A Beginner’s Guide to Device Trees in Embedded Linux: Understanding the Basics

In the world of embedded Linux, the device tree plays a crucial role in defining hardware configurations. For beginners, understanding device trees can seem complex, but it’s essential for anyone working with custom hardware and Linux systems. A device tree is essentially a data structure used by the Linux kernel to describe the hardware components of a system. Instead of hard-coding the hardware details into the kernel, device trees provide a flexible and scalable way to represent this information.

In this guide, we’ll break down the basics of device trees in embedded Linux, how they work, and how to use them in your embedded projects.

Why Are Device Trees Important?

The device tree simplifies the process of defining hardware configurations, especially for embedded systems, where hardware may vary significantly across devices. Before device trees, hardware descriptions were hardcoded into the Linux kernel. This made it difficult to maintain and port Linux to different platforms. With device trees, the hardware information is separated from the kernel, making it easier to support new hardware without modifying the kernel itself.

For example, in an embedded system with multiple hardware components (such as GPIOs, UART, SPI), the device tree allows the kernel to understand how these components are connected and configured.

Device Tree Structure

A device tree is structured in a hierarchical, human-readable format. It’s written in a text-based file format known as Device Tree Source (DTS), which is then compiled into a Device Tree Blob (DTB). The kernel uses this compiled DTB to understand the hardware layout of the system during boot time.

Here’s an example of a simple device tree structure:

/ {
    compatible = "my-embedded-device";
    model = "Custom Embedded Board";

    cpu@0 {
        compatible = "arm,cortex-a9";
        reg = <0x0>;
    };

    memory@10000000 {
        device_type = "memory";
        reg = <0x10000000 0x2000000>;
    };

    uart0: serial@101f1000 {
        compatible = "ns16550a";
        reg = <0x101f1000 0x1000>;
    };
};

In this example:

• / refers to the root node representing the whole system.
• cpu@0 defines a CPU component.
• memory@10000000 specifies the memory configuration.
• uart0: serial@101f1000 defines a UART serial device.

Each node in the device tree represents a hardware component, and its properties (like compatible, reg, and device_type) provide essential details that the Linux kernel needs to configure and use the hardware.

How to Use a Device Tree in Embedded Linux

1. Writing a Device Tree Source (DTS) File
   The first step in using a device tree is to write a DTS file that describes the hardware of your system. This file is typically located in the arch/arm/boot/dts/ directory in the Linux source tree.
2. Compiling the Device Tree
   After creating the DTS file, it needs to be compiled into a Device Tree Blob (DTB) using the Device Tree Compiler (DTC). The DTB file is what the kernel reads at boot time to understand the hardware configuration. To compile a DTS file into DTB:

   dtc -I dts -O dtb -o mydevice.dtb mydevice.dts

3. Loading the Device Tree
   Once compiled, the DTB file is passed to the bootloader (such as U-Boot) to be loaded by the kernel during boot. The bootloader ensures the kernel reads the device tree and configures the hardware accordingly. In U-Boot, you can load a device tree using the following command:

   load mmc 0:1 0x83000000 mydevice.dtb

4. Testing and Debugging
   After loading the device tree, you can boot into Linux and verify if the hardware is correctly recognized. You can check the device tree structure from within Linux using:

   cat /proc/device-tree

Common Use Cases for Device Trees in Embedded Linux

1. Custom Hardware Platforms
   When developing custom hardware, you need to describe all peripherals, interfaces, and devices connected to the CPU. The device tree provides a clear and scalable way to do this without modifying the kernel source code.
2. Multiple Configurations
   Device trees allow different hardware configurations without recompiling the kernel. For example, if you have multiple variants of a product, each with different peripherals, you can use different device trees for each variant while keeping the same kernel.
3. Faster Development and Porting
   By separating the hardware description from the kernel, developers can quickly adapt Linux to new hardware platforms by updating the device tree, rather than diving deep into kernel modifications.

Conclusion: Understanding Device Trees is Key in Embedded Linux Development

Device trees in embedded Linux provide a flexible, maintainable, and scalable way to describe hardware to the Linux kernel. For beginners, understanding device trees might seem daunting at first, but with some practice, you’ll find that it simplifies hardware development and configuration significantly. By using device trees, you can ensure that your embedded Linux projects are easier to maintain, extend, and scale.