it seems like there are 2 solutions for this problem, but sometimes none of these work. I tried changing the boot options (editing-updating grub and stuff) and that didn't work. As I have seen in many forums (and in my case too), the change in the xorg.conf file won't work because there is no such file... so?
After searching a couple hours, I found something interesting:
those lines supposedly get the driver working as it should, but the original problem remains. The important thing is that those lines CREATE the xorg.conf file, so now it is possible to apply the second solution (adding the
Option "RegistryDwords" "EnableBrightnessControl=1" line to the file).
Hope this helps someone, I used this on a Samsung R580, with a Nvidia GeForce310m and worked just fine.
By the way, I used
sudo gedit /etc/X11/xorg.conf instead of the
nano command, it's just a matter of tastes I guess...