From 4d56ed5a009b7d31ecae1dd26c047b8bb0dd9287 Mon Sep 17 00:00:00 2001 From: Philipp Zabel Date: Tue, 25 Feb 2014 15:44:49 +0100 Subject: [PATCH] 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 Acked-by: Tomi Valkeinen Acked-by: Mauro Carvalho Chehab Acked-by: Sylwester Nawrocki --- Documentation/devicetree/bindings/graph.txt | 129 ++++++++++++++++++++ 1 file changed, 129 insertions(+) create mode 100644 Documentation/devicetree/bindings/graph.txt diff --git a/Documentation/devicetree/bindings/graph.txt b/Documentation/devicetree/bindings/graph.txt new file mode 100644 index 000000000000..1a69c078adf2 --- /dev/null +++ b/Documentation/devicetree/bindings/graph.txt @@ -0,0 +1,129 @@ +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. +