Linux设备树（Device Tree）

一、什么是设备树（Device Tree）
二、设备树的相关名词
三、DTS 文件格式
四、DTB文件格式
五、实例分析（根据dtb文件结构）
参考资料

一、什么是设备树（Device Tree）

它是一种描述硬件资源的数据结构，可以通过 bootloader 将它传给内核，内核（Kernal）使用它对硬件进行初始化，好处是使得内核和硬件资源描述相对独立，不需要太多的硬编码。

二、设备树的相关名词

DTS（Device Tree Source）：.dts文件是一种ASCII文本对Device Tree的描述，位于linux-x.xx/arch/arm64/boot/dts目录下。
DTC（Device Tree Compiler）：DTC为编译工具，它可以将.dts文件编译成.dtb 文件，DTC 的源码位于 linux-x.xx/scripts/dtc目录下。
DTB（Device Tree Blob）：DTC编译.dts 生成的二进制文件（.dtb）（实际上是字节码），bootloader 在加载内核时，也会同时把.dtb加载到内存，后面传递给内核使用。

三、DTS 文件格式

/*
* Device Tree Source for OMAP2 SoC
*
* Copyright (C) 2011 Texas Instruments Incorporated - http://www.ti.com/
*
* This file is licensed under the terms of the GNU General Public License
* version 2.  This program is licensed "as is" without any warranty of any
* kind, whether express or implied.
*/
/include/ "skeleton.dtsi"            /*需要引用的dts*/
/ {
    compatible = "ti,omap2430", "ti,omap2420", "ti,omap2";
    aliases {
        serial0 = &uart1;
        serial1 = &uart2;
        serial2 = &uart3;
    };
    cpus {
        cpu@0 {
            compatible = "arm,arm1136jf-s";
        };
    };
    soc {
        compatible = "ti,omap-infra";
        mpu {
            compatible = "ti,omap2-mpu";
            ti,hwmods = "mpu";
        };
    };
    ocp {
        compatible = "simple-bus";
        #address-cells = <1>;
        #size-cells = <1>;
        ranges;
        ti,hwmods = "l3_main";
        intc: interrupt-controller@1 {
            compatible = "ti,omap2-intc";
            interrupt-controller;
            #interrupt-cells = <1>;
        };
        uart1: serial@4806a000 {
            compatible = "ti,omap2-uart";
            ti,hwmods = "uart1";
            clock-frequency = <48000000>;
        };
        uart2: serial@4806c000 {
            compatible = "ti,omap2-uart";
            ti,hwmods = "uart2";
            clock-frequency = <48000000>;
        };
        uart3: serial@4806e000 {
            compatible = "ti,omap2-uart";
            ti,hwmods = "uart3";
            clock-frequency = <48000000>;
        };
    };
};

“/“ 为 root 节点，在一个. dts 文件中，有且仅有一个 root 节点，/include/ "skeleton.dtsi"和C代码中的 include 头文件的作用差不多，也就是把/include/ "skeleton.dtsi"定义的 device tree 节点包含到 omap2.dtsi 中，虽然omap2.dtsi文件中也会有一个 “/“，但是 dtc 编译时，会把它们合并成一个。

/*
* Copyright (C) 2011 Texas Instruments Incorporated - http://www.ti.com/
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
/dts-v1/;                        /*dts的版本*/
/include/ "omap4.dtsi"            /*需要引用的dts*/
/ {
    model = "TI OMAP4 PandaBoard";
    compatible = "ti,omap4-panda", "ti,omap4430", "ti,omap4";
    memory {
        device_type = "memory";
        reg = <0x80000000 0x40000000>; /* 1 GB */
    };
};

1）aliases node

 aliases {
         serial0 = &uart0;
         serial1 = &uart1;
         serial2 = &uart2;
     };

aliases 节点定义了一些别名。为何要定义这个 node 呢？因为 Device tree 是树状结构，当要引用一个 node 的时候要指明相对于 root node 的 full path。例如

uart1: serial@4806a000 {
  compatible = "ti,omap2-uart";
  ti,hwmods = "uart1";
  clock-frequency = <48000000>;
};

serial1 = &uart1 所以 serial1就是 serial@4806a000 的一个别名，uart1是一个 lable，也是 serial@4806a000，使用 lable 需要在前面加上 & 。例如

