Assembling custom kernel for a smartphone is a process that opens up limitless possibilities for the owner of the device. You can not only optimize power consumption, but also introduce support for the latest features that the manufacturer might not have added to the stock firmware. This is a difficult path that requires a deep understanding of the Linux architecture and build tools, but the result is worth it.
Many enthusiasts are afraid to start this path because of the apparent complexity and the risk of turning the phone into a “brick.” However, if you have basic knowledge of the Linux command line and are willing to follow the instructions carefully, success is guaranteed. In this article we will analyze all the stages: from preparing the workstation to the final firmware of the assembled image zImage or Image.gz-dtb into your device.
Why is it even worth building the kernel yourself? First, this is the only way to gain full control over the task scheduler and hardware drivers. Secondly, the community often releases security patches faster than official vendors. And third, you can enable experimental features, such as new processor frequency control algorithms or improved display drivers.
Preparing the environment and choosing tools
The first and most critical step is creating the right environment for compilation. Most developers use the operating system Ubuntu or Debianas they provide the best compatibility with Android build scripts. You will need to install the package manager apt and download the required set of utilities, including git, curl, make and libncurses-dev.
Particular attention should be paid to the choice of toolchain (toolchain). It is a set of compilers, linkers and libraries that translate C source code into machine code for your processor. For modern ARM64-based devices, GNU GCC or LLVM Clang is most often used. The wrong choice of compiler version can lead to the fact that the kernel simply will not load or will work unstable.
⚠️ Attention: Never use a compiler whose version is newer than the one with which the original kernel from the manufacturer was compiled, unless you are sure of backward compatibility. This is a common cause of module errors.
The dependency installation process may vary depending on the Linux distribution you are using. Below is a list of the main packages that need to be installed in the terminal before starting work:
- 📦
build-essential— a basic set of compilation tools - 📦
libssl-dev— libraries to support cryptography - 📦
bc—an arbitrary precision calculator required for kernel scripts - 📦
lzop—a compression utility often used in bootloaders
After installing the packages, make sure that the environment variables are configured correctly. You should add the path to your toolchain to a variable PATHso that the system can find compiler executables without specifying the full path. This will make it easier to enter commands later.
Use a virtual machine or Docker container to build the kernel. This isolates the environment and makes it easy to rollback changes if you accidentally damage the host system.
Obtaining the kernel source code
The kernel source code is the foundation of your future firmware. It must be obtained from a reliable source. Typically, device developers release code on platforms like GitHub or GitLab in accordance with the GPL license. You need to find a repository that matches the codename of your device (codename) and Android version.
The repository is cloned through the terminal using the utility git. The command looks simple, but it hides the download of hundreds of megabytes of data. Make sure you have a stable internet connection, as interrupted downloads can damage your directory structure.
git clone https://github.com/manufacturer/device-kernel.git -b android-12.0It is important to understand the difference between branches and tags. Branches contain the current state of development, which may be unstable. Tags are tied to specific releases. For the first build, it is best to use a tag that corresponds to the latest stable version of the software for your smartphone.
After cloning, go to the project directory. Here you will see many configuration files, build scripts and a folder driverswhere hardware drivers are stored. Learning the directory structure will take some time, but it is necessary to understand where to make changes.
Kernel configuration before compilation
Before starting the build process, you need to generate a configuration file .config. This file determines which drivers will be included in the kernel, which file systems are supported, and which security features are enabled. Errors at this stage can lead to the phone not seeing the Wi-Fi module or touch screen.
Most often, developers take as a basis the ready-made config provided by the manufacturer (usually it is in the source archive or in the folder configs). You can copy it to the root of the kernel directory under the name .config. After this, you can launch the configuration menu for fine-tuning.
To work with the configuration, use the utility menuconfig or nconfig. It provides a text interface where you can enable and disable options. Beginners are advised to be careful: do not disable options whose purpose you do not understand, especially those related to critical subsystems.
| Configuration parameter | Description | Recommended value |
|---|---|---|
CONFIG_LOCALVERSION |
Kernel version suffix | -custom (for difference from stock) |
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE |
Code optimization | y (for speed) or n (for size) |
CONFIG_DEBUG_INFO |
Debugging information | n (increases the size, only needed for debugging) |
CONFIG_SWAP |
Paging file support | y (useful for devices with low RAM) |
If you want to enable support for specific features, such as CPU overclocking or new scheduler algorithms, you will have to find the appropriate patches and apply them before the configuration stage. Patches are applied by the command git apply.
What is Device Tree?
Device Tree (DT) is a data structure that describes the device hardware for the kernel. It tells the kernel which processor pins are used for the screen, buttons, and sensors. Without the correct DT, the kernel will not start.
The compilation process and error elimination
The compilation process itself is launched with one command make, but behind this lies a complex chain of actions. The compiler goes through thousands of source code files, converting them into object files, and then the linker assembles them into a single binary image. On a powerful PC, this process takes from 10 to 40 minutes, depending on the number of processor cores.
