Difficulty¶

To make it easier task are categorized into different levels:

Starter: Good tasks to get started with the DRM subsystem.

Intermediate: Tasks which need some experience with working in the DRMsubsystem, or some specific GPU/display graphics knowledge. For debugging issueit’s good to have the relevant hardware (or a virtual driver set up) availablefor testing.

Advanced: Tricky tasks that need fairly good understanding of the DRM subsystemand graphics topics. Generally need the relevant hardware for development andtesting.

Remove custom dumb_map_offset implementations¶

All GEM based drivers should be usingdrm_gem_create_mmap_offset() instead.Audit each individual driver, make sure it’ll work with the genericimplementation (there’s lots of outdated locking leftovers in variousimplementations), and then remove it.

Contact: Daniel Vetter, respective driver maintainers

Level: Intermediate

Convert existing KMS drivers to atomic modesetting¶

3.19 has the atomic modeset interfaces and helpers, so drivers can now beconverted over. Modern compositors like Wayland or Surfaceflinger on Androidreally want an atomic modeset interface, so this is all about the brightfuture.

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

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

Contact: Daniel Vetter, respective driver maintainers

Level: Advanced

Clean up the clipped coordination confusion around planes¶

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

Contact: Ville Syrjälä, Daniel Vetter, driver maintainers

Level: Advanced

Improve plane atomic_check helpers¶

Aside from the clipped coordinates right above there’s a few suboptimal thingswith the current helpers:

• drm_plane_helper_funcs->atomic_check gets called for enabled or disabledplanes. At best this seems to confuse drivers, worst it means they blow upwhen the plane is disabled without the CRTC. The only special handling isresetting values in the plane state structures, which instead should be movedinto the drm_plane_funcs->atomic_duplicate_state functions.
• Once that’s done, helpers could stop calling ->atomic_check for disabledplanes.
• Then we could go through all the drivers and remove the more-or-less confusedchecks for plane_state->fb and plane_state->crtc.

Contact: Daniel Vetter

Level: Advanced

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 fixednow, but there’s still a pile of existing drivers that easily could beconverted over to the new infrastructure.

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

Contact: Daniel Vetter, respective driver maintainers

Level: Advanced

Fallout from atomic KMS¶

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

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

  Except for some driver code this is done. This task should be finished byadding WARN_ON(!drm_drv_uses_atomic_modeset) indrm_modeset_lock_all().

• A bunch of the vtable hooks are now in the wrong place: DRM has a splitbetween core vfunc tables (nameddrm_foo_funcs), which are used toimplement the userspace ABI. And then there’s the optional hooks for thehelper libraries (namedrm_foo_helper_funcs), which are purely forinternal use. Some of these hooks should be move from_funcs to_helper_funcs since they are not part of the core ABI. There’s aFIXME comment in the kerneldoc for each such case indrm_crtc.h.

Contact: Daniel Vetter

Level: Intermediate

Get rid of dev->struct_mutex from GEM drivers¶

dev->struct_mutex is the Big DRM Lock from legacy days and infestedeverything. Nowadays in modern drivers the only bit where it’s mandatory isserializing GEM buffer object destruction. Which unfortunately means drivershave to keep track of that lock and either callunreference orunreference_locked depending upon context.

Core GEM doesn’t have a need forstruct_mutex any more since kernel 4.8,and there’s agem_free_object_unlocked callback for any drivers which areentirelystruct_mutex free.

For drivers that needstruct_mutex it should be replaced with a driver-private lock. The tricky part is the BO free functions, since those can’treliably take that lock any more. Instead state needs to be protected withsuitable subordinate locks or some cleanup work pushed to a worker thread. Forperformance-critical drivers it might also be better to go with a morefine-grained per-buffer object and per-context lockings scheme. Currently onlythemsm andi915 drivers usestruct_mutex.

Contact: Daniel Vetter, respective driver maintainers

Level: Advanced

Convert logging to drm_* functions with drm_device paramater¶

For drivers which could have multiple instances, it is necessary todifferentiate between which is which in the logs. Since DRM_INFO/WARN/ERRORdon’t do this, drivers used dev_info/warn/err to make this differentiation. Wenow have drm_* variants of the drm print functions, so we can start to convertthose drivers back to using drm-formatted specific log messages.

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

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

Level: Starter

Convert drivers to use simple modeset suspend/resume¶

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

Contact: Maintainer of the driver you plan to convert

Level: Intermediate

Convert drivers to use drm_fbdev_generic_setup()¶

Most drivers can usedrm_fbdev_generic_setup(). Driver have to implementatomic modesetting and GEM vmap support. Current generic fbdev emulationexpects the framebuffer in system memory (or system-like memory).

Contact: Maintainer of the driver you plan to convert

Level: Intermediate

drm_framebuffer_funcs and drm_mode_config_funcs.fb_create cleanup¶

A lot more drivers could be switched over to the drm_gem_framebuffer helpers.Various hold-ups:

• Need to switch over to the generic dirty tracking code usingdrm_atomic_helper_dirtyfb first (e.g. qxl).
• Need to switch todrm_fbdev_generic_setup(), otherwise a lot of the custom fbsetup code can’t be deleted.
• Many drivers wrapdrm_gem_fb_create() only to check for valid formats. Foratomic drivers we could check for valid formats by callingdrm_plane_check_pixel_format() against all planes, and pass if any planesupports the format. For non-atomic that’s not possible since like the formatlist for the primary plane is fake and we’d therefor reject valid formats.
• Many drivers subclass drm_framebuffer, we’d need a embedding compatibleversion of the varios drm_gem_fb_create functions. Maybe calleddrm_gem_fb_create/_with_dirty/_with_funcs as needed.

Contact: Daniel Vetter

Level: Intermediate

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.There’sdrm_gem_prime_mmap() for this now, but still needs to be rolled out.

Contact: Daniel Vetter

Level: Intermediate

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. The mainissue is that it uses some fields in struct page itself, which breaks shmemgem objects (and other things). To support defio, affected drivers requirethe use of a shadow buffer, which may add CPU and memory overhead.

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

• In the drm_fbdev_fb_mmap helper, if we need defio, change thedefault 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 corefbdev defio stuff. These should all work on plain ptes, they don’t actuallyrequire a struct page. uff. These should all work on plain ptes, they don’tactually require a struct page.

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

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

Contact: Daniel Vetter, Noralf Tronnes

Level: Advanced

idr_init_base()¶

DRM core&drivers uses a lot of idr (integer lookup directories) for mappinguserspace IDs to internal objects, and in most places ID=0 means NULL and henceis never used. Switching toidr_init_base() for these would make the idr moreefficient.

Contact: Daniel Vetter

Level: Starter

struct drm_gem_object_funcs¶

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

We also need a 2nd version of the CMA define that doesn’t require thevmapping to be present (different hook for prime importing). Plus this needs tobe rolled out to all drivers using their own implementations, too.

Level: Intermediate

Use DRM_MODESET_LOCK_ALL_* helpers instead of boilerplate¶

For cases where drivers are attempting to grab the modeset locks with a localacquire context. Replace the boilerplate code surroundingdrm_modeset_lock_all_ctx() withDRM_MODESET_LOCK_ALL_BEGIN() andDRM_MODESET_LOCK_ALL_END() instead.

This should also be done for all places wheredrm_modeset_lock_all() is stillused.

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

Contact: Sean Paul, respective driver maintainers

Level: Starter

Rename CMA helpers to DMA helpers¶

CMA (standing for contiguous memory allocator) is really a bit an accident ofwhat these were used for first, a much better name would be DMA helpers. In thetext these should even be called coherent DMA memory helpers (so maybe CDM, butno one knows what that means) since underneath they just use dma_alloc_coherent.

Contact: Laurent Pinchart, Daniel Vetter

Level: Intermediate (mostly because it is a huge tasks without good partialmilestones, not technically itself that challenging)

connector register/unregister fixes¶

• For most connectors it’s a no-op to call drm_connector_register/unregisterdirectly from driver code, drm_dev_register/unregister take care of thisalready. We can remove all of them.
• For dp drivers it’s a bit more a mess, since we need the connector to beregistered when calling drm_dp_aux_register. Fix this by instead callingdrm_dp_aux_init, and moving the actual registering into a late_registercallback as recommended in the kerneldoc.

Level: Intermediate

Remove load/unload callbacks from all non-DRIVER_LEGACY drivers¶

The load/unload callbacks in struct &drm_driver are very much midlayers, plusfor historical reasons they get the ordering wrong (and we can’t fix that)between setting up the &drm_driver structure and callingdrm_dev_register().

• Rework drivers to no longer use the load/unload callbacks, directly coding theload/unload sequence into the driver’s probe function.
• Once all non-DRIVER_LEGACY drivers are converted, disallow the load/unloadcallbacks for all modern drivers.

