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