[i915] Backlight brighter since 3.9.0

[Daniel Vetter]

On Mon, Jun 03, 2013 at 09:13:18AM -0700, Aaron Plattner wrote:
> On 05/20/2013 02:55 PM, Daniel Vetter wrote:
> >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.
> 
> This property seems like it's generally useful for all GPUs that
> support range compression.  Has anyone started the process of adding
> it to randrproto.txt as an official property?
> 
> http://cgit.freedesktop.org/xorg/proto/randrproto/tree/randrproto.txt#n1723

Oops, I didn't know that we have some properties standardized there,
especially since the existing pile of drm/kms drivers seem to only lously
follow them. Should we move this into the kernel since that's essentially
the place that defines them?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch