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diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst new file mode 100644 index 000000000..a7c150d6b --- /dev/null +++ b/Documentation/gpu/todo.rst @@ -0,0 +1,479 @@ +.. _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. + +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. + +* 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``. + +* There's a new helper ``drm_atomic_helper_best_encoder()`` which could be + used by all atomic drivers which don't select the encoder for a given + connector at runtime. That's almost all of them, and would allow us to get + rid of a lot of ``best_encoder`` boilerplate in drivers. + + This was almost done, but new drivers added a few more cases again. + +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 the +following drivers still use ``struct_mutex``: ``msm``, ``omapdrm`` and +``udl``. + +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(). + +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 +---------------------------- + +Currently the DRM subsystem has only one global header, ``drmP.h``. This is +used both for functions exported to helper libraries and drivers and functions +only used internally in the ``drm.ko`` module. The goal would be to move all +header declarations not needed outside of ``drm.ko`` into +``drivers/gpu/drm/drm_*_internal.h`` header files. ``EXPORT_SYMBOL`` also +needs to be dropped for these functions. + +This would nicely tie in with the below task to create kerneldoc after the API +is cleaned up. Or with the "hide legacy cruft better" task. + +Note that this is well in progress, but ``drmP.h`` is still huge. The updated +plan is to switch to per-file driver API headers, which will also structure +the kerneldoc better. This should also allow more fine-grained ``#include`` +directives. + +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 + +Hide legacy cruft better +------------------------ + +Way back DRM supported only drivers which shadow-attached to PCI devices with +userspace or fbdev drivers setting up outputs. Modern DRM drivers take charge +of the entire device, you can spot them with the DRIVER_MODESET flag. + +Unfortunately there's still large piles of legacy code around which needs to +be hidden so that driver writers don't accidentally end up using it. And to +prevent security issues in those legacy IOCTLs from being exploited on modern +drivers. This has multiple possible subtasks: + +* Extract support code for legacy features into a ``drm-legacy.ko`` kernel + module and compile it only when one of the legacy drivers is enabled. + +This is mostly done, the only thing left is to split up ``drm_irq.c`` into +legacy cruft and the parts needed by modern KMS drivers. + +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. + +Contact: Daniel Vetter + +Create a virtual KMS driver for testing (vkms) +---------------------------------------------- + +With all the latest helpers it should be fairly simple to create a virtual KMS +driver useful for testing, or for running X or similar on headless machines +(to be able to still use the GPU). This would be similar to vgem, but aimed at +the modeset side. + +Once the basics are there there's tons of possibilities to extend it. + +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. + +See drivers/gpu/drm/amd/display/TODO for tasks. + +Contact: Harry Wentland, Alex Deucher + +i915 +---- + +- Our early/late pm callbacks could be removed in favour of using + device_link_add to model the dependency between i915 and snd_had. See + https://dri.freedesktop.org/docs/drm/driver-api/device_link.html + +Outside DRM +=========== |