I have a Dell XPS 13. I can get the backlight to work if I merely do the following
echo "0" > /sys/class/backlight/intel_backlight/brightness
When my laptop suspends, I loose control of the backlight and have to repeat the command above. I also created a service to issue that command on boot via systemd, but that has no effect until I am in X and issue a
systemctl restart backlightfix
Where backlightfix is the following, and the 'fixbacklight.sh' is the echo command above.
[Unit] Description=Fix Backlight [Service] Type=oneshot RemainAfterExit=yes ExecStart=/usr/local/bin/fixbacklight.sh [Install] WantedBy=multi-user.target