[Intel-gfx] [PATCH 1/2] drm/i915: enable deepest RC6 state

[Ben Widawsky]

On Tue, Nov 29, 2011 at 01:01:31PM +0000, Chris Wilson wrote:
> On Tue, 29 Nov 2011 10:55:04 -0200, Eugeni Dodonov <eugeni.dodonov at intel.com> wrote:
> > This should allow even more energy saving when GPU is not in use.
> > According to the testing, this state results in around 0.1 - 0.4 W better
> > power usage.
> > 
> > No issues or regressions observed so far, but additional testing is
> > certainly welcome.
> 
> The docs I saw said "not implemented; do not use". Do we have it on good
> authority that this is safe and useful to enable? And doesn't it
> require programming of more transition thresholds?
> -Chris

Yes, I think we do have to program more stuff to make this work. Perhaps
the BIOS puts in decent values for these registers though? We would have
to restore those on reset and resume I'd guess. If BIOS doesn't use
anything there, you probably aren't even entering these states.

Also, for posterity, there are 3 rc6 states, rc6, deep rc6, and deepest
rc6. I think deepest rc6 was recommended to avoid (though I don't recall
a specific root caused issue, just some data from Jesse, from the
windows team that it didn't seem stable). And I think deepest rc6 is
also referred to as rc7 sometimes.

IIRC, Jesse had patches for deep rc6, but it was very minimal
performance gain. Someone should check if the windows driver uses it.

Ben