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Documentation / devicetree / bindings / graph.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.

1	Common bindings for device graphs
2	
3	General concept
4	---------------
5	
6	The hierarchical organisation of the device tree is well suited to describe
7	control flow to devices, but there can be more complex connections between
8	devices that work together to form a logical compound device, following an
9	arbitrarily complex graph.
10	There already is a simple directed graph between devices tree nodes using
11	phandle properties pointing to other nodes to describe connections that
12	can not be inferred from device tree parent-child relationships. The device
13	tree graph bindings described herein abstract more complex devices that can
14	have multiple specifiable ports, each of which can be linked to one or more
15	ports of other devices.
16	
17	These common bindings do not contain any information about the direction or
18	type of the connections, they just map their existence. Specific properties
19	may be described by specialized bindings depending on the type of connection.
20	
21	To see how this binding applies to video pipelines, for example, see
22	Documentation/device-tree/bindings/media/video-interfaces.txt.
23	Here the ports describe data interfaces, and the links between them are
24	the connecting data buses. A single port with multiple connections can
25	correspond to multiple devices being connected to the same physical bus.
26	
27	Organisation of ports and endpoints
28	-----------------------------------
29	
30	Ports are described by child 'port' nodes contained in the device node.
31	Each port node contains an 'endpoint' subnode for each remote device port
32	connected to this port. If a single port is connected to more than one
33	remote device, an 'endpoint' child node must be provided for each link.
34	If more than one port is present in a device node or there is more than one
35	endpoint at a port, or a port node needs to be associated with a selected
36	hardware interface, a common scheme using '#address-cells', '#size-cells'
37	and 'reg' properties is used number the nodes.
38	
39	device {
40	        ...
41	        #address-cells = <1>;
42	        #size-cells = <0>;
43	
44	        port@0 {
45		        #address-cells = <1>;
46		        #size-cells = <0>;
47			reg = <0>;
48	
49	                endpoint@0 {
50				reg = <0>;
51				...
52			};
53	                endpoint@1 {
54				reg = <1>;
55				...
56			};
57	        };
58	
59	        port@1 {
60			reg = <1>;
61	
62			endpoint { ... };
63		};
64	};
65	
66	All 'port' nodes can be grouped under an optional 'ports' node, which
67	allows to specify #address-cells, #size-cells properties for the 'port'
68	nodes independently from any other child device nodes a device might
69	have.
70	
71	device {
72	        ...
73	        ports {
74	                #address-cells = <1>;
75	                #size-cells = <0>;
76	
77	                port@0 {
78	                        ...
79	                        endpoint@0 { ... };
80	                        endpoint@1 { ... };
81	                };
82	
83	                port@1 { ... };
84	        };
85	};
86	
87	Links between endpoints
88	-----------------------
89	
90	Each endpoint should contain a 'remote-endpoint' phandle property that points
91	to the corresponding endpoint in the port of the remote device. In turn, the
92	remote endpoint should contain a 'remote-endpoint' property. If it has one,
93	it must not point to another than the local endpoint. Two endpoints with their
94	'remote-endpoint' phandles pointing at each other form a link between the
95	containing ports.
96	
97	device-1 {
98	        port {
99	                device_1_output: endpoint {
100	                        remote-endpoint = <&device_2_input>;
101	                };
102	        };
103	};
104	
105	device-2 {
106	        port {
107	                device_2_input: endpoint {
108	                        remote-endpoint = <&device_1_output>;
109	                };
110	        };
111	};
112	
113	
114	Required properties
115	-------------------
116	
117	If there is more than one 'port' or more than one 'endpoint' node or 'reg'
118	property is present in port and/or endpoint nodes the following properties
119	are required in a relevant parent node:
120	
121	 - #address-cells : number of cells required to define port/endpoint
122	                    identifier, should be 1.
123	 - #size-cells    : should be zero.
124	
125	Optional endpoint properties
126	----------------------------
127	
128	- remote-endpoint: phandle to an 'endpoint' subnode of a remote device node.
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