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.
josh