This is a script to control screen brightness on Lenovo ThinkPad X220 and similar after you changed PWM frequency by writing a custom value to 0xC8254 register. I use it with acpid.
The script reads current PWM frequency from the register, calculates correct new brightness level for
/sys/class/backlight/intel_backlight/brightness and writes it.
To increase brightness:
/etc/acpi/default.sh on my Gentoo box:
case "$group" in video) case "$action" in brightnessup) /usr/local/bin/write-intel-brightness + ;; brightnessdown) /usr/local/bin/write-intel-brightness - ;; esac ;; esac