EBC Exercise 08a Cross-Compiling

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This class is about developing software for embedded Linux. So far we have been doing all of our development on the Beagle. This works well for small (and not so small) programs. However, we are now moving into kernel development and that's best done on a more powerful host computer. Here you learned how to download and install the cross-compilers and the source for kernel and u-boot. Now we'll use those tools.

First we'll check everything by compiling the Hello World program, then we'll try the kernel and u-boot.

Cross-compiling Hello World

Listing 2-4 on page 29 of the text is an embedded version of Hello World. If you've set up your git repository you will find it in helloWorld.c when you do a git pull. Compile and run it on your host to be sure it works.

host$ gcc helloWorld.c
host$ ./a.out
Hello, World! Main is executing at 0x400524
This address (0x7fff8260bdf8) is in our stack frame
This address (0x601038) is in our bss section
This address (0x601020) is in our data section

Now that you know it's working, let's cross compile it. First set the paths to find the cross-compilers. Put the following in a file, call it ~/.oe/crossCompileEnv.sh. Make sure the path is correct for your system. This is for a 32-bit linux.

# add cross tools to your path
export ARM_TOOLCHAIN_PATH=~/BeagleBoard/oe/build/tmp-angstrom_2010_x-eglibc/sysroots/i686-linux/usr/bin/armv7a-angstrom-linux-gnueabi
export ARCH=arm
export CROSS_COMPILE=arm-angstrom-linux-gnueabi-

Here it is for 64-bit

# add cross tools to your path
export ARM_TOOLCHAIN_PATH=~/BeagleBoard/oe/build/tmp-angstrom_2010_x-eglibc/sysroots/x86_64-linux/usr/bin/armv7a-angstrom-linux-gnueabi
export ARCH=arm
export CROSS_COMPILE=arm-angstrom-linux-gnueabi-

Now source the file and compile again. (Note: you only have to source once per terminal session.)

host$ source ~/.oe/crossCompileEnv.sh
host$ ${CROSS_COMPILE}gcc helloWorld.c
host$ file a.out
a.out: ELF 32-bit LSB executable, ARM, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.16, not stripped

The file command tells what's in the file. In this case we have an ARM executable. Success! Now copy to your Beagle and run

host$ scp a.out root@beagle:.
host$ ssh root@beagle ./a.out
Hello, World! Main is executing at 0x8374
This address (0xbeb32d4c) is in our stack frame
This address (0x10650) is in our bss section
This address (0x10648) is in our data section

The scp copies a.out to the beagle and the ssh runs the a.out on the beagle. Notice the address are very different from the host version.

Finding the Kernel and Installing It

After doing this exercise you have a Beagle kernel on your host computer. Let's see if it works.

Once compiled the kernel is put in a file called uImage. You may have seen it in the FAT partition on your SD card. Let's find it on your host. One way to find it is

host$ cd ~/BeagleBoard/oe
host$ find . -name "*uImage*" | grep 2.6.32 | grep boot

We are looking for uImage, but we only want the 2.6.32 version. The last grep file the file in the boot directory. Several lines are printed, but the one I'm interested in is:


The first part of the path (in italics) is the path to the kernel. Check it out.

host$ cd build/tmp-angstrom_2010_x-eglibc/work/beagleboard-angstrom-linux-gnueabi/linux-omap-psp-2.6.32-r110b+gitr5fc29e7b2a76a64a739f857858ef0b98294aa155/git
host$ ls -F
arch/           drivers/   Kbuild       modules.order   samples/    usr/
block/          firmware/  kernel/      Module.symvers  scripts/    virt/
COPYING         fs/        lib/         net/            security/   vmlinux*
CREDITS         include/   MAINTAINERS  patches/        sound/      vmlinux.o
crypto/         init/      Makefile     README          System.map
Documentation/  ipc/       mm/          REPORTING-BUGS  tools/

We'll be learning what's in many of these over the next couple of weeks. Remember this location. Let's find uImage.

