[Debian] Fix the backlight function keys on Intel GPU laptop

*UPDATE* March 10th, 2019

I have to update my post because the previous fix does not work anymore with recent versions of debian + GNOME 3, on the same laptop as before (sandybridge core i3).

Now, you have to follow this suggestion given by people all around the world: edit the /etc/default/grub file in super user mode and insert acpi_backlight=none to the GRUB_CMDLINE_LINUX_DEFAULT variable. For example, here is what I have:

GRUB_CMDLINE_LINUX_DEFAULT="quiet acpi_backlight=none"

After editing this file, run sudo update-grub and reboot. Backlight keys should work now.

You can get rid of the 20-intel.conf file if you have used it.

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Tired

Time spent to get the info: one day. Length of the fix: a few lines. Haha, this is so ridiculous it makes me smile a little.

Okay, so you are:

1. on fresh Debian Jessie (GNOME 3),
2. on a laptop,
3. with an integrated graphics unit,
4. and it is an Intel CPU. In my case, I have a Core i3 Sandy Bridge CPU with HD Graphics 3000.

The backlight function keys:

1. are working so you have an on-screen display feedback,
2. are not changing the effective brightness of your screen,
3. are changing the values of this file: /sys/class/backlight/acpi_video0/brightness (close and reopen in gedit to see the value changing or check by command line).

You have this folder: /sys/class/backlight/intel_backlight.

Yay! You may have a solution!

1. Create a file:

   sudo gedit /usr/share/X11/xorg.conf.d/20-intel.conf

2. Add this in the file:

   Section "Device"
           Identifier "card0"
           Driver "intel"
           Option "Backlight" "intel_backlight"
           BusID "PCI:0:2:0"
   EndSection

3. Save file.
4. Close gedit.
5. Restart the computer.
6. It should work now!

If this fix does not work on your laptop, I am sorry to hear that but I cannot help you further. Don’t ask the noob. I have just followed guidance on linux threads such as here: link to Ubuntu’s launchpad website. Good luck!