&uart1 {
     status = "okay";
};

就是把serial@4806a000节点里面的 status 属性改成 okay

2）memory node

    memory {
        device_type = "memory";
        reg = <0x80000000 0x40000000>; /* 1 GB */
    };

对于 memory node，device_type 必须为 memory，memory device node 是所有设备树文件的必备节点，它定义了系统物理内存的 layout。
reg 描述了 memory-mapped IO register 的 offset 和 length。对于 memory node，定义了该 memory 的起始地址和长度，这里的0x80000000是起始地址，0x40000000是内存的大小（长度）。

 #address-cells = <1>;
 #size-cells = <1>;

为什么是 0 0x40000000 表示起始地址，因为 root 节点 #address-cells = <2>; 表示用两个 cell （32 位），同样的 #size-cells = <2> 表示用两个 cell （32 位）。

每个 node 用节点名字（node name）标识，节点名字的格式是 node-name@unit-address。如果该 node 没有 reg 属性，那么该节点名字中必须不能包括 @和 unit-address。unit-address 的具体格式是和设备挂在那个 bus 上相关。

所以上面 memory 的描述是，起始地址是 0x 80000000

3）chosen node

34     chosen {
35         stdout-path = "serial0:921600n8";
36     };

chosen node 主要用来描述由系统指定的 runtime parameter，它并没有描述任何硬件设备节点信息。原先通过 tag list 传递的一些 linux kernel 运行的参数，可以通过 chosen 节点来传递。如 command line 可以通过 bootargs 这个 property 来传递。如果存在 chosen node，它的 parent 节点必须为 “/” 根节点。

四、DTB文件格式

1、dtb文件结构

DTB文件结构

2、fdt_header

struct fdt_header {
    uint32_t magic;             /* magic word FDT_MAGIC */
    uint32_t totalsize;         /* total size of DT block */
    uint32_t off_dt_struct;         /* offset to structure */
    uint32_t off_dt_strings;     /* offset to strings */
    uint32_t off_mem_rsvmap;     /* offset to memory reserve map */
    uint32_t version;         /* format version */
    uint32_t last_comp_version;     /* last compatible version */
    /* version 2 fields below */
    uint32_t boot_cpuid_phys;     /* Which physical CPU id we're
                        booting on */
    /* version 3 fields below */
    uint32_t size_dt_strings;     /* size of the strings block */
    /* version 17 fields below */
    uint32_t size_dt_struct;     /* size of the structure block */
};

off_dt_struct 是到dt_struct 结构块的偏移量（相对于文件起始位置），off_dt_strings 是到dt_strings字符串块的偏移量（相对于文件起始位置），off_mem_rsvmap 是到memory reserve map 区域的偏移量（相对于文件起始位置）。头部一共40字节，即off_mem_rsvmap 的值为40。

3、memory reserve map

该区域保存的数据会4字节对齐

4、dt_struct

结构块里面保存了dts 里面描述的设备信息，和dts 里面写的内容一致，只不过转成了另一种数据格式。节点的开始和结束，属性的开始用下面定义的标识。

#define FDT_MAGIC        0xd00dfeed    /* 4: version, 4: total size */
#define FDT_TAGSIZE        sizeof(uint32_t)
#define FDT_BEGIN_NODE    0x1        /* Start node: full name */
#define FDT_END_NODE    0x2        /* End node */
#define FDT_PROP        0x3        /* Property: content_size, name off(in_dt_string), content */
#define FDT_NOP            0x4        /* nop */
#define FDT_END            0x9

5、off_dt_strings

字符串块保存的是dts中属性的名字，因为在不同的节点中会用到相同的属性名，为了减少保存重复的属性名字符串，所以把它们放在字符串块中，每个字符串是以\0为结束标识。

6、dtb文件生成与查看

使用dtc工具将dts转为dtb，命令如下：

mate@Vostro:~/Documents/.../arch/arm/boot/dts$ dtc -I dts omap4-panda.dts -O dtb -o omap4-panda.dtb
#或
mate@Vostro:~/Documents/.../arch/arm/boot/dts$ dtc -I dts -O dtb omap4-panda.dts -o omap4-panda.dtb

