Device Tree

Device tree is for describe the peripheral HW. It not noly can describe the detectable devices, but also can be used for non-detectable devices

non-detectable device: Like memory information, flash information, UARTs, GPIOs …etc.

Device Tree Source (.dts)

root

• /: This is root for device tree.

/ {
    
    name = "device-tree-example";
    model = "device model name";
    compatiable = "deive-tree";
    #address-cells = <2>;
    #size-cells = <2>;
    # linux,phandle = <0>;
}

node

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

[]: means option

• [aliae_name:] node-name[@unit-address] :
  • unit-address: indicate the address mapping to device.
  • if the node doesn't have "reg" property, the unit-address can't be exist.
• properties definitions:

  • describe the node property

  • the property can be no assigned value. e.g properties = [value]

    • value :
      • can be a value or multiple value.
      ​​​​​​​​​​​​address = <1>;
      ​​​​​​​​​​​​address2 = [0x00001 0x20000];
      

      • can be string lists
      ​​​​​​​​​​​​device_type = "memory";
      ​​​​​​​​​​​​name = "PowerPC,970";
      

      Property value

  • properties attirbute:
    Standard Properties
    Here are list some special properites.

    1. address-cells and #size-cells :

    The #address-cells and #size-cells properties may be used in any device node that has children in the devicetree hierarchy and describes how child device nodes should be addressed.

    The #address-cells property defines the number of <u32> cells used to encode the address field in a child node’s reg property.

    The #size-cells property defines the number of <u32> cells used to encode the size field in a child node’s reg property

    ​​​​​​​​    e.g: 
    ​​​​​​​​    #address-cells = <2>;
    ​​​​​​​​    #size-celss = <2>;
    ​​​​​​​​    memory@0 {
    ​​​​​​​​        device_type = "memory";
    ​​​​​​​​        reg = <0x000000000 0x00000000 0x00000000 0x80000000>;
    ​​​​​​​​    };
    

    reg will use <address1, size1, address2, size2, …>

    2. phandle:

    A phandle value is a way to reference another node in the devicetree

    1. In an array, the "&" reference will expand to a phandle.

    ​​​​​​​​mydevice: mydev {
    ​​​​​​​​    compatiable = "mydevice_list";
    ​​​​​​​​    pincontrol = <&my_pin_ctrl>;
    ​​​​​​​​}
    ​​​​​​​​
    ​​​​​​​​maybe in another dtsi
    ​​​​​​​​
    ​​​​​​​​my_pin_ctrl: my_pin_ctrl {
    ​​​​​​​​    ....
    ​​​​​​​​};
    

    2. Outside an arry, the "&" reference will expand to the path of the node you're referring to.

    ​​​​​​​​label:node {
    ​​​​​​​​    #address-cell = <1>;
    ​​​​​​​​    #size-cells = <0>;
    ​​​​​​​​}
    
    ​​​​​​​​&label {
    ​​​​​​​​    ...;
    ​​​​​​​​};
    

    This means

    ​​​​​​​​label:node {
    ​​​​​​​​    #address-cell = <1>;
    ​​​​​​​​    #size-cells = <0>;
    ​​​​​​​​    ....;
    ​​​​​​​​}
    

Device tree code trace and How to load into kernel

[TBD]

how to build

$ dtc -@ -I dts -O dtb -o <output>.dtbo <src>.dts

ref

Introduction to Linux kernel
driver programming
Device Tree（二）：基本概念
Device Tree（三）：代码分析
Device Tree（四）：文件结构解析
example in linux kernel

• dtsi example
  
• dts example
  
• Device Tree PIN defineitions
  
• Device match to device driver