[Intel-gfx] LVDS refresh rate power scaling power savings

[Keith Packard]

Matthew Garrett and I were sitting around today and experimenting with
LVDS refresh rates, measuring system power savings. Here's what we
measured:

60Hz	9W
45Hz	8.5W
15Hz	8W

At 15Hz, the display was flickering badly, but at 45Hz, there was no
discernible effect. These measurements were all taken using powertop,
and waiting for the measured power to stabilize, so they're probably not
very accurate. Even still, it looks like there is some significant power
to save here.

I think we should automatically scale the refresh rate to suit the
current usage. We can reduce the refresh rate when the server is idle,
and then bump it back up again when the server gets busy drawing again.
Of course, if DRM is busy, we'll want to leave the refresh high as well.

-- 
keith.packard at intel.com
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