host$ cd arch/arm/boot; ls -shF
total 13M
4.0K bootp/       6.3M Image*      4.0K Makefile  3.0M zImage*
4.0K compressed/  4.0K install.sh  3.0M uImage
host$ ls -l uImage
-rw-r--r-- 1 beagle beagle 3144300 2011-12-08 01:02 uImage

It should have the date that you did the bitbake. Let's see if it runs on the Beagle

host$ scp uImage root@beagle:.
host$ ssh root@beagle
beagle$ cd /boot; ls -F
total 18M
4.0K MLO@
372K Module.symvers-2.6.32
1.4M System.map-2.6.32
 80K config-2.6.32
4.0K u-boot.bin@
   0 uImage@
3.1M uImage-2.6.32
8.9M vmlinux-2.6.32
beagle$ ls -l uImage
lrwxrwxrwx 1 www-data www-data 13 May 12  2011 uImage -> uImage-2.6.32

So the uImage that is there is a symbolic link to uImage-2.6.32. Let's save the working uImage in a difference file and copy our new uImage in.

beagle$ mv uImage-2.6.32 uImage-2.6.32.orig
beagle$ mv ~/uImage uImage-2.6.32

And then reboot.

beagle$ shutdown -r now

After a couple of minutes your should be back and running again. Check and see if you are really running the new kernel.

beagle$ uname -a
Linux beagleboard 2.6.32 #3 PREEMPT Thu Dec 8 01:02:13 EST 2011 armv7l GNU/Linux

It worked! That's the date I compiled mine on.

Installing a New U-boot

While we're at it, let's install a new U-boot. Note: The new U-boot runs for me, but doesn't boot the kernel, yet.

host$ cd ~/BeagleBoard/oe/build/tmp-angstrom_2010_x-eglibc/work/beagleboard-angstrom-linux-gnueabi/
host$ ls -F

There's the U-boot directory.

host$ cd u-boot-2011.09-r4/git/; ls -F
api/        COPYING    fs/          mkconfig*     post/            tools/
arch/       CREDITS    include/     mmc_spl/      README           u-boot*
board/      disk/      lib/         nand_spl/     rules.mk         u-boot.bin
boards.cfg  doc/       MAINTAINERS  net/          snapshot.commit  u-boot.lds
common/     drivers/   MAKEALL*     onenand_ipl/  spl/             u-boot.map
config.mk   examples/  Makefile     patches/      System.map       u-boot.srec

The file we want is right on the top level.

host$ scp u-boot.bin root@beagle:.
host$ ssh -X root@beagle
beagle$ cd /media/mmcblk0p1/; ls -F
MLO*         u-boot.bin.broken*  uEnv.txt*
U-BOOT.BIN*  UIMAGE*             uEnv/

Be sure you are logged into the Beagle via the serial port, rather than using ssh. Otherwise you won't see the boot sequence that appears before the kernel is running.

Here we've changed to the FAT partition which is where u-boot lives. Back it up and install the new one.

beagle$ mv U-BOOT-BIN u-boot-bin.orig
beagle$ mv ~/.u-boot-bin .
beagle$ shutdown -r now

You should now see the new u-boot running if you are logged in via the serial port.

Compile via make

When you use bitbake it sets up all the paths to use the correct cross compilers. You can also build the kernel or u-boot by using make if you set the paths like we did above.

If you haven't already, source the file and cd to the kernel directory and try a make.

host$ source ~/.oe/crossCompileEnv.sh
host$ cd ~/BeagleBoard/oe/build/tmp-angstrom_2010_x-eglibc/work/beagleboard-angstrom-linux-gnueabi/linux-omap-psp-2.6.32-r110b+gitr5fc29e7b2a76a64a739f857858ef0b98294aa155/git
host$ make xconfig

(EBC Exercise 13 Configuring the Kernel has details on configuring the kernel.)

To make the kernel run

host$ make uImage

Now, follow the instructions above to install your freshly compiled kernel.