How to compile an Android kernel

By nathanchance, Recognized Developer.

Introduction

Hello everyone, I will be going over how to compile a kernel from beginning to end!

Prerequisites:

• A Linux environment (preferably 64-bit)
• Knowledge of how to navigate the command line
• Common sense
• A learning spirit, there will be no spoonfeeding here

What this guide will cover:

1. Downloading the source
2. Downloading a cross compiler
3. Building the kernel
4. Flashing the kernel

What this guide will NOT cover:

• Setting up a build environment (plenty of existing Linux installation guides)
• Adding features to the kernel (plenty of git cherry-picking guides)

I know this has been done before but on a cursory search, I have not seen a guide that was recently updated at all.

1. Downloading the source

If you have a custom kernel you want to build, move along after cloning the kernel using the git clone command below.

If you are compiling your stock kernel, it is ultimately up to you to know where to get your kernel source from but here are some common places:

Google: https://android.googlesource.com/kernel/msm/ (pick your architecture and look at the branches)
LG: http://opensource.lge.com/index
Samsung: http://opensource.samsung.com/reception.do
HTC: https://www.htcdev.com/devcenter/downloads
OnePlus: https://github.com/OnePlusOSS
Motorola: https://github.com/MotorolaMobilityLLC
Sony: https://github.com/sonyxperiadev/kernel

To download the kernel, you can either use git clone or download the tarball and extract it:

Code:

git clone -b <branch_to_checkout> <url> <desired_folder_name>

OR

tar -xvf <filename>

For example, if I wanted to grab the latest Nexus 6P from Google above:

Code:

git clone -b android-msm-angler-3.10-nougat-mr2 https://android.googlesource.com/kernel/msm/ angler

This will clone the kernel/msm repo into an angler folder and checkout the android-msm-angler-3.10-nougat-mr2 automatically.

I can try and help you locate your source if necessary.

2. Downloading a cross compiler

Since most Android devices are ARM based, we need a compiler that is targeting ARM devices. A host (or native) compiler will not work unless you are compiling on another ARM device.

You can either compile one yourself if you know how (crosstool-NG is a great tool for this) or download a prebuilt one. Luckily Google provides a high quality toolchain for this, in both an arm (32-bit) and arm64 (64-bit). It's up to you to know the architecture of your device. Typically speaking, most devices in the past two-three years are 64-bit.

Another popular toolchain is UberTC, which can be found here: https://bitbucket.org/matthewdalex/. Most kernels will need patches for anything higher than 4.9 and while I don't mind assisting with finding them, you should compile with Google's toolchain first.

Once you have decided, clone the toolchain:

Code:

git clone <url>

3. Compile the kernel

1. Point the Makefile to your compiler (run this from within the toolchain folder!!)

Code:

export CROSS_COMPILE=$(pwd)/bin/<toolchain_prefix>-

Example:

Code:

export CROSS_COMPILE=$(pwd)/bin/aarch64-linux-android-

NOTE #1: For kernels that can be compiled with Clang (like the Pixel 2), see this guide. I will support it here if there are any questions.

NOTE #2: Pixel and Pixel 2 users, you will need to follow these steps as well if compiling for Android Pie.

2. Tell the Makefile the architecture of the device

Code:

export ARCH=<arch> && export SUBARCH=<arch>

Example:

Code:

export ARCH=arm64 && export SUBARCH=arm64

3. Locate your proper defconfig
Navigate to the arch/<arch>/configs folder within the kernel source (e.g. arch/arm64/configs) and locate your device's or custom kernel developer's proper config file. For example, it will often be in the form of <codename>_defconfig or <kernel_name>_defconfig. Generic Qualcomm configs may be used as well (msm-perf_defconfig, msmcortex-perf_defconfig). When in doubt, ask here if you are confused. A defconfig tells the compiler what options to add to the kernel.

4. Build the kernel

Code:

make clean
make mrproper
make <defconfig_name>
make -j$(nproc --all)

If those commands succeed, you will have an Image, Image-dtb, Image.gz, or Image.gz-dtb file at the end.

If it failed, as was pointed out to me by @flar2 while making a complete idiot of myself, you may need to specify an output directory while making new CAF based kernels, like so:

Code:

mkdir -p out
make O=out clean
make O=out mrproper
make O=out <defconfig_name>
make O=out -j$(nproc --all)

If after that something is still broken, you may need to fix some headers or other issues. If it is a custom kernel, bring it up with your developer.
If it's an OEM, it's up to you to try and fix it, which we can assist with.

4. Flash the kernel

Assuming you were able to compile the kernel successfully, you now need to flash it! I will be covering two different ways to flash a compiled kernel: unpacking and repacking the boot image by hand using Android Image Kitchen or AnyKernel2, both by the brilliant @osm0sis. If there are any per-device nuances, please let me know and I'll add them here! Additionally, this section can vary drastically by device, you may need to consult developers of your device for assistance if necessary.

Android Image Kitchen

1. Pull your device's boot image from the latest image available for your device (whether it be a ROM or stock)
2. Download the latest Android Image Kitchen from this thread.
3. Run the following with the boot image:
   
   Code:

   unpackimg.sh <image_name>.img

4. Locate the zImage file and replace it with your kernel image (rename it to what came out of the boot image)
5. Run the following to repack:
   
   Code:

   repackimg.sh

6. Flash the new boot image with fastboot or TWRP!

Receiving help

I am happy to answer anything that I touched on in this guide. I may point you to another thread if it's better suited but I don't mind off topic (within reason) within the thread. I also want this to be a collaborative effort; other developers, if you have something to add, correct, or improve upon, please let me know!

I am particular in how people ask for help. I do NOT respond to posts asking for a hand out ("How do I fix this?", "Please fix this!", etc.). I only respond to posts with clear logs and steps that you have tried. Basically, show me that you have read this guide and have a specific issue. I am not here to hold your hand through this, this is a developers' forum.

Source: https://forum.xda-developers.com/android/software-hacking/reference-how-to-compile-android-kernel-t3627297