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

[Paul Bolle]

On Wed, 2014-02-19 at 21:20 -0500, Josh Boyer wrote:
> 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.

On an (rather old) ThinkPad X41, which also uses i915, brightness
adjustments stopped working altogether in v3.14-rc1 (I haven't used its
docking station in the v3.14 release cycle). In v3.13.y things behave as
expected. So perhaps there's actually a more general problem here.

> I can see that the value of
> /sys/class/backlight/acpi_video0/brightness

On the X41 I check /proc/acpi/ibm/brightness ...

>  increases/decreases but
> /sys/class/backlight/intel_backlight/brightness doesn't reflect any
> changes.

but otherwise things look similar.

>  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.

Echoing values into /sys/class/backlight/intel_backlight/brightness
works too (that's a new trick for me!). But, again, no docking station
is required, so the problem looks less odd than the problem on that
x220.

> 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.

I haven't yet tried bisecting.

> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1067071


Paul Bolle