Here is a workaround from Aaron Lu on linux-acpi: specify the backlight option in xorg.conf.
Section "Device" Identifier "Default Device" Option "Backlight" "intel_backlight" Driver "intel" BusID "PCI:0:2:0" EndSection
Here is a workaround from Aaron Lu on linux-acpi: specify the backlight option in xorg.conf.
Section "Device"
Identifier "Default Device"
Option "Backlight" "intel_backlight"
Driver "intel"
BusID "PCI:0:2:0"
EndSection