Fan speed control not working on Pi 5 under Ubuntu 23.10

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Status changed to 'Confirmed' because the bug affects multiple users.

This affects at least the pwm-fan and gpio-fan overlays as both are using the step_wise governor.

The problem supposedly has been fixed in raspberrypi rpi-6.5.y: https://github.com/raspberrypi/linux/commit/51c8b070cbf3f5df97a633c5203f729013ab60d0

The step-wise governor certainly appears to have some issues. It would appear our version is reading trip_temp before it is ever initialized (https://git.launchpad.net/ubuntu/+source/linux-raspi/tree/drivers/thermal/gov_step_wise.c#n82). The patch linked from the forum post in the description corrects this; will build a test kernel to see if this fixes the issue.

Incidentally, a work-around to make things more bearable until this can be fixed is as follows:

  $ echo 2 | sudo tee /sys/class/thermal/cooling_device0/cur_state

Where 2 is a medium (and in my opinion, largely silent) fan speed. Valid values are between 0 (off) and 4 (full speed).

@juergh Confirmed that the patch to drivers/thermal/gov_step_wise.c is indeed the root cause. In thermal_zone_trip_update, I changed:

  hyst_temp = trip.temperature

To:

  hyst_temp = trip_temp = trip.temperature;

This ensures trip_temp is not uninitialized when it's read. Also removed the (redundantly added) second call to get_tz_trend; fan operated correctly after installing recompiled kernel.

I've applied the patch to a kernel build in ppa:waveform/fan-fix (https://launchpad.net/~waveform/+archive/ubuntu/fan-fix) which should serve as a temporary fix until the next kernel release.