ACPI DSDT has actually two paths for changing backlight.
One is simple to understand and is using SMI Command Port B2, but only works 
when booting with BIOS.
The other is quite complicated (for me acpi noob) and is using a ressource 
called "smbus-ddcblc".
I don't know if the backlight acpi is failing because it doesn't follow the 
right code path (in which case acpi
think it's bios where it isn't) or because memory is not mapped as acpi thinks 
it should be, or... ?

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https://bugs.launchpad.net/bugs/818826

Title:
  backlight brightness control won't work in 2011 iMac 27"

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