TODO list

This section contains a list of smaller janitorial tasks in the kernel DRM graphics subsystem useful as newbie projects. Or for slow rainy days.

Subsystem-wide refactorings

Remove custom dumb_map_offset implementations

All GEM based drivers should be using drm_gem_create_mmap_offset() instead. Audit each individual driver, make sure it’ll work with the generic implementation (there’s lots of outdated locking leftovers in various implementations), and then remove it.

Contact: Daniel Vetter, respective driver maintainers

Convert existing KMS drivers to atomic modesetting

3.19 has the atomic modeset interfaces and helpers, so drivers can now be converted over. Modern compositors like Wayland or Surfaceflinger on Android really want an atomic modeset interface, so this is all about the bright future.

There is a conversion guide for atomic and all you need is a GPU for a non-converted driver (again virtual HW drivers for KVM are still all suitable).

As part of this drivers also need to convert to universal plane (which means exposing primary & cursor as proper plane objects). But that’s much easier to do by directly using the new atomic helper driver callbacks.

Contact: Daniel Vetter, respective driver maintainers

Clean up the clipped coordination confusion around planes

We have a helper to get this right with drm_plane_helper_check_update(), but it’s not consistently used. This should be fixed, preferrably in the atomic helpers (and drivers then moved over to clipped coordinates). Probably the helper should also be moved from drm_plane_helper.c to the atomic helpers, to avoid confusion - the other helpers in that file are all deprecated legacy helpers.

Contact: Ville Syrjälä, Daniel Vetter, driver maintainers

Convert early atomic drivers to async commit helpers

For the first year the atomic modeset helpers didn’t support asynchronous / nonblocking commits, and every driver had to hand-roll them. This is fixed now, but there’s still a pile of existing drivers that easily could be converted over to the new infrastructure.

One issue with the helpers is that they require that drivers handle completion events for atomic commits correctly. But fixing these bugs is good anyway.

Contact: Daniel Vetter, respective driver maintainers

Fallout from atomic KMS

drm_atomic_helper.c provides a batch of functions which implement legacy IOCTLs on top of the new atomic driver interface. Which is really nice for gradual conversion of drivers, but unfortunately the semantic mismatches are a bit too severe. So there’s some follow-up work to adjust the function interfaces to fix these issues:

  • atomic needs the lock acquire context. At the moment that’s passed around implicitly with some horrible hacks, and it’s also allocate with GFP_NOFAIL behind the scenes. All legacy paths need to start allocating the acquire context explicitly on stack and then also pass it down into drivers explicitly so that the legacy-on-atomic functions can use them.

    Except for some driver code this is done. This task should be finished by adding WARN_ON(!drm_drv_uses_atomic_modeset) in drm_modeset_lock_all().

  • A bunch of the vtable hooks are now in the wrong place: DRM has a split between core vfunc tables (named drm_foo_funcs), which are used to implement the userspace ABI. And then there’s the optional hooks for the helper libraries (name drm_foo_helper_funcs), which are purely for internal use. Some of these hooks should be move from _funcs to _helper_funcs since they are not part of the core ABI. There’s a FIXME comment in the kerneldoc for each such case in drm_crtc.h.

Contact: Daniel Vetter

Get rid of dev->struct_mutex from GEM drivers

dev->struct_mutex is the Big DRM Lock from legacy days and infested everything. Nowadays in modern drivers the only bit where it’s mandatory is serializing GEM buffer object destruction. Which unfortunately means drivers have to keep track of that lock and either call unreference or unreference_locked depending upon context.

Core GEM doesn’t have a need for struct_mutex any more since kernel 4.8, and there’s a gem_free_object_unlocked callback for any drivers which are entirely struct_mutex free.

For drivers that need struct_mutex it should be replaced with a driver- private lock. The tricky part is the BO free functions, since those can’t reliably take that lock any more. Instead state needs to be protected with suitable subordinate locks or some cleanup work pushed to a worker thread. For performance-critical drivers it might also be better to go with a more fine-grained per-buffer object and per-context lockings scheme. Currently only the msm driver still use struct_mutex.

Contact: Daniel Vetter, respective driver maintainers

Convert instances of dev_info/dev_err/dev_warn to their DRM_DEV_* equivalent

For drivers which could have multiple instances, it is necessary to differentiate between which is which in the logs. Since DRM_INFO/WARN/ERROR don’t do this, drivers used dev_info/warn/err to make this differentiation. We now have DRM_DEV_* variants of the drm print macros, so we can start to convert those drivers back to using drm-formwatted specific log messages.

Before you start this conversion please contact the relevant maintainers to make sure your work will be merged - not everyone agrees that the DRM dmesg macros are better.

Contact: Sean Paul, Maintainer of the driver you plan to convert

Convert drivers to use simple modeset suspend/resume

Most drivers (except i915 and nouveau) that use drm_atomic_helper_suspend/resume() can probably be converted to use drm_mode_config_helper_suspend/resume(). Also there’s still open-coded version of the atomic suspend/resume code in older atomic modeset drivers.

