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.
Paul Bolle