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

[Eric Rannaud]

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?

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