linux/Documentation/devicetree/bindings/gpio/gpio-fan.txt
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   1Bindings for fan connected to GPIO lines
   2
   3Required properties:
   4- compatible : "gpio-fan"
   5- gpios: Specifies the pins that map to bits in the control value,
   6  ordered MSB-->LSB.
   7- gpio-fan,speed-map: A mapping of possible fan RPM speeds and the
   8  control value that should be set to achieve them. This array
   9  must have the RPM values in ascending order.
  10
  11Optional properties:
  12- alarm-gpios: This pin going active indicates something is wrong with
  13  the fan, and a udev event will be fired.
  14
  15Examples:
  16
  17        gpio_fan {
  18                compatible = "gpio-fan";
  19                gpios = <&gpio1 14 1
  20                         &gpio1 13 1>;
  21                gpio-fan,speed-map = <0    0
  22                                      3000 1
  23                                      6000 2>;
  24                alarm-gpios = <&gpio1 15 1>;
  25        };
  26