How to Tune CPU FAN Speed With PWM on Armbian (cooling-levels based on Temperature)

Armbian images for many SBCs expose a PWM-controlled fan through the Linux thermal framework and a pwm-fan device tree node. If you want the fan to spin up earlier, run quieter, or use your own temperature→PWM mapping without a userspace script, the clean approach is to create a Device Tree Overlay that modifies:

• cooling-levels — PWM values (0–255)
• rockchip,temp-trips — temperature thresholds mapped to those levels (on many Rockchip-based boards)

This post shows a practical, repeatable way to create, install, and enable an overlay on Armbian.

Note: Not every Armbian board uses rockchip,temp-trips. This guide applies to systems where you can find cooling-levels and rockchip,temp-trips under the pwm-fan node in the running device tree.

1) Confirm you have PWM fan control in Armbian

Check sysfs for PWM control:

ls -R /sys/class/pwm 2>/dev/null
grep -R . /sys/class/hwmon/hwmon*/name 2>/dev/null
ls /sys/class/hwmon/hwmon*/pwm* 2>/dev/null

If you see a pwm1 file under an hwmon device (often named pwmfan), the kernel exposes PWM control.

2) Dump the running device tree and locate the fan node

Install the device tree compiler:

sudo apt update
sudo apt install -y device-tree-compiler

Dump the live device tree (best source of truth because it reflects overlays already applied):

sudo dtc -I fs -O dts -o /tmp/running.dts /proc/device-tree

Search for the fan node and curve properties:

grep -nEi 'pwm-fan|pwmfan|cooling-levels|rockchip,temp-trips|temp-trips' /tmp/running.dts | head -120

You’re looking for something like:

• pwm-fan { ... }
• cooling-levels = <...>;
• rockchip,temp-trips = <...>;

Also identify the node path for the fan. On many systems it’s /pwm-fan, but don’t assume—verify in running.dts.

3) Understand what you’re editing

cooling-levels

A list of PWM duty values (0–255). Example:

• <0 50 100 150 200 255>

Each entry is a cooling “state”.

rockchip,temp-trips

Pairs of:

• temperature (millidegrees C) and
• cooling level index (usually 1..N, where 0 would be off)

Example:

• <40000 1 42000 2 45000 3>

Meaning:

• at 40°C use cooling level 1
• at 42°C use cooling level 2
• at 45°C use cooling level 3

Temperatures are in millidegrees C: 45000 = 45°C.

4) Create a Device Tree Overlay to set your fan curve

A) Choose your fan node path

From /tmp/running.dts, determine the correct path for the fan node. In the example below we use:

• /pwm-fan

If your fan node path differs, update target-path = "/pwm-fan"; accordingly.

B) Write the overlay source (.dtso)

This example sets the curve:

• 40°C → PWM 200
• 42°C → PWM 225
• 45°C+ → PWM 255

sudo tee /root/fan-curve.dtso >/dev/null <<'EOF'
/dts-v1/;
/plugin/;

&{/} {
  fragment@0 {
    target-path = "/pwm-fan";
    __overlay__ {
      /*
       * Fan PWM steps (0..255):
       * level 0 = off
       * level 1 = 200
       * level 2 = 225
       * level 3 = 255
       */
      cooling-levels = <0 200 225 255>;

      /*
       * Temp (milli°C) -> cooling level index
       * 40°C -> level 1 (200)
       * 42°C -> level 2 (225)
       * 45°C -> level 3 (255)
       */
      rockchip,temp-trips = <40000 1 42000 2 45000 3>;
    };
  };
};
EOF

5) Compile the overlay to .dtbo

Armbian typically supports user overlays in /boot/overlay-user/:

sudo mkdir -p /boot/overlay-user
sudo dtc -@ -I dts -O dtb -o /boot/overlay-user/fan-curve.dtbo /root/fan-curve.dtso

Verify the file exists:

ls -l /boot/overlay-user/fan-curve.dtbo

6) Enable the overlay in Armbian

Edit Armbian’s boot environment file:

sudo nano /boot/armbianEnv.txt

Add or update:

user_overlays=fan-curve

If you already have overlays enabled, append it space-separated:

user_overlays=existing-overlay another-overlay fan-curve

Save the file and reboot:

sudo reboot

7) Verify the overlay applied

After reboot, dump the live device tree again:

sudo dtc -I fs -O dts -o /tmp/running2.dts /proc/device-tree
grep -n "cooling-levels" /tmp/running2.dts | head -40
grep -n "rockchip,temp-trips" /tmp/running2.dts | head -40

You should see your updated values (sometimes displayed in hex).

8) Watch PWM change in real time

Find your PWM hwmon device:

for d in /sys/class/hwmon/hwmon*; do
  n=$(cat "$d/name" 2>/dev/null)
  if echo "$n" | grep -qi pwm; then
    echo "$d -> $n"
    ls "$d"/pwm* 2>/dev/null
  fi
done

Then watch a thermal zone temperature and the PWM value together (adjust paths for your system):

watch -n 1 'cat /sys/class/thermal/thermal_zone0/temp; cat /sys/class/hwmon/hwmon8/pwm1'