Contact: Maintainer of the driver you plan to convert

Convert drivers to use drm_fb_helper_fbdev_setup/teardown()

Most drivers can use drm_fb_helper_fbdev_setup() except maybe:

  • amdgpu which has special logic to decide whether to call drm_helper_disable_unused_functions()
  • armada which isn’t atomic and doesn’t call drm_helper_disable_unused_functions()
  • i915 which calls drm_fb_helper_initial_config() in a worker

Drivers that use drm_framebuffer_remove() to clean up the fbdev framebuffer can probably use drm_fb_helper_fbdev_teardown().

Contact: Maintainer of the driver you plan to convert

Clean up mmap forwarding

A lot of drivers forward gem mmap calls to dma-buf mmap for imported buffers. And also a lot of them forward dma-buf mmap to the gem mmap implementations. Would be great to refactor this all into a set of small common helpers.

Contact: Daniel Vetter

Generic fbdev defio support

The defio support code in the fbdev core has some very specific requirements, which means drivers need to have a special framebuffer for fbdev. Which prevents us from using the generic fbdev emulation code everywhere. The main issue is that it uses some fields in struct page itself, which breaks shmem gem objects (and other things).

Possible solution would be to write our own defio mmap code in the drm fbdev emulation. It would need to fully wrap the existing mmap ops, forwarding everything after it has done the write-protect/mkwrite trickery:

  • In the drm_fbdev_fb_mmap helper, if we need defio, change the default page prots to write-protected with something like this:

    vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
    
  • Set the mkwrite and fsync callbacks with similar implementions to the core fbdev defio stuff. These should all work on plain ptes, they don’t actually require a struct page. uff. These should all work on plain ptes, they don’t actually require a struct page.

  • Track the dirty pages in a separate structure (bitfield with one bit per page should work) to avoid clobbering struct page.

Might be good to also have some igt testcases for this.

Contact: Daniel Vetter, Noralf Tronnes

Remove the ->gem_prime_res_obj callback

The ->gem_prime_res_obj callback can be removed from drivers by using the reservation_object in the drm_gem_object. It may also be possible to use the generic drm_gem_reservation_object_wait helper for waiting for a bo.

Contact: Daniel Vetter

idr_init_base()

DRM core&drivers uses a lot of idr (integer lookup directories) for mapping userspace IDs to internal objects, and in most places ID=0 means NULL and hence is never used. Switching to idr_init_base() for these would make the idr more efficient.

Contact: Daniel Vetter

Defaults for .gem_prime_import and export

Most drivers don’t need to set drm_driver->gem_prime_import and ->gem_prime_export now that drm_gem_prime_import() and drm_gem_prime_export() are the default.

struct drm_gem_object_funcs

GEM objects can now have a function table instead of having the callbacks on the DRM driver struct. This is now the preferred way and drivers can be moved over.

DRM_GEM_CMA_VMAP_DRIVER_OPS, DRM_GEM_SHMEM_DRIVER_OPS already support this, but DRM_GEM_VRAM_DRIVER_PRIME does not yet and needs to be aligned with the previous two. We also need a 2nd version of the CMA define that doesn’t require the vmapping to be present (different hook for prime importing). Plus this needs to be rolled out to all drivers using their own implementations, too.

Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate

For cases where drivers are attempting to grab the modeset locks with a local acquire context. Replace the boilerplate code surrounding drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and DRM_MODESET_LOCK_ALL_END() instead.

This should also be done for all places where drm_modest_lock_all() is still used.

As a reference, take a look at the conversions already completed in drm core.

Contact: Sean Paul, respective driver maintainers

Rename CMA helpers to DMA helpers

CMA (standing for contiguous memory allocator) is really a bit an accident of what these were used for first, a much better name would be DMA helpers. In the text these should even be called coherent DMA memory helpers (so maybe CDM, but no one knows what that means) since underneath they just use dma_alloc_coherent.

Contact: Laurent Pinchart, Daniel Vetter

Convert direct mode.vrefresh accesses to use drm_mode_vrefresh()

drm_display_mode.vrefresh isn’t guaranteed to be populated. As such, using it is risky and has been known to cause div-by-zero bugs. Fortunately, drm core has helper which will use mode.vrefresh if it’s !0 and will calculate it from the timings when it’s 0.

Use simple search/replace, or (more fun) cocci to replace instances of direct vrefresh access with a call to the helper. Check out https://lists.freedesktop.org/archives/dri-devel/2019-January/205186.html for inspiration.

Once all instances of vrefresh have been converted, remove vrefresh from drm_display_mode to avoid future use.

Contact: Sean Paul

Remove drm_display_mode.hsync

We have drm_mode_hsync() to calculate this from hsync_start/end, since drivers shouldn’t/don’t use this, remove this member to avoid any temptations to use it in the future. If there is any debug code using drm_display_mode.hsync, convert it to use drm_mode_hsync() instead.

