0. Introduction

Following is a writeup on how to compile Linux kernel& BusyBox for ARM architecture and to load a simple device driver on the emulated system.

Prerequisites

    Ubuntu Linux machine with build utils and build essentials like make etc.
    Working internet Connection.

The Shell script myEmu.sh in my repository automates the below outlined process and boots up a linux kernel running on ARM processor (of course this is NOT a silent run, passwords have to be entered and menuconfigs configured).

1. Compiling Linux from Source for ARM architecture

Get Linux Kernel source

    wget https://www.kernel.org/pub/linux/kernel/v3.0/linux-3.10.tar.bz2

Get cross compilation tool-chain for ARM architecture

    sudo apt-get install gcc-arm-linux-gnueabi

Extract the linux source from the Gzipped tarball

    tar xjvf linux-3.10.tar.bz2

Set environment variables to tell the Linux Build system to build for ARM and use a specific cross-compiler.

    export ARCH=arm
    export CROSS_COMPILE=arm-linux-gnueabi-

Note the hyphen at the end, the CROSS_COMPILE env variable is a prefix added to the default compiler to get the cross compiler.

Configure Linux Build system to compile for the versatile express family of boards.

    cd linux-3.10
    make vexpress_defconfig

This creates a .config hidden file containing all the build configurations.

Actually Build the Linux Kernel Code

    make -j 4 all

The -j 4 option is to enable parallelism during compilation. Once the Build is Complete, the linux Kernel Image for ARM architecture is saved as zImage under linux-3.10/arch/arm/boot/

2. Compiling BusyBox from source for ARM architecture

Get BusyBox source

    wget http://www.busybox.net/downloads/busybox-1.21.1.tar.bz2

Extract source from Gzipped tarball

    tar xjvf busybox-1.21.1.tar.bz2

Configure the BusyBox Build system using the default configurations.

    cd busybox-1.21.1
    make defconfig

Additionally use a GUI driven build configuration settings page to tell BusyBox to compile everything statically and leave out certain unwanted and troublesome modules.

    make menuconfig

Traverse in the GUI

Busybox Settings ==> Build Options

SELECT Build BusyBox as a static binary(no shared libs)

Network Utilities==> Omit the Setup RPC Utilities (Optional, compiling with RPC might fail on some systems.

If you are getting an error that curses.h is missing install ncurses-dev package.

    sudo apt-get install libncurses5-dev

Actually Build BusyBox Code

    make -j 4 install

Once the build is complete, a folder named _install is created. This folder contains a bare structure of the linux root file system. As you can see some important folder like proc, dev, sys etc are missing. So lets go ahead and create them.

    cd _install
    mkdir proc sys dev etc etc/init.d

It is not enough that we just create the special directories, we have to tell the kernel to mount special services to their respective directories.

Create etc/init.d/rcS file and enter the following shell code

#!/bin/sh
mount -t proc none /proc
mount -t sysfs none /sys
/sbin/mdev -s

/sbin/init is usually the first program run by the linux kernel and its default behaviour is to execute the /etc/init.d/rcS file.

Mark rcS file as executable

    chmod +x etc/init.d/rcS  

Copy our Custom Memory Device Driver to filesystem (Optional)

Since we are planning to install our simple memory driver on the emulated ARM Linux system, copy the driver files to any folder in this location, preferably create a new one.

Copy Driver *.ko to a new folder, memDriver

    mkdir memDrive/
    cp <path to driver files> memDriver/

Create the root filesystem image with the cpio tool.

    find . | cpio -o --format=newc > ../rootfs.img

The root FileSystem should be create by the name rootfs.img inside the busybox-1.21.1 folder.

3. Running Linux with BusyBox on Linux for ARM on QEMU

Installing QEMU

    sudo apt-get install qemu

Start QEMU for ARM using our custom, Kernel and BusyBox.

    qemu-system-arm -M vexpress-a9 -m 256M -kernel linux-3.10/arch/arm/boot/zImage -initrd busybox-1.21.1/rootfs.img -append "root=/dev/ram rdinit=/sbin/init"

A QEMU window should open up with kernel initialization messages and finally a message asking, press Enter to activate console. When you hit enter a root prompt is received and now you are running Linux Kernel on an emulated ARM processor.

4. Loading a Memory Driver on the QEMU Installation (Optional)

Create a character device file with Major Number as 60 and minor number as 0.

    mknod /dev/mymem c 60 0

Insert our driver module into the kernel.

    cd memDriver/
    insmod memory.ko

printk messages in module_init function should be now seen in dmesg|tail This should enable us to now read/write a single byte of data from/to memory.

Write to device

echo -n 4 > /dev/mymem

Read from device

cat /dev/mymem