1、This page walks through how to write a device tree for a new machine. It is intended to provide an overview of device tree concepts and how they are used to describe a machine.本文将介绍如何为一个新机器编写设备树。我们准备提供一个有关设备树概念的概述和如何使用这些设备树来描述一个机器。For a full technical description of device tree data format, refer to
2、 the ePAPR specification. The ePAPR specification covers a lot more detail than the basic topics covered on this page, please refer to it for more advanced usage that isnt covered by this page.完整的设备树数据格式的技术说明书请参考 ePAPR 规范。ePAPR 规范涵盖了比本文基本主题更丰富的细节，要查阅本文没有涉及到的高级用法请参考该规范。-The device tree is a simple tr
3、ee structure of nodes and properties. Properties are key-value pairs, and node may contain both properties and child nodes. For example, the following is a simple tree in the .dts format:设备树是一个包含节点和属性的简单树状结构。属性就是键值对，而节点可以同时包含属性和子节点。例如，以下就是一个 .dts 格式的简单树：这棵树显然是没什么用的，因为它并没有描述任何东西，但它确实体现了节点的一些属性： a sin
4、gle root node: “/“一个单独的根节点：“/” a couple of child nodes: “node1“ and “node2“两个子节点：“node1”和“node2” a couple of children for node1: “child-node1“ and “child-node2“两个 node1 的子节点：“child-node1”和“child-node2” a bunch of properties scattered through the tree.一堆分散在树里的属性。Properties are simple key-value pairs
5、where the value can either be empty or contain an arbitrary byte stream. While data types are not encoded into the data structure, there are a few fundamental data representations that can be expressed in a device tree source file.Device Tree U属性是简单的键值对，它的值可以为空或者包含一个任意字节流。虽然数据类型并没有编码进数据结构，但在设备树源文件中任
6、有几个基本的数据表示形式。 Text strings (null terminated) are represented with double quotes:文本字符串（无结束符）可以用双引号表示：1. string-property = “a string“ Cells are 32 bit unsigned integers delimited by angle brackets:Cells是 32 位无符号整数，用尖括号限定：1. cell-property = binary data is delimited with square brackets:二进制数据用方括号限定：1. b
7、inary-property = 0x01 0x23 0x45 0x67; Data of differing representations can be concatenated together using a comma:不同表示形式的数据可以使用逗号连在一起：1. mixed-property = “a string“, 0x01 0x23 0x45 0x67, ; Commas are also used to create lists of strings:逗号也可用于创建字符串列表：1. string-list = “red fish“, “blue fish“;Basic C
8、oncepts基本概念-To understand how the device tree is used, we will start with a simple machine and build up a device tree to describe it step by step.我们将以一个简单机开始，然后通过一步步的建立一个描述这个简单机的设备树，来了解如何使用设备树。Sample Machine模型机Consider the following imaginary machine (loosely based on ARM Versatile), manufactured by
9、 “Acme“ and named “Coyotes Revenge“:考虑下面这个假想的机器（大致基于ARM Versatile），制造商为“Acme”，并命名为“Coyotes Revenge”： One 32bit ARM CPU一个 32 位 ARM CPU processor local bus attached to memory mapped serial port, spi bus controller, i2c controller, interrupt controller, and external bus bridge 256MB of SDRAM based at 0
10、256MB SDRAM 起始地址为 0 2 Serial ports based at 0x101F1000 and 0x101F2000两个串口起始地址：0x101F1000 和 0x101F2000 GPIO controller based at 0x101F3000GPIO 控制器起始地址：0x101F3000 SPI controller based at 0x10170000 with following devices带有以下设备的 SPI 控制器起始地址：0x10170000 MMC slot with SS pin attached to GPIO #1MMC 插槽的 SS
11、管脚连接至 GPIO #1 External bus bridge with following devices外部总线桥挂载以下设备 SMC SMC91111 Ethernet device attached to external bus based at 0x10100000SMC SMC91111 以太网设备连接到外部总线，起始地址：0x10100000 i2c controller based at 0x10160000 with following devicesi2c 控制器起始地址：0x10160000，并挂载以下设备 Maxim DS1338 real time clock.
