[Intel-gfx] [PATCH] drm/i915/execlists: Inhibit context save/restore for the fake preempt context

[Chris Wilson]

Quoting Chris Wilson (2018-01-23 17:05:57)
> We only use the preempt context to inject an idle point into execlists.
> We never need to reference its logical state, so tell the GPU never to
> load it or save it.
> 
> Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Michal Winiarski <michal.winiarski at intel.com>
> Cc: Michel Thierry <michel.thierry at intel.com>
> Cc: Michal Wajdeczko <michal.wajdeczko at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> ---
> Please check the register definition. I know the save-inhibit bit used
> to exist, I don't know if it is BIT(1) in RING_CONTEXT_CONTROL -- it
> feels like it should be, and the numbers indicate that it does
> something beneficial.

Fwiw, seems to be within normal variation. Just had a string of bad runs
on drm-tip contrasting with a set of good runs with the patch. With a
large enough sample size, no significance. :( Maybe with a bit more tuning
this will have larger impact. Still, I think it's the right thing to do.
-Chris