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一. 设备树简介

        Linux设备树，英文称为Devicetree，设备树的源文件称为DTS。设备树是用于描述系统的硬件资源。例如在Linux2.x时，我们写驱动程序需要在代码中写死我们需要使用的GPIO等资源，但是当硬件改板或是开发另一款设备时，GPIO有所改动时，我们就要修改驱动源码了，这样修改有可能会遗漏，特别GPIO改动太大时。Linux3.x内核出现设备树时，我们就可以在可以在一个设备树文件中把GPIO资源都修改好，不仅提高了效率，而且也避免了出错。

二. DTS文件

1. DTS文件类型

        设备树文件有两种文件，以.dts和.dtsi结尾的文件。dtsi文件通常是用来描述CPU所支持的外设 ，dts文件通常是用来描述具体单板的硬件资源使用。所以dtsi文件是用来被dts文件包含的。它们的关系类似于C语言中的.c和.h文件的关系。

2. DTS文件布局

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

dtsi示例代码：

/ {#address-cells = <1>;#size-cells = <1>;compatible = "ralink,mtk7620n-soc";cpus {cpu@0 {compatible = "mips,mips24KEc";};};chosen {bootargs = "console=ttyS0,57600";};cpuintc: cpuintc@0 {#address-cells = <0>;#interrupt-cells = <1>;interrupt-controller;compatible = "mti,cpu-interrupt-controller";};palmbus@10000000 {compatible = "palmbus";reg = <0x10000000 0x200000>;ranges = <0x0 0x10000000 0x1FFFFF>;#address-cells = <1>;#size-cells = <1>;sysc@0 {compatible = "ralink,mt7620a-sysc", "ralink,rt3050-sysc";reg = <0x0 0x100>;};timer@100 {compatible = "ralink,mt7620a-timer", "ralink,rt2880-timer";reg = <0x100 0x20>;interrupt-parent = <&intc>;interrupts = <1>;};watchdog@120 {compatible = "ralink,mt7620a-wdt", "ralink,rt2880-wdt";reg = <0x120 0x10>;resets = <&rstctrl 8>;reset-names = "wdt";interrupt-parent = <&intc>;interrupts = <1>;};intc: intc@200 {compatible = "ralink,mt7620a-intc", "ralink,rt2880-intc";reg = <0x200 0x100>;resets = <&rstctrl 19>;reset-names = "intc";interrupt-controller;#interrupt-cells = <1>;interrupt-parent = <&cpuintc>;interrupts = <2>;};memc@300 {compatible = "ralink,mt7620a-memc", "ralink,rt3050-memc";reg = <0x300 0x100>;resets = <&rstctrl 20>;reset-names = "mc";interrupt-parent = <&intc>;interrupts = <3>;};gpio0: gpio@600 {compatible = "ralink,mt7620a-gpio", "ralink,rt2880-gpio";reg = <0x600 0x34>;resets = <&rstctrl 13>;reset-names = "pio";interrupt-parent = <&intc>;interrupts = <6>;gpio-controller;#gpio-cells = <2>;ralink,gpio-base = <0>;ralink,num-gpios = <24>;ralink,register-map = [ 00 04 08 0c20 24 28 2c30 34 ];};gpio1: gpio@638 {compatible = "ralink,mt7620a-gpio", "ralink,rt2880-gpio";reg = <0x638 0x24>;interrupt-parent = <&intc>;interrupts = <6>;gpio-controller;#gpio-cells = <2>;ralink,gpio-base = <24>;ralink,num-gpios = <16>;ralink,register-map = [ 00 04 08 0c10 14 18 1c20 24 ];#status = "disabled";status = "okay";};gpio2: gpio@660 {compatible = "ralink,mt7620a-gpio", "ralink,rt2880-gpio";reg = <0x660 0x24>;interrupt-parent = <&intc>;interrupts = <6>;gpio-controller;#gpio-cells = <2>;ralink,gpio-base = <40>;ralink,num-gpios = <32>;ralink,register-map = [ 00 04 08 0c10 14 18 1c20 24 ];status = "disabled";};gpio3: gpio@688 {compatible = "ralink,mt7620a-gpio", "ralink,rt2880-gpio";reg = <0x688 0x24>;interrupt-parent = <&intc>;interrupts = <6>;gpio-controller;#gpio-cells = <2>;ralink,gpio-base = <72>;ralink,num-gpios = <1>;ralink,register-map = [ 00 04 08 0c10 14 18 1c20 24 ];status = "disabled";};i2c: i2c@900 {compatible = "ralink,rt2880-i2c";reg = <0x900 0x100>;resets = <&rstctrl 16>;reset-names = "i2c";#address-cells = <1>;#size-cells = <0>;pinctrl-names = "default";pinctrl-0 = <&i2c_pins>;};spi@b00 {compatible = "ralink,mt7620a-spi", "ralink,rt2880-spi";reg = <0xb00 0x100>;resets = <&rstctrl 18>;reset-names = "spi";#address-cells = <1>;#size-cells = <1>;pinctrl-names = "default";pinctrl-0 = <&spi_pins>;};uartlite@c00 {compatible = "ralink,mt7620a-uart", "ralink,rt2880-uart", "ns16550a";reg = <0xc00 0x100>;resets = <&rstctrl 19>;reset-names = "uartl";interrupt-parent = <&intc>;interrupts = <12>;reg-shift = <2>;pinctrl-names = "default";pinctrl-0 = <&uartlite_pins>;};systick@d00 {compatible = "ralink,mt7620a-systick", "ralink,cevt-systick";reg = <0xd00 0x10>;resets = <&rstctrl 28>;reset-names = "intc";interrupt-parent = <&cpuintc>;interrupts = <7>;};};pinctrl {compatible = "ralink,rt2880-pinmux";pinctrl-names = "default";pinctrl-0 = <&state_default>;state_default: pinctrl0 {};spi_pins: spi {spi {ralink,group = "spi";ralink,function = "spi";};};uartlite_pins: uartlite {uart {ralink,group = "uartlite";ralink,function = "uartlite";};};};rstctrl: rstctrl {compatible = "ralink,mt7620a-reset", "ralink,rt2880-reset";#reset-cells = <1>;};usbphy: usbphy {compatible = "ralink,mt7620a-usbphy";#phy-cells = <1>;resets = <&rstctrl 22 &rstctrl 25>;reset-names = "host", "device";};ethernet@10100000 {compatible = "ralink,mt7620a-eth";reg = <0x10100000 10000>;#address-cells = <1>;#size-cells = <0>;interrupt-parent = <&cpuintc>;interrupts = <5>;resets = <&rstctrl 21 &rstctrl 23>;reset-names = "fe", "esw";mdio-bus {#address-cells = <1>;#size-cells = <0>;status = "disabled";};};gsw@10110000 {compatible = "ralink,mt7620a-gsw";reg = <0x10110000 8000>;resets = <&rstctrl 23>;reset-names = "esw";interrupt-parent = <&intc>;interrupts = <17>;ralink,port4 = "gmac";};ehci@101c0000 {compatible = "ralink,rt3xxx-ehci";reg = <0x101c0000 0x1000>;interrupt-parent = <&intc>;interrupts = <18>;phys = <&usbphy 1>;phy-names = "usb";status = "disabled";};ohci@101c1000 {compatible = "ralink,rt3xxx-ohci";reg = <0x101c1000 0x1000>;phys = <&usbphy 1>;phy-names = "usb";interrupt-parent = <&intc>;interrupts = <18>;status = "disabled";};wmac@10180000 {compatible = "ralink,rt7620-wmac", "ralink,rt2880-wmac";reg = <0x10180000 40000>;interrupt-parent = <&cpuintc>;interrupts = <6>;ralink,eeprom = "soc_wmac.eeprom";};
};

