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

[Eric Rannaud]

On Tue, Aug 26, 2014 at 1:59 PM, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>> Forcing FBC with i915.enable_fbc=1 brings the idle power consumption
>> back to under 7W, however.
>> This is all on 3.15.4-ARCH-00041-gf4db98240ac2.
>
> Any significant changes in package C state as reported in powertop?
> Indeed fairly impressive how much fbc saves here ...

Not that I can tell.
    Powertop report with FBC: http://pastebin.com/5qfJKpTQ
    Without FBC: http://pastebin.com/NaYkR4n0

Some highly uneducated guesses on what could explain a +4W jump with no FBC:

#1- The higher DRAM and bus duty cycle during scanout is enough to
prevent some DRAM subsystems from sleeping, by crossing some tight
threshold (maybe Apple has FBC enabled, so parameters somewhere in
firmware are tuned for the lower level of background activity they
expect with FBC on?). Not much we can do about that, unless such
parameters can be tweaked by us.

#2- Without FBC, the FB doesn't fit in L3 (i7-4750HQ has 6MB,
2880x1800 compressed at least 1:4 fits), keeping the DRAM awake more.

#3- Disabling FBC somehow affects the layout of the framebuffer in
DRAM, keeping more of the DRAM active and awake during scanout.
Different tiling, swizzling, etc. parameters? Is it worth looking at
the code for that kind of thing? To be clear, I'm (blindly) suggesting
that it might be possible to increase the locality of the framebuffer
in physical DRAM, even without compression enabled.

Actually, with an image of white-noise displayed fullscreen while
powertop takes a 20 second measurement, the idle power consumption
shoots up to 11W, with FBC enabled. This experiment actually
invalidates my guess #3. The white noise image will not compress much
at all, while my typical test screen is a couple of static
black-and-white terminal windows (filling up the screen), which will
compress well.

So it would appear 4W is the actual cost of having a full-size 20MB
framebuffer on this system.

Anything an outsider can do to help getting FBC enabled by default again?

Thanks,
Eric