[Intel-gfx] [PATCH] drm/i915/gt: Track engine round-trip times

[Joonas Lahtinen]

Quoting Chris Wilson (2019-12-19 12:50:43)
> Knowing the round trip time of an engine is useful for tracking the
> health of the system as well as providing a metric for the baseline
> responsiveness of the engine. We can use the latter metric for
> automatically tuning our waits in selftests and when idling so we don't
> confuse a slower system with a dead one.
> 
> Upon idling the engine, we send one last pulse to switch the context
> away from precious user state to the volatile kernel context. We know
> the engine is idle at this point, and the pulse is non-preemptible, so
> this provides us with a good measurement of the round trip time. It also
> provides us with faster engine parking for ringbuffer submission, which
> is a welcome bonus (e.g. softer-rc6).
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Stuart Summers <stuart.summers at intel.com>

<SNIP>

> +/* A simple estimator for the round-trip latency of an engine */
> +DECLARE_EWMA(delay, 6, 4)

i915_delay as the minimum to emphasis that this is specific tous,
with that name clarified;

Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>

Regards, Joonas