My screen brightness doesn't work as many others have reported. So I dug around...
ls /sys/class/backlight generates two entries:
This does nothing:
echo 20 > /sys/class/backlight/acpi_video0/brightness
This however does:
echo 20 > /sys/class/backlight/radeon_bl0/brightness
It makes me think the brightness control is targeting the wrong one. How can I fix this?