Contact: Sean Paul

drm_fb_helper tasks

  • drm_fb_helper_restore_fbdev_mode_unlocked() should call restore_fbdev_mode() not the _force variant so it can bail out if there is a master. But first these igt tests need to be fixed: kms_fbcon_fbt@psr and kms_fbcon_fbt@psr-suspend.
  • The max connector argument for drm_fb_helper_init() and drm_fb_helper_fbdev_setup() isn’t used anymore and can be removed.
  • The helper doesn’t keep an array of connectors anymore so these can be removed: drm_fb_helper_single_add_all_connectors(), drm_fb_helper_add_one_connector() and drm_fb_helper_remove_one_connector().

Core refactorings

Clean up the DRM header mess

The DRM subsystem originally had only one huge global header, drmP.h. This is now split up, but many source files still include it. The remaining part of the cleanup work here is to replace any #include <drm/drmP.h> by only the headers needed (and fixing up any missing pre-declarations in the headers).

In the end no .c file should need to include drmP.h anymore.

Contact: Daniel Vetter

Add missing kerneldoc for exported functions

The DRM reference documentation is still lacking kerneldoc in a few areas. The task would be to clean up interfaces like moving functions around between files to better group them and improving the interfaces like dropping return values for functions that never fail. Then write kerneldoc for all exported functions and an overview section and integrate it all into the drm book.

See https://dri.freedesktop.org/docs/drm/ for what’s there already.

Contact: Daniel Vetter

Make panic handling work

This is a really varied tasks with lots of little bits and pieces:

  • The panic path can’t be tested currently, leading to constant breaking. The main issue here is that panics can be triggered from hardirq contexts and hence all panic related callback can run in hardirq context. It would be awesome if we could test at least the fbdev helper code and driver code by e.g. trigger calls through drm debugfs files. hardirq context could be achieved by using an IPI to the local processor.
  • There’s a massive confusion of different panic handlers. DRM fbdev emulation helpers have one, but on top of that the fbcon code itself also has one. We need to make sure that they stop fighting over each another.
  • drm_can_sleep() is a mess. It hides real bugs in normal operations and isn’t a full solution for panic paths. We need to make sure that it only returns true if there’s a panic going on for real, and fix up all the fallout.
  • The panic handler must never sleep, which also means it can’t ever mutex_lock(). Also it can’t grab any other lock unconditionally, not even spinlocks (because NMI and hardirq can panic too). We need to either make sure to not call such paths, or trylock everything. Really tricky.
  • For the above locking troubles reasons it’s pretty much impossible to attempt a synchronous modeset from panic handlers. The only thing we could try to achive is an atomic set_base of the primary plane, and hope that it shows up. Everything else probably needs to be delayed to some worker or something else which happens later on. Otherwise it just kills the box harder, prevent the panic from going out on e.g. netconsole.
  • There’s also proposal for a simplied DRM console instead of the full-blown fbcon and DRM fbdev emulation. Any kind of panic handling tricks should obviously work for both console, in case we ever get kmslog merged.

Contact: Daniel Vetter

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.

Contact: Daniel Vetter

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.

Better Testing

Enable trinity for DRM

And fix up the fallout. Should be really interesting …

Make KMS tests in i-g-t generic

The i915 driver team maintains an extensive testsuite for the i915 DRM driver, including tons of testcases for corner-cases in the modesetting API. It would be awesome if those tests (at least the ones not relying on Intel-specific GEM features) could be made to run on any KMS driver.

Basic work to run i-g-t tests on non-i915 is done, what’s now missing is mass- converting things over. For modeset tests we also first need a bit of infrastructure to use dumb buffers for untiled buffers, to be able to run all the non-i915 specific modeset tests.

Extend virtual test driver (VKMS)

See the documentation of 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

Driver Specific

tinydrm

Tinydrm is the helper driver for really simple fb drivers. The goal is to make those drivers as simple as possible, so lots of room for refactoring:

  • backlight helpers, probably best to put them into a new drm_backlight.c. This is because drivers/video is de-facto unmaintained. We could also move drivers/video/backlight to drivers/gpu/backlight and take it all over within drm-misc, but that’s more work. Backlight helpers require a fair bit of reworking and refactoring. A simple example is the enabling of a backlight. Tinydrm has helpers for this. It would be good if other drivers can also use the helper. However, there are various cases we need to consider i.e different drivers seem to have different ways of enabling/disabling a backlight. We also need to consider the backlight drivers (like gpio_backlight). The situation is further complicated by the fact that the backlight is tied to fbdev via fb_notifier_callback() which has complicated logic. For further details, refer to the following discussion thread: https://groups.google.com/forum/#!topic/outreachy-kernel/8rBe30lwtdA
  • spi helpers, probably best put into spi core/helper code. Thierry said the spi maintainer is fast&reactive, so shouldn’t be a big issue.
  • extract the mipi-dbi helper (well, the non-tinydrm specific parts at least) into a separate helper, like we have for mipi-dsi already. Or follow one of the ideas for having a shared dsi/dbi helper, abstracting away the transport details more.

Contact: Noralf Trønnes, Daniel Vetter

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

i915

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.

Contact: Sam Ravnborg

Outside DRM