Linux Node Properties

This article talks about the properties of Linux device tree nodes.

For related subjects, please refer to the SOC Table of Contents.

In order to define a node for device trees, it will need to follow the following format:

[label:]node-name[@unit-address]{
        [properties definitions]
        [child nodes]
};

Note that the portions in square brackets [] can be left blank. Including the label makes it easier for it to be referenced by other nodes in the .dts file.

node-name is the name of the node and can be any combination of ASCII characters forming a string; it is recommended for its name to describe its function, for example naming a UART1 peripheral node uart1.

unit-address represents the device’s address or base address, this can be left blank of the node doesn’t have it, such as cpu@0 or the interrupt nodes.

As stated before, nodes can have parent/child relations. An example is shown below:

cpus{
        #address-cells = <1>;
        #size-cells = <0>;

        cpu0:cpu@0{
               compatible = “arm,cortex-a9”;
               device_type = “cpu”;
               reg = <0>;
               clocks = <&clkc 3>;
               clock-latency = <1000>;
               cpu0-supply = <&regulator_vccpint>;
               operating-points = <
                       /* kHz uV */
                       666667 1000000
                       333334 1000000
                       >;
        };

        cpu1:cpu@1{
               compatible = “arm,cortex-a9”;
               device_type = “cpu”;
               reg = <1>;
               clocks = <&clkc 3>;
        };
};

Note that the cpus node at the start of the code block only has the node name. The cpus node has two properties: #address-cells and #size-cells, with values of <1> and <0> respectively.

We will proceed to go over each other node property.

Node properties can be any of the following six types:

String

compatible = “arm,cortex-a9”;

The string is enveloped in double quotation marks “”. As we can see from the example above, the property value of compatible is “arm,cortex-a9”.

32-bit unsigned integer

clock-latency = <1000>;

reg = <0x00000000 0x00500000>;

The unsigned integer is enveloped in open angle brackets <>. As we can see from the example above, clock-latency has a 32-bit unsigned int value of 1000, whereas reg has two pieces of data separated by a space, which we can identify as an array.

Binary

local-mac-address = [00 0a 35 00 le 53];

The binary value is enveloped in square brackets []. As we can see from the example above, it’s an array consisting of binary values.

String array

compatible = “n25q512a”,“micron,m25p80”;

Note that properties can also be lists of strings, as shown above. The strings are separated by commas.

Mixed values

mixed-property = “string”,[0x01 0x23 0x45 0x67],<0x12345678>;

Note that properties can also be a combination of different types, where they are separated by commas.

Node referencing

clocks = <&clkc 3>;

Note that &clkc references the node clkc, and the entire operation is also enveloped by open angle brackets.