Using QEMU for Embedded Systems Development, Part 1

Techies who work in the embedded domain must be familiar with the ARM (Advanced RISC Machine) architecture. In the modern era, our lives have been taken over by mobile devices like phones, PDAs, MP3 players and GPS devices that use this architecture. ARM has cemented its place in the embedded devices market because of its low cost, lower power requirements, less heat dissipation and good performance.

Purchasing ARM development hardware can be an expensive proposition. Thankfully, the QEMU developers have added the functionality of emulating the ARM processor to QEMU. You can use QEMU for two purposes in this arena — to run an ARM program, and to boot and run the ARM kernel.

In the first case, you can run and test ARM programs without installing ARM OS or its kernel. This feature is very helpful and time-saving. In the second case, you can try to boot the Linux kernel for ARM, and test it.

Compiling QEMU for ARM

In the last article, we compiled QEMU for x86. This time let’s compile it for ARM. Download the QEMU source, if you don’t have it already. Extract the tarball, change to the extracted directory, configure and build it as follows:

$ tar -zxvf qemu-0.14.0.tar.gz
$ cd qemu-0.14.0
$ ./configure –target-list=arm-softmmu
$ make
$ su
# make install

You will find two output binaries, qemu-arm and qemu-system-arm, in the source code directory. The first is used to execute ARM binary files, and the second to boot the ARM OS.

Obtaining an ARM tool-chain

Let’s develop a small test program. Just as you need the x86 tool-chain to develop programs for Intel, you need the ARM tool-chain for ARM program development. You can download it from here.
Extract the archive’s contents, and view a list of the available binaries:

$ tar -jxvf arm-2010.09-50-arm-none-linux-gnueabi-i686-pc-linux-gnu.tar.bz2
$ cd arm-2010.09/bin/
$ ls
-rwxr-xr-x 1 root root 569820 Nov 7 22:23 arm-none-linux-gnueabi-addr2line
-rwxr-xr-x 2 root root 593236 Nov 7 22:23 arm-none-linux-gnueabi-ar
-rwxr-xr-x 2 root root 1046336 Nov 7 22:23 arm-none-linux-gnueabi-as
-rwxr-xr-x 2 root root 225860 Nov 7 22:23 arm-none-linux-gnueabi-c++
-rwxr-xr-x 1 root root 572028 Nov 7 22:23 arm-none-linux-gnueabi-c++filt
-rwxr-xr-x 1 root root 224196 Nov 7 22:23 arm-none-linux-gnueabi-cpp
-rwxr-xr-x 1 root root 18612 Nov 7 22:23 arm-none-linux-gnueabi-elfedit
-rwxr-xr-x 2 root root 225860 Nov 7 22:23 arm-none-linux-gnueabi-g++
-rwxr-xr-x 2 root root 222948 Nov 7 22:23 arm-none-linux-gnueabi-gcc

Cross-compiling and running the test program for ARM

Now use the arm-none-linux-gnueabi-gcc tool to compile a test C program. Before proceeding, you should add the ARM tool-chain to your PATH:

# PATH=/(Your-path)/arm-2010.09/bin:$PATH

Create a small test program, test.c, with the basic “Hello world”:

#include<stdio.h>
int main(){
    printf("Welcome to Open World\n");
}

Use the ARM compiler to compile this program:

# arm-none-linux-gnueabi-gcc test.c -o test

Once the file is compiled successfully, check the properties of the output file, showing that the output executable is built for ARM:

# file test 
test: ELF 32-bit LSB executable, ARM, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.16, not stripped

Run the test program:

#qemu-arm -L /your-path/arm-2010.09/arm-none-linux-gnueabi/libc ./test 
Welcome to Open World

While executing the program, you must link it to the ARM library. The option -L is used for this purpose.

Building the Linux kernel for ARM

So, you are now done with the ARM tool-chain and qemu-arm. The next step is to build the Linux kernel for ARM. The mainstream Linux kernel already contains supporting files and code for ARM; you need not patch it, as you used to do some years ago.

Download latest version of Linux from kernel.org (v2.6.37 as of this writing), and extract the tarball, enter the extracted directory, and configure the kernel for ARM:

# tar -jxvf linux-2.6.37.tar.bz2
# cd linux-2.6.37
# make menuconfig ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi-

Here, specify the architecture as ARM, and invoke the ARM tool-chain to build the kernel. In the configuration window, navigate to “Kernel Features”, and enable “Use the ARM EABI to compile the kernel”. (EABI is Embedded Application Binary Interface.) Without this option, the kernel won’t be able to load your test program.

Modified kernel for u-boot

In subsequent articles, we will be doing lots of testing on u-boot — and for that, we need a modified kernel. The kernel zImage files are not compatible with u-boot, so let’s use uImage instead, which is a kernel image with the header modified to support u-boot. Compile the kernel, while electing to build a uImage for u-boot. Once again, specify the architecture and use the ARM tool-chain:

# make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi- uImage -s 
  Generating include/generated/mach-types.h 
arch/arm/mm/alignment.c: In function 'do_alignment': 
arch/arm/mm/alignment.c:720:21: warning: 'offset.un' may be used uninitialized in this function 
.
.
.
  Kernel: arch/arm/boot/Image is ready 
  Kernel: arch/arm/boot/zImage is ready 
Image Name:   Linux-2.6.37 
Created:      Thu May  5 16:59:28 2011 
Image Type:   ARM Linux Kernel Image (uncompressed) 
Data Size:    1575492 Bytes = 1538.57 kB = 1.50 MB 
Load Address: 00008000 
Entry Point:  00008000 
  Image arch/arm/boot/uImage is ready

After the compilation step, the uImage is ready. Check the file’s properties:

# file arch/arm/boot /uImage 
uImage: u-boot legacy uImage, Linux-2.6.37, Linux/ARM, OS Kernel Image (Not compressed), 1575492 bytes, Thu May  5 17:11:30 2011, Load Address: 0x00008000, Entry Point: 0x00008000, Header CRC: 0xFC6898D9, Data CRC: 0x5D0E1B70 

Now test this image on QEMU; the result is shown in Figure 1:

#  qemu-system-arm -M versatilepb -m 128M -kernel /home/manoj/Downloads/linux-2.6.37/arch/arm/boot/uImage

The kernel will crash at the point where it searches for a root filesystem, which you didn’t specify in the above command.

The next task is to develop a dummy filesystem for your testing. It’s very simple — develop a small test C program hello.c, and use it to build a small dummy filesystem:

#include<stdio.h>
int main(){
    while(1){
        printf("Hello Open World\n");
        getchar();
    }

The endless loop (while(1)) will print a message when the user presses a key. Compile this program for ARM, but compile it statically; as you are trying to create a small dummy filesystem, you will not use any library in it. In GCC, the -static option does this for you:

# arm-none-linux-gnueabi-gcc hello.c -static -o hello

Use the output file to create a root filesystem. The command cpio is used for this purpose. Execute the following command:

# echo hello | cpio -o --format=newc > rootfs
1269 blocks

Check the output file:

# file rootfs
rootfs: ASCII cpio archive (SVR4 with no CRC)

You now have a dummy filesystem ready for testing with this command:

# qemu-system-arm -M versatilepb -m 128M -kernel /home/manoj/Downloads/
linux-2.6.37/arch/arm/boot/uImage -initrd rootfs -append "root=/dev/ram rdinit=/hello"

When the kernel boots, it mounts rootfs as its filesystem, and starts the hello program as init. So now you are able to run ARM programs, and boot the ARM kernel inside QEMU.

The next step would be to use u-boot on QEMU. An array of testing is ahead of us, which we will cover in a forthcoming article.