Contact: Daniel Vetter

Level: Intermediate

Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi¶

Once EDID is parsed, the monitor HDMI support information is available throughdrm_display_info.is_hdmi. Many drivers still calldrm_detect_hdmi_monitor() toretrieve the same information, which is less efficient.

Audit each individual driver callingdrm_detect_hdmi_monitor() and switch todrm_display_info.is_hdmi if applicable.

Contact: Laurent Pinchart, respective driver maintainers

Level: Intermediate

Consolidate custom driver modeset properties¶

Before atomic modeset took place, many drivers where creating their ownproperties. 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 onesif available:

A quick, unconfirmed, examples list.

Introduce core helpers:- 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

Level: Intermediate

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. Themain issue here is that panics can be triggered from hardirq contexts andhence all panic related callback can run in hardirq context. It would beawesome if we could test at least the fbdev helper code and driver code bye.g. trigger calls through drm debugfs files. hardirq context could beachieved by using an IPI to the local processor.
• There’s a massive confusion of different panic handlers. DRM fbdev emulationhelpers have one, but on top of that the fbcon code itself also has one. Weneed to make sure that they stop fighting over each another.
• drm_can_sleep() is a mess. It hides real bugs in normal operations andisn’t a full solution for panic paths. We need to make sure that it onlyreturns true if there’s a panic going on for real, and fix up all thefallout.
• The panic handler must never sleep, which also means it can’t evermutex_lock(). Also it can’t grab any other lock unconditionally, noteven spinlocks (because NMI and hardirq can panic too). We need to eithermake sure to not call such paths, or trylock everything. Really tricky.
• For the above locking troubles reasons it’s pretty much impossible toattempt a synchronous modeset from panic handlers. The only thing we couldtry to achive is an atomicset_base of the primary plane, and hope thatit shows up. Everything else probably needs to be delayed to some worker orsomething else which happens later on. Otherwise it just kills the boxharder, prevent the panic from going out on e.g. netconsole.
• There’s also proposal for a simplied DRM console instead of the full-blownfbcon and DRM fbdev emulation. Any kind of panic handling tricks shouldobviously 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 drmstructure for you. This is lazy.
• We probably want to have some support for debugfs files on crtc/connectors andmaybe other kms objects directly in core. There’s even drm_print support inthe 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. Foranything 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 oldmidlayered load sequence. DRM debugfs should work more like sysfs, where youcan create properties/files for an object anytime you want, and the coretakes care of publishing/unpuplishing all the files at register/unregistertime. Drivers shouldn’t need to worry about these technicalities, and fixingthis (together with the drm_minor->drm_device move) would allow us to removedebugfs_init.
• Drop the return code and error checking from all debugfs functions. Greg KH isworking 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 emptyfunction tables in drivers, but before we can remove them we need to make surethat all the users in helpers and drivers do correctly check for a NULLvtable.
• Cleanup up the various ->destroy callbacks. A lot of them just wrapt thedrm_*_cleanup implementations and can be removed. Some tack akfree() at theend, for which we could add drm_*_cleanup_kfree(). And then there’s the (forhistorical reasons) misnameddrm_primary_helper_destroy() function.

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 wouldbe awesome if those tests (at least the ones not relying on Intel-specific GEMfeatures) 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 ofinfrastructure to use dumb buffers for untiled buffers, to be able to run allthe non-i915 specific modeset tests.

Level: Advanced

Extend virtual test driver (VKMS)¶

See the documentation ofVKMS for more details. This is an idealinternship task, since it only requires a virtual machine and can be sized tofit 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 outbacklight_enable() andbacklight_disable() helpers everywhere. Thishas 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

AMD DC Display Driver¶

AMD DC is the display driver for AMD devices starting with Vega. There has beena 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

Convert fbdev drivers to DRM¶

There are plenty of fbdev drivers for older hardware. Some hwardware hasbecome obsolete, but some still provides good(-enough) framebuffers. Thedrivers that are still useful should be converted to DRM and afterwardsremoved from fbdev.

Very simple fbdev drivers can best be converted by starting with a newDRM driver. Simple KMS helpers and SHMEM should be able to handle anyexisting hardware. The new driver’s call-back functions are filled fromexisting fbdev code.

More complex fbdev drivers can be refactored step-by-step into a DRMdriver with the help of the DRM fbconv helpers. [1] These helpers providethe transition layer between the DRM core infrastructure and the fbdevdriver 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 forseveral fbdev drivers are available at [1] and a tutorial of this processavailable at [2]. The result is a primitive DRM driver that can run X11and 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