Here are my notes about the process I used to build a basic ARM linux system testable with QEMU. This knowledged has been gathered from the balau82.wordpress.com blog and from a Pierre Ficheux’s article introducing BuildRoot and published in Linux Magazine special issue n°47.
Here is a summary of the environment I will use:
- Ubuntu 10.04 development host
- ELDK 4.2 ARM cross compilation toolchain
- Linux kernel 2.6.37
- BuildRoot 2010.11
- QEMU 0.13
- Emulated hardware target: Versatile/PB development platform
Installing a cross compilation toolchain
The Embedded Linux Development Toolkit (ELDK) contains a GCC cross-compilation toolchain and a number of pre-built target tools and libraries. It is available for ARM, PPC and PPC64 targets (Documentation and ELDK 4.2 ARM ISO).
The ELDK 4.2 toolchain is EABI (the ELDK 4.1 was OABI).
A full install of the ELDK 4.2 development tools and target components for the ARM target require about 2GB of disk space. Only installing the development tools will require just 71MB. To avoid installing the target components you need to empty the etc/rpm_arm.list file provided in the ISO distribution.
# Mount the ISO and copy its content in the current directory $> sudo mkdir /mnt/tmp $> sudo mount –o loop –t iso9660 arm-2008-11-24.iso /mnt/eldk42 $> cp –r /mnt/eldk42 . $> unmount /mnt/eldk42 # Patch the rpm_arm.list file $> cd eldk42/etc $> mv rpm_arm.list rpm_arm.list.original $> touch rpm_arm.list $> cd .. # Install the toolchain $> sudo mkdir –p /opt/ELDK42/arm $> ./install –d /opt/ELDK42/arm arm
The toolchain is installed in /opt/ELDK42/arm.
Cross compilation environment variables
I use a shell script to set default environment variables required to cross compile the kernel.
#!/bin/sh PATH=/opt/ELDK42/arm/usr/bin:$PATH ARCH=arm CROSS_COMPILE=arm-linux- export PATH ARCH CROSS_COMPILE
Execute this script before any call to the kernel makefile.
Kernel configuration and build
To use the default kernel configuration for the Versatile/PB platform and then access the configuration menu:
$> curl –O http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.37.tar.bz2 $> tar xjvf linux-2.6.37.tar.bz2 $> cd linux-2.6.37 $> ../set_env_arm.sh $> make versatile_defconfig $> make menuconfig
In the configuration menu, I modify the following settings:
- Disable the module support
- Kernel Features / Use the ARM EABI
The ARM EABI option is required because of the ELDK 4.2 toolchain configuration.
Then to build the kernel:
The resulting kernel image is in the file arch/arm/boot/zImage.
Building QEMU from the sources
Compiling QEMU from source requires the following packages to be installed in Ubuntu: zliblg-dev libsdl-dev.
$> curl –O http://wiki.qemu.org/download/qemu-0.13.0.tar.gz $> tar xzvf qemu-0.13.0.tar.gz $> cd qemu-0.13.0 $> ./configure --target-list=arm-softmmu --enable-sdl $> make
The generated executable is arm-softmmu/qemu-system-arm.
To get a list of the supported machines:
$> arm-softmmu/qemu-system-arm –M ? ... realview-pb-a8 ARM RealView Platform Baseboard for Cortex-A8 realview-pbx-a9 ARM RealView Platform Baseboard Explore for Cortex-A9 versatilepb ARM Versatile/PB (ARM926EJ-S) ...
Executing the linux kernel in QEMU
The QEMU emulator is started by specifiying a machine to emulate and a Linux compressed image.
$> cd arm-softmmu $> ./qemu-system-arm -M versatilepb –m 128M -kernel ../../linux-2.6.37/arch/arm/boot/zImage
The-m option specifies the RAM size available for the versatilepb machine. It defaults to 16MB if not specified.
The following capture shows the kernel boot in QEMU. The kernel boot ends with a kernel panic because we don’t have provided an init filesystem.
Building a root filesystem with BuildRoot
Buildroot is a set of makefile that helps to build complete embedded Linux system. It can compiles a GCC cross-compilation toolchain, a kernel and a root filesystem with selectable applications and packages.
I already have a compiled kernel thus I will use BuildRoot to only build a basic root filesystem containing the Busybox shell. This root filesystem will be a simple compressed CPIO archive.
$> curl –O http://buildroot.uclibc.org/downloads/buildroot-2010.11.tar.bz2 $> tar xjvf buildroot-2010.11.tar.bz2 $> cd buildroot-2010.11.tar.bz2 $> make menuconfig
I set the following options:
- Target Architecture: arm
- Target Architecture Variant: arm926t
- Target ABI: EABI
- Toolchain/Kernel Headers: 2.6.36.x (I could specify a path to my kernel sources)
- Package Selection for the Target/Busybox 1.17.x
- Target Fileystem Options/cpio the root filesystem and Compression method: gzip
Some dependencies you may need to install in Ubuntu: g++ flex msgfmt(gettext) patch subversion.
To start the BuildRoot execution (with optional V=1 to enable the verbose mode):
$> make V=1
BuildRoot automatically download the required source packages from their original servers, applies patches if necessary and compile them. It then builds a filesystem with the built packages installed.
Executing the kernel with the BuildRoot root filesystem
$> cd qemu-0.13.0/arm-softmmu $> ./qemu-system-arm -M versatilepb –m 128M -kernel ../../linux-2.6.37/arch/arm/boot/zImage –initrd ../../buildroot-2010.11/output/images/rootfs.cpio.gz
The initrd option specifies a filesystem image to load as the initial ram disk. During boot the kernel will mount this ram disk as its root filesystem and look for an init executable to start. In our case the init executable is provided by Busybox.
Here a capture of the result. I am able to loginp as root and then do a ls on the root of the filesystem.
Quitting QEMU when the mouse has been grabbed
The QEMU graphical window can grab the mouse pointer to interact with the virtual machine display. Ctrl-Alt should exit the mouse grab but in my case (QEMU executed by Ubuntu emulated by OS X/VMWare Fusion) it does not work so I am unable to get back the mouse when I want to end my session.
So I use the console accessed through Ctrl-Alt-2 and the quit command.
compat_monitor0 console QEMU 0.13.0 monitor - type 'help' for more information (qemu) quit