before atomic modeset took place, many drivers where creating their own
|
properties. Among other things, atomic brought the requirement that custom,
|
driver specific properties should not be used.
|
|
For this task, we aim to introduce core helpers or reuse the existing ones
|
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)
|
- brightness, contrast, etc (armada, nouveau) - overlay only (?)
|
- broadcast rgb (gma500, intel)
|
- colorkey (armada, nouveau, rcar) - overlay only (?)
|
- dither (amdgpu, nouveau, radeon) - varies across drivers
|
- underscan family (amdgpu, radeon, nouveau)
|
|
Already in core:
|
- colorspace (sti)
|
- tv format names, enhancements (gma500, intel)
|
- tv overscan, margins, etc. (gma500, intel)
|
- zorder (omapdrm) - same as zpos (?)
|
|
|
Contact: Emil Velikov, respective driver maintainers
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|
Level: Intermediate
|
|
Plumb drm_atomic_state all over
|
-------------------------------
|
|
Currently various atomic functions take just a single or a handful of
|
object states (eg. plane state). While that single object state can
|
suffice for some simple cases, we often have to dig out additional
|
object states for dealing with various dependencies between the individual
|
objects or the hardware they represent. The process of digging out the
|
additional states is rather non-intuitive and error prone.
|
|
To fix that most functions should rather take the overall
|
drm_atomic_state as one of their parameters. The other parameters
|
would generally be the object(s) we mainly want to interact with.
|
|
For example, instead of
|
|
.. code-block:: c
|
|
int (*atomic_check)(struct drm_plane *plane, struct drm_plane_state *state);
|
|
we would have something like
|
|
.. code-block:: c
|
|
int (*atomic_check)(struct drm_plane *plane, struct drm_atomic_state *state);
|
|
The implementation can then trivially gain access to any required object
|
state(s) via drm_atomic_get_plane_state(), drm_atomic_get_new_plane_state(),
|
drm_atomic_get_old_plane_state(), and their equivalents for
|
other object types.
|
|
Additionally many drivers currently access the object->state pointer
|
directly in their commit functions. That is not going to work if we
|
eg. want to allow deeper commit pipelines as those pointers could
|
then point to the states corresponding to a future commit instead of
|
the current commit we're trying to process. Also non-blocking commits
|
execute locklessly so there are serious concerns with dereferencing
|
the object->state pointers without holding the locks that protect them.
|
Use of drm_atomic_get_new_plane_state(), drm_atomic_get_old_plane_state(),
|
etc. avoids these problems as well since they relate to a specific
|
commit via the passed in drm_atomic_state.
|
|
Contact: Ville Syrjälä, Daniel Vetter
|
|
Level: Intermediate
|
|
|
Core refactorings
|
=================
|
|
Make panic handling work
|
------------------------
|
|
This is a really varied tasks with lots of little bits and pieces:
|
|
* The panic path can't be tested currently, leading to constant breaking. The
|
main issue here is that panics can be triggered from hardirq contexts and
|
hence all panic related callback can run in hardirq context. It would be
|
awesome if we could test at least the fbdev helper code and driver code by
|
e.g. trigger calls through drm debugfs files. hardirq context could be
|
achieved by using an IPI to the local processor.
|
|
* There's a massive confusion of different panic handlers. DRM fbdev emulation
|
helpers have one, but on top of that the fbcon code itself also has one. We
|
need to make sure that they stop fighting over each another.
|
|
* ``drm_can_sleep()`` is a mess. It hides real bugs in normal operations and
|
isn't a full solution for panic paths. We need to make sure that it only
|
returns true if there's a panic going on for real, and fix up all the
|
fallout.
|
|
* The panic handler must never sleep, which also means it can't ever
|
``mutex_lock()``. Also it can't grab any other lock unconditionally, not
|
even spinlocks (because NMI and hardirq can panic too). We need to either
|
make sure to not call such paths, or trylock everything. Really tricky.
|
|
* For the above locking troubles reasons it's pretty much impossible to
|
attempt a synchronous modeset from panic handlers. The only thing we could
|
try to achive is an atomic ``set_base`` of the primary plane, and hope that
|
it shows up. Everything else probably needs to be delayed to some worker or
|
something else which happens later on. Otherwise it just kills the box
|
harder, prevent the panic from going out on e.g. netconsole.
|
|
* There's also proposal for a simplied DRM console instead of the full-blown
|
fbcon and DRM fbdev emulation. Any kind of panic handling tricks should
|
obviously work for both console, in case we ever get kmslog merged.
|
|
Contact: Daniel Vetter
|
|
Level: Advanced
|
|
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.
|
|
- Drop the return code and error checking from all debugfs functions. Greg KH is
|
working on this already.
|
|
Contact: Daniel Vetter
|
|
Level: Intermediate
|
|
KMS cleanups
|
------------
|
|
Some of these date from the very introduction of KMS in 2008 ...
|
|
- Make ->funcs and ->helper_private vtables optional. There's a bunch of empty
|
function tables in drivers, but before we can remove them we need to make sure
|
that all the users in helpers and drivers do correctly check for a NULL
|
vtable.
|
|
- Cleanup up the various ->destroy callbacks. A lot of them just wrapt the
|
drm_*_cleanup implementations and can be removed. Some tack a kfree() at the
|
end, for which we could add drm_*_cleanup_kfree(). And then there's the (for
|
historical reasons) misnamed drm_primary_helper_destroy() function.
|
|
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
|
==============
|
|
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.
|
|
Contact: Daniel Vetter
|
|
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
|
|
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/
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- [RFC PATCH v2 00/13] Kernel based bootsplash
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https://lkml.org/lkml/2017/12/13/764
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Contact: Sam Ravnborg
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Level: Advanced
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Outside DRM
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===========
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Convert fbdev drivers to DRM
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----------------------------
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There are plenty of fbdev drivers for older hardware. Some hwardware has
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become obsolete, but some still provides good(-enough) framebuffers. The
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drivers that are still useful should be converted to DRM and afterwards
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removed from fbdev.
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Very simple fbdev drivers can best be converted by starting with a new
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DRM driver. Simple KMS helpers and SHMEM should be able to handle any
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existing hardware. The new driver's call-back functions are filled from
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existing fbdev code.
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More complex fbdev drivers can be refactored step-by-step into a DRM
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driver with the help of the DRM fbconv helpers. [1] These helpers provide
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the transition layer between the DRM core infrastructure and the fbdev
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driver interface. Create a new DRM driver on top of the fbconv helpers,
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copy over the fbdev driver, and hook it up to the DRM code. Examples for
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several fbdev drivers are available at [1] and a tutorial of this process
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available at [2]. The result is a primitive DRM driver that can run X11
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and Weston.
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- [1] https://gitlab.freedesktop.org/tzimmermann/linux/tree/fbconv
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- [2] https://gitlab.freedesktop.org/tzimmermann/linux/blob/fbconv/drivers/gpu/drm/drm_fbconv_helper.c
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Contact: Thomas Zimmermann <tzimmermann@suse.de>
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Level: Advanced
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