Both backlight acpi and apple_bl drivers fail to set brightness when running under EFI (it has no effect). Both work well under BIOS on the same machine.
Hardware : imac12,2 (2011 27", graphic card 1002:6740)
modprobe apple_bl debug=1
apple_backlight: read brightness of 0
video/backlight/apple_bl.c says :
"Check that the hardware responds - this may not work under EFI"
but why ? Is it simply because B2/B3 io ports are setup by BIOS only ?
I do not get it to work under refit either, but is that the same bug?
i do not think it is a "linux" bug. Anyway maybe smarter people will figure out how to change backlight through radeon. eDP 1.2 specifies a way to do it, maybe apple did implement it on that hardware ?
I have the same issue on MacBookPro 7,1, with nvidia hardware rather than radeon.
apple_bl doesn't work when booting using grub-efi. And nouveau's nv_backlight doesn't work properly either (but that is a separate bug).
Created attachment 73481 [details]
While apple_bl doesn't work with EFI/linux 3.12,
radeon_bl0 has been working well for a while now (at least kernel 3.10).
Matthew Garrett (the author of the driver) says that the SMI-based backlight control was only meant to work with BIOS, not UEFI. You should be able to get native backlight control from the GPU driver.
Does the native GPU driver work?