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Change the behavior of the PWM fan

By default, the speed of the fan is dynamically adjusted according to the CPU temperature. If you need to let the fan run, you can modify the script /usr/bin/start-rk3399-pwm-fan.sh to the following:

echo 0 > /sys/class/pwm/pwmchip1/export
sleep 1
echo 0 > /sys/class/pwm/pwmchip1/pwm0/enable
echo 50000 > /sys/class/pwm/pwmchip1/pwm0/period
echo 1 > /sys/class/pwm/pwmchip1/pwm0/enable
echo 45000 > /sys/class/pwm/pwmchip1/pwm0/duty_cycle