[i915] Backlight brighter since 3.9.0

[Aaron Plattner]

On 06/03/2013 12:36 PM, Daniel Vetter wrote:
> 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?

Maybe?  I think I'm the only one who even tries to follow those, so 
"SHOULD" and "MUST" don't really mean a whole lot right now.  One option 
would be to just abandon the idea of standardizing properties, but I do 
think standardization is good.  Where that standard should live, though, 
is a another question.  The kernel doesn't seem like the right place 
since RandR properties are useful on lots of platforms other than Linux.

> -Daniel

-- 
Aaron