After updating from linux 3.12.8 to 3.13 my display backlight 
switches off when starting X. 
The kernel doesn't  crash, it just switched off the backlight.

Hardware name: Acer Aspire 3820T/JM31_CP, BIOS V1.19 10/27/2010

This is a notebook with an intel/radeon hybrid graphics. 
I use  intel for my X. I report this as radeon problem because 
git bisect (on linux-stable tree) showed the following commit 
as the cause of my problem:

10ebc0bc09344ab6310309169efc73dfe6c23d72 is the first bad commit
commit 10ebc0bc09344ab6310309169efc73dfe6c23d72
Author: Dave Airlie <airlied at redhat.com>
Date:   Mon Sep 17 14:40:31 2012 +1000

    drm/radeon: add runtime PM support (v2)

    This hooks radeon up to the runtime PM system to enable
    dynamic power management for secondary GPUs in switchable
    and powerxpress laptops.

    v2: agd5f: clean up, add module parameter

    Signed-off-by: Dave Airlie <airlied at redhat.com>
    Signed-off-by: Alex Deucher <alexander.deucher at amd.com>


Please let me know if you need addititonal information.