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ISSUE: Backlight brightness does not change.

More Detail: Brightness will not change, using both 'System Settings->Screen', or FN keys (Brightness bar shows and moves, but screen brightness does not change).

Notcied a post in this thread (http://ubuntuforums.org/showthread.php?t=1866283) about having multiple folders in Sys->Class->Backlight...

I HAVE TWO FOLDERS TOO!

'intel_backlight' and 'acpi_video0'

Using the function keys, alters the value in the acpi_video0's 'Brightness' file - But doesn't actually alter the brightness of the screen.

If I add 'backlight=vendor' in Grub, my function keys then edit the value in the 'Intel_Backlight brightness file. - But again doesnt actually change the brightness of the screen.

Computer: Fujitsu Siemans Pi2515, Intel Integrated Graphics, No hdd partition.

Already Tried:

-Editing grub to contain: acpi_osi=Linux acpi_backlight=vendor

-http://ubuntuguide.net/change-screen-brightness-with-fn-key-in-ubuntu-11-0410-10

-sudo apt-get install acpi

-$ sudo setpci -s 00:02.0 F4.B=20

-Brightness does not adjust in fallback mode either.

-Reinstalling OS, Using Linux Mint (Same problem).

-Upgrading and downgrading BIOS.

Many thanks for reading, I understand this problem may need a bit of a Linux pro to sort. If anyones up for the challenge i'll spend any amount of time being walked through this, posting results. Don't want to give up here!

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3 Answers

I solved this problem by editing the following file as root:

/etc/default/grub

GRUB_CMDLINE_LINUX="acpi_backlight=vendor"

Then as root run:

/usr/sbin/update-grub

Then reboot and your brightness keys should function as expected.

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I have a very similar problem with a Toshiba Portege R935 in 12.04. I can't change the brightness after suspending and resuming.

The Grub line didn't help either (actually with that line, it stopped working even before suspending).

In the meantime, I discovered this workaround:

  • as root (not with sudo but actually changing user: su root), change the value in sys/class/backlight/intel_backlight/brightness

    for instance:

    echo 2937 > /sys/class/backlight/intel_backlight/brightness
    

Warning: these values are not between 0 and 7 (as the ones in the other directory), but between 284 and 4539, or something like that.

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If you tried everything else and your system's backlight non-responsive and if you are using an Intel 915 or descendent thereof ("945GSE", etc.) then your issue may be backlight inversion confusion on gen4.

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