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Asked: 2012-05-11 05:27:23 +0800 CST

Why there are two brightness control file (/sys/class/) in my system

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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

/sys/class/backlight/acpi_video0/brightness
/sys/class/backlight/intel_backlight/brightness

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 12.04 nvidia-optimus

1 Answers

  1. Best Answer

    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.

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