I had some progress here. I attached Vista to WinDbg and traced inside the 
kernel and drivers. I found what it does to change the backlight, but I need 
some more work in order to port this to Linux. The actual change happens with a 
call to WRITE_REGISTER_ULONG in a kernel virtual address (9XXXXXXX) with values 
from 800000f0 (low) to 80000401(full). It seems prety straighforward. There is 
no port IO as in the pommed driver, and I dont know how this address (9XXXXXXX) 
is translated to nvidia register addresses and/or physical bus addresses. There 
are calls to HalTranslateBusAddress and mmMapIoSpace at the initialization of 
the driver, I'll have to trace those and see where they map the used address 
spaces. I will work some more on it and post the results. 
 
If anyone is experienced with memory mapped I/O stuff in both Win and Linux, 
please help!

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No Screen Backlight Control; Sony Vaio with nvidia 8 series graphics
https://bugs.launchpad.net/bugs/95444
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