General Concept

Capturing video or picture usually consists of

1. external devices e.g. camera sensors
2. internal SOC blocks e.g. ISP's or video DMA engines or video processors

All of these need to be represented correctly in the Devicetree for the devices to work.

For External devices (camera sensors)

SoC internal Blocks are represented as devicetree nodes and the external devices connected to them are represented as their child nodes of their respective bus controller nodes. e.g. For MIPI CSI camera-sensors I2C, For USB camera-sensors USB.

Data interfaces on camera-sensors are represented as port child nodes. And the devices that this data interface gets connected to needs to be represented as endpoint child nodes of the port node.

Since these devices are the part of internal SoC Blocks they have their own devicetree nodes. Which will contain an 'port' node and an 'endpoint' node.

We need to link the 2 endpoint nodes to each other using 'remote-endpoint' phandles for them to work.

For Example:

camera-device@<i2c-addr> {
        ...
        ports {
                #address-cells = <1>;
                #size-cells = <0>;

                port@0 {
                        ...
                        endpoint@0 { ... };
                        endpoint@1 { ... };
                };
                port@1 { ... };
        };
};

Key Rules to keep in mind while writing devicetrees for camera-devices

1. If the camera-device connects to more than one internal SoC blocks on the same bus then separate endpoint nodes need to be provided for each internal SoC block.

2. All 'port' nodes can be grouped under optional 'ports' node, which allows to specify #address-cells, #size-cells properties independently for the 'port' and 'endpoint' nodes and any child device nodes a device might have.

Further Reference Links

1. https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/media/video-interfaces.txt

Example devicetree

i2c0: i2c@fff20000 {
        ...
        imx074: camera@1a {
                
                /* Compatible property shows compatibility */
                compatible = "sony,imx074";
                
                /* I2C device address */
                reg = <0x1a>;
                
                /* Power supply devicetree nodes  */
                vddio-supply = <&regulator1>;
                vddcore-supply = <&regulator2>;

                /* Shared clock with ov772x_1 */
                clock-frequency = <30000000>;   
                
                /* */
                clocks = <&mclk 0>;
                clock-names = "sysclk";

                /* MIPI CSI Port connections */
                port {
                        /* Data Endpoint */
                        imx074_1: endpoint {
                                clock-lanes = <0>;
                                /* MIPI CSI data lanes */
                                data-lanes = <1 2>;
                                /* Remote endpoints */
                                remote-endpoint = <&csi2_1>;
                        };
                };
        };
};

/* MIPI CSI controller node*/
csi2: csi2@ffc90000 {
        compatible = "renesas,sh-mobile-csi2";
        reg = <0xffc90000 0x1000>;
        interrupts = <0x17a0>;
        #address-cells = <1>;
        #size-cells = <0>;

        port@1 {
                /* One of CSI2I and CSI2C. */
                compatible = "renesas,csi2c";   
                reg = <1>;                      
                csi2_1: endpoint {
                        clock-lanes = <0>;
                        data-lanes = <2 1>;
                        remote-endpoint = <&imx074_1>;
                };
        };
};