使用二进制文件查看工具hexdump查看编译后的dtb文件，如下：

mate@Vostro:~/Documents/linux-A3352W-4G-SPI-CAN-GPIO-IIC_20210622/linux-A3352W-4G-SPI-CAN-GPIO-IIC/arch/arm/boot/dts$ hexdump -C omap4-panda.dtb
00000000  d0 0d fe ed 00 00 06 3c  00 00 00 48 00 00 05 80  |.......<...H....|
00000010  00 00 00 28 00 00 00 11  00 00 00 10 00 00 00 00  |...(............|
00000020  00 00 00 bc 00 00 05 38  00 00 00 00 9d 00 00 00  |.......8........|
00000030  00 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
00000040  00 00 00 00 00 00 00 00  00 00 00 01 00 00 00 00  |................|
00000050  00 00 00 03 00 00 00 04  00 00 00 00 00 00 00 01  |................|
00000060  00 00 00 03 00 00 00 04  00 00 00 0f 00 00 00 01  |................|
00000070  00 00 00 03 00 00 00 24  00 00 00 1b 74 69 2c 6f  |.......$....ti,o|
00000080  6d 61 70 34 2d 70 61 6e  64 61 00 74 69 2c 6f 6d  |map4-panda.ti,om|
00000090  61 70 34 34 33 30 00 74  69 2c 6f 6d 61 70 34 00  |ap4430.ti,omap4.|
000000a0  00 00 00 03 00 00 00 04  00 00 00 26 00 00 00 01  |...........&....|
000000b0  00 00 00 03 00 00 00 14  00 00 00 37 54 49 20 4f  |...........7TI O|
000000c0  4d 41 50 34 20 50 61 6e  64 61 42 6f 61 72 64 00  |MAP4 PandaBoard.|
000000d0  00 00 00 01 63 68 6f 73  65 6e 00 00 00 00 00 02  |....chosen......|
000000e0  00 00 00 01 61 6c 69 61  73 65 73 00 00 00 00 03  |....aliases.....|
000000f0  00 00 00 17 00 00 00 3d  2f 6f 63 70 2f 73 65 72  |.......=/ocp/ser|
00000100  69 61 6c 40 30 78 34 38  30 36 61 30 30 30 00 00  |ial@0x4806a000..|
00000110  00 00 00 03 00 00 00 17  00 00 00 45 2f 6f 63 70  |...........E/ocp|
00000120  2f 73 65 72 69 61 6c 40  30 78 34 38 30 36 63 30  |/serial@0x4806c0|
00000130  30 30 00 00 00 00 00 03  00 00 00 17 00 00 00 4d  |00.............M|
00000140  2f 6f 63 70 2f 73 65 72  69 61 6c 40 30 78 34 38  |/ocp/serial@0x48|
00000150  30 32 30 30 30 30 00 00  00 00 00 03 00 00 00 17  |020000..........|
00000160  00 00 00 55 2f 6f 63 70  2f 73 65 72 69 61 6c 40  |...U/ocp/serial@|
00000170  30 78 34 38 30 36 65 30  30 30 00 00 00 00 00 02  |0x4806e000......|
00000180  00 00 00 01 6d 65 6d 6f  72 79 00 00 00 00 00 03  |....memory......|
00000190  00 00 00 07 00 00 00 5d  6d 65 6d 6f 72 79 00 00  |.......]memory..|
000001a0  00 00 00 03 00 00 00 08  00 00 00 69 80 00 00 00  |...........i....|
000001b0  40 00 00 00 00 00 00 02  00 00 00 01 63 70 75 73  |@...........cpus|
000001c0  00 00 00 00 00 00 00 01  63 70 75 40 30 00 00 00  |........cpu@0...|
000001d0  00 00 00 03 00 00 00 0e  00 00 00 1b 61 72 6d 2c  |............arm,|
000001e0  63 6f 72 74 65 78 2d 61  39 00 00 00 00 00 00 02  |cortex-a9.......|
000001f0  00 00 00 01 63 70 75 40  31 00 00 00 00 00 00 03  |....cpu@1.......|
