try modifying:
/etc/default/grub
add:
acpi_backlight=vendor
to the line:
GRUB_CMDLINE_LINUX_DEFAULT=”quiet splash”
to get:
GRUB_CMDLINE_LINUX_DEFAULT=”quiet splash acpi_backlight=vendor”
save and execute:
sudo update-grub
restart
try modifying:
/etc/default/grub
add:
acpi_backlight=vendor
to the line:
GRUB_CMDLINE_LINUX_DEFAULT=”quiet splash”
to get:
GRUB_CMDLINE_LINUX_DEFAULT=”quiet splash acpi_backlight=vendor”
save and execute:
sudo update-grub
restart