No working hysteresis support in gov_step_wise.c => fan switching on/off very frequently.

[jkreileder]

Describe the bug

gov_step_wise.c has hysteresis code but this actually does nothing as ops->get_trip_hyst is an optional operation and there's no implementation of it.
On the pi5 hyst is set to 5000 for all trip points but because of the missing get_trip_hyst, it's basically always 0 in gov_step_wise.c.

The result are very frequent switches around trip temperatures. E.g. on my pi5 I see more than 1000 on/off switches around 50 degrees in 1 hour. Sometimes the fan only stops for a fraction of a second before spinning up again. Not sure if that is good for the fan in the long term.

Note that Linux 6.7 removes the get_trip_hyst operation altogether: torvalds/linux@35d8dbb
Apparently the hyst values should be read directly from the tables.

Steps to reproduce the behaviour

On a PI 5:

echo 'file gov_step_wise.c +p' | sudo tee /sys/kernel/debug/dynamic_debug/control
dmesg

=> temp and hyst (which is/should be "temp - configured hyst" value here) are always the same.

Device (s)

Other

System

Ubuntu 23.10
Linux pi5 6.5.0-1006-raspi #9~fanfix1-Ubuntu SMP PREEMPT_DYNAMIC Wed Nov 15 15:03:29 UTC 20 aarch64 aarch64 aarch64 GNU/Linux

Logs

No response

Additional context

No response

[pelwell]

This is clearly a problem that will need to be resolved before we move to 6.6 - the next LTS kernel.