[Intel-gfx] [PATCH] drm/i915: Only emit one semaphore per request

[Chris Wilson]

Quoting Chris Wilson (2019-04-01 15:18:38)
> Ideally we only need one semaphore per ring to accommodate waiting on
> multiple engines in parallel. However, since we do not know which fences
> we will finally be waiting on, we emit a semaphore for every fence. It
> turns out to be quite easy to trick ourselves into exhausting our
> ringbuffer causing an error, just by feeding in a batch that depends on
> several thousand contexts.
> 
> Since we never can be waiting on more than one semaphore in parallel
> (other than perhaps the desire to busywait on multiple engines), just
> pick the first fence for our semaphore. If we pick the wrong fence to
> busywait on, we just miss an opportunity to reduce latency.
> 
> An adaption might be to use sched.flags as either a semaphore counter,
> or to track the first busywait on each engine, converting it back to a
> single use bit prior to closing the request.

Grrr, this disables semaphores for basic-sequential gem_exec_nop. Maybe
allow semaphores for the same GEM context regardless.
-Chris