Bug 37352 - apple_backlight is indeed not working under EFI, but why ?
apple_backlight is indeed not working under EFI, but why ?
Product: EFI
Classification: Unclassified
Component: Video
All Linux
: P1 normal
Assigned To: EFI Virtual User
Depends on:
  Show dependency treegraph
Reported: 2011-06-12 21:00 UTC by Jérémy Lal
Modified: 2014-04-10 11:15 UTC (History)
5 users (show)

See Also:
Kernel Version: 3.0.0-3.3.0
Tree: Mainline
Regression: No

DSDT imac12,2 (187.11 KB, text/x-dsl)
2012-05-31 22:17 UTC, Jérémy Lal

Description Jérémy Lal 2011-06-12 21:00:30 UTC
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 ?
Comment 1 Andreas Tunek 2011-08-10 15:24:48 UTC
I do not get it to work under refit either, but is that the same bug?
Comment 2 Jérémy Lal 2011-08-10 17:12:15 UTC
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 ?
Comment 3 Mourad De Clerck 2011-08-22 15:04:58 UTC
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).
Comment 4 Jérémy Lal 2012-05-31 22:17:52 UTC
Created attachment 73481 [details]
DSDT imac12,2
Comment 5 Jérémy Lal 2013-12-25 20:12:30 UTC
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).
Comment 6 Matt Fleming 2014-01-07 15:57:38 UTC
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.
Comment 7 Matt Fleming 2014-03-11 20:39:50 UTC
Does the native GPU driver work?

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