3.13 i915 brightness settings broken when going from docked -> undocked

[Josh Boyer]

On Wed, Feb 19, 2014 at 9:20 PM, Josh Boyer <jwboyer at fedoraproject.org> wrote:
> Hi All,
>
> We've had a rather weird report[1] of the brightness adjustments being
> broken in a specific case with Thinkpad x220 hardware (SandyBridge
> based).  If you boot the machine with it in a dock and then undock,
> the brightness adjustments do not work.  That is with either the FN
> keys or the GNOME brightness slider.
>
> I can see that the value of
> /sys/class/backlight/acpi_video0/brightness increases/decreases but
> /sys/class/backlight/intel_backlight/brightness doesn't reflect any
> changes.  With 3.12 this works, and oddly with 3.14-rc1 it works
> (specifically, it starts working around v3.13-10231-g53d8ab2 which is
> right after the first DRM merge for 3.14).  With 3.13, if I undock and
> echo a higher value in the intel_backlight_brightness sysfs entry, the
> brightness will actually increase so it can be done manually, but it
> does not work as you'd expect.
>
> I'm in the middle of trying to do a reverse bisect for which patch
> fixes it in the 3.14-rcX series, but that's taking a while.  I thought
> I'd email and see if anyone already knows about this situation, what
> patch in 3.13 broke this, and which one then fixed it again.  Thus far
> all I've gathered is that backlight handling is confusing.

The reverse bisect between 3.13 and 3.14-rc1 didn't prove fruitful,
either because I messed it up or there's a combination of things that
fix the issue.  So instead I did a regular git bisect between 3.12 and
3.13 to see which commit _broke_ things and caused the above behavior.
 That landed me at:

Author: Jesse Barnes <jbarnes at virtuousgeek.org>
Date:   Thu Oct 31 18:55:49 2013 +0200

    drm/i915: make backlight functions take a connector

I have no idea if that makes sense or not, but it's at least something
that seems to be in a relevant area of code.  Does anyone involved in
that know why it would cause the above symptoms on 3.13, and which
commit(s) fix it in 3.14-rc1?

josh