Quoting Chris Wilson (2019-12-04 21:17:03)
> We want the bonded request to have the same scheduler properties as its
> master so that it is placed at the same depth in the queue. For example,
> consider we have requests A, B and B', where B & B' are a bonded pair to
> run in parallel on two engines.
>
> A -> B
> \- B'
>
> B will run after A and so may be scheduled on an idle engine and wait on
> A using a semaphore. B' sees B being executed and so enters the queue on
> the same engine as A. As B' did not inherit the semaphore-chain from B,
> it may have higher precedence than A and so preempts execution. However,
> B' then sits on a semaphore waiting for B, who is waiting for A, who is
> blocked by B.
>
> Ergo B' needs to inherit the scheduler properties from B (i.e. the
> semaphore chain) so that it is scheduled with the same priority as B and
> will not be executed ahead of Bs dependencies.
>
> Furthermore, to prevent the priorities changing via the expose fence on
> B', we need to couple in the dependencies for PI. This requires us to
> relax our sanity-checks that dependencies are strictly in order.
>
> Fixes: ee1136908e9b ("drm/i915/execlists: Virtual engine bonding")
> Testcase: igt/gem_exec_balancer/bonded-chain
> Testcase: igt/gem_exec_balancer/bonded-semaphore
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Closes: https://gitlab.freedesktop.org/drm/intel/issues/464
-Chris
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