Hi, it took me quite some time to get here, but hopefully you can still solve 
my problem.
I'm running a 2006 Intel iMac (Core Duo) under Unbuntu 12.04LTS server, which 
sits on my desk and almost never log to the console, which would normally go 
blank a few minutes after boot.

There is strong evidence that this patch found its way to my "stock" kernel 
back in september 2014, and since then my iMac exhibits the following behaviour 
after boot:
- whenever the console blanking activates, backlight goes to MAX (with my LCD 
screen going all strange and overheating — even causing fans to blow)
- only way out is to echo 0 > /sys/class/backlight/radeon_bl0/brightness — 
which I 'crontabbed' but is not exactly satisfactory.
- any other value causes the backlight to go all the way up and overheat.

I did try to "echo 1 > /sys/class/backlight/acpi_video0/brightness"
today, and that caused the iMac to reboot.

I have not experimented anything else, but would be happy to help.

uname is "Linux xxx 3.13.0-49-generic #81~precise1-Ubuntu SMP Wed Mar 25
16:32:40 UTC 2015 i686 i686 i386 GNU/Linux"

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https://bugs.launchpad.net/bugs/1363623

Title:
  [ASUS-A7K laptop] screen brightness dimmed after kernel update

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