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+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 = <0xc 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>;
+ };
+};