linux-kernel - Re: [PATCH v2] apple-gmux: preserve brightness using EFI

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Message-ID: <20260204182816.1179-1-atharvatiwarilinuxdev@gmail.com>
Date: Wed,  4 Feb 2026 18:28:15 +0000
From: Atharva Tiwari <atharvatiwarilinuxdev@...il.com>
To: lukas@...ner.de
Cc: atharvatiwarilinuxdev@...il.com,
	hansg@...nel.org,
	ilpo.jarvinen@...ux.intel.com,
	linux-kernel@...r.kernel.org,
	platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v2] apple-gmux: preserve brightness using EFI

> Older Intel Macs with a gmux can be booted in CSM mode (bypassing EFI).
> I'm wondering what happens on those with the above change?
> Do they only get the warning or do they crash?
> The warning is probably uncalled for on CSM-booted Macs.

I guess CSM-booted Macs would just trigger a warning when EFI isn’t enabled,
as efi_rt_services_supported(EFI_RT_SUPPORTED_SET_VARIABLE) will return 0.

> On my MacBookPro9,1 (which does have a gmux), the backlight-level variable
> has 6 bytes which currently contain (hexdump): 07 00 00 80 35 00
> I guess this is actually a struct of a 32-bit and a 16-bit value
> or alternatively three 16-bit values.  Yet you're only writing a
> single 16-bit value to the variable.  How many bytes does the variable
> have on your iMac and what are their contents?  I'm worried that
> your change may not be compatible with all Macs which have this variable.

This function actually writes 6 bytes, which is u32 efi_attr (first 32-bits)
and u16 efi_data (last 16-bits), but im confused about the fact, that the
hexdumps show that ur attr are different. the attr should be 07 00 00 00 but its
07 00 00 80, i think we should retrieve the attr in the probe function and use that for
writing. to fix this problem

> Finally, there are Macs which don't have a gmux but which do have a
> backlight.  They usually control brightness through i915 I think.
> It would be nice to save brightness to the EFI variable on those as well.

There should be something for Amdgpus aswell for mac pro, but i cant test i915 nor amdgpu,
as i dont have these devices. so i think we should currently drop this idea.