dts文件示例：

/dts-v1/;/include/ "mt7620n.dtsi"/ {compatible = "mediatek,hiwooya", "ralink,mt7620n-soc";model = "Wooya IOT Smart 7620";palmbus@10000000 {gpio0: gpio@600 {status = "okay";};gpio2: gpio@660 {status = "okay";};gpio3: gpio@688 {status = "okay";};spi@b00 {status = "okay";m25p80@0 {#address-cells = <1>;#size-cells = <1>;compatible = "mx25l12805d";reg = <0 0>;linux,modalias = "m25p80", "w25q128";spi-max-frequency = <10000000>;partition@0 {label = "u-boot";reg = <0x0 0x30000>;read-only;};partition@30000 {label = "u-boot-env";reg = <0x30000 0x10000>;read-only;};factory: partition@40000 {label = "factory";reg = <0x40000 0x10000>;read-only;};partition@50000 {label = "firmware";reg = <0x50000 0xdb0000>;};partition@ff0000 {label = "config";reg = <0xe00000 0x100000>;};};};};lte-leds {compatible = "gpio-leds";red-led {label = "red";gpios = <&gpio0 7 1>;linux,default-trigger = "none";};green-led {label = "green";gpios = <&gpio0 1 1>;};blue-led {label = "blue";gpios = <&gpio1 14 1>;linux,default-trigger = "none";};l1 {label = "led1";gpios = <&gpio2 6 1>;linux,default-trigger = "none";};l2 {label = "led2";gpios = <&gpio2 11 1>;linux,default-trigger = "none";};l3 {label = "led3";gpios = <&gpio2 10 1>;linux,default-trigger = "none";};l4 {label = "led4";gpios = <&gpio2 19 1>;linux,default-trigger = "none";};};gpio-keys-polled {compatible = "gpio-keys-polled";#address-cells = <1>;#size-cells = <0>;poll-interval = <20>;reset {label = "reset";gpios = <&gpio1 13 1>;linux,code = <0x198>;};}; ehci@101c0000 {status = "okay";};ohci@101c1000 {status = "okay";};ethernet@10100000 {mtd-mac-address = <&factory 0x4>;ralink,port-map = "llllw";};wmac@10180000 {ralink,mtd-eeprom = <&factory 0>;};pinctrl {state_default: pinctrl0 {default {ralink,group = "pa", "spi refclk", "wdt", "uartf", "nd_sd";ralink,function = "gpio";};i2c_pins: i2c_pins {i2c_pins {ralink,group = "i2c";ralink,function = "i2c";};};};};};

