Comment 55 for bug 969489

I'm on a Dell XPS 15z. I detail below other attempts (all pretty much taken from this thread but unsuccessful for my setup); instead, what works for me is to (1) change in /etc/default/grub the variable GRUB_CMDLINE_LINUX_DEFAULT to "acpi_backlight=vendor dell_laptop.backlight=0 quiet splash", i.e., adding the entire part beginning "acpi_backlight..." ahead of "quiet splash", as suggested in

https://wiki.ubuntu.com/HardwareSupport/Machines/Laptops/Dell/XPS/15z

(thanks).

What also worked was (2) replace the Display Manager lightdm with gdm, i.e., in a terminal do

sudo apt-get install gdm

and then select gdm, although obviously that's not a fix for a lightdm problem. [Alternatively, "sudo dpkg-reconfigure gdm" if you're, luckily, already running lightdm or some other DM.]

Trials that failed for me:

A. In /etc/init/lighdm.conf
Add line "sleep 10" just before line "exec lightdm"
Add flag "-d" to "exec lightdm"
Add line "and stopped udevtrigger" just after line "and started dbus"
Add line "respawn" right before "emits login-session start"
(whether in combination or singly).

B. In a terminal "sudo restart lightdm" or "sudo service lightdm restart" or "sudo service lightdm --full-restart".