Ever since I upgrade the kernel to version 4.0.5 I can no longer adjust the brightness of my laptop, using fn+f2 or fn+f3, using the slider in the KDE taskbar (shows 0% at all times), or using xbacklight (No outputs have backlight property). However I can still adjust birghtness by editing the value in /sys/class/backlight/radeon_bl0/brightness. This all started right after the upgrade to 4.0.5, nothing else changed. I have looked through the config several times, but I can't find anything that might cause this, my .config is here: https://bpaste.net/show/9a44cb2dc1e9
I have been stuck with this for several weeks now, and I hope someone here might have an answer.

There are a few kernel parameters you can try.
https://wiki.archlinux.org/index.php...d-line_options
https://github.com/torvalds/linux/bl...parameters.txt

It is more likely that this is an XWindows/XOrg [i](in other words, "the GUI") issue. Maybe your changes actually affected some setting there ... or maybe the GUI software needs to be upgraded to match the new kernel.

The GUI, of course, like KDE, is "strictly userland." The Linux kernel neither knows nor cares if you're using a GUI.

(Guess?)

While these links are useful, I can't find my answer here, the directory /sys/class/backlight, just contains the radeon folder.

I had guessed that as well, but I can't find anything in config files that might be causing this, besides ACPI is supposed to adjust brightness with the fn+f2/f3 keys independent of X/KDE etc, or so I've been told.