Massive power regression going 3.4->3.5

[James Bottomley]

On Tue, 2012-07-31 at 16:09 +0100, James Bottomley wrote:
> On Tue, 2012-07-31 at 07:27 -0700, Keith Packard wrote:
> > James Bottomley <James.Bottomley at HansenPartnership.com> writes:
> > 
> > > on 3.5 killing X causes idle power to go 14W -> 5.9W
> > > on 3.4.6 killing X causes idle power to go 6.8W -> 5.7W
> > 
> > That's actually pretty good news -- you're just not getting to RC6
> > when X is running, but RC6 is otherwise working. And, yes, the GPU
> > really can suck that much power. The debug info that Chris pointed you
> > at should tell a more complete story. For comparison, on my sandybridge
> > box this morning:
> 
> Well, I don't know what it means, but I have a culprit from bisect (I
> managed to manually verify the bisection heads which would step back
> into 3.3).
> 
> 2911a35b2e4eb87ec48d03aeb11f019e51ae3c0d is the first bad commit
> commit 2911a35b2e4eb87ec48d03aeb11f019e51ae3c0d
> Author: Ben Widawsky <ben at bwidawsk.net>
> Date:   Thu Apr 5 14:47:36 2012 -0700
> 
>     drm/i915: use semaphores for the display plane
> 
> I'm going to try building 3.5 with this reverted (it doesn't revert
> cleanly).

OK, I confirm that reverting this patch in 3.5 recovers a ~6.5W idle
power.

James