To speed up the build, it is recommended to use the parameter -j, indicating the number of threads. The optimal value is usually equal to the number of cores in your processor plus one. For example, for an 8-core processor the command will look like make -j9. This will allow the system to load to the maximum.
⚠️ Attention: If an error occurs during the build process, carefully read the last 10 lines of the output. Often the problem lies in the absence of a certain library or incompatibility of syntax with the compiler version.
During compilation, you may encounter warnings. In most cases they can be ignored, but if there are too many warnings, it may indicate problems in the code. Errors are critical and stop the process. A common mistake is missing header files or incorrect paths in build scripts.
The result of successful compilation will be a kernel file. Depending on the architecture and bootloader requirements of your device, this may be a file zImage, Image.gz or a combined file Image.gz-dtb. Kernel modules will also be compiled if they were included in the configuration.
☑️ Pre-firmware checklist
Packaging and flashing the image into the device
The compiled kernel itself is often not ready for flashing. It must be packaged in a format that is understandable to the bootloader of your smartphone. For Samsung devices, this is the format .tar or .img with a header. For devices with an unlocked bootloader (Xiaomi, OnePlus, Pixel), a clean image or an image with Device Tree attached is often sufficient.
There are utilities such as mkbootimg or specific scripts from the community (for example, AnyKernel3), which help create a ready-made zip archive for installation via custom recovery (TWRP). The script AnyKernel3 automatically replaces only the kernel file in the boot partition without affecting the system partition, which reduces the risk of data loss.
The firmware process is carried out through Fastboot mode or through the Recovery menu. If you are using Fastboot, the command will look like this:
fastboot flash boot boot.imgAfter executing the command, you must reboot the device. The first startup may take longer than usual as the kernel initializes hardware and creates new cache files. If your phone is stuck at the logo, it means the kernel is not compatible with your hardware or configuration.
If the boot fails, you will have to return the stock kernel. That is why having a backup of the original boot.img is a prerequisite before starting experiments. You can return it with the same command fastboot flash boot stock_boot.img.
Using AnyKernel3 scripts is the safest method of flashing a custom kernel, as it minimizes the risk of damage to the system partition.
Debugging and performance optimization
After the successful launch of the new kernel, the fine-tuning stage begins. You can check the operation of the kernel using applications like CPU Thermo Monitor or Kernel Adiutor. These utilities allow you to view processor frequencies, sensor temperatures, and scheduler activity in real time.
One of the main goals of building a custom kernel is to improve autonomy. You can change the operating parameters CPU Governor (frequency manager) so that the processor resets frequencies faster when idle. You can also configure the operating parameters of Wi-Fi and Bluetooth modules to reduce power consumption in the background.
If you find a memory leak or unstable operation of any application, you will need to enable kernel debugging and collect logs. The command dmesg in the Android terminal will show kernel messages, which often indicate the cause of driver failures or kernel panics.
⚠️ Attention: Kernel management interfaces (sysfs) may differ in different versions of Android. What worked on Android 10 may not be available or moved to Android 13.
Optimization is an iterative process. You will have to repeatedly rebuild the kernel, changing configuration parameters, to find the perfect balance between performance and stability. Don't be afraid to experiment, but always record the changes you make to the code or config.
To analyze energy consumption, use the batterystats utility via ADB. It will show which processes and kernel drivers consume the most power.
Frequently asked questions (FAQ)
Is it possible to build the kernel directly on a smartphone?
Technically, this is possible using applications like Termux and installing a compiler, but it is highly not recommended. Building the kernel requires significant CPU and RAM resources. The smartphone will become very hot, and the process will take 5-10 times longer than on a PC. In addition, the risk of overheating and throttling can lead to a compilation error at later stages.
What to do if the phone goes into an endless reboot (bootloop)?
Don't panic. Boot into Recovery mode (usually by holding the Volume Up + Power buttons). If you have custom recovery installed, just wipe the cache (Wipe Cache/Dalvik) and try to flash the kernel again. If this does not help, connect the phone to the PC in Fastboot mode and flash back the saved backup of the stock image boot.img.
Do you need to unlock the bootloader to build the kernel?
To build the kernel on the computer itself, unlocking the bootloader is not required. You can compile the code as much as you like. However, to install (firmware) the assembled kernel into the device, an unlocked bootloader is required. Without this, the manufacturer-signed kernel will not accept an image you have not signed.
Which compiler is better: GCC or Clang?
Clang (from LLVM) has become the standard for building the Android kernel in recent years, starting with Android 11. It provides faster compilation and often generates more optimized code for modern ARM processors. GCC is still used for older devices, but for new projects it is preferable to choose Clang if the source code allows it.
Will I lose data when flashing the kernel?
When flashing only the partition boot (where the kernel is located), user data (photos, contacts, applications) are not deleted. However, if you make a mistake in the commands and flash the partition system or do a complete wipe in recovery, the data will be lost. Always make a backup copy of important data before any manipulations with system partitions.