Build AOSP Kernel
More recent kernel sources can be acquired using repo and be built without further configuration by running
build/build.sh
from the root of your source checkout. The root of the kernel source checkout contains
build/build.sh
. The Android tree contains only prebuilt kernel binaries. The kernel trees contain the kernel sources and all tools to build the kernels, including this script. For older kernels or kernels not listed below, refer to the instructions on how to build legacy kernels.
Downloading sources and build tools
For recent kernels, the sources, toolchain and build scripts can be downloaded with
repo
. Some kernels (e.g. the Pixel 3 kernels) require sources from multiple git repositories, while others like the common kernels require only a single one. Using the repo
approach ensures a correct source directory setup. Download the sources for the appropriate branch:
repo init -u https://android.googlesource.com/kernel/manifest -b BRANCH
repo sync
The following table lists the BRANCH names for kernels available through this method:
Device
Binary path in AOSP tree
Repo branches
Pixel 3 (blueline)
device/google/crosshatch-kernel
android-msm-crosshatch-4.9-pie-qpr2
Pixel 3 XL (crosshatch)
Pixel 2 (walleye)
device/google/wahoo-kernel
android-msm-wahoo-4.4-pie-qpr2
Pixel 2 XL (taimen)
Pixel (sailfish)
device/google/marlin-kernel
android-msm-marlin-3.18-pie-qpr2
Pixel XL (marlin)
Hikey
device/linaro/hikey-kernel
android-hikey-linaro-4.4 android-hikey-linaro-4.9 android-hikey-linaro-4.14 android-hikey-linaro-4.19
Hikey960
device/linaro/hikey-kernel
android-hikey960-linaro-4.4 android-hikey960-linaro-4.9 android-hikey960-linaro-4.14 android-hikey960-linaro-4.19
Android Common Kernel
N/A
common-android-4.4 common-android-4.9 common-android-4.14 common-android-4.19
Building the kernel
Then build the kernel with:
build/build.sh
Note: Common kernels are generic, customizable kernels and therefore do not define a default configuration. Please see the customize build section to find out how to specify the
BUILD_CONFIG
for common kernels. The kernel binary, modules and corresponding image are located in the
out/BRANCH/dist
directory. Running the kernel
There are multiple ways of running a custom-built kernel. The following are known ways suitable for various development scenarios:
Embedding into the Android image build
Copy the
Image.lz4-dtb
to the respective kernel binary location within the AOSP tree and rebuild the boot image. Alternatively, you can define the
TARGET_PREBUILT_KERNEL
variable while using make bootimage
(or any other make
command line that builds a boot image). This variable is supported by all devices as it is set up via device/common/populate-new-device.sh
. For example: export TARGET_PREBUILT_KERNEL=DIST_DIR/Image.lz4-dtb
Flashing and booting kernels with fastboot
Most recent devices have a bootloader extension to streamline the process of generating and booting a bootimage. To boot the kernel without flashing:
adb reboot bootloader
fastboot boot Image.lz4-dtb
Using this method, the kernel is not actually flashed and hence won't persist across a reboot.
Note: Kernel names differ by device. To locate the correct filename for your kernel, refer to device/VENDOR/NAME-kernel
in the AOSP tree.
Customize the kernel build
The build process and outcome can be influenced by environment variables. Most of them are optional and each kernel branch should come with a proper default configuration. The most frequently used ones are listed here. For a complete (and up-to-date) list, refer to build/build.sh
.
Environment variable | Description | Example |
---|---|---|
BUILD_CONFIG |
Build config file to initialize the build environment from. The location is to be defined relative to the repo root directory. Defaults to 'build.config'. Mandatory for common kernels! |
BUILD_CONFIG=common/build.config.cuttlefish.x86_64 |
OUT_DIR |
Base output directory for the kernel build. | OUT_DIR=/path/to/my/out |
DIST_DIR |
Base output directory for the kernel distribution. | OUT_DIR=/path/to/my/dist |
CC |
Override compiler to be used. Falls back to the default compiler defined by build.config. | CC=clang |
SKIP_MRPROPER |
Skip make mrproper |
SKIP_MRPROPER=1 |
SKIP_DEFCONFIG |
Skip make defconfig |
SKIP_DEFCONFIG=1 |
Custom kernel config for local builds
If you require to regularly switch a kernel configuration option, e.g. when working on a feature, or need an option to be set for development purposes, you can achieve that flexibility by maintaining a local modification or copy of the build config.
Set the variable POST_DEFCONFIG_CMDS to a statement that is evaluated right after the usual make defconfig
step has been done. As the build.config
files are sourced into the build environment, functions defined in build.config
can be called as part of the post-defconfig commands.
A common example is disabling LTO (Link Time Optimization) for crosshatch kernels during development. While LTO is beneficial for released kernels, the overhead at build time can be significant. The following snippet added to the local build.config
will disable LTO persistently when using build/build.sh
.
POST_DEFCONFIG_CMDS="check_defconfig && update_debug_config"
function update_debug_config() {
${KERNEL_DIR}/scripts/config --file ${OUT_DIR}/.config \
-d LTO \
-d LTO_CLANG \
-d CFI \
-d CFI_PERMISSIVE \
-d CFI_CLANG
(cd ${OUT_DIR} && \
make O=${OUT_DIR} $archsubarch CROSS_COMPILE=${CROSS_COMPILE} olddefconfig)
}
Identifying kernel versions
There are multiple ways of identifying the right version to build.
Kernel version from AOSP tree
The AOSP tree contains prebuilt kernel versions. Most of the time the git log reveals the correct version as part of the commit message:
cd $AOSP/device/VENDOR/NAME
git log --max-count=1
Kernel version from system image
To determine the kernel version used in a system image, run the following command against the kernel file:
file kernel
For
Image.lz4-dtb
files, run: grep -a 'Linux version' Image.lz4-dtb