12、 Responds to slave address 1101000 (0x58)Maxim DS1338 实时时钟。响应至从地址 1101000 (0x58) 64MB of NOR flash based at 0x30000000Initial structure初始结构The first step is to lay down a skeleton structure for the machine. This is the bare minimum structure required for a valid device tree. At this stage you want t
13、o uniquely identify the machine.1. / 2. compatible = “acme,coyotes-revenge“;3. ;compatible specifies the name of the system. It contains a string in the form “,. It is important to specify the exact device, and to include the manufacturer name to avoid namespace collisions. Since the operating syste
14、m will use the compatible value to make decisions about how to run on the machine, it is very important to put correct data into this patible 指定了系统的名称。它包含了一个“,”形式的字符串。重要的是要指定一个确切的设备，并且包括制造商的名子，以避免命名空间冲突。由于操作系统会使用 compatible 的值来决定如何在机器上运行，所以正确的设置这个属性变得非常重要。Theoretically, compatible is all the data an
15、 OS needs to uniquely identify a machine. If all the machine details are hard coded, then the OS could look specifically for “acme,coyotes-revenge“ in the top level compatible property.CPUs中央处理器Next step is to describe for each of the CPUs. A container node named “cpus“ is added with a child node fo
16、r each CPU. In this case the system is a dual-core Cortex A9 system from ARM. / compatible = “acme,coyotes-revenge“;cpus cpu0 compatible = “arm,cortex-a9“; ; cpu1 compatible = “arm,cortex-a9“; ; ; ;The compatible property in each cpu node is a string that specifies the exact cpu model in the form ,
17、just like the compatible property at the top level.每个 cpu 节点的 compatible 属性是一个“,”形式的字符串，并指定了确切的 cpu，就像顶层的 compatible 属性一样。More properties will be added to the cpu nodes later, but we first need to talk about more of the basic concepts.Node Names节点名称It is worth taking a moment to talk about naming co
18、nventions. Every node must have a name in the form .现在应该花点时间来讨论命名约定了。每个节点必须有一个“”形式的名字。is a simple ascii string and can be up to 31 characters in length. In general, nodes are named according to what kind of device it represents. ie. A node for a 3com Ethernet adapter would be use the name ethernet,
19、not 3com509.就是一个不超过31位的简单 ascii 字符串。通常，节点的命名应该根据它所体现的是什么样的设备。比如一个 3com 以太网适配器的节点就应该命名为 ethernet，而不应该是 3com509。The unit-address is included if the node describes a device with an address. In general, the unit address is the primary address used to access the device, and is listed in the nodes reg pro
20、perty. Well cover the reg property later in this document.如果该节点描述的设备有一个地址的话就还应该加上设备地址（unit-address）。通常，设备地址就是用来访问该设备的主地址，并且该地址也在节点的 reg 属性中列出。本文档中我们将在稍后涉及到 reg 属性。Sibling nodes must be uniquely named, but it is normal for more than one node to use the same generic name so long as the address is diff
21、erent (ie, serial101f1000 cpus cpu0 compatible = “arm,cortex-a9“; ; cpu1 compatible = “arm,cortex-a9“; ; ; serial101F0000 compatible = “arm,pl011“; ; serial101F2000 compatible = “arm,pl011“; ; gpio101F3000 compatible = “arm,pl061“; ; interrupt-controller10140000 compatible = “arm,pl190“; ; spi101150
22、00 compatible = “arm,pl022“; ; external-bus ethernet0,0 compatible = “smc,smc91c111“; ; i2c1,0 compatible = “acme,a1234-i2c-bus“; rtc58 compatible = “maxim,ds1338“; ; ; flash2,0 compatible = “samsung,k8f1315ebm“, “cfi-flash“; ; ; ;In this tree, a node has been added for each device in the system, an
23、d the hierarchy reflects the how devices are connected to the system. ie. devices on the extern bus are children of the external bus node, and i2c devices are children of the i2c bus controller node. In general, the hierarchy represents the view of the system from the perspective of the CPU.在此树中，已经为
