0006-t2.patch appears to route non-T2 `MacBookAir6,2` onto the ACPI `applesmc` path and breaks keyboard backlight

[jtbrough]

Hello,

I have run into what feels like an unintended non-T2 regression from the applesmc changes in 6.19/0006-t2.patch (possibly earlier; I have not bisected when this started and only noticed it with these kernels).

Hardware:

• Apple MacBookAir6,2
• non-T2 machine

Observed behavior:

• On Arch 6.18.13-arch1-1, keyboard backlight works normally.
• On linux-cachyos 6.19.3-2, the keyboard backlight does not visibly change.
• On the failing kernel, writes to /sys/class/leds/smc::kbd_backlight/brightness succeed and read back correctly, but the physical keyboard backlight does not visibly respond.

Important runtime difference:

• Working Arch kernel:
  • smc::kbd_backlight is under the legacy/platform applesmc path
  • modalias/path shows platform:applesmc
• Failing linux-cachyos 6.19.3-2 kernel:
  • smc::kbd_backlight is under ACPI APP0001:00
  • driver path is /sys/bus/acpi/drivers/applesmc

From source inspection, 6.19/0006-t2.patch rewrites drivers/hwmon/applesmc.c from the legacy platform-driver model to an ACPI APP0001 model and adds the new resource/MMIO path. It still appears to keep the broad Apple DMI whitelist, including older non-T2 Macs like MacBookAir.

That makes me suspect the rewritten applesmc path is being applied too broadly on non-T2 hardware. In particular, it looks like MacBookAir6,2 is getting routed onto the new ACPI/T2-style applesmc path when it should likely stay on the old legacy/platform path.

related: https://discuss.cachyos.org/t/macbook-air-keyboard-backlight-not-working/5388/15

[1Naim]

I assume the backlight works on the rc kernel? Since the t2 patch isn't applied there yet.

[jtbrough]

I assume the backlight works on the rc kernel? Since the t2 patch isn't applied there yet.

Correct. On 7.0.0-rc2-1-cachyos-rc backlight controls work perfectly.

[fnimick]

I can confirm this same problem on a MacbookAir7,2 and that 7.0.0-rc3-1 does not have the backlight issue.

[1Naim]

Can anyone affected test the linux-t2 kernel? https://github.com/NoaHimesaka1873/linux-t2-arch/releases/tag/v6.19.6

[fnimick]

Can anyone affected test the linux-t2 kernel? https://github.com/NoaHimesaka1873/linux-t2-arch/releases/tag/v6.19.6

Installed on my MacbookAir7,2; keyboard brightness still does not work.

[AdityaGarg8]

Could someone share their dmesg and journalctl -k from the affected kernels?

[fnimick]

dmesg.txt

journalctl.txt

From kernel 6.19.6-2-cachyos

[AdityaGarg8]

Also please share ls /sys/class/leds

[fnimick]

ls /sys/class/leds:

lrwxrwxrwx - root 11 Mar 15:41 input6::capslock -> ../../devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5:1.1/0003:05AC:0291.0002/input/input6/input6::capslock
lrwxrwxrwx - root 11 Mar 15:41 input6::compose -> ../../devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5:1.1/0003:05AC:0291.0002/input/input6/input6::compose
lrwxrwxrwx - root 11 Mar 15:41 input6::kana -> ../../devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5:1.1/0003:05AC:0291.0002/input/input6/input6::kana
lrwxrwxrwx - root 11 Mar 15:41 input6::numlock -> ../../devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5:1.1/0003:05AC:0291.0002/input/input6/input6::numlock
lrwxrwxrwx - root 11 Mar 15:41 input6::scrolllock -> ../../devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5:1.1/0003:05AC:0291.0002/input/input6/input6::scrolllock
lrwxrwxrwx - root 11 Mar 15:41 smc::kbd_backlight -> ../../devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A08:00/device:0c/APP0001:00/leds/smc::kbd_backlight

[AdityaGarg8]

Could you share:

ls /sys/class/leds/smc::kbd_backlight, cat /sys/class/leds/smc::kbd_backlight/brightness and cat /sys/class/leds/smc::kbd_backlight/max_brightness

[fnimick]

ls /sys/class/leds/smc::kbd_backlight:

lrwxrwxrwx    - root 12 Mar 08:14  device -> ../../../APP0001:00
drwxr-xr-x    - root 12 Mar 08:14  power
lrwxrwxrwx    - root 12 Mar 08:14  subsystem -> ../../../../../../../../class/leds
.rw-r--r-- 4.1k root 12 Mar 08:14 󰡯 brightness
.r--r--r-- 4.1k root 12 Mar 08:14 󰡯 max_brightness
.rw-r--r--    0 root 12 Mar 08:14 󰡯 trigger
.rw-r--r-- 4.1k root 12 Mar 08:14 󰡯 uevent

cat /sys/class/leds/smc::kbd_backlight/brightness: ranges from 0 to 255 when I attempt to adjust

cat /sys/class/leds/smc::kbd_backlight/max_brightness: 255

[AdityaGarg8]

https://github.com/AdityaGarg8/applesmc-t2/tree/debug

Install the version of applesmc here (in the debug branch)

Then try adjusting the backlight and after a few tries at adjusting, share sudo dmesg and journalctl -k again.

[fnimick]

I'm a novice at linux kernel stuff - is it as simple as make and make install? How can I confirm that your debug module was loaded? (and then how do I switch back to upstream afterward?)

[AdityaGarg8]

I'm a novice at linux kernel stuff - is it as simple as make and make install? How can I confirm that your debug module was loaded? (and then how do I switch back to upstream afterward?)

git clone -b debug https://github.com/AdityaGarg8/applesmc-t2/
cd debug
make
sudo modprobe -r applesmc && sleep 2 && sudo modprobe ./applesmc.ko

This should temporarily load the test driver without messing up anything.

[fnimick]

This is probably a very basic question, but how can I get make to use clang instead of gcc? I assume that is the cause of these errors, I tried export CC=clang with no effect:

~/applesmc-test/applesmc-t2 debug
❯ CC=clang make
make -C /lib/modules/6.19.6-2-cachyos/build M=/home/francis/applesmc-test/applesmc-t2 modules
make[1]: Entering directory '/usr/lib/modules/6.19.6-2-cachyos/build'
make[2]: Entering directory '/home/francis/applesmc-test/applesmc-t2'
warning: the compiler differs from the one used to build the kernel
  The kernel was built by: clang version 21.1.8
  You are using:           gcc (GCC) 15.2.1 20260209
  CC [M]  applesmc.o
gcc: error: unrecognized command-line option ‘-mstack-alignment=8’
gcc: error: unrecognized command-line option ‘-mretpoline-external-thunk’
gcc: error: unrecognized command-line option ‘-fsplit-lto-unit’
gcc: error: unrecognized command-line option ‘-fdebug-info-for-profiling’
gcc: error: unrecognized command-line option ‘-mllvm’
gcc: error: unrecognized command-line option ‘-mllvm’
gcc: error: unrecognized command-line option ‘-improved-fs-discriminator=true’
make[4]: *** [/usr/lib/modules/6.19.6-2-cachyos/build/scripts/Makefile.build:289: applesmc.o] Error 1
make[3]: *** [/usr/lib/modules/6.19.6-2-cachyos/build/Makefile:2069: .] Error 2
make[2]: *** [/usr/lib/modules/6.19.6-2-cachyos/build/Makefile:248: __sub-make] Error 2