Docs: dt: add generic MSI bindings

This document describes the generic device tree binding for MSI controllers and

their master(s).

Message Signaled Interrupts (MSIs) are a class of interrupts generated by a

write to an MMIO address.

MSIs were originally specified by PCI (and are used with PCIe), but may also be

used with other busses, and hence a mechanism is required to relate devices on

those busses to the MSI controllers which they are capable of using,

potentially including additional information.

MSIs are distinguished by some combination of:

- The doorbell (the MMIO address written to).

  Devices may be configured by software to write to arbitrary doorbells which

  they can address. An MSI controller may feature a number of doorbells.

- The payload (the value written to the doorbell).

  Devices may be configured to write an arbitrary payload chosen by software.

  MSI controllers may have restrictions on permitted payloads.

- Sideband information accompanying the write.

  Typically this is neither configurable nor probeable, and depends on the path

  taken through the memory system (i.e. it is a property of the combination of

  MSI controller and device rather than a property of either in isolation).

MSI controllers:

================

An MSI controller signals interrupts to a CPU when a write is made to an MMIO

address by some master. An MSI controller may feature a number of doorbells.

Required properties:

--------------------

- msi-controller: Identifies the node as an MSI controller.

Optional properties:

--------------------

- #msi-cells: The number of cells in an msi-specifier, required if not zero.

  Typically this will encode information related to sideband data, and will

  not encode doorbells or payloads as these can be configured dynamically.

  The meaning of the msi-specifier is defined by the device tree binding of

  the specific MSI controller. 

MSI clients

===========

MSI clients are devices which generate MSIs. For each MSI they wish to

generate, the doorbell and payload may be configured, though sideband

information may not be configurable.

Required properties:

--------------------

- msi-parent: A list of phandle + msi-specifier pairs, one for each MSI

  controller which the device is capable of using.

  This property is unordered, and MSIs may be allocated from any combination of

  MSI controllers listed in the msi-parent property.

  If a device has restrictions on the allocation of MSIs, these restrictions

  must be described with additional properties.

  When #msi-cells is non-zero, busses with an msi-parent will require

  additional properties to describe the relationship between devices on the bus

  and the set of MSIs they can potentially generate.

Example

=======

/ {

	#address-cells = <1>;

	#size-cells = <1>;

	msi_a: msi-controller@a {

		reg = <0xa 0xf00>;

		compatible = "vendor-a,some-controller";

		msi-controller;

		/* No sideband data, so #msi-cells omitted */

	};

	msi_b: msi-controller@b {

		reg = <0xb 0xf00>;

		compatible = "vendor-b,another-controller";

		msi-controller;

		/* Each device has some unique ID */

		#msi-cells = <1>;

	};

	msi_c: msi-controller@c {

		reg = <0xb 0xf00>;

		compatible = "vendor-b,another-controller";

		msi-controller;

		/* Each device has some unique ID */

		#msi-cells = <1>;

	};

	dev@0 {

		reg = <0x0 0xf00>;

		compatible = "vendor-c,some-device";

		/* Can only generate MSIs to msi_a */

		msi-parent = <&msi_a>;

	};

	dev@1 {

		reg = <0x1 0xf00>;

		compatible = "vendor-c,some-device";

		/* 

		 * Can generate MSIs to either A or B.

		 */

		msi-parent = <&msi_a>, <&msi_b 0x17>;

	};

	dev@2 {

		reg = <0x2 0xf00>;

		compatible = "vendor-c,some-device";

		/*

		 * Has different IDs at each MSI controller.

		 * Can generate MSIs to all of the MSI controllers.

		 */

		msi-parent = <&msi_a>, <&msi_b 0x17>, <&msi_c 0x53>;

	};

};