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Specifying GPIO information for devices
============================================

1) gpios property
-----------------

Nodes that makes use of GPIOs should specify them using one or more
properties, each containing a 'gpio-list':

	gpio-list ::= <single-gpio> [gpio-list]
	single-gpio ::= <gpio-phandle> <gpio-specifier>
	gpio-phandle : phandle to gpio controller node
	gpio-specifier : Array of #gpio-cells specifying specific gpio
			 (controller specific)

GPIO properties should be named "[<name>-]gpios".  Exact
meaning of each gpios property must be documented in the device tree
binding for each device.

For example, the following could be used to describe gpios pins to use
as chip select lines; with chip selects 0, 1 and 3 populated, and chip
select 2 left empty:

	gpio1: gpio1 {
		gpio-controller
		 #gpio-cells = <2>;
	};
	gpio2: gpio2 {
		gpio-controller
		 #gpio-cells = <1>;
	};
	[...]
	 chipsel-gpios = <&gpio1 12 0>,
			 <&gpio1 13 0>,
			 <0>, /* holes are permitted, means no GPIO 2 */
			 <&gpio2 2>;

Note that gpio-specifier length is controller dependent.  In the
above example, &gpio1 uses 2 cells to specify a gpio, while &gpio2
only uses one.

gpio-specifier may encode: bank, pin position inside the bank,
whether pin is open-drain and whether pin is logically inverted.
Exact meaning of each specifier cell is controller specific, and must
be documented in the device tree binding for the device.

Example of the node using GPIOs:

	node {
		gpios = <&qe_pio_e 18 0>;
	};

In this example gpio-specifier is "18 0" and encodes GPIO pin number,
and empty GPIO flags as accepted by the "qe_pio_e" gpio-controller.

2) gpio-controller nodes
------------------------

Every GPIO controller node must both an empty "gpio-controller"
property, and have #gpio-cells contain the size of the gpio-specifier.

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

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

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

2.1) gpio-controller and pinctrl subsystem
------------------------------------------

gpio-controller on a SOC might be tightly coupled with the pinctrl
subsystem, in the sense that the pins can be used by other functions
together with optional gpio feature.

While the pin allocation is totally managed by the pin ctrl subsystem,
gpio (under gpiolib) is still maintained by gpio drivers. It may happen
that different pin ranges in a SoC is managed by different gpio drivers.

This makes it logical to let gpio drivers announce their pin ranges to
the pin ctrl subsystem and call 'pinctrl_request_gpio' in order to
request the corresponding pin before any gpio usage.

For this, the gpio controller can use a pinctrl phandle and pins to
announce the pinrange to the pin ctrl subsystem. For example,

	qe_pio_e: gpio-controller@1460 {
		#gpio-cells = <2>;
		compatible = "fsl,qe-pario-bank-e", "fsl,qe-pario-bank";
		reg = <0x1460 0x18>;
		gpio-controller;
		gpio-ranges = <&pinctrl1 0 20 10>, <&pinctrl2 10 50 20>;

    }

where,
   &pinctrl1 and &pinctrl2 is the phandle to the pinctrl DT node.

   Next values specify the base pin and number of pins for the range
   handled by 'qe_pio_e' gpio. In the given example from base pin 20 to
   pin 29 under pinctrl1 with gpio offset 0 and pin 50 to pin 69 under
   pinctrl2 with gpio offset 10 is handled by this gpio controller.

The pinctrl node must have "#gpio-range-cells" property to show number of
arguments to pass with phandle from gpio controllers node.

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