In this tutorial, we will see a solution that may render your brightness
keys workable again in a laptop/notebook running Ubuntu/Linux Mint.
Before starting, you should bear in mind that this solution is not meant
to fix all laptop or notebook models and is dedicated for users of
intel cards for adjusting brightness levels. Also, this tip is only
workable on Ubuntu/Linux Mint with GRUB 2.
Getting Started
You can check if you have an intel card by running this command:
ls /sys/class/backlight/
If the returned outputs show this:
(...) intel_backlight
Then this solution may work for you. In the same terminal window, enter this command:
sudo gedit /etc/default/grub
Find this line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
Change it to this one:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Save your file and exit, then run this command to update GRUB 2:
sudo update-grub
Finally, reboot your system.
Good luck!