How to compile the Android kernel for a smartphone

Building your own operating system kernel is the pinnacle for Android enthusiasts. This process opens up access to deep tuning of device performance, power saving and functionality at a level not available to ordinary users of stock firmware. Compiling the kernel allows you to implement your own patches, improve the performance of drivers and optimize the code for the specific hardware configuration of your gadget.

However, before diving into the world of source code i compilers, you need to understand the scale of the task. It's not just about installing an app from the store; This means working with the console, gigantic amounts of data and complex dependencies. An error at any stage can result in the device turning into a “brick”, and correcting the error will take hours of debugging. However, the result in the form of a completely customized system is worth the effort.

In this guide we will analyze the entire process from preparing the workstation to obtaining the finished file. boot.img. We'll look at selecting the correct version Toolchain, setting environment variables, and directly starting the build process. It is important to approach the matter methodically, since haste here is the main enemy of successful compilation.

Preparing the working environment and choosing an OS

For comfortable work with the source code of the Linux kernel on which Android is based, it is strongly recommended to use an operating system of the Unix family. Although it is theoretically possible to build on Windows via WSL or Cygwin, the native environment provides maximum compatibility and speed of file operations. Most build scripts and documentation are aimed specifically at Linux. Ubuntu or Debian provides maximum compatibility and speed of file operations. Most build scripts and documentation are aimed specifically at Linux.

You will need to install a basic set of development tools. Without them, the process will not even start, since the system will have nothing to process scripts and compile code. Make sure you have enough free disk space, as the kernel sources and device tree can take up tens of gigabytes. It is also critical to have a stable internet connection to initially download the repositories.

To install the required packages on Debian-based distributions, run the command in the terminal. This set includes a compiler, a version control system, and utilities for working with images: After installing the packages, check their functionality by entering the command. If the system displays the compiler version number, then the base environment is ready. Ignoring this step can lead to strange errors in the middle of the build process, when the system suddenly reports that the executable file is missing. gcc, version control system git and utilities for working with images:

sudo apt-get install git gnupg flex bison gperf build-essential zip curl zlib1g-dev gcc-multilib g++-multilib libc6-dev-i386 lib32ncurses5-dev x11proto-core-dev libx11-dev lib32z-dev ccache libgl1-mesa-dev libxml2-utils xsltproc unzip

After installing the packages, check their functionality by entering the command gcc --version. If the system displays the compiler version number, then the base environment is ready. Ignoring this step can lead to strange errors in the middle of the build process when the system suddenly reports that the executable file is missing.

⚠️ Attention: Compiler versions matter. Modern Android kernels may require a specific version Clang or GCCdifferent from the one installed on the system by default. Always check the device manufacturer's documentation (for example, the README file of the source code).

Downloading the kernel and tree source code devices

The kernel source code is not a single monolith for all devices. Each smartphone or tablet has its own set of drivers and configurations, combined into the so-called Device Tree (DTS). So the first step is to find the exact repository for your device model. Typically, this data is hosted by manufacturers on platforms like GitHub or GitLab as part of open source apps.

The upload process is carried out through the system git. You need to clone two main components: the kernel itself and the device tree. Sometimes you also need to download separate repositories for vendor blobs (proprietary drivers) if they are not included in the main code. The folder structure must strictly correspond to the expectations of the build scripts.

Use the following sequence of commands to initialize the workspace. Replace the URLs with the actual links for your device:

git clone https://android.googlesource.com/kernel/common.git kernel-source
git clone https://github.com/manufacturer/device-tree.git device-tree
cd kernel-source
git checkout android-13.0

Pay attention to the branch you select. It must match the version of Android installed on your device, or the one for which you are building the kernel. A mismatch between the kernel and userspace versions will lead to critical loading errors, since ABI (the binary compatibility interface) may be broken.

Where to look for the source code?

Official repositories can often be found on the manufacturers' websites (Samsung Open Source, Xiaomi MIUI Source) or on the CodeAurora Forum portal. There are also communities on XDA Developers, where enthusiasts post adapted sources for popular models.

Setting up Toolchain and environment variables

Toolchain is a set of tools, including a compiler, assembler and linker, necessary to convert source code into machine instructions for a specific processor. For the ARM or ARM64 architecture used in most smartphones, a regular system compiler may not be suitable. A specialized cross-compiler optimized for mobile platforms is required.

Modern versions of Android are increasingly switching to the compiler Clang from Google, which often provides better optimization and support for new C/C++ standards compared to GCC. However, many older devices and some manufacturers still rely on GNU GCC. Selecting the wrong toolchain will result in compilation errors or a broken kernel.

To configure the environment, you need to export variables that tell the system where to look for the compiler and archiver. This can be done manually before each build run, or added to the shell profile file. Example setup for 64-bit architecture:

export ARCH=arm64
export CROSS_COMPILE=aarch64-linux-android-
export CC=clang

Make sure that the path to the compiler binaries is added to the variable PATH. If you downloaded the prebuilt version of Toolchain from Google (Android Prebuilts), unpack it and specify the full path. An error in the path is one of the most common reasons why a build script crashes with the message "command not found".

