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IS IT DIFFICULT TO MAKE A DEVICE TREE FOR A NEW DEVICE?

Is It Difficult To Make A Device Tree For A New Device?

Creating a device tree for a new smartphone is a complex technical undertaking that requires deep knowledge of embedded Linux systems, kernel compilation, and hardware abstraction. We will explore the multifaceted challenges involved in this process, providing a comprehensive guide for developers and enthusiasts who are considering building a custom ROM for a device with no existing support.

Understanding the Fundamentals of Device Trees

To grasp the difficulty of creating a device tree, one must first understand what a device tree actually is. In the Linux kernel, a Device Tree (specifically Device Tree Blob or DTB) is a data structure that describes the hardware configuration of a system. Unlike older x86 architectures where the kernel could probe for hardware, ARM-based SoCs (System on Chips) used in modern smartphones require a precise map of the hardware components, their addresses, and interrupt lines. This map is the device tree.

The Role of DTS and DTC

The source code for a device tree is written in a human-readable format with the extension .dts (Device Tree Source). This file is compiled using the Device Tree Compiler (DTC) into a binary format (.dtb) that the bootloader passes to the Linux kernel at boot time. The kernel uses this DTB to initialize drivers and populate the platform device structure. Without an accurate device tree, the kernel cannot communicate with essential hardware like the display, touch controller, storage, or sensors.

Hardware Abstraction and Linux

Linux relies on the device tree to bridge the gap between generic kernel code and specific hardware implementations. The Device Tree Overlay (DTO) mechanism allows for dynamic modifications to the base device tree, which is useful for modules and overlays, but for a new phone, we are usually dealing with creating the base definitions from scratch.

The Inherent Difficulty of Device Tree Creation

We must be frank: creating a device tree for a completely new device is extremely difficult. It is not merely a task of copying and pasting files; it is an exercise in reverse engineering and low-level system programming.

Hardware Complexity and Vendor Dependencies

Modern smartphones are incredibly complex systems. A single device tree file must describe:

The difficulty multiplies when the System on Chip (SoC) vendor provides incomplete or proprietary documentation. Vendors often release BSP (Board Support Package) kernels, which are monolithic and difficult to adapt to mainline Linux standards.

The “Same SoC” Advantage

As mentioned in the context provided, having a device with the same SoC as an already supported device is a massive advantage. If your device uses a Snapdragon 778G and there is already a custom ROM running on a different phone with that exact chip, you have a reference point. You can extract the device tree from the working device, modify the specific hardware definitions (like GPIO pins for the PMIC or display panel), and attempt to boot.

Without this reference, you are essentially flying blind. You must identify every component on the board, its manufacturer, and its connection to the SoC.

Prerequisites for Building a Device Tree

Before touching a single line of code, we must establish a robust development environment. The difficulty of the task is heavily influenced by the tools at your disposal.

Development Environment Setup

Extracting Stock Partitions

To understand the hardware, we often need to dump the partitions from the stock ROM. Using tools like dd or custom recovery scripts, we extract:

We analyze these binaries using tools like dumpDtbs (to extract .dtb files from images) and fdtdump (to inspect the structure).

The Step-by-Step Process of Creating a Device Tree

We will now outline the practical steps involved in building a device tree. This process highlights where the technical hurdles typically lie.

1. Reverse Engineering the Hardware

If no source code is available, we must identify the hardware components. This involves:

2. Setting Up the Device Tree Source (DTS) Structure

A standard device tree structure for a mobile device includes:

Example of a Node Definition:

&soc {
    i2c_4: i2c@7af5000 {
        compatible = "qcom,i2c-geni";
        reg = <0x0 0x7af5000>;
        interrupts = <GIC_SPI 350 IRQ_TYPE_LEVEL_HIGH>;
        clocks = <&gcc GCC_I2C4_AHB_CLK>, <&gcc GCC_I2C4_SUSPEND_CLK>;
        status = "okay";
        
        /* Touchscreen Controller */
        synaptics-dsi-ts@20 {
            compatible = "synaptics,dsx";
            reg = <0x20>;
            interrupt-parent = <&tlmm>;
            interrupts = <65 IRQ_TYPE_EDGE_FALLING>;
            vdd-supply = <&vdd_vreg>;
            vio-supply = <&vio_vreg>;
        };
    };
};

Modifying these parameters—clock frequencies, interrupt lines, and voltage supplies—is where errors commonly occur. A single wrong hex value can prevent the device from booting.

3. Kernel Configuration (Defconfig)

The device tree describes the hardware, but the kernel configuration determines which drivers are compiled. We must modify the kernel’s .config or defconfig to enable:

We often encounter the issue where the stock kernel uses modules (.ko files) for drivers, while custom ROMs prefer compiling them directly into the kernel (built-in) or as modules loaded via init.rc scripts. Balancing this is difficult.

