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

Table of Contents

1. Introduction


Ubuntu Linux machine with build essentials like make, gcc, etc. or equivalent. Run the following command if you’re not sure.

sudo apt-get install build-essential

The Shell script myEmu.sh 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).

2. Compile Linux 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 environment variable is a prefix added to the default compiler executable to form the cross compiler executable.

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/

3. Compile BusyBox 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

We want to tell BusyBox to compile everything statically and leave out certain unwanted and troublesome modules. Let’s fire up menuconfig:

make menuconfig

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

sudo apt-get install libncurses5-dev

In the UI, traverse:

  • Busybox Settings => Build Options, and select, Build BusyBox as a static binary(no shared libs)
  • In Network Utilities, select, Omit the Setup RPC Utilities
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
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.

mkdir -p memDriver/
cp src/memory.c memDriver/
cp src/Makefile memDriver/
cd memDriver/
make -C ../linux-3.10 M=./memDriver modules
mkdir ../busybox-1.21.1/memDriver/
cp memory.ko ../busybox-1.21.1/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.

4. Running 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 on an emulated ARM processor.

5. Loading the Memory Driver (Optional)

Create a character device file

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 memory_init function should be now seen in dmesg | tail. Now we can read/write a single byte of data from/to memory.

Write to device
echo -n 4 > /dev/mymem
Read from device
cat /dev/mymem