Q:

Backlight broken after upgrading to Ubuntu 18.04

Yesterday, I upgraded from Ubuntu 16.04 LTS to Ubuntu 18.04 LTS. Now it seems the backlight is no longer adjustable. There is no change in the screen brightness when using either my Laptop's hotkeys or the slider in settings -> power -> screen brightness. 
I have searched for solutions, and tried various things (such as the suggestions in this question: Brightness problem Ubuntu 18.04 LTS) with no avail. 
I have a System 76 Serval WS laptop. If it helps, here is the output of 
$ lspci | grep VGA
01:00.0 VGA compatible controller: NVIDIA Corporation GM204M [GeForce GTX 970M] (rev a1)

and the output of 
$ ll /sys/class/backlight/
total 0
drwxr-xr-x  2 root root 0 Jul 13 11:24 ./
drwxr-xr-x 72 root root 0 Jul 13 11:24 ../
lrwxrwxrwx  1 root root 0 Jul 13 11:24 acpi_video0 -> ../../devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0/

I would really appreciate any help with solving this problem. Currently the brightness is so high it gives me a headache when looking at me laptop screen for too long. I would also appreciate explanations with any answer, as I am still a novice with Linux (even though I've used Ubuntu for years).

A:

To get my backlight (and HDMI port) working I needed to install the system76 driver: https://support.system76.com/articles/system76-driver/. I guess the driver needed for my system76 laptop didn't carry over during the update from Ubuntu 16.04 to 18.04. 

