Comment # 20 on bug 907368 from Andrej Lazo

(In reply to Takashi Iwai from comment #15)
> 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.

do you mean this?
#######################
linux-b4vl:~ # evtest /dev/input/event5
Input driver version is 1.0.1
Input device ID: bus 0x19 vendor 0x0 product 0x6 version 0x0
Input device name: "Video Bus"
Supported events:
  Event type 0 (EV_SYN)
  Event type 1 (EV_KEY)
    Event code 224 (KEY_BRIGHTNESSDOWN)
    Event code 225 (KEY_BRIGHTNESSUP)
    Event code 227 (KEY_SWITCHVIDEOMODE)
    Event code 241 (KEY_VIDEO_NEXT)
    Event code 242 (KEY_VIDEO_PREV)
    Event code 243 (KEY_BRIGHTNESS_CYCLE)
    Event code 244 (KEY_BRIGHTNESS_ZERO)
    Event code 245 (KEY_DISPLAY_OFF)
Properties:
Testing ... (interrupt to exit)

##########

doesn't register