[Intel-gfx] i915: Regression: +4W in idle power use on Macbook Pro 15 (late 2013)

[Ville Syrjälä]

On Mon, Aug 25, 2014 at 05:19:49AM -0700, Eric Rannaud wrote:
> Hi Jani,
> 
> Is there a way to restore the prior lower power consumption when
> idling? As I understand it, FBC should not have a direct effect on
> power consumption when idle, only when the FB is actively refreshed.
> Is it understood why no FBC would have such a dramatic impact (+4W) on
> a system sitting idle?

FBC works best when the screen contents don't change. The more activity
on the screen the less effective FBC becomes. 4W sounds way too much for
FBC however. 0.4W is closer to what one might expect from FBC based on my
observations. 4W sounds more like the difference between min vs. max
display brightness to me.

> 
> I'm aware of i915.i915_enable_rc6=1 and i915.lvds_downclock=1, which I
> haven't yet tried. Is there anything else I should also try?
> 
> Thanks,
> Eric
> 
> On Mon, Aug 25, 2014 at 3:22 AM, Jani Nikula <jani.nikula at intel.com> wrote:
> >
> > [just moving from lkml to intel-gfx for a better fitting audience]
> >
> > On Mon, 25 Aug 2014, Jani Nikula <jani.nikula at intel.com> wrote:
> >> On Fri, 22 Aug 2014, Eric Rannaud <eric.rannaud at gmail.com> wrote:
> >>> Hi,
> >>>
> >>> Between 3.15.4 and 3.15.8, there was an increase in idle power consumption on
> >>> Apple Macbook Pro 15 (late 2013) on a freshly booted system (no wifi driver
> >>> loaded; brightness set to 4/100; X running; no desktop environment, except
> >>> Awesome), from 6.5W to about 10.5W, as reported by powertop.
> >>>
> >>> In the stable tree, it bisects to:
> >>>         commit f4db98240ac2c6d9d2118c6f82d483ff5293f1ed
> >>>         Author: Chris Wilson <chris at chris-wilson.co.uk>
> >>>         Date:   Fri Jun 6 10:37:11 2014 +0100
> >>>
> >>>             drm/i915: Disable FBC by default also on Haswell and later
> >>>
> >>>             commit 0368920e51ae0cded0eb518c340a4dd17764d461 upstream.
> >>>
> >>>             It causes black screen on bootup and is approximately 100x
> >>> slower than
> >>>             running with FBC disabled, so the GPU runs at a high
> >>> frequency for much
> >>>             longer - completely contrary to the power saving claims.
> >>> It also still
> >>>             has mutex deadlocks in multi-head scenarios, which can lead to a
> >>>             system/X lockup. These bugs were known before FBC was
> >>> enabled by default
> >>>             on Haswell and still have not been fixed.
> >>>
> >>> The issue is still present in Linus' tree (v3.17-rc1-22-g480cadc2b7e0).
> >>>
> >>> With a 75Wh battery, that's a significant loss in battery life in normal use.
> >>>
> >>> I'll be happy to help test any potential fix.
> >>
> >> The earlier regression trumps, and in this case it was enabling FBC by
> >> default on Haswell. Sorry.
> >>
> >> You can enable FBC with i915.enable_fbc=1 module parameter, but all bets
> >> are off. See the commit message you quoted above. I don't recommend.
> >>
> >> BR,
> >> Jani.
> >>
> >>
> >> --
> >> Jani Nikula, Intel Open Source Technology Center
> >
> > --
> > Jani Nikula, Intel Open Source Technology Center
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-- 
Ville Syrjälä
Intel OTC