https://bugzilla.kernel.org/show_bug.cgi?id=60659
--- Comment #4 from Charles-Antoine Couret renault@fedoraproject.org --- With default option, after boot :
[root@DiNozzo ~]# cat /sys/class/backlight/acpi_video0/brightness 20
Same conditions, after pressure of FN down brightness key :
[root@DiNozzo ~]# cat /sys/class/backlight/acpi_video0/brightness 10
I suppose the driver can write this file after that. ;)