Fan working strategy

EC-A1684JD4 fans work in 4 levels:

The higher the working level of the fan, the faster the speed. If the
current temperature of the main controller is lower than 33°C, the fan
is turned off by default.

Users can view the current working temperature of the main controller
by executing the following command:

   cat /sys/class/thermal/thermal_zone1/temp

For example the following is 45.5°C:

   linaro@bm1684:~$ cat /sys/class/thermal/thermal_zone1/temp
   45500

In addition, the user can also manually turn on the fan by himself
(write 0-4, 0 is off, 4 is the maximum level):

   sudo -i
   echo 4 > /sys/class/thermal/cooling_device0/cur_state