Previously on my Samsung QX412-S01AU laptop I had been able to get brightness controls working, see this question: Brightness Hotkeys Only Slightly Work.
After a recent automatic update, which upgraded the kernel, my brightness has stopped working and is on an almost unusably low setting. The keyboard shortcuts for brightness can adjust the screen between about 0 and 30%. The slider using the keyboard shortcuts then ceases to move up, every button press just keeps the level at 30%. "Brightness and Lock" in the settings allows you to move the slider the whole way, but never gives you more than the same 30% brightness.
Here's what I've tried so far:
echo "0" | sudo tee /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0/brightness
echo "7" | sudo tee /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0/brightness
These used to work, but now have no effect at all.xbacklight -set 100
This only allows you to change from about 0-30, as above.- I tried to re-apply this answer, but the
acpi_backlight=vendor bit was still in my grub config. - The output of
ls /sys/class/backlight/*/brightnessused to be this:
- Used to be this:
/sys/class/backlight/intel_backlight/brightness - After the update is this:
/sys/class/backlight/intel_backlight/brightness /sys/class/backlight/samsung/brightness
- Used to be this:
The kernel version that came through in the update was linux-image-3.2.0-27-generic:amd64 (3.2.0-27.43).
EDIT- (Ian.B.) Just wanted to add that I have the same problem (Samsung QX411, NVIDIA Optimus) and have removed the acpi_backlight=kernel from the grub and it doesn't change anything. I also tried upgrading to a mainline 3.4 kernel with no effect (with or without the acpi key in grub). Interestingly if I cat the parameters in /sys/class/samsung/brightness/ with the power cord in/out I get:
actual_brightness 8/1
bl_power 0
brightness 4/2
max_brightness 8
Don't know if it's a clue that the brightness and actual brightness keys aren't in sync.
