[Intel-gfx] [PATCH] drm/i915: Don't update the render-clock for every bo.

[Eric Anholt]

On Mon, 30 Nov 2009 01:08:56 +0000, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> Only update the render-clock on transition from busy to idle and vice
> versa, or else we burn a significant percentage of the cpu just rewriting
> the register -- not quite as power-friendly as intended ;-)
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Jesse Barnes <jbarnes at virtuousgeek.org>

Mostly looks good.

With this change, though, there seems to be a race where the idle timer
could fire making busy = false, new work comes in and marks busy = true
(setting render clock), then the scheduled work for idling the GPU
happens and busy is untouched but the clocks get dropped.  Any following
work would be done at low render clock, until busy = false happens again
and we get another chance to wake up.