Add "acpi_osi=Linux" in your boot options. For example, modify /etc/default/grub like this:
GRUB_CMDLINE_LINUX_DEFAULT=“quiet splash acpi_osi=Linux”
then run update-grub and reboot.
After boot you will be enable to change backlight power with notebook hardware keys.
Another workaround:
Add "acpi_osi=Linux" in your boot options. For example, modify /etc/default/grub like this: LINUX_DEFAULT= “quiet splash acpi_osi=Linux”
GRUB_CMDLINE_
then run update-grub and reboot.
After boot you will be enable to change backlight power with notebook hardware keys.