drm/msm: Devfreq tuning

drm/msm: Devfreq tuning

This adds a few things to try and make frequency scaling better match
the workload:

1) Longer polling interval to avoid whip-lashing between too-high and
   too-low frequencies in certain workloads, like mobile games which
   throttle themselves to 30fps.

   Previously our polling interval was short enough to let things
   ramp down to minimum freq in the "off" frame, but long enough to
   not react quickly enough when rendering started on the next frame,
   leading to uneven frame times.  (Ie. rather than a consistent 33ms
   it would alternate between 16/33/48ms.)

2) Awareness of when the GPU is active vs idle.  Since we know when
   the GPU is active vs idle, we can clamp the frequency down to the
   minimum while it is idle.  (If it is idle for long enough, then
   the autosuspend delay will eventually kick in and power down the
   GPU.)

   Since devfreq has no knowledge of powered-but-idle, this takes a
   small bit of trickery to maintain a "fake" frequency while idle.
   This, combined with the longer polling period allows devfreq to
   arrive at a reasonable "active" frequency, while still clamping
   to minimum freq when idle to reduce power draw.

3) Boost.  Because simple_ondemand needs to see a certain threshold
   of busyness to ramp up, we could end up needing multiple polling
   cycles before it reacts appropriately on interactive workloads
   (ex. scrolling a web page after reading for some time), on top
   of the already lengthened polling interval, when we see a idle
   to active transition after a period of idle time we boost the
   frequency that we return to.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210726144653.2180096-4-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>