[i915] Backlight brighter since 3.9.0

[Daniel Vetter]

On Sat, May 18, 2013 at 12:39:14AM +0200, Jan Hinnerk Stosch wrote:
> Hallo,
> 
> I hope this is the right place to ask, because I actually don't know
> whether it is a bug or a feature that I'm experiencing since linux 3.9:
> When I boot my system the backlight gets extremely bright compared to older
> kernel versions. It is most obvious when I leave X (more a yellow than a
> black background), but I have the impression, that the colors in X are
> brighter than usual, too.
> I used my spare time this afternoon to do a kernel bisect and learned that
> the first "bad" commit is 55bc60db5988c8366751d3d04dd690698a53412c. As I
> don't have insight or understanding of the code: Is this behaviour intended
> and how could I change it to the old state or is it a bug and should I
> report it somewhere?
> My system is as follows:
> Intel i5-3570k with Intel HD 4000
> my monitor is connected via HDMI.
> If you need any more information just tell me.

Yeah, this is a feature. HDMI has (for oddball backwards compat with
analog TV signals) a special mode which reduces the useable RGB value
range by chopping off about 10% at the bottom and top end. This results in
light colors getting brighter and dark colors getting darker.

The above mentioned commit tries (to the best of our knowledge) to
auto-set the option which most likely fits what the hdmi sink will do with
the color data. You can either fix this up in the hdmi sink with the
on-screen menu or by manually setting the "RBG Broadcast" property for the
relevant hdmi connector to the setting you want.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch