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

[Eugeni Dodonov]

On Tue, Nov 29, 2011 at 19:42, Ben Widawsky <ben at bwidawsk.net> wrote:

> 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.
>

We already setup the variables for both deep and deepest rc6 in our driver
(GEN6_RC6p_* and GEN6_RC6pp_*), but we weren't using this additional state
previously - if I understood the documentation and the code correctly, we
do enable plain rc6 and deep rc6 currently. I haven't found any indications
which would tell to avoid it in the latest docs, and I also haven't seen
any regressions or issues with it being enabled on any of the machines, so
I thought it would be worth trying that additional state as well.

>From the testing which QA did for this patch, looks like we save between
0.1 and 0.4W when compared to what we had with i915_enable_rc6=1.

But in any case, this is all highly experimental and I'll do more testing
with it :).

-- 
Eugeni Dodonov
<http://eugeni.dodonov.net/>
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