Xubuntu 18.04: make Fn brightness keys work with /sys/class/backlight/nv_backlight/

The nouveau driver now (some time ago it didn't) creates the /sys/class/backlight/nv_backlight and it works. So passing acpi_backlight=vendor to the kernel disables the /sys/class/backlight/acpi_video0. Without acpi_video0, the backlight_helper_get_best_backlight() will return the nv_backlight out of the box.

So, open /etc/default/grub, edit:

GRUB_CMDLINE_LINUX_DEFAULT="[...] acpi_backlight=vendor"

Save and sudo update-grub.

In case somebody still has issues with Xubuntu 18.04 backlight keys, which enable Brightness popup however don't increase/decrease brightness bar, here is the way which helped me. It appears that brightness keys were simply disabled in XFCE4 power manager configuration:

xfconf-query -c xfce4-power-manager -p /xfce4-power-manager/handle-brightness-keys
=> false

Therefore this fixed the situation:

xfconf-query -c xfce4-power-manager -p /xfce4-power-manager/handle-brightness-keys --create -t bool -s true