Hello,
I am using a Compute Module 5 (CM5) inside the official CM5 IO board case with the bundled 8K PWM fan.
Power supply: official Raspberry Pi PSU.
Software: Debian Bookworm, kernel 6.12.34+rpt-rpi-2712 (2025-06-26), aarch64.
What I am seeing
sensors shows the fan RPM sometimes around 3100–3200 RPM, sometimes suddenly drops to 0 RPM, even though I keep forcing PWM values.
Temperatures are stable (~55–70°C), so cooling works when the fan spins, but the fan control logic seems unstable.
Sometimes after reboot it spins, sometimes not.
Things I tried
Checked /boot/firmware/config.txt and /etc/thermal.d/ configs
Removed custom lines, commented everything out, so no override should exist.
Manual writes to sysfs
echo 1 | sudo tee /sys/class/hwmon/hwmon*/pwm1_enable
echo 255 | sudo tee /sys/class/hwmon/hwmon*/pwm1
→ sometimes works (fan spins full), sometimes reports 0 RPM in sensors while the fan is visibly spinning.
Verified module loaded
lsmod | grep pwm
pwm_fan 49152 0
Blacklisting check
Removed /etc/modprobe.d/blacklist-pwmfan.conf just in case.
Ran update-initramfs -u.
Systemd service approach
Wrote a service + script (fanlock.sh) that continuously forces manual mode and PWM=220.
Added “kick-start” (set 255 for 2s, then drop to 220).
Logs show script running, but still sometimes fan goes to 0 RPM.
Inotifywatch on pwm1
Checked if something else resets the value. Could not clearly identify a culprit.
Sensors output
Example:
pwmfan-isa-0000
Adapter: ISA adapter
fan1: 3151 RPM
…then suddenly:
pwmfan-isa-0000
Adapter: ISA adapter
fan1: 0 RPM
Question
Is this a driver / kernel regression with pwm_fan on CM5 IO board, or am I missing a new overlay/config requirement?
Since I am on CM5 + official IO case + official PSU + bundled fan, I would expect it to “just work”.
Any guidance on:
Correct overlay or config.txt setting required?
Known issues with tach feedback on the 8K fan?
Way to permanently force manual PWM without kernel/firmware fighting back?
Thanks in advance for any help!
I am using a Compute Module 5 (CM5) inside the official CM5 IO board case with the bundled 8K PWM fan.
Power supply: official Raspberry Pi PSU.
Software: Debian Bookworm, kernel 6.12.34+rpt-rpi-2712 (2025-06-26), aarch64.
What I am seeing
sensors shows the fan RPM sometimes around 3100–3200 RPM, sometimes suddenly drops to 0 RPM, even though I keep forcing PWM values.
Temperatures are stable (~55–70°C), so cooling works when the fan spins, but the fan control logic seems unstable.
Sometimes after reboot it spins, sometimes not.
Things I tried
Checked /boot/firmware/config.txt and /etc/thermal.d/ configs
Removed custom lines, commented everything out, so no override should exist.
Manual writes to sysfs
echo 1 | sudo tee /sys/class/hwmon/hwmon*/pwm1_enable
echo 255 | sudo tee /sys/class/hwmon/hwmon*/pwm1
→ sometimes works (fan spins full), sometimes reports 0 RPM in sensors while the fan is visibly spinning.
Verified module loaded
lsmod | grep pwm
pwm_fan 49152 0
Blacklisting check
Removed /etc/modprobe.d/blacklist-pwmfan.conf just in case.
Ran update-initramfs -u.
Systemd service approach
Wrote a service + script (fanlock.sh) that continuously forces manual mode and PWM=220.
Added “kick-start” (set 255 for 2s, then drop to 220).
Logs show script running, but still sometimes fan goes to 0 RPM.
Inotifywatch on pwm1
Checked if something else resets the value. Could not clearly identify a culprit.
Sensors output
Example:
pwmfan-isa-0000
Adapter: ISA adapter
fan1: 3151 RPM
…then suddenly:
pwmfan-isa-0000
Adapter: ISA adapter
fan1: 0 RPM
Question
Is this a driver / kernel regression with pwm_fan on CM5 IO board, or am I missing a new overlay/config requirement?
Since I am on CM5 + official IO case + official PSU + bundled fan, I would expect it to “just work”.
Any guidance on:
Correct overlay or config.txt setting required?
Known issues with tach feedback on the 8K fan?
Way to permanently force manual PWM without kernel/firmware fighting back?
Thanks in advance for any help!
Statistics: Posted by dogukansahil — Mon Sep 15, 2025 6:26 am