[Intel-gfx] [RFC] Hack to avoid C-state reduction during graphics activity.

[Eric Anholt]

This is a series I came up with as a result of KS discussions.  The
plan was to pin the CPU C state to keep the GPU going as fast as
possible while it was active, since there's some relation between the
two (we don't know for sure yet, per chipset, whether it's due to
latency of DMA writes having to bring CPU state up, or reduced memory
bandwidth due to bus frequency reduction, or other).  The actual patch
series doesn't achieve that -- a hint is provided to the scheduler and
cpufreq/cpuidle, which may choose not to sleep as deeply as it would
otherwise.

I found a 1% performance difference in nexuiz on my Ironlake system
from the last patch in this series (HEAD~1 didn't have a statistically
significant effect).  If that's all the difference, I'll drop this.
It may pay off higher for non-Ironlake, in which case I'll work on a
"proper" solution of actually clamping C-states while graphics is
active.