I cannot adjust the brightness of my Samsung R540 laptop for a long time. A year ago, when I had Ubuntu 10.10, installing the ATI Driver fixed my problem. But now I am using Arch Linux and my eyes are burning with the brightness of a thousand suns.
When I plug in the AC adapter, it goes to max brightness.
When I do not, it goes to min.
I have the packages samsung-laptop-git and samsung-tools installed from the AUR. I added samsung-tools to rc.conf.
I did not do any further configuration.
I can echo anything to /sys/class/backlight/acpi_video0/brightness, but it does not make a difference. And even root can't make modifications to other files like max_brightness and actual_brightness.
I also have the open-source ati drivers installed.
I think that's everything.
Thanks in advance

Uninstall samsung-laptop-git and use the mainline version from the kernel (3.5 in testing right now or 3.4.6)
Try adding:

acpi_osi=Linux acpi_backlight=vendor in grub2 (search on google)

And see what happens.

It worked!!! I am overwhelmed with joy, crying on the floor.
My eyes thank you, honorable sir!

Added
GRUB_CMDLINE_LINUX_DEFAULT="quiet acpi_osi=Linux acpi_backlight=vendor"
to /etc/default/grub
than ran update-grub

And I recently learned that you need to select the core kernel (3.4.6) at boot menu, instead of the long-term-support one (which is 3.0.something). These were enough to fix the backlight problem on my Samsung R540.