Re: i915: Regression: +4W in idle power use on Macbook Pro 15 (late 2013)

[Jani Nikula <jani.nikula@intel.com>]

On Fri, 22 Aug 2014, Eric Rannaud <eric.rannaud@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@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