My brightness function keys always worked, but suddenly they stopped working as well as the “Reduce backlight brightness” option in Power Management Preferences :( Every time I unplugged my power cord the monitor dimmed automatically. Now not even with the function keys I get my brightness down.

This happened in my HP Pavilion dv5-1020ep running Ubuntu 8.10 64 bits.

Here some additional info after searching in the Ubuntu forums and Google:

grep . -r /var/lib/acpi-support/*-*
/var/lib/acpi-support/bios-version:F.20
/var/lib/acpi-support/system-manufacturer:Hewlett-Packard 
/var/lib/acpi-support/system-product-name:HP Pavilion dv5 Notebook PC 
/var/lib/acpi-support/system-version:Rev 1
more /proc/acpi/video/DVGA/LCD/brightness
<not supported>
sudo echo -n 5 > /proc/acpi/video/DVGA/LCD/brightness
bash: /proc/acpi/video/DVGA/LCD/brightness: Permission denied

This happened either after a kernel or hal update…

I found a workaround, that allow the brightness to be changed. This isn’t the fix, if I unplug the power cord the brightness keeps it’s value. Still, the function keys do their job…

Just add acpi_osi=“Linux” to your grub menu:

sudo vi /boot/grub/menu.ls
title           Ubuntu 8.10, kernel 2.6.27-11-generic
root            (hd0,2)
kernel          /boot/vmlinuz-2.6.27-11-generic root=UUID=cba317f7-1d97-4cf8-9ac8-a4a6aaf0ea81 ro quiet splash acpi_osi="Linux"
initrd          /boot/initrd.img-2.6.27-11-generic
quiet