24、系统中的每个设备添加了节点，而且这个层次结构也反映了设备与系统的连接方式。例如，外部总线上的设备就是外部总线节点的子节点，i2c 设备就是i2c 总线节点的子节点。通常，这个层次结构表现的是 CPU 视角的系统视图。This tree isnt valid at this point. It is missing information about connections between devices. That data will be added later.现在这棵树还是无效的，因为它缺少关于设备之间互联的信息。稍后将添加这些信息。Some things to notice in th
25、is tree:在这颗树中，应该注意这些事情： Every device node has a compatible property.每个设备节点都拥有一个 compatible 属性。 The flash node has 2 strings in the compatible property. Read on to the next section to learn why.闪存（flash）节点的 compatible 属性由两个字符串构成。欲知为何，请阅读下一节。 As mentioned earlier, node names reflect the type of device
26、, not the particular model. See section 2.2.2 of the ePAPR spec for a list of defined generic node names that should be used wherever possible.正如前面所述，节点的命名应当反映设备的类型而不是特定的型号。请查阅 ePAPR 规范第 2.2.2 节里定义的通用节点名，应当优先使用这些节点名。Understanding the compatible Property理解 compatible 属性Every node in the tree that rep
27、resents a device is required to have the compatible property. compatible is the key an operating system uses to decide which device driver to bind to a device.树中每个表示一个设备的节点都需要一个 compatible 属性。compatible 属性是操作系统用来决定使用哪个设备驱动来绑定到一个设备上的关键因素。compatible is a list of strings. The first string in the list s
28、pecifies the exact device that the node represents in the form “,“. The following strings represent other devices that the device is compatible patible 是一个字符串列表，之中第一个字符串指定了这个节点所表示的确切的设备，该字符串的格式为：“,“。剩下的字符串的则表示其它与之相兼容的设备。For example, the Freescale MPC8349 System on Chip (SoC) has a serial device whic
29、h implements the National Semiconductor ns16550 register interface. The compatible property for the MPC8349 serial device should therefore be: compatible = “fsl,mpc8349-uart“, “ns16550“. In this case, fsl,mpc8349-uart specifies the exact device, and ns16550 states that it is register-level compatibl
30、e with a National Semiconductor 16550 UART.例如，Freescale MPC8349 片上系统（SoC）拥有一个实现了美国国家半导体ns16550 的寄存器接口的串行设备，那么 MPC8349 的串行设备的 compatible 属性就应该是：compatible = “fsl,mpc8349-uart“, “ns16550“。在这里，mpc8349-uart 指定了确切的设备，而 ns16550 则说明这是与美国国家半导体 ns16550 UART 的寄存器级兼容。Note: ns16550 doesnt have a manufacturer pr
31、efix purely for historical reasons. All new compatible values should use the manufacturer prefix.注：ns16550 并没有制造商前缀，这仅仅是历史原因造成的。所有的新 compatible 值都应该使用制造商前缀。This practice allows existing device drivers to be bound to a newer device, while still uniquely identifying the exact hardware.这种做法可以使现有的设备驱动能够
32、绑定到新设备上，并仍然唯一的指定确切的设备。Warning: Dont use wildcard compatible values, like “fsl,mpc83xx-uart“ or similar. Silicon vendors will invariably make a change that breaks your wildcard assumptions the moment it is too late to change it. Instead, choose a specific silicon implementations and make all subseque
33、nt silicon compatible with it.警告：不要使用带通配符的 compatible 值，比如“fsl,mpc83xx-uart”或类似情况。芯片提供商无不会做出一些能够轻易打破你通配符猜想的变化，这时候在修改已经为时已晚了。相反，应该选择一个特定的芯片然后是所有后续芯片都与之兼容。How Addressing Works如何编址-Devices that are addressable use the following properties to encode address information into the device tree:可编址设备使用以下属性将地
34、址信息编码进设备树： reg #address-cells #size-cellsEach addressable device gets a reg which is a list of tuples in the form reg = . Each tuple represents an address range used by the device. Each address value is a list of one or more 32 bit integers called cells. Similarly, the length value can either be a l
35、ist of cells, or empty.每个可编址设备都有一个元组列表的 reg，元组的形式为：reg = 。每个元组都表示一个该设备使用的地址范围。每个地址值是一个或多个 32 位整型数列表，称为 cell。同样，长度值也可以是一个 cell 列表或者为空。Since both the address and length fields are variable of variable size, the #address-cells and #size-cells properties in the parent node are used to state how many cel