00000200  00 00 00 0e 00 00 00 1b  61 72 6d 2c 63 6f 72 74  |........arm,cort|
00000210  65 78 2d 61 39 00 00 00  00 00 00 02 00 00 00 02  |ex-a9...........|
00000220  00 00 00 01 73 6f 63 00  00 00 00 03 00 00 00 0e  |....soc.........|
00000230  00 00 00 1b 74 69 2c 6f  6d 61 70 2d 69 6e 66 72  |....ti,omap-infr|
00000240  61 00 00 00 00 00 00 01  6d 70 75 00 00 00 00 03  |a.......mpu.....|
00000250  00 00 00 0d 00 00 00 1b  74 69 2c 6f 6d 61 70 34  |........ti,omap4|
00000260  2d 6d 70 75 00 00 00 00  00 00 00 03 00 00 00 04  |-mpu............|
00000270  00 00 00 6d 6d 70 75 00  00 00 00 02 00 00 00 01  |...mmpu.........|
00000280  64 73 70 00 00 00 00 03  00 00 00 0d 00 00 00 1b  |dsp.............|
00000290  74 69 2c 6f 6d 61 70 33  2d 63 36 34 00 00 00 00  |ti,omap3-c64....|
000002a0  00 00 00 03 00 00 00 04  00 00 00 6d 64 73 70 00  |...........mdsp.|
000002b0  00 00 00 02 00 00 00 01  69 76 61 00 00 00 00 03  |........iva.....|
000002c0  00 00 00 09 00 00 00 1b  74 69 2c 69 76 61 68 64  |........ti,ivahd|
000002d0  00 00 00 00 00 00 00 03  00 00 00 04 00 00 00 6d  |...............m|
000002e0  69 76 61 00 00 00 00 02  00 00 00 02 00 00 00 01  |iva.............|
000002f0  6f 63 70 00 00 00 00 03  00 00 00 1b 00 00 00 1b  |ocp.............|
00000300  74 69 2c 6f 6d 61 70 34  2d 6c 33 2d 6e 6f 63 00  |ti,omap4-l3-noc.|
00000310  73 69 6d 70 6c 65 2d 62  75 73 00 00 00 00 00 03  |simple-bus......|
00000320  00 00 00 04 00 00 00 00  00 00 00 01 00 00 00 03  |................|
00000330  00 00 00 04 00 00 00 0f  00 00 00 01 00 00 00 03  |................|
00000340  00 00 00 00 00 00 00 77  00 00 00 03 00 00 00 1e  |.......w........|
00000350  00 00 00 6d 6c 33 5f 6d  61 69 6e 5f 31 00 6c 33  |...ml3_main_1.l3|
00000360  5f 6d 61 69 6e 5f 32 00  6c 33 5f 6d 61 69 6e 5f  |_main_2.l3_main_|
00000370  33 00 00 00 00 00 00 01  69 6e 74 65 72 72 75 70  |3.......interrup|
00000380  74 2d 63 6f 6e 74 72 6f  6c 6c 65 72 40 34 38 32  |t-controller@482|
00000390  34 31 30 30 30 00 00 00  00 00 00 03 00 00 00 12  |41000...........|
000003a0  00 00 00 1b 61 72 6d 2c  63 6f 72 74 65 78 2d 61  |....arm,cortex-a|
000003b0  39 2d 67 69 63 00 00 00  00 00 00 03 00 00 00 00  |9-gic...........|
000003c0  00 00 00 7e 00 00 00 03  00 00 00 04 00 00 00 93  |...~............|
000003d0  00 00 00 01 00 00 00 03  00 00 00 10 00 00 00 69  |...............i|
000003e0  48 24 10 00 00 00 10 00  48 24 01 00 00 00 01 00  |H$......H$......|
000003f0  00 00 00 03 00 00 00 04  00 00 00 a4 00 00 00 01  |................|
00000400  00 00 00 02 00 00 00 01  73 65 72 69 61 6c 40 30  |........serial@0|
00000410  78 34 38 30 36 61 30 30  30 00 00 00 00 00 00 03  |x4806a000.......|
