[Intel-gfx] [PATCH 2/3] drm/i915: Boost RPS frequency for CPU stalls

[Jesse Barnes]

On Wed, 2 Oct 2013 01:33:24 +0100
Chris Wilson <chris at chris-wilson.co.uk> wrote:

> On Tue, Oct 01, 2013 at 03:39:40PM -0700, Jesse Barnes wrote:
> > On Tue, 1 Oct 2013 23:23:32 +0100
> > Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > 
> > > On Tue, Oct 01, 2013 at 02:54:26PM -0700, Jesse Barnes wrote:
> > > > On Wed, 25 Sep 2013 17:34:56 +0100
> > > > Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > > > 
> > > > > +void gen6_rps_idle(struct drm_i915_private *dev_priv)
> > > > > +{
> > > > > +	mutex_lock(&dev_priv->rps.hw_lock);
> > > > > +	if (dev_priv->info->is_valleyview)
> > > > > +		valleyview_set_rps(dev_priv->dev, dev_priv->rps.min_delay);
> > > > > +	else
> > > > > +		gen6_set_rps(dev_priv->dev, dev_priv->rps.min_delay);
> > > > > +	mutex_unlock(&dev_priv->rps.hw_lock);
> > > > > +}
> > > > 
> > > > Looks pretty good, but I think these should be rpe_delay instead.  Not
> > > > much point in going down to a less efficient frequency...
> > > 
> > > Less efficient for what? My concern here is only with power draw when
> > > idle. As soon as we start to render again (well very shortly afterwards
> > > with this particular iteration) we bump up to rpe and then beyond.
> > > 
> > > Correct me if I am wrong but rpe is an inflection point rather than a
> > > minumum?
> > 
> > So yes, running at a lower than RPe freq will use less power, but it'll
> > also be less efficient (perf/power) than doing the same rendering at
> > RPe.
> 
> Right, so I think so long as autotuning works, we can run at low power,
> low efficiency for as long as that is capable of sustaining the desired
> throughput and latency. And the heuristics we have here are pretty good
> at detecting when more power is required.

The downside is that we'll take longer to enter RC6 if we run at the
slower speed.   And supposedly, running at RPe so we get to RC6 a
little faster ends up saving more power than running slower, even
though running slower uses less power while active.

Or have you taken that into account here?

Jesse