I own an dell xps 15. I guess the following is because I have hybrid graphics system (nvidia optimus).

When I run command to find out lcd brightness path using following command,

ls /sys/class/backlight/*/brightness

I receive the following output


Now if I echo a value say 5 to either of them, LCD brightness changes. Only thing is intel_backlight path brightness is very low compared to acpi_video0.

Is it normal? I want to know what is the difference (if any) and whether for this two different paths gnome fails to save my brightness settings on reboot.


Brightness could be controlled by ACPI or by graphic driver. I suppose the one that controls your brightness is intel_backlight. However, you could easy find it out adding to your /etc/rc.local (before exit 0):

echo 5 > /sys/class/backlight/acpi_video0/brightness

or, alternatively:

echo 5 > /sys/class/backlight/intel_backlight/brightness

It seems that Fn-Keys do change the value of acpi_video0, the problem is that the controlling item is intel_backlight.

If the system starts with the kernel parameter acpi_backlight=vendor, the item acpi_video0 is replaced by the item intel, but then the Fn-Keys can not change the value of this item.

  • Changing kernel parameter actually worked. I got more steps in brightness control switch. – Web-E May 19 '12 at 11:18

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