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Drivers that can delegate waits to the firmware/GPU pass the scheduled
fence to drm_sched_job_add_dependency(), and issue wait commands to
the firmware/GPU at job submission time. For this to be possible, they
need all their 'native' dependencies to have a valid parent since this
is where the actual HW fence information are encoded.
In drm_sched_main(), we currently call drm_sched_fence_set_parent()
after drm_sched_fence_scheduled(), leaving a short period of time
during which the job depending on this fence can be submitted.
Since setting parent and signaling the fence are two things that are
kinda related (you can't have a parent if the job hasn't been scheduled),
it probably makes sense to pass the parent fence to
drm_sched_fence_scheduled() and let it call drm_sched_fence_set_parent()
before it signals the scheduled fence.
Here is a detailed description of the race we are fixing here:
Thread A Thread B
- calls drm_sched_fence_scheduled()
- signals s_fence->scheduled which
wakes up thread B
- entity dep signaled, checking
the next dep
- no more deps waiting
- entity is picked for job
submission by drm_gpu_scheduler
- run_job() is called
- run_job() tries to
collect native fence info from
s_fence->parent, but it's
NULL =>
BOOM, we can't do our native
wait
- calls drm_sched_fence_set_parent()
v2:
* Fix commit message
v3:
* Add a detailed description of the race to the commit message
* Add Luben's R-b
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Frank Binns <frank.binns@imgtec.com>
Cc: Sarah Walker <sarah.walker@imgtec.com>
Cc: Donald Robson <donald.robson@imgtec.com>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230623075204.382350-1-boris.brezillon@collabora.com
db8b4968a8