36、ls are in each field. Or in other words, interpreting a reg property correctly requires the parent nodes #address-cells and #size-cells values. To see how this all works, lets add the addressing properties to the sample device tree, starting with the CPUs.由于地址和长度字段都是可变大小的变量，那么父节点的 #address-cells 和 #
37、size-cells 属性就用来声明各个字段的 cell 的数量。换句话说，正确解释一个 reg 属性需要用到父节点的 #address-cells 和 #size-cells 的值。要知道这一切是如何运作的，我们将给模型机添加编址属性，就从 CPU 开始。CPU addressingCPU编址The CPU nodes represent the simplest case when talking about addressing. Each CPU is assigned a single unique ID, and there is no size associated with C
38、PU ids.CPU 节点表示了一个关于编址的最简单的例子。每个 CPU 都分配了一个唯一的 ID，并且没有 CPU id 相关的大小信息。1. cpus 2. #address-cells = ;3. #size-cells = ;4. cpu0 5. compatible = “arm,cortex-a9“;6. reg = ;7. ;8. cpu1 9. compatible = “arm,cortex-a9“;10. reg = ;11. ;12. ;In the cpus node, #address-cells is set to 1, and #size-cells is set
39、 to 0. This means that child reg values are a single uint32 that represent the address with no size field. In this case, the two cpus are assigned addresses 0 and 1. #size-cells is 0 for cpu nodes because each cpu is only assigned a single address.在 cpu 节点中，#address-cells 设置为 1，#size-cells 设置为 0。这意味
40、着子节点的reg 值是一个单一的 uint32，这是一个不包含大小字段的地址，为这两个 cpu 分配的地址是 0 和 1。cpu 节点的 #size-cells 为 0 是因为只为每个 cpu 分配一个单独的地址。Youll also notice that the reg value matches the value in the node name. By convention, if a node has a reg property, then the node name must include the unit-address, which is the first addres
41、s value in the reg property.你可能还会注意到 reg 的值和节点名字是相同的。按照惯例，如果一个节点有 reg 属性，那么该节点的名字就必须包含设备地址，这个设备地址就是 reg 属性里第一个地址值。Memory Mapped Devices内存映射设备Instead of single address values like found in the cpu nodes, a memory mapped device is assigned a range of addresses that it will respond to. #size-cells is u
42、sed to state how large the length field is in each child reg tuple. In the following example, each address value is 1 cell (32 bits), and each length value is also 1 cell, which is typical on 32 bit systems. 64 bit machines may use a value of 2 for #address-cells and #size-cells to get 64 bit addres
43、sing in the device tree.与 cpu 节点里单一地址值不同，应该分配给内存映射设备一个地址范围。#size-cells 声明每个子节点的 reg 元组中长度字段的大小。在接下来的例子中，每个地址值是 1 cell（32 位），每个长度值也是 1 cell，这是典型的 32 位系统。64 位的机器则可以使用值为 2 的 #address-cells 和 #size-cells 来获得在设备树中的 64 位编址。 / #address-cells = ; #size-cells = ; . serial101f0000 compatible = “arm,pl011“; re
44、g = ; ; serial101f2000 compatible = “arm,pl011“; reg = ; ; gpio101f3000 compatible = “arm,pl061“; reg = ; ; interrupt-controller10140000 compatible = “arm,pl190“; reg = ; ; spi10115000 compatible = “arm,pl022“; reg = ; ; . ;Each device is assigned a base address, and the size of the region it is ass
45、igned. The GPIO device address in this example is assigned two address ranges; 0x101f3000.0x101f3fff and 0x101f40000x101f400f.每个设备都被分配了一个基址以及该区域的大小。这个例子中为 GPIO 分配了两个地址范围：0x101f3000.0x101f3fff 和 0x101f40000x101f400f。Some devices live on a bus with a different addressing scheme. For example, a device
46、can be attached to an external bus with discrete chip select lines. Since each parent node defines the addressing domain for its children, the address mapping can be chosen to best describe the system. The code below show address assignment for devices attached to the external bus with the chip sele
47、ct number encoded into the address.一些挂在总线上的设备有不同的编址方案。例如一个带独立片选线的设备也可以连接至外部总线。由于父节点会为其子节点定义地址域，所以可以选择不同的地址映射来最恰当的描述该系统。下面的代码展示了设备连接至外部总线并将其片选号编码进地址的地址分配。 external-bus #address-cells = #size-cells = ; ethernet0,0 compatible = “smc,smc91c111“; reg = ; ; i2c1,0 compatible = “acme,a1234-i2c-bus“; reg = ; rtc58 compatible = “maxim,ds1338“; ; ; flash2,0 compatible = “samsung,k8f1315ebm“, “cfi-flash“; reg = ; ; ;The external-bus uses 2 cells for the address v