Architecture	Prefix CROSS_COMPILE	Recommended compiler	Note
ARM (32-bit)	arm-linux-android-	GCC 4.9 / Clang	Legacy devices
ARM64	aarch64-linux-android-	Clang 14+	Standard for Android 10+
x86_64	x86_64-linux-android-	Clang	Emulators and tablets

Kernel configuration: .config and defconfig

Before compilation begins, the kernel must know which functions to enable and which to disable. This information is stored in the configuration file .config. Creating one from scratch manually is almost impossible due to the thousands of options available. Instead, the base file defconfigprovided by the manufacturer for a specific device model is used.

The file defconfig is located in the directory arch/arm64/configs/ (path may vary depending on the architecture). It contains the minimum set of settings required to boot the device. Your task is to apply this file to the current assembly using the utility make. This will create a file .config at the root of the source tree.

Run the command to generate the configuration:

make O=out/ vendor_defconfig_name_defconfig

After creating the basic configuration, you can modify it by enabling additional drivers or changing task scheduler settings. To do this, it is convenient to use the text interface menuconfig, which allows you to navigate through the options using the keyboard. Changes made here will directly affect the size of the final image and its functionality.

⚠️ Attention: Enabling experimental drivers or disabling critical modules (such as power management or the file system) may make the kernel unable to boot. Keep a copy of the original .config before making changes.

Compilation process and error elimination

The compilation process itself is launched with one command, but can take from 15 minutes to several hours depending on the power of your processor and the number of cores. Use argument -j to specify the number of compilation threads. The optimal value is usually equal to the number of logical cores of your processor plus one (for example -j9 for an 8-core CPU).

Running the build looks like this. The output will be streamed, showing each file being compiled:

make -j$(nproc --all) O=out/

You may encounter errors during compilation. The most common of them are related to incompatibility between compiler and source code versions, missing header files, or syntax errors in patches. Read the terminal output carefully: lines starting with error:indicate the file and line number where the problem occurred.

If the build is interrupted, you do not always need to start over. The system is make smart and will continue compilation from where it stopped after the error is corrected. However, if you have changed the configuration or header files, it is better to run the command make clean before restarting to avoid object file conflicts.

Successful completion of the process will be indicated by the appearance of a file Image or Image.gz-dtb in the output directory. This is the compiled kernel, ready to be packaged into a boot image. The absence of this file means that the process failed, even if the last lines of the log did not look like a fatal error.

Packaging the image and flashing the firmware onto the device

The raw kernel (Image) is usually not ready for direct loading by the bootloader of the smartphone. It must be combined with a Device Tree Blob (DTB) and possibly with a ramdisk image containing the initial filesystem. The result of this operation is a file boot.img, which can already be flashed through Fastboot mode.

The utility mkbootimg (or its modern analogues, such as AVB 2.0 tools) is used for packaging. You need to know the boot addresses (base addresses) for the kernel, ramdisk and other components. These addresses are specific to each device and can be found from the existing boot.img using the utility unpackbootimg or look in the manufacturer's documentation.

Example command to create an image:

mkbootimg --kernel out/arch/arm64/boot/Image.gz --dtb out/arch/arm64/boot/dtb/vendor/device.dtb --ramdisk ramdisk.cpio.gz --output boot.img --base 0x00000000 --pagesize 2048

After receiving the file boot.img connect the device in mode Fastboot and execute the firmware command. At this stage it is critical not to interrupt the connection. If the image is assembled incorrectly, the device will go into a cyclic reboot (bootloop), and you will have to restore the stock image using the same Fastboot.

⚠️ Attention: On devices with an unlocked bootloader and integrity check enabled (AVB/Verified Boot), you may need to disable verification or re-sign the image with developer keys, otherwise the system will refuse to load the custom kernel.

Frequently asked questions (FAQ)

How long does it take to compile a kernel for the first time?

The time depends on the power of your PC and the size of the source code. On a modern 8-core processor, a complete clean build takes from 20 to 40 minutes. Rebuilding with minor changes may only take a couple of minutes.

Is it possible to compile the kernel directly on a smartphone?

Technically, this is possible using applications like Termux and installing the necessary packages, but it is highly not recommended. The process is resource intensive, heats up the device, and can take several hours to complete, draining the battery. In addition, mobile processors are not designed for such an intensive multi-threaded load.

What to do if the device does not turn on after flashing the firmware?

Boot the device into Fastboot mode (usually by holding the volume down and power button) and flash back the stock image boot.imgthat you should have saved before the experiments. Command: fastboot flash boot stock_boot.img.

What is the difference between Clang and GCC for kernel building?

Clang is a modern compiler that often provides faster compilation and better code optimization for newer versions of Android. GCC is a classic compiler that may be needed to support very old devices or specific drivers that are not compatible with Clang.

Where can I find the base addresses for mkbootimg?

They can be extracted from the original boot.img of your firmware using the utility unpackbootimg. Also, this data is often published in device development threads on the XDA Developers forum in the "Kernel Source" or "Development" section.