00000420  00 00 00 0e 00 00 00 1b  74 69 2c 6f 6d 61 70 34  |........ti,omap4|
00000430  2d 75 61 72 74 00 00 00  00 00 00 03 00 00 00 06  |-uart...........|
00000440  00 00 00 6d 75 61 72 74  31 00 00 00 00 00 00 03  |...muart1.......|
00000450  00 00 00 04 00 00 00 ac  02 dc 6c 00 00 00 00 02  |..........l.....|
00000460  00 00 00 01 73 65 72 69  61 6c 40 30 78 34 38 30  |....serial@0x480|
00000470  36 63 30 30 30 00 00 00  00 00 00 03 00 00 00 0e  |6c000...........|
00000480  00 00 00 1b 74 69 2c 6f  6d 61 70 34 2d 75 61 72  |....ti,omap4-uar|
00000490  74 00 00 00 00 00 00 03  00 00 00 06 00 00 00 6d  |t..............m|
000004a0  75 61 72 74 32 00 00 00  00 00 00 03 00 00 00 04  |uart2...........|
000004b0  00 00 00 ac 02 dc 6c 00  00 00 00 02 00 00 00 01  |......l.........|
000004c0  73 65 72 69 61 6c 40 30  78 34 38 30 32 30 30 30  |serial@0x4802000|
000004d0  30 00 00 00 00 00 00 03  00 00 00 0e 00 00 00 1b  |0...............|
000004e0  74 69 2c 6f 6d 61 70 34  2d 75 61 72 74 00 00 00  |ti,omap4-uart...|
000004f0  00 00 00 03 00 00 00 06  00 00 00 6d 75 61 72 74  |...........muart|
00000500  33 00 00 00 00 00 00 03  00 00 00 04 00 00 00 ac  |3...............|
00000510  02 dc 6c 00 00 00 00 02  00 00 00 01 73 65 72 69  |..l.........seri|
00000520  61 6c 40 30 78 34 38 30  36 65 30 30 30 00 00 00  |al@0x4806e000...|
00000530  00 00 00 03 00 00 00 0e  00 00 00 1b 74 69 2c 6f  |............ti,o|
00000540  6d 61 70 34 2d 75 61 72  74 00 00 00 00 00 00 03  |map4-uart.......|
00000550  00 00 00 06 00 00 00 6d  75 61 72 74 34 00 00 00  |.......muart4...|
00000560  00 00 00 03 00 00 00 04  00 00 00 ac 02 dc 6c 00  |..............l.|
00000570  00 00 00 02 00 00 00 02  00 00 00 02 00 00 00 09  |................|
00000580  23 61 64 64 72 65 73 73  2d 63 65 6c 6c 73 00 23  |#address-cells.#|
00000590  73 69 7a 65 2d 63 65 6c  6c 73 00 63 6f 6d 70 61  |size-cells.compa|
000005a0  74 69 62 6c 65 00 69 6e  74 65 72 72 75 70 74 2d  |tible.interrupt-|
000005b0  70 61 72 65 6e 74 00 6d  6f 64 65 6c 00 73 65 72  |parent.model.ser|
000005c0  69 61 6c 30 00 73 65 72  69 61 6c 31 00 73 65 72  |ial0.serial1.ser|
000005d0  69 61 6c 32 00 73 65 72  69 61 6c 33 00 64 65 76  |ial2.serial3.dev|
000005e0  69 63 65 5f 74 79 70 65  00 72 65 67 00 74 69 2c  |ice_type.reg.ti,|
000005f0  68 77 6d 6f 64 73 00 72  61 6e 67 65 73 00 69 6e  |hwmods.ranges.in|
00000600  74 65 72 72 75 70 74 2d  63 6f 6e 74 72 6f 6c 6c  |terrupt-controll|
00000610  65 72 00 23 69 6e 74 65  72 72 75 70 74 2d 63 65  |er.#interrupt-ce|
00000620  6c 6c 73 00 70 68 61 6e  64 6c 65 00 63 6c 6f 63  |lls.phandle.cloc|
00000630  6b 2d 66 72 65 71 75 65  6e 63 79 00              |k-frequency.|
0000063c

