Building Android's Kernel

Linux Kernel

For the most part, developing your device drivers is the same as developing a typical Linux device driver. Android uses a specialized version of the Linux kernel with a few special additions such as wakelocks, a memory management system that is more aggressive in preserving memory, the Binder IPC driver, and other features that are important for a mobile embedded platform like Android. These additions have less to do with driver development than with the system's functionality. You can use any version of the kernel that you want as long as it supports the required features, such as the binder driver. However, we recommend using the latest version of the Android kernel. For the latest Android kernel, see Building Kernels.

Figuring out which kernel to build

This table lists the name and locations of the kernel sources and binaries:

Device	Binary location	Source location	Build configuration
hammerhead	device/lge/hammerhead-kernel	kernel/msm	hammerhead_defconfig
flo	device/asus/flo-kernel/kernel	kernel/msm	flo_defconfig
deb	device/asus/flo-kernel/kernel	kernel/msm	flo_defconfig
manta	device/samsung/manta/kernel	kernel/exynos	manta_defconfig
mako	device/lge/mako-kernel/kernel	kernel/msm	mako_defconfig
grouper	device/asus/grouper/kernel	kernel/tegra	tegra3_android_defconfig
tilapia	device/asus/grouper/kernel	kernel/tegra	tegra3_android_defconfig
maguro	device/samsung/tuna/kernel	kernel/omap	tuna_defconfig
toro	device/samsung/tuna/kernel	kernel/omap	tuna_defconfig
panda	device/ti/panda/kernel	kernel/omap	panda_defconfig
stingray	device/moto/wingray/kernel	kernel/tegra	stingray_defconfig
wingray	device/moto/wingray/kernel	kernel/tegra	stingray_defconfig
crespo	device/samsung/crespo/kernel	kernel/samsung	herring_defconfig
crespo4g	device/samsung/crespo/kernel	kernel/samsung	herring_defconfig

You will want to look at the git log for the kernel binary in the device project that you are interested in.

Device projects are of the form device//.

$ git clone https://android.googlesource.com/device/ti/panda
$ cd panda
$ git log --max-count=1 kernel

The commit message for the kernel binary contains a partial git log of the kernel sources that were used to build the binary in question. 

Identifying kernel version

$ dd if=kernel bs=1 skip=$(LC_ALL=C grep -a -b -o $'\x1f\x8b\x08\x00\x00\x00\x00\x00' kernel | cut -d ':' -f 1) | zgrep -a 'Linux version'

$ bzgrep -a 'Linux version' vmlinux.bz2

Downloading sources

Depending on which kernel you want,

$ git clone https://android.googlesource.com/kernel/common.git
$ git clone https://android.googlesource.com/kernel/exynos.git
$ git clone https://android.googlesource.com/kernel/goldfish.git
$ git clone https://android.googlesource.com/kernel/msm.git
$ git clone https://android.googlesource.com/kernel/omap.git
$ git clone https://android.googlesource.com/kernel/samsung.git
$ git clone https://android.googlesource.com/kernel/tegra.git

• The goldfish project contains the kernel sources for the emulated platforms.
• The msm project has the sources for ADP1, ADP2, Nexus One, Nexus 4, and can be used as a starting point for work on Qualcomm MSM chipsets.
• The omap project is used for PandaBoard and Galaxy Nexus, and can be used as a starting point for work on TI OMAP chipsets.
• The samsung project is used for Nexus S, and can be used as a starting point for work on Samsung Hummingbird chipsets.
• The tegra project is for Xoom and Nexus 7, and can be used as a starting point for work on NVIDIA Tegra chipsets.
• The exynos project has the kernel sources for Nexus 10, and can be used as a starting point for work on Samsung Exynos chipsets.

Downloading a prebuilt gcc

Ensure that the prebuilt toolchain is in your path.

$ export PATH=$(pwd)/prebuilts/gcc/linux-x86/arm/arm-eabi-4.6/bin:$PATH

On a linux host, if you don't have an Android source tree, you can download the prebuilt toolchain from:

$ git clone https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/arm/arm-eabi-4.6

Building

As an example, we would build the panda kernel using the following commands:

$ export ARCH=arm
$ export SUBARCH=arm
$ export CROSS_COMPILE=arm-eabi-
$ cd omap
$ git checkout 
$ make panda_defconfig
$ make

To build the tuna kernel, you may run the previous commands replacing all instances of "panda" with "tuna".

The kernel binary is output as: `arch/arm/boot/zImage` It can be copied into the Android source tree in order to build the matching boot image.

Or you can include the TARGET_PREBUILT_KERNEL variable while using make bootimage or any other make command line that builds a boot image.

$ export TARGET_PREBUILT_KERNEL=$your_kernel_path/arch/arm/boot/zImage

That variable is supported by all devices as it is set up via device/common/populate-new-device.sh

Creating Your Own Androidized Kernel (http://www.linaro.org/linaro-blog/2012/03/20/androidization-of-linux-kernel/)

To add the Android patches to 3.2 Linux kernel for Vexpress-rtsm.

How to create an Androidized kernel using the git rebase command. Androidization process was just 4 step process:

1. Clone the Linux kernel and create a branch for androidization :

    To clone the repository then work on the remote branch "arm/vexpressdt-rtsm" locally:

git clone http://git.linaro.org/git/people/dmart/linux-3-arm.git   -b   arm/vexpressdt-rtsm

    Create an switch to the local branch android:

git   checkout   -b   android

2. Add the remote topic branch:

    To add the remote server repository "androidization":

git   remote   add   androidization   git://git.linaro.org/landing-teams/working/ti/kernel.git

3. Fetch and rebase the kernel:

    To fetch everything the remote server "androidization"  has:

git fetch androidization

    To rebase
git   rebase   remotes/androidization/linaro-androidization-tracking

4. Add the necessary configs to the board-defconfig file to enable Android components in the kernel:

CONFIG_ASHMEM=y
CONFIG_STAGING=y
CONFIG_ANDROID=y
CONFIG_ANDROID_BINDER_IPC=y
CONFIG_ANDROID_LOGGER=y
CONFIG_ANDROID_RAM_CONSOLE=y
CONFIG_ANDROID_LOW_MEMORY_KILLER=y

Additionally I had to set “CONFIG_VMSPLIT_3G=y” for Android to boot on vexpress RTSM/Fastmodel.

The androidization patches are usually provided by Google but was not available for 3.2 kernel. For people working on 3.3 kernel, androidization patches are available from Google at:
https://android.googlesource.com/kernel/common.git for the Android-3.3 branch.