Building the Linux Kernel

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This page covers the process of configuring and compiling the Linux kernel using the EMAC kernel build script. This process assumes that you have already acquired the following software:

1. EMAC Software Development Kit SDK
2. Linux kernel source for target hardware (provided via EMAC public SVN server )
3. Kernel build script

The example below will assume that a kernel image for the SoM-9307 module will be created, although the instructions apply to other hardware as well assuming that the correct SDK, kernel tree, and build script is used.

Setup

The steps below assume that the kernel-build-cross.sh script is located in the same directory as the kernel tree. Be sure to modify the environment.cfg.sh script so that the SDKLOC variable references the location of the SDK and that the appropriate ARCH_FLAGS and CROSS variables are used.

Configuring the Kernel

The first step for building the kernel is to configure it as desired. EMAC provides default configuration files as a part of the kernel tree. For the SoM-9307, this file is linux-2.6.25-ep93xx/arch/arm/configs/som9307_defconfig. The following are steps to configure the kernel:

1. Copy the default configuration file to the kernel source tree and rename it .config.

   developer@ldc:~$ cp linux-2.6.25-ep93xx/arch/arm/configs/som9307_defconfig linux-2.6.25-ep93xx/.config
   

2. The kernel-build-cross script accepts the SOURCE_TREE as the first argument and either config or build as the second argument. Optionally, a third argument, BUILD_SUFFIX may be supplied as a suffix to add to the build directory.BUILD_SUFFIX is commonly used to add a date tag or machine name to a build.

   developer@ldc:~$ ./kernel-build-cross.sh linux-2.6.25-ep93xx config som9307
   

3. The kernel menu-driven configuration utility will be displayed. Features can be selected/deselected to be built into the kernel. Some features can be built as a loadable module, denoted by < >, and not built directly into the kernel.

   Disabling or modularizing some kernel features may prevent the kernel from starting correctly or at all.

   Use the space bar to select an option or the 'm' key to configure the selected option as a module. Select Exit to close the kernel configuration menu and save the configuration to the newly created build directory. When the same build-suffix is used for subsequent builds, this configuration will be used.

Building the Kernel

1. Run the kernel-build-cross script again with the build option this time using the same build-suffix used in the configuration step.

   developer@ldc:~$ ./kernel-build-cross.sh linux-2.6.25-ep93xx build som9307
   

2. The kernel will begin compiling now. This will take several minutes to complete depending the kernel configuration and the speed of the development machine. Only move on to the next step if the build completes with no errors.

3. The new kernel image will be in the build-2.6.25-BUILD_SUFFIX/arch/arm/boot directory called uImage. This is the image that will get loaded onto the board and executed by the bootloader. To load the new kernel onto an ARM target machine, see the Loading Linux Kernels Onto a Board page. For an x86 based machine simply copy the new kernel to the /boot/ directory and create a symlink that is called bzImage that points to the new kernel. Then follow the appropriate steps for Installing LILO.

   The build script will also create an archive of all of the modules created during the build process and place it in the build directory. The archive will be called build-2.6.25-BUILD_SUFFIX/kernel-2.6.25.tar.gz.

Loading Kernel Modules

If the kernel is recompiled without changing the configuration or source code for any modules, it is not necessary to reload the modules archive. Alternatively, if a module was modified or added, it is only necessary to reload the modules archive. To reload the modules:

1. Copy the archive to the root of the filesystem of the target machine

   developer@ldc:~$ scp build-2.6.25-som9307/kernel-2.6.25.tar.gz root@1IP_ADDRESS:/
   

2. Log onto the