Re: [PATCH v4 1/2] efi: Save Brightness using EFI on Macs

[Aditya Garg]

On 3/17/26 15:08, Aditya Garg wrote:
> > On 16 Mar 2026, at 9:03 PM, Atharva Tiwari <atharvatiwarilinuxdev@xxxxxxxxx> wrote:
> >
> > Currently when a Mac reboots, the brightness is not the same
> > as the previous boot instead its the same as the last time the
> > Mac booted macOS.
> >
> > We can fix this issue by saving the brightness level to the efivar
> > backlight-level.
> >
> > (tested on iMac20,1)
> >
> > Suggested-by: Lukas Wunner <lukas@xxxxxxxxx>
> > Signed-off-by: Atharva Tiwari <atharvatiwarilinuxdev@xxxxxxxxx>
> > ---
> > drivers/firmware/efi/Kconfig                  | 10 +++
> > drivers/firmware/efi/Makefile                 |  1 +
> > drivers/firmware/efi/apple-brightness.c       | 67 +++++++++++++++++++
> > .../linux/platform_data/apple-brightness.h    | 20 ++++++
> > 4 files changed, 98 insertions(+)
> > create mode 100644 drivers/firmware/efi/apple-brightness.c
> > create mode 100644 include/linux/platform_data/apple-brightness.h
>
> I tested this patch on a MacBookPro16,1, which is a T2 Intel Mac, and the behaviour is very weird:
>
> 1. macOS will never respect any brightness set by Linux
> 2. The Max brightness set by macOS is not the max brightness on Linux. I get these values on Linux:
>
> aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
> GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
> Name: "backlight-level"
> Attributes:
> 	Non-Volatile
> 	Boot Service Access
> 	Runtime Service Access
> Value:
> 00000000  f1 02                                             |..              |
>
> aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
> 48238
>
> 3. I set the Max brightness on Linux, and restarted. I get this:
>
> aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
> GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
> Name: "backlight-level"
> Attributes:
> 	Non-Volatile
> 	Boot Service Access
> 	Runtime Service Access
> Value:
> 00000000  7a ff                                             |z.              |
>
> aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
> 65402
>
>
> 4. Finally, any attempt to reduce the brightness seems worthless as unless booted back to macOS, Linux always boots at max brightness, but the nvram variable is changed successfully:
>
> aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
> GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
> Name: "backlight-level"
> Attributes:
> 	Non-Volatile
> 	Boot Service Access
> 	Runtime Service Access
> Value:
> 00000000  eb 41                                             |.A              |
>
> aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
> 65402
>
>
> Overall, it’s very inconsistent. Also, I’ve noticed Windows doesn't mess around with efivars, rather stores the brightness locally and implements it on every boot, something which systemd should do on Linux.

I've done another test as well:

1. I booted into macOS, and set a normal brightness, not too low, not 
too high.
2. I booted back to Linux. The brightness was consistent. These are the 
values:

aditya@MacBook:~$ efivar -p -n 
'7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "backlight-level"
Attributes:
	Non-Volatile
	Boot Service Access
	Runtime Service Access
Value:
00000000  92 00                                             |.. 
    |

aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
7792

This is the journalctl of that boot: https://termbin.com/p0vf

3. I slightly modified the brightness and restarted. Upon a restart, the 
brightness was max again:

aditya@MacBook:~$ efivar -p -n 
'7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "backlight-level"
Attributes:
	Non-Volatile
	Boot Service Access
	Runtime Service Access
Value:
00000000  51 3a                                             |Q: 
    |

aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
63589

This is the journalctl of that boot: https://termbin.com/1z9r