Comment # 15 on bug 907368 from Takashi Iwai

WMI hotkey is an unlikely target.  It might provide something about brightness
but only for old models.  The newer models should give the key either via ACPI
video bus or the normal i8042 serial I/O (aka PS/2 port).
On openSUSE 13.1, it looks like that the former one took.  But the behavior
depends on BIOS, i.e. ACPI OSI string may influence somehow, so it's not really
sure.

Try to find out the event device for video bus, and check whether the key comes
with it.

Also, check the value in
/sys/module/video/parameters/brightness_switch_enabled.
If it's N, try to turn it on:
  echo 1 > /sys/module/video/parameters/brightness_switch_enabled
and retest.