I recently acquired an 8Gb Raspberry Pi 4 and promptly installed Ubuntu 20.04 on it to see how it worked. Right away, I was excited that the PoE Hat worked and that it booted without any problems, except that the fan on the Hat was no longer controllable the way that it used to be.
It appears that the old way of using config.txt and friends to override those settings no longer works.
Here’s what you need to do:
sudo nano /etc/udev/rules.d/50-rpi-fan.rulesPut this in it to start with, and tweak as needed for your situation:
SUBSYSTEM=="thermal"
KERNEL=="thermal_zone3"
# If the temp hits 81C, highest RPM
ATTR{trip_point_0_temp}="82000"
ATTR{trip_point_0_hyst}="3000"
#
# If the temp hits 80C, higher RPM
ATTR{trip_point_1_temp}="81000"
ATTR{trip_point_1_hyst}="2000"
#
# If the temp hits 70C, higher RPM
ATTR{trip_point_2_temp}="71000"
ATTR{trip_point_2_hyst}="3000"
#
# If the temp hits 60C, turn on the fan
ATTR{trip_point_3_temp}="61000"
ATTR{trip_point_3_hyst}="5000"
#
# Fan is off otherwise