[Linux] GPIO 고정하기

$ cd linux
$ cat Documentation/devicetree/bindings/gpio/gpio.txt

The GPIO chip may contain GPIO hog definitions. GPIO hogging is a mechanism
providing automatic GPIO request and configuration as part of the 
gpio-controller's driver probe function.

Each GPIO hog definition is represented as a child node of the GPIO controller.
Required properties:
- gpio-hog:   A property specifying that this child node represents a GPIO hog.
- gpios:      Store the GPIO information (id, flags, ...) for each GPIO to
              affect. Shall contain an integer multiple of the number of cells
              specified in its parent node (GPIO controller node).
Only one of the following properties scanned in the order shown below.
This means that when multiple properties are present they will be searched
in the order presented below and the first match is taken as the intended
configuration.
- input:      A property specifying to set the GPIO direction as input.
- output-low  A property specifying to set the GPIO direction as output with
              the value low.
- output-high A property specifying to set the GPIO direction as output with
              the value high.

Optional properties:
- line-name:  The GPIO label name. If not present the node name is used.

Example of two SOC GPIO banks defined as gpio-controller nodes:

        qe_pio_a: gpio-controller@1400 {
                compatible = "fsl,qe-pario-bank-a", "fsl,qe-pario-bank";
                reg = <0x1400 0x18>;
                gpio-controller;
                #gpio-cells = <2>;

                line_b {
                        gpio-hog;
                        gpios = <6 0>; 
                        output-low;
                        line-name = "foo-bar-gpio";
                };
        };

        qe_pio_e: gpio-controller@1460 {
                compatible = "fsl,qe-pario-bank-e", "fsl,qe-pario-bank";
                reg = <0x1460 0x18>;
                gpio-controller;
                #gpio-cells = <2>;
        };

...
#include <dt-bindings/gpio/gpio.h>

...

	gpio3: gpio3@xxxxxxxx {
		compatible = "...";
		gpio-controller;
		...

		always_low {
			gpio-hog;
			gpios = <5 GPIO_ACTIVE_HIGH>;
			output-low;
		};
	};