[Intel-gfx] [PATCH] drm/i915: Prevent bonded requests from overtaking each other on preemption

[Chris Wilson]

Quoting Chris Wilson (2019-09-20 09:36:24)
> Force bonded requests to run on distinct engines so that they cannot be
> shuffled onto the same engine where timeslicing will reverse the order.
> A bonded request will often wait on a semaphore signaled by its master,
> creating an implicit dependency -- if we ignore that implicit dependency
> and allow the bonded request to run on the same engine and before its
> master, we will cause a GPU hang.

Thinking more, it should not directly cause a GPU hang, as the stuck request
should be timesliced away, and each preemption should be enough to keep
hangcheck at bay (though we have evidence it may not). So at best it runs
at half-speed, at worst a third (if my model is correct).
-Chris