        从上面设备的设备树代码来看，设备树dts文件需要指定dts的版本号（/dts-v1/;），包含了什么dtsi文件（/include/ "mt7620n.dtsi"），以/开始的根节点和节点名开始的子节点。简化代码如下

/dts-v1/;/include/ "mt7620n.dtsi"/ {compatible = "mediatek,hiwooya", "ralink,mt7620n-soc";model = "Wooya IOT Smart 7620";palmbus@10000000 {........};lte-leds {compatible = "gpio-leds";red-led {........};};gpio-keys-polled {compatible = "gpio-keys-polled";........}; ohci@101c1000 {status = "okay";};
};

        从上面代码可以看出设备树的节点是以node-name@unit-address { }组成，node-name一般以字母数字等ASCII码组成，根节点是一种特殊节点。有些节点带有unit-address，而有些没有，节点都有一个compatible属性等。接下来详细介绍。        

三. DTS设备节点属性介绍

1. compatible

        compatible可以包含一个或者多个字符串，中间以逗号隔开。主要用来指定驱动代码设备的型号名称。当驱动代码中的名称和设备树的某个节点compatible属性名称一样，驱动代码则调用probe函数。示例如下：

compatible = "fsl,mpc8641", "ns16550";

2. model

        主要用来指定产商的设备型号。示例如下

model = "fsl,MPC8349EMITX";

3. phandle

        phandle属性指定一个设备树中唯一的数字标识。带有phandle属性的节点可以被其他节点引用，示例如下

pic@10000000 {phandle = <1>;interrupt-controller;
};another-device-node {interrupt-parent = <1>;
};

4. status

        status属性指示设备的可操作的状态，可用的状态如下：

okay ：表示设备可以操作

disabled ：表示设备当前不可操作，但未来可能可以操作

fail ：表示设备不可操作，检测到严重的错误，如果不修复无法操作。

fail-sss ：和fail的含义类型

5. #address-cells和#size-cells

        #address-cells和#size-cells属性可用于任何在设备树层次结构中拥有子节点的设备节点，并描述子设备节点应该如何被寻址。#address-cells属性定义了该节点的子节点中的reg属性该用几个u32类型的数字编码其地址字段。#size-cells则定义了reg属性的size字段需要使用的u32类型的数字的数量。#address-cells和#size-cells都不从父节点那继承，需要明确的定义。示例如下

soc {#address-cells = <1>;#size-cells = <1>;serial {compatible = "ns16550";reg = <0x4600 0x100>;clock-frequency = <0>;interrupts = <0xA 0x8>;interrupt-parent = <&ipic>;};
};

6. reg

        reg属性描述了在父总线定义的地址空间中设备资源的地址。通常表示寄存器映射在内存中的起始地址和长度，或者CPU寄存器地址的真实地址。

        当寄存器在内存中有两段地址时，reg需要指定两对#address-cells和#size-cells，示例如下

reg = <0x3000 0x20 0xFE00 0x100>;

7. ranges

        以后补充

8. dma-ranges

        以后补充

四. DTS设备节点类型介绍

1. 根节点（/）

        设备树有一个根节点，其余节点都是子节点，根节点使用/表示，示意代码如下

/ {};

2. /aliases

        aliases节点是用来定义一个设备节点的别名的，aliases节点应该在设备树的根节点中使用。

aliases的属性名是需要指定的别名，属性值是设备树中某个节点的完整路径，示例如下

aliases {serial0 = "/simple-bus@fe000000/serial@llc500";ethernet0 = "/simple-bus@fe000000/ethernet@31c000";
};

3. /memory

       memory节点是用于描述系统的物理内存的的布局的，如果系统有多个地址范围的内存，则多个memory节点需要创建，或者在reg节点中指定多个范围。示例如下

memory@0 {device_type = "memory";reg = <0x000000000 0x00000000 0x00000000 0x800000000x000000001 0x00000000 0x00000001 0x00000000>;
};

memory@0 {device_type = "memory";reg = <0x000000000 0x00000000 0x00000000 0x80000000>;
};
memory@100000000 {device_type = "memory";reg = <0x000000001 0x00000000 0x00000001 0x00000000>;
};

4. /chosen

        chosen节点不用来描述一个真实的设备，而是描述设备的启动和运行参数，例如bootargs，示例如下

chosen {bootargs = "root=/dev/nfs rw nfsroot=192.168.1.1 console=ttyS0,115200";
};

5. /cpus

        后面补充

6. /cpus/cpu*

        后面补充

五. 总结

        本文主要介绍了设备树的格式布局，属性名，设备节点名的使用。