五、实例分析（根据dtb文件结构）

1、fdt_header

struct fdt_header {
    uint32_t magic;                     /* magic word FDT_MAGIC */
    uint32_t totalsize;                 /* total size of DT block */
    uint32_t off_dt_struct;            /* offset to structure */
    uint32_t off_dt_strings;         /* offset to strings */
    uint32_t off_mem_rsvmap;         /* offset to memory reserve map */
    uint32_t version;                 /* format version */
    uint32_t last_comp_version;         /* last compatible version */
    /* version 2 fields below */
    uint32_t boot_cpuid_phys;         /* Which physical CPU id we'rebooting on */
    /* version 3 fields below */
    uint32_t size_dt_strings;         /* size of the strings block */
    /* version 17 fields below */
    uint32_t size_dt_struct;         /* size of the structure block */
};

00000000  d0 0d fe ed                     00 00 06 3c                  00 00 00 48                        00 00 05 80
          /*magic_num*/                 /*total = 1596*/        /*off_struct = 72*/     /*off_str = 1408*/
00000010  00 00 00 28                     00 00 00 11                  00 00 00 10                     00 00 00 00
          /*off_rsvmap = 40*/     /*version = 17*/        /*last_comp = 16*/         /*boot_cpuid = 0*/
00000020  00 00 00 bc                     00 00 05 38
          /*size_str = 188*/        /*size_dt_struct = 1336*/

2、memory reserve map

00000020  -----------------------  00 00 00 00 9d 00 00 00  |.......8........|
00000030  00 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
00000040  00 00 00 00 00 00 00 00

3、dt_struct

（节点、属性等四字节对齐）

①：标志一个节点的开始（0x01，四字节对齐）；
②：节点的名称（memory，四字节对齐）；
③：标志一个属性的开始；
④：属性内容的长度（sizeof(“memory”) = 7）；
⑤：属性名称在dt_string中的偏移量（偏移93字节：device_type）；
⑥：属性的内容（memory，四字节对齐）;
⑦：标志节点的结束（0x02，四字节对齐）。

#define FDT_BEGIN_NODE    0x1        /* Start node: full name */
#define FDT_END_NODE    0x2        /* End node */
#define FDT_PROP        0x3        /* Property: content_size, name off(in_dt_string), content */
#define FDT_NOP            0x4        /* nop */
#define FDT_END            0x9

4、dt_string

（字符串紧密排列，相当于属性名称的库。利用dt_struct中的属性名称偏移量检索，至“\0”截止）

00000580  23 61 64 64 72 65 73 73  2d 63 65 6c 6c 73 00 23  |#address-cells.#|
00000590  73 69 7a 65 2d 63 65 6c  6c 73 00 63 6f 6d 70 61  |size-cells.compa|
000005a0  74 69 62 6c 65 00 69 6e  74 65 72 72 75 70 74 2d  |tible.interrupt-|
000005b0  70 61 72 65 6e 74 00 6d  6f 64 65 6c 00 73 65 72  |parent.model.ser|
000005c0  69 61 6c 30 00 73 65 72  69 61 6c 31 00 73 65 72  |ial0.serial1.ser|
000005d0  69 61 6c 32 00 73 65 72  69 61 6c 33 00 64 65 76  |ial2.serial3.dev|
000005e0  69 63 65 5f 74 79 70 65  00 72 65 67 00 74 69 2c  |ice_type.reg.ti,|
000005f0  68 77 6d 6f 64 73 00 72  61 6e 67 65 73 00 69 6e  |hwmods.ranges.in|
00000600  74 65 72 72 75 70 74 2d  63 6f 6e 74 72 6f 6c 6c  |terrupt-controll|
00000610  65 72 00 23 69 6e 74 65  72 72 75 70 74 2d 63 65  |er.#interrupt-ce|
00000620  6c 6c 73 00 70 68 61 6e  64 6c 65 00 63 6c 6f 63  |lls.phandle.cloc|
00000630  6b 2d 66 72 65 71 75 65  6e 63 79 00              |k-frequency.|