android kernel

Kernel in Action

Hi 🙂 You may be one of those Android developers or users who enjoy divergent applications available on the market and may be playing with different applications all day long, but actually what you were doing is playing with the kernel. 🙂 Um just kidding Kernel is common to all Oss. Kernel acts as a bridge between software and hardware. For each action you make on the device, the application sends a request to the kernel in the form of a system call. Kernel will communicate with hardware through device drivers, to do what you want it to do.

In developer perspective, let’s assume we created a simple hello world. Android applications do not directly interact with the kernel anyway. Hence what we do is calling an API method available in the Android Application Framework. This API method call will, then make a system call on the kernel. Ultimately kernel will get the job done for you. 🙂 What if there is no such thing called “Kernel” there? Then the poor developer will have to write code for each event for every single piece of hardware in the device.

What is a Kernel?

At the very basic level what controls your mobile device is the operating system (OS). All operating systems have a kernel, which is known as the most important or the central part of the OS. Kernel itself is a program. What it does is controlling the hardware of the device (Ex. Changes to brightness, volume level) and manages the system resources (Ex. Power Management, Memory Management, I/O management, process management). When the system boots kernel program loads to the RAM and starts providing its facilities.

Android Kernel:

Android OS has a Linux kernel. But Android kernel and Linux kernel are not exactly the same. Android uses a modified version of Linux kernel to support the device architecture, since Android is a mobile device and not a PC. When considering a mobile device there are hardware imitations as well as new features comparing to a desktop PC. Ex. Drivers need for a mobile device small keypads, 3G/4G cards, SIM cards, etc.

Following are some new changes on Android Kernel which are not available in Linux Kernel.

IPC Binder : Allow processes to provide services to other processes through inter process communication.

Ashmem (Anonymous Shared MEMory)

pmem

logger

wakelocks

Android Debug Bridge

RAM console and Log Device

Changes to YAFFS2 (Yet Another Flash File Systems 2^nd Edition) files

Changes to Bluetooth files, Scheduler, Power Management

Keypad light controls, management to TCP networking.

You can find different kernel sources available at the Google repository: Here https://android.googlesource.com/?format=HTML

Here you can start building the android kernel source:

Hi 🙂 This is my first attempt building an android kernel source. I just thought of sharing my experiences so far. This will be useful for a beginner. 🙂

Prerequisites:

Linux installed on a PC or on a Virtual Box
Android Kernel Source
A Toolchain

I have installed Ubuntu on Oracle VM VirtualBox.

Getting Android Kernel Source

I am using the kernel source of “omap” project. According to the android official web page the project is used for PandaBoard and Galaxy Nexus, and can be used as a starting point for work on TI OMAP chipsets.

1. Download the “omap.git-android-omap-3.0.tar.gz” from Google repository https://android.googlesource.com/

Goto ->https://android.googlesource.com/kernel/omap.git
Select android-omap-3.0

2. Extract source:

Open a new Terminal and enter following command.

Create a directory structure as follows and move the downloaded omap.git-android-omap-3.0.tar.gz file into that folder

:~$mkdir -p ~/android/kernel

Extract it:

:~/android/kernel$tar xvf omap.git-android-omap-3.0.tar.gz

Once you extracted the source: Goto ->android–>kernel–>arch–>arm–>configs folder. You can find the panda_defconfig file there.

Getting prebuilt toolchain

You can get prebuilt toolchain from the same Google repository.

1. Goto –> https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/arm/arm-eabi-4.6

2. Download the master branch and extract the source in to a new folder call toolchain, following the same steps as above.

:~$mkdir -p ~/android/toolchain

:~/android/toolchain$tar xvf arm-eabi-4.6-master.tar.gz

3. Extract source:

Set the path to toolchain

Here in my example the path to toolchain is : /home/tharaka/android/toolchain/bin. You can check the folder where it contains arm-linux-androideabi-gcc

Open Terminal and enter command: gedit ~/.bashrc

It will open up the text editor:

export PATH=$PATH:/home/tharaka/android/toolchain/bin

CROSS_COMPILE kernel

Kernels are compiled with a program called gcc. The gcc comes with toolchain is an optimized version of gcc. Therefore here we do a cross-compile.

1. Go inside the kernel source folder and enter following command

make ARCH=arm CROSS_COMPILE=arm-linux-androideabi- panda_defconfig

Build the Kernel

Command: make

Compiling….

While compiling it gives messages and warnings. We can ignore warnings but when there are errors compilation stops and terminate and we will have to fix the issuses.