computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...

, a devicetree (also written device tree) is a

data structure In computer science, a data structure is a data organization and storage format that is usually chosen for Efficiency, efficient Data access, access to data. More precisely, a data structure is a collection of data values, the relationships amo ...

describing the hardware components of a particular computer so that the

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

's kernel can use and manage those components, including the CPU 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 remembe ...

, the

buses A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a motor vehicle that carries significantly more passengers than an average car or van, but fewer than the average rail transport. It is most commonly used ...

and the integrated peripherals. The device tree was derived from SPARC-based and

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 Inc., App ...

-based computers via the Open Firmware project. The current Devicetree specification is targeted at smaller systems and embedded systems, but is still used with some server-class systems (for instance, those described by the Power Architecture Platform Reference).

Personal computer A personal computer, commonly referred to as PC or computer, is a computer designed for individual use. It is typically used for tasks such as Word processor, word processing, web browser, internet browsing, email, multimedia playback, and PC ...

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 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...

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 EEPROM, or stored in NAND device like eUFS) 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 Computer hardware, hardware such as a physical button on the computer or by a software command. After it is switched on, a computer's central processing unit (CPU) h ...

. As an example, Das U-Boot and kexec 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 software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

supports this approach. The Devicetree specification is currently managed by a community name
devicetree.org
which is associated with, among others, Linaro and Arm.

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 stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, e.g., including only woody plants with secondary growth, only ...

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: Philosophy and science * Property (philosophy), in philosophy and logic, an abstraction characterizing an ...

. Nodes contain properties and child nodes, while properties are name–value pairs. Device trees have both a binary format 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

for the ARC, ARM, C6x,

H8/300 The Hitachi H8 is a large family of 8-bit, 16-bit and 32-bit microcontrollers made by Renesas Technology, originating in the early 1990s within Hitachi Semiconductor. The original design, the H8/300, was an 8-bit processor that had a 16-bit ...

, 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 ...

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 ...

Power ISA Power ISA is a reduced instruction set computer (RISC) instruction set architecture (ISA) currently developed by the OpenPOWER Foundation, led by IBM. It was originally developed by IBM and the now-defunct Power.org industry group. Power IS ...

RISC-V RISC-V (pronounced "risk-five") is an open standard instruction set architecture (ISA) based on established reduced instruction set computer (RISC) principles. The project commenced in 2010 at the University of California, Berkeley. It transfer ...

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 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 (gene), CISH * SOCS1 * SOCS2 * SOCS3 * SOCS4 * SOCS5 * SOCS6 * SOCS7 Structure All SOCS have certai ...

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 bootstrap loader, is a computer program that is responsible for booting a computer and booting an operating system. If it also provides an interactive menu with multiple boot choices then it's o ...

, 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 (also known as the flattened device tree) 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 that includes the Linux kernel for its kernel functionality. Although the name does not imply product distribution per se, a distro—if distributed on its own—is oft ...

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 collaboration with Broadcom Inc., Broadcom. To commercialize the product and support its growing demand, the ...

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 is commonly used even though its use is not required. is a Latin phrase (literally " of fact"), here meaning "in practice but not necessarily ordained by law" or "in practice or actuality, ...

among Allwinner SoCs. Devicetree is widely used for ARM-based Android devices.

Windows

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

Apple

On the boot process of

iOS Ios, Io or Nio (, ; ; locally Nios, Νιός) is a Greek island in the Cyclades group in the Aegean Sea. Ios is a hilly island with cliffs down to the sea on most sides. It is situated halfway between Naxos and Santorini. It is about long an ...

iPadOS iPadOS is a mobile operating system developed by Apple for its iPad line of tablet computers. It was given a name distinct from iOS, the operating system used by Apple's iPhones to reflect the diverging features of the two product lines, suc ...

and ARM

macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...

, the Low-Level Bootloader (LLB) will load Apple-encrypted devicetree to main memory, then loads iBoot.

Coreboot

The coreboot 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 (SoC) is an integrated circuit that combines most or all key components of a computer or Electronics, electronic system onto a single microchip. Typically, an SoC includes a central processing unit (CPU) with computer 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 resource 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. 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.

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 Embedded systems