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drm/doc: Document submission error signaling

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Different approaches have been tried to signal resets and other errors in
vendor specific ways which not only resulted in a wide variety of
implementations but also repeating the same bugs and problems over different
drivers.

Document that drivers should use dma_fence based error signaling which is
vendor agnostic and allows userspace to query submission errors in generic
non-vendor specific code.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240826122541.85663-3-christian.koenig@amd.com
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Christian König 2024-08-26 14:25:40 +02:00 committed by Christian König
parent a401bd1264
commit f07a0d1bf7
1 changed files with 20 additions and 7 deletions
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27
Documentation/gpu/drm-uapi.rst
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@ -305,13 +305,26 @@ Kernel Mode Driver
------------------
The KMD is responsible for checking if the device needs a reset, and to perform
it as needed. Usually a hang is detected when a job gets stuck executing. KMD
should keep track of resets, because userspace can query any time about the
reset status for a specific context. This is needed to propagate to the rest of
the stack that a reset has happened. Currently, this is implemented by each
driver separately, with no common DRM interface. Ideally this should be properly
integrated at DRM scheduler to provide a common ground for all drivers. After a
reset, KMD should reject new command submissions for affected contexts.
it as needed. Usually a hang is detected when a job gets stuck executing.
Propagation of errors to userspace has proven to be tricky since it goes in
the opposite direction of the usual flow of commands. Because of this vendor
independent error handling was added to the &dma_fence object, this way drivers
can add an error code to their fences before signaling them. See function
dma_fence_set_error() on how to do this and for examples of error codes to use.
The DRM scheduler also allows setting error codes on all pending fences when
hardware submissions are restarted after an reset. Error codes are also
forwarded from the hardware fence to the scheduler fence to bubble up errors
to the higher levels of the stack and eventually userspace.
Fence errors can be queried by userspace through the generic SYNC_IOC_FILE_INFO
IOCTL as well as through driver specific interfaces.
Additional to setting fence errors drivers should also keep track of resets per
context, the DRM scheduler provides the drm_sched_entity_error() function as
helper for this use case. After a reset, KMD should reject new command
submissions for affected contexts.
User Mode Driver
----------------