backlight control lost after resume - screen black after hibernate

[Stefan Rutzinger <rutzi@fs.tum.de>]

Hello,

I run Wheezy on a quite new sony vaio PCG laptop with GeForce GT 555M 
(GF106) and a second intel 2D video card and nouveau FB.

After resume from suspend or hibernate, the backlight control gets lost. I 
still can echo and cat to /sys/class/backlight/acpi_video[0,1]/ but it has 
no effect. After a fresh boot, everything behaves as expected and e.g.
FN-Brightness keys work well.

This is especially annoying since at hibernate-resume the backlight is 
switched _off_ and there is no way to switch it back on anymore. It is 
actually off which can be seen when the memory is restored from disk and 
the cursor is blinking in the corner, one can see a slight shine at the 
edges of the display which disapperas immediately after.

The system however is resumed and working, it can be "shutdown -r now" 
'blind' or be accessed by ssh.

There are a lot of descriptions about the backlight not being restored to 
the correct value but there is nothing about the /sys/../backlight branch 
being without function after resume at all.

Any Ideas?
Stefan