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In computing, a devicetree (also written device tree) is a
data structure In computer science, a data structure is a data organization, management, and storage format that is usually chosen for efficient access to data. More precisely, a data structure is a collection of data values, the relationships among them, a ...
describing the hardware components of a particular computer so that the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
's
kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learni ...
can use and manage those components, including the
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...
or CPUs, the
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered, ...
, the
buses A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for char ...
and the
integrated peripheral A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...
s. The device tree was derived from
SPARC SPARC (Scalable Processor Architecture) is a reduced instruction set computer (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system developed i ...
-based computers via the
Open Firmware Open Firmware is a standard defining the interfaces of a computer firmware system, formerly endorsed by the Institute of Electrical and Electronics Engineers (IEEE). It originated at Sun Microsystems, where it was known as OpenBoot, and has bee ...
project. The current Devicetree specification is targeted at smaller systems, but is still used with some server-class systems (for instance, those described by the
Power Architecture Platform Reference Power Architecture Platform Reference (PAPR) is an initiative from Power.org to make a new open computing platform based on Power ISA processors. It follows two previous attempts made in the 1990s, PReP and CHRP. The PAPR specification provides t ...
).
Personal computer A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or techn ...
s with the
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introd ...
architecture generally do not use device trees, relying instead on various auto configuration protocols (e.g. ACPI) to discover hardware. Systems which use device trees usually pass a static device tree (perhaps stored in
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * R ...
) to the operating system, but can also generate a device tree in the early stages of
booting In computing, booting is the process of starting a computer as initiated via hardware such as a button or by a software command. After it is switched on, a computer's central processing unit (CPU) has no software in its main memory, so some ...
. As an example,
Das U-Boot Das U-Boot (subtitled "the Universal Boot Loader" and often shortened to U-Boot; see ''History'' for more about the name) is an open-source, primary boot loader used in embedded devices to package the instructions to boot the device's operating ...
and
kexec kexec, abbreviated from ''kernel execute'' and analogous to the Unix/Linux kernel call '' exec'', is a mechanism of the Linux kernel that allows booting of a new kernel from the currently running one. Essentially, kexec skips the bootloader stage ...
can pass a device tree when launching a new operating system. On systems with a boot loader that does not support device trees, a static device tree may be installed along with the operating system; the
Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...
supports this approach. The Devicetree specification is currently managed by a community named devicetree.org, which is associated with, among others,
Linaro Linaro is an engineering organization that works on free and open-source software such as the Linux kernel, the GNU Compiler Collection (GCC), QEMU, power management, graphics and multimedia interfaces for the ARM family of instruction sets and i ...
and
Arm In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...
.


Formats

A device tree can hold any kind of data as internally it is a
tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondar ...
of named nodes and
properties Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property. Property may also refer to: Mathematics * Property (mathematics) Philosophy and science * Property (philosophy), in philosophy and ...
. Nodes contain properties and child nodes, while properties are name–value pairs. Device trees have both a
binary format A binary file is a computer file that is not a text file. The term "binary file" is often used as a term meaning "non-text file". Many binary file formats contain parts that can be interpreted as text; for example, some computer document file ...
for operating systems to use and a textual format for convenient editing and management.


Usage


Linux

Given the correct device tree, the same compiled kernel can support different hardware configurations within a wider architecture family. The
Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...
for the ARC,
ARM In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...
, C6x,
H8/300 H8 is the name of a large family of 8-bit, 16-bit and 32-bit microcontrollers made by Renesas Technology, originating in the early 1990s within Hitachi Semiconductor. An administrator on the Renesas user community boards commented in 2011 that ...
, MicroBlaze, MIPS, NDS32,
Nios II Nios II is a 32-bit embedded processor architecture designed specifically for the Altera family of field-programmable gate array (FPGA) integrated circuits. Nios II incorporates many enhancements over the original Nios architecture, making it mo ...
,
OpenRISC OpenRISC is a project to develop a series of open-source hardware based central processing units (CPUs) on established reduced instruction set computer (RISC) principles. It includes an instruction set architecture (ISA) using an open-source lic ...
,
PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple– IBM� ...
,
RISC-V RISC-V (pronounced "risk-five" where five refers to the number of generations of RISC architecture that were developed at the University of California, Berkeley since 1981) is an open standard instruction set architecture (ISA) based on establis ...
,
SuperH SuperH (or SH) is a 32-bit reduced instruction set computing (RISC) instruction set architecture (ISA) developed by Hitachi and currently produced by Renesas. It is implemented by microcontrollers and microprocessors for embedded systems. At the ...
, and
Xtensa Tensilica was a company based in Silicon Valley in the semiconductor intellectual property core business. It is now a part of Cadence Design Systems. Tensilica is known for its customizable Xtensa microprocessor core. Other products include: HiF ...
architectures reads device tree information; on ARM, device trees have been mandatory for all new
SoCs SOCS (suppressor of cytokine signaling proteins) refers to a family of genes involved in inhibiting the JAK-STAT signaling pathway. Genes * CISH * SOCS1 * SOCS2 * SOCS3 * SOCS4 * SOCS5 * SOCS6 * SOCS7 Suppressor of cytokine signaling 7 is a pr ...
since 2012. This can be seen as a remedy to the vast number of forks (of Linux and Das U-Boot) that have historically been created to support (marginally) different ARM boards. The purpose is to move a significant part of the hardware description out of the kernel binary, and into the compiled device tree blob, which is handed to the kernel by the
boot loader A bootloader, also spelled as boot loader or called boot manager and bootstrap loader, is a computer program that is responsible for booting a computer. When a computer is turned off, its softwareincluding operating systems, application code, an ...
, replacing a range of board-specific C source files and compile-time options in the kernel. It is specified in a Devicetree Source file (.dts) and is compiled into a Devicetree Blob or device tree binary (.dtb) file through the Devicetree compiler (DTC). Device tree source files can include other files, referred to as device tree source includes. It has been customary for ARM-based
Linux distribution A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...
s to include a boot loader, that necessarily was customized for specific boards, for example
Raspberry Pi Raspberry Pi () is a series of small single-board computers (SBCs) developed in the United Kingdom by the Raspberry Pi Foundation in association with Broadcom. The Raspberry Pi project originally leaned towards the promotion of teaching basic ...
or Hackberry A10. This has created problems for the creators of Linux distributions as some part of the operating system must be compiled specifically for every board variant, or updated to support new boards. However, some modern SoCs (for example, Freescale i.MX6) have a vendor-provided boot loader with device tree on a separate chip from the operating system. A proprietary configuration file format used for similar purposes, the FEX file format, is a
de facto standard A ''de facto'' standard is a custom or convention that has achieved a dominant position by public acceptance or market forces (for example, by early entrance to the market). is a Latin phrase (literally " in fact"), here meaning "in practice b ...
among Allwinner SoCs.


Windows

Windows does not use DeviceTree (DTS file) as described here. Instead, it uses ACPI (ASL file) to discover and manage devices.


Coreboot

The
coreboot coreboot, formerly known as LinuxBIOS, is a software project aimed at replacing proprietary firmware (BIOS or UEFI) found in most computers with a lightweight firmware designed to perform only the minimum number of tasks necessary to load and r ...
project makes use of device trees, but they are different from the flattened device trees used in the Linux kernel.


Example

Example of Devicetree Source (DTS) format: /dts-v1/; / ; In the example above, the line /dts-v1/; signifies version 1 of the DTS syntax. The tree has four nodes: / (root node), soc (stands for "
system on a chip A system on a chip or system-on-chip (SoC ; pl. ''SoCs'' ) is an integrated circuit that integrates most or all components of a computer or other electronic system. These components almost always include a central processing unit (CPU), memory ...
"), flash-controller@4001e000 and flash@0 (instance of flash which uses the flash controller). Besides these node names, the latter two nodes have ''labels'' flash_controller and flash0 respectively. The latter two nodes have ''properties'', which represent name/value pairs. Property label has string type, property erase-block has integer type and property reg is an array of integers (32-bit unsigned values). Property values can refer to other nodes in the devicetree by their ''phandles''. Phandle for a node with label flash0 would be written as &flash0. Phandles are also 32-bit values. Parts of the node names after the "at" sign (@) are ''unit addresses''. Unit addresses specify a node's address in the address space of its parent node. The above tree could be compiled by the standard DTC compiler to binary DTB format or
assembly Assembly may refer to: Organisations and meetings * Deliberative assembly, a gathering of members who use parliamentary procedure for making decisions * General assembly, an official meeting of the members of an organization or of their representa ...
. In Zephyr RTOS, however, DTS files are compiled into C header files (.h), which are then used by the build system to compile code for a specific
board Board or Boards may refer to: Flat surface * Lumber, or other rigid material, milled or sawn flat ** Plank (wood) ** Cutting board ** Sounding board, of a musical instrument * Cardboard (paper product) * Paperboard * Fiberboard ** Hardboard, a ty ...
.


See also

*
PCI configuration space PCI configuration space is the underlying way that the Conventional PCI, PCI-X and PCI Express perform auto configuration of the cards inserted into their bus. Overview PCI devices have a set of registers referred to as ''configuration space' ...
*
Hardware abstraction Hardware abstractions are sets of routines in software that provide programs with access to hardware resources through programming interfaces. The programming interface allows all devices in a particular class ''C'' of hardware devices to be acce ...
*
Open Firmware Open Firmware is a standard defining the interfaces of a computer firmware system, formerly endorsed by the Institute of Electrical and Electronics Engineers (IEEE). It originated at Sun Microsystems, where it was known as OpenBoot, and has bee ...


References


External links

*
Device Tree Reference eLinux.org

Embedded Power Architecture Platform Requirements (ePAPR)

About The Device Tree
{{Firmware and booting Data structures by computing platform Operating system technology Firmware ARM architecture