On my late 2013 Macbook Pro, I have a couple of scripts that set the
fans to auto or full-speed:

fan-hi:
#!/bin/sh
sudo sh -c 'echo 1 > /sys/devices/platform/applesmc.768/fan1_manual
             echo 1 > /sys/devices/platform/applesmc.768/fan2_manual
             cat /sys/devices/platform/applesmc.768/fan1_max > /sys/devices/platform/applesmc.768/fan1_output
             cat /sys/devices/platform/applesmc.768/fan2_max > /sys/devices/platform/applesmc.768/fan2_output'

fan-auto:
#!/bin/sh
sudo sh -c 'echo 0 > /sys/devices/platform/applesmc.768/fan1_manual
             echo 0 > /sys/devices/platform/applesmc.768/fan2_manual'

Running ./fan-hi and then ./fan-auto on Linux v5.6 works and doesn't
cause any problems, but after updating to v5.9 I see this in dmesg:

[Nov 6 17:24] applesmc: send_byte(0x01, 0x0300) fail: 0x40
[  +0.000005] applesmc: FS! : write data fail
[  +0.191777] applesmc: send_byte(0x30, 0x0300) fail: 0x40
[  +0.000009] applesmc: F0Tg: write data fail
[  +7.097416] applesmc: send_byte(0x00, 0x0300) fail: 0x40
[  +0.000006] applesmc: FS! : write data fail

and the fan controls don't work.

Googling turned up this [1] which looks like the same problem. They said
it began occurring between v5.7 and v5.8, so I looked and found this
commit.

After reverting commit fff2d0f701e6753591609739f8ab9be1c8e80ebb from
v5.9, I no longer see the errors in dmesg and the fan controls work
again.

Any ideas what the problem is?

Thanks,
Matt

[1] https://stackoverflow.com/questions/63505469/cant-write-data-to-applesmc-error-after-upgrade-to-arch-linux-kernel-5-8-1