Since the upgrade to Ubuntu kernel 3.13.0-35 (from 3.13.0-34), the screen is
very dark to the point it's unusable.
Backlight seems completely turned off. Using the brightness keys doesn't help.
It was working fine with 3.13.0-34.
I found that the offending patch is "drm/radeon: set default bl level to
something reasonable".
I reverted this patch and recompiled 3.13.0-35, and it solved the problem.
I also checked that this problem also affects mainline kernel 3.17rc3.
More technical details can be found on Ubuntu's bugtracker :
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1364148