dt: pinctrl: Document device tree binding

== Introduction ==

Hardware modules that control pin multiplexing or configuration parameters

such as pull-up/down, tri-state, drive-strength etc are designated as pin

controllers. Each pin controller must be represented as a node in device tree,

just like any other hardware module.

Hardware modules whose signals are affected by pin configuration are

designated client devices. Again, each client device must be represented as a

node in device tree, just like any other hardware module.

For a client device to operate correctly, certain pin controllers must

set up certain specific pin configurations. Some client devices need a

single static pin configuration, e.g. set up during initialization. Others

need to reconfigure pins at run-time, for example to tri-state pins when the

device is inactive. Hence, each client device can define a set of named

states. The number and names of those states is defined by the client device's

own binding.

The common pinctrl bindings defined in this file provide an infrastructure

for client device device tree nodes to map those state names to the pin

configuration used by those states.

Note that pin controllers themselves may also be client devices of themselves.

For example, a pin controller may set up its own "active" state when the

driver loads. This would allow representing a board's static pin configuration

in a single place, rather than splitting it across multiple client device

nodes. The decision to do this or not somewhat rests with the author of

individual board device tree files, and any requirements imposed by the

bindings for the individual client devices in use by that board, i.e. whether

they require certain specific named states for dynamic pin configuration.

== Pinctrl client devices ==

For each client device individually, every pin state is assigned an integer

ID. These numbers start at 0, and are contiguous. For each state ID, a unique

property exists to define the pin configuration. Each state may also be

assigned a name. When names are used, another property exists to map from

those names to the integer IDs.

Each client device's own binding determines the set of states the must be

defined in its device tree node, and whether to define the set of state

IDs that must be provided, or whether to define the set of state names that

must be provided.

Required properties:

pinctrl-0:	List of phandles, each pointing at a pin configuration

		node. These referenced pin configuration nodes must be child

		nodes of the pin controller that they configure. Multiple

		entries may exist in this list so that multiple pin

		controllers may be configured, or so that a state may be built

		from multiple nodes for a single pin controller, each

		contributing part of the overall configuration. See the next

		section of this document for details of the format of these

		pin configuration nodes.

		In some cases, it may be useful to define a state, but for it

		to be empty. This may be required when a common IP block is

		used in an SoC either without a pin controller, or where the

		pin controller does not affect the HW module in question. If

		the binding for that IP block requires certain pin states to

		exist, they must still be defined, but may be left empty.

Optional properties:

pinctrl-1:	List of phandles, each pointing at a pin configuration

		node within a pin controller.

...

pinctrl-n:	List of phandles, each pointing at a pin configuration

		node within a pin controller.

pinctrl-names:	The list of names to assign states. List entry 0 defines the

		name for integer state ID 0, list entry 1 for state ID 1, and

		so on.

For example:

	/* For a client device requiring named states */

	device {

		pinctrl-names = "active", "idle";

		pinctrl-0 = <&state_0_node_a>;

		pinctrl-1 = <&state_1_node_a &state_1_node_b>;

	};

	/* For the same device if using state IDs */

	device {

		pinctrl-0 = <&state_0_node_a>;

		pinctrl-1 = <&state_1_node_a &state_1_node_b>;

	};

	/*

	 * For an IP block whose binding supports pin configuration,

	 * but in use on an SoC that doesn't have any pin control hardware

	 */

	device {

		pinctrl-names = "active", "idle";

		pinctrl-0 = <>;

		pinctrl-1 = <>;

	};

== Pin controller devices ==

Pin controller devices should contain the pin configuration nodes that client

devices reference.

For example:

	pincontroller {

		... /* Standard DT properties for the device itself elided */

		state_0_node_a {

			...

		};

		state_1_node_a {

			...

		};

		state_1_node_b {

			...

		};

	}

The contents of each of those pin configuration child nodes is defined

entirely by the binding for the individual pin controller device. There

exists no common standard for this content.

The pin configuration nodes need not be direct children of the pin controller

device; they may be grandchildren, for example. Whether this is legal, and

whether there is any interaction between the child and intermediate parent

nodes, is again defined entirely by the binding for the individual pin

controller device.