# TODO リスト¶

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¶

### De-midlayer drivers¶

With the recent drm_bus cleanup patches for 3.17 it is no longer required to have a drm_bus structure set up. Drivers can directly set up the drm_device structure instead of relying on bus methods in drm_usb.c and drm_pci.c. The goal is to get rid of the driver's ->load / ->unload callbacks and open-code the load/unload sequence properly, using the new two-stage drm_device setup/teardown.

Once all existing drivers are converted we can also remove those bus support files for USB and platform devices.

All you need is a GPU for a non-converted driver (currently almost all of them, but also all the virtual ones used by KVM, so everyone qualifies).

Contact: Daniel Vetter, Thierry Reding, respective driver maintainers

### Switch from reference/unreference to get/put¶

For some reason DRM core uses reference/unreference suffixes for refcounting functions, but kernel uses get/put (e.g. kref_get/put()). It would be good to switch over for consistency, and it's shorter. Needs to be done in 3 steps for each pair of functions:

• Create new get/put functions, define the old names as compatibility wrappers
• Switch over each file/driver using a cocci-generated spatch.
• Once all users of the old names are gone, remove them.

This way drivers/patches in the progress of getting merged won't break.

Contact: Daniel Vetter

### 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

### Better manual-upload support for atomic¶

This would be especially useful for tinydrm:

• Add a struct drm_rect dirty_clip to drm_crtc_state. When duplicating the crtc state, clear that to the max values, x/y = 0 and w/h = MAX_INT, in __drm_atomic_helper_crtc_duplicate_state().
• Move tinydrm_merge_clips into drm_framebuffer.c, dropping the tinydrm_ prefix ofc and using drm_fb_. drm_framebuffer.c makes sense since this is a function useful to implement the fb->dirty function.
• Create a new drm_fb_dirty function which does essentially what e.g. mipi_dbi_fb_dirty does. You can use e.g. drm_atomic_helper_update_plane as the template. But instead of doing a simple full-screen plane update, this new helper also sets crtc_state->dirty_clip to the right coordinates. And of course it needs to check whether the fb is actually active (and maybe where), so there's some book-keeping involved. There's also some good fun involved in scaling things appropriately. For that case we might simply give up and declare the entire area covered by the plane as dirty.

Contact: Noralf Trønnes, Daniel Vetter

### 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

### Put a reservation_object into drm_gem_object¶

This would remove the need for the ->gem_prime_res_obj callback. It would also allow us to implement generic helpers for waiting for a bo, allowing for quite a bit of refactoring in the various wait ioctl implementations.

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

## 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 ...

• drm_mode_config.crtc_idr is misnamed, since it contains all KMS object. Should be renamed to drm_mode_config.object_idr.
• drm_display_mode doesn't need to be derived from drm_mode_object. That's leftovers from older (never merged into upstream) KMS designs where modes where set using their ID, including support to add/remove modes.

## 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.
• tinydrm_gem_cma_prime_import_sg_table should probably go into the cma helpers, as a _vmapped variant (since not every driver needs the vmap). And tinydrm_gem_cma_free_object could the be merged into drm_gem_cma_free_object().
• tinydrm_fb_create we could move into drm_simple_pipe, only need to add the fb_create hook to drm_simple_pipe_funcs, which would again simplify a bunch of things (since it gives you a one-stop vfunc for simple drivers).
• Quick aside: The unregister devm stuff is kinda getting the lifetimes of a drm_device wrong. Doesn't matter, since everyone else gets it wrong too :-)
• also rework the drm_framebuffer_funcs->dirty hook wire-up, see above.

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.