Documentation: of: Document graph bindings

The device tree graph bindings as used by V4L2 and documented in
Documentation/device-tree/bindings/media/video-interfaces.txt contain
generic parts that are not media specific but could be useful for any
subsystem with data flow between multiple devices. This document
describes the generic bindings.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>

Documentation/devicetree/bindings/graph.txt

+Common bindings for device graphs
+
+General concept
+---------------
+
+The hierarchical organisation of the device tree is well suited to describe
+control flow to devices, but there can be more complex connections between
+devices that work together to form a logical compound device, following an
+arbitrarily complex graph.
+There already is a simple directed graph between devices tree nodes using
+phandle properties pointing to other nodes to describe connections that
+can not be inferred from device tree parent-child relationships. The device
+tree graph bindings described herein abstract more complex devices that can
+have multiple specifiable ports, each of which can be linked to one or more
+ports of other devices.
+
+These common bindings do not contain any information about the direction or
+type of the connections, they just map their existence. Specific properties
+may be described by specialized bindings depending on the type of connection.
+
+To see how this binding applies to video pipelines, for example, see
+Documentation/device-tree/bindings/media/video-interfaces.txt.
+Here the ports describe data interfaces, and the links between them are
+the connecting data buses. A single port with multiple connections can
+correspond to multiple devices being connected to the same physical bus.
+
+Organisation of ports and endpoints
+-----------------------------------
+
+Ports are described by child 'port' nodes contained in the device node.
+Each port node contains an 'endpoint' subnode for each remote device port
+connected to this port. If a single port is connected to more than one
+remote device, an 'endpoint' child node must be provided for each link.
+If more than one port is present in a device node or there is more than one
+endpoint at a port, or a port node needs to be associated with a selected
+hardware interface, a common scheme using '#address-cells', '#size-cells'
+and 'reg' properties is used number the nodes.
+
+device {
+        ...
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        port@0 {
+	        #address-cells = <1>;
+	        #size-cells = <0>;
+		reg = <0>;
+
+                endpoint@0 {
+			reg = <0>;
+			...
+		};
+                endpoint@1 {
+			reg = <1>;
+			...
+		};
+        };
+
+        port@1 {
+		reg = <1>;
+
+		endpoint { ... };
+	};
+};
+
+All 'port' nodes can be grouped under an optional 'ports' node, which
+allows to specify #address-cells, #size-cells properties for the 'port'
+nodes independently from any other child device nodes a device might
+have.
+
+device {
+        ...
+        ports {
+                #address-cells = <1>;
+                #size-cells = <0>;
+
+                port@0 {
+                        ...
+                        endpoint@0 { ... };
+                        endpoint@1 { ... };
+                };
+
+                port@1 { ... };
+        };
+};
+
+Links between endpoints
+-----------------------
+
+Each endpoint should contain a 'remote-endpoint' phandle property that points
+to the corresponding endpoint in the port of the remote device. In turn, the
+remote endpoint should contain a 'remote-endpoint' property. If it has one,
+it must not point to another than the local endpoint. Two endpoints with their
+'remote-endpoint' phandles pointing at each other form a link between the
+containing ports.
+
+device-1 {
+        port {
+                device_1_output: endpoint {
+                        remote-endpoint = <&device_2_input>;
+                };
+        };
+};
+
+device-2 {
+        port {
+                device_2_input: endpoint {
+                        remote-endpoint = <&device_1_output>;
+                };
+        };
+};
+
+
+Required properties
+-------------------
+
+If there is more than one 'port' or more than one 'endpoint' node or 'reg'
+property is present in port and/or endpoint nodes the following properties
+are required in a relevant parent node:
+
+ - #address-cells : number of cells required to define port/endpoint
+                    identifier, should be 1.
+ - #size-cells    : should be zero.
+
+Optional endpoint properties
+----------------------------
+
+- remote-endpoint: phandle to an 'endpoint' subnode of a remote device node.
+