[Intel-gfx] [PATCH 7/8] drm/i915/pmu: Wire up engine busy stats to PMU

[Chris Wilson]

Quoting Tvrtko Ursulin (2017-09-26 13:32:25)
> 
> On 25/09/2017 18:48, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2017-09-25 16:15:42)
> >> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>
> >> We can use engine busy stats instead of the MMIO sampling timer
> >> for better efficiency.
> >>
> >> As minimum this saves period * num_engines / sec mmio reads,
> >> and in a better case, when only engine busy samplers are active,
> >> it enables us to not kick off the sampling timer at all.
> > 
> > Or you could inspect port_isset(execlists.port).
> > You can avoid the mmio for this case also by just using HWSP. It's just
> > that I never enabled busy tracking in isolation, so I always ended up
> > using the mmio.
> 
> This would be for execlists only. I could change the main patch to do 
> this, you think it is worth it?

I'm thinking we do the busy = last_seqno != current_seqno approach for
the first patch. Then this patch is purely about the accuracy
improvement for execlists, taking advantage of the existing interrupts.

We could do something similar by forcing the user interrupt and treating
that as context-out, likely to be much more fiddly than execlists and
those processors did not take kindly to a flood of interrupts from the
gpu. (Or that may just be a side-effect of the interrupt handler from a
few years ago.)
-Chris