Arch Linux

Description:
Since the 2.6.38.2 kernel, I can't change my laptop's brightness. It worked in 2.6.37.6 version.

I try to change it by using three ways :
- Fn + brightness up/down keys : it doesn't work
- echo <number> > /sys/class/backlight/acpi_video0/brightness : it doesn't work (but the file exists)
- xbacklight command : it prints "No outputs have backlight property"

I have updated my kernel and I have tried to change the acpi_osi and acpi_backlight boot options, but it didn't work. I have posted a message on ArchLinux BBS (https://bbs.archlinux.org/viewtopic.php?pid=960197), but nobody could help me.

Additional info:
Kernel version : 2.6.39.3
Laptop : HP Probook 4515s
Graphic card : 01:05.0 VGA compatible controller [0300]: ATI Technologies Inc RS780M/RS780MN [Radeon HD 3200 Graphics] [1002:9612]
Driver : xf86-video-ati (radeon)
xorg-server version : 1.10.3-1
Desktop environment : KDE

I have attached my xorg.conf file. Tell me if you need additional information.

Steps to reproduce:
Try to change the brightness as said in the description.

P.S. : Please forgive my bad English, but I'm not a native speaker.