4. Compiling and Testing

Compilation is performed using the kernel source tree:

make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- <your_device_defconfig>
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j$(nproc)

The output includes the Image.gz (kernel) and the compiled .dtb files.

Testing Phase: This is the most volatile part of the process. We flash the new boot image (kernel + dtb + ramdisk) via fastboot:

fastboot flash boot boot.img
fastboot reboot

If the device fails to boot, it often results in a “hard brick” state (requiring EDL mode to recover) or a “bootloop.” Debugging requires:

Common Obstacles and Challenges

We encounter several specific hurdles that make this process difficult.

Proprietary Blobs and Firmware

Even with a perfect device tree, the device may not boot due to missing firmware. The Linux kernel requires firmware blobs (usually found in /lib/firmware) to load the microcode for:

Extracting these from the stock ROM and placing them in the correct directory structure is tedious and error-prone.

Display Panels and DRM (Direct Rendering Manager)

Configuring the display is notoriously difficult. It involves defining the MIPI DSI timings, the physical dimensions of the panel, and the GPIO pins for the backlight. An incorrect timing sequence will result in a black screen or visual artifacts (tearing, incorrect colors). We often need to probe the panel ID via I2C to ensure the correct configuration is activated.

Power Management (PMIC)

The Power Management Integrated Circuit controls voltage rails for all components. The device tree must map the PMIC nodes correctly to the SoC’s SPMI (System Power Management Interface) bus. If the voltage rails are not defined or enabled in the correct sequence, the device will reset immediately upon booting the kernel.

Strategies to Reduce Difficulty

While the task is hard, there are ways to mitigate the complexity.

Leveraging Existing Trees

As the original user noted, finding a device with the same SoC is the best starting point. We can clone the repository of a working device (e.g., from LineageOS or AOSP repositories) and perform a “device port.”

  1. Identify Deltas: Compare the hardware of the new device with the reference device.
  2. Update Nodes: Change the I2C addresses, GPIO assignments, and panel definitions to match the new hardware.
  3. Vendor specificities: If the reference is a Xiaomi device and the new device is a OnePlus, you may need to switch from a Qualcomm vendor kernel to a CAF (Code Aurora Forum) base, or vice versa.

Using Tools for Automated Extraction

Tools like AnyKernel3 and Magisk Module templates can help package the device tree and kernel for testing. For development, using a minimal initramfs (initial RAM filesystem) helps isolate kernel issues from userspace issues. We often use a generic recovery image (like TWRP) to test if the kernel and device tree can mount the system partitions.

Community Collaboration

Reverse engineering is rarely a solo mission. Engaging with communities like XDA Developers or specific Telegram groups is vital. Often, someone else has already reverse-engineered a specific component (like the touch driver or audio routing) for the same SoC. Sharing logs (dmesg, logcat) is essential for collaborative debugging.

The Role of Magisk Modules in Device Development

At Magisk Modules (Magisk Module Repository), we recognize that device tree issues often persist even after a custom ROM is installed. For instance, a user might successfully boot a ROM but suffer from broken audio or camera functionality. This is where Magisk modules become a powerful tool for developers and users.

Patching Hardware Properties

Magisk allows for runtime modification of the system. If a device tree lacks a specific property or has an incorrect one, a Magisk module can inject the correct values via service.sh or by overlaying files in the /system partition.

Vendor Overlay and HAL Fixes

Sometimes, the hardware abstraction layer (HAL) in the ROM is incompatible with the device tree nodes. We can package a Magisk module that replaces specific HAL libraries (.so files) or adds missing overlay files to /vendor/etc. This is particularly useful for fixing camera HALs or fingerprint sensors without needing to rebuild the entire ROM.

Conclusion: Is It Difficult?

To answer the question directly: Yes, creating a device tree for a new device is highly difficult. It requires a strong grasp of embedded systems, patience for trial and error, and access to hardware documentation or existing codebases. It is a journey that involves navigating the complexities of the Linux kernel, proprietary vendor blobs, and specific hardware quirks.

However, with the right tools, a reference device with the same SoC, and a methodical approach to reverse engineering, it is a solvable challenge. For enthusiasts who are willing to invest the time, the reward is a fully functional custom ROM and a deeper understanding of how their device operates. At Magisk Modules, we support the developer community by providing the tools needed to tweak and perfect these device trees, ensuring that every device can reach its full potential.

Whether you are extracting blobs, modifying kernel configurations, or debugging bootloops, remember that every complex device tree started as a single line of code. With persistence and the right technical resources, you can bridge the gap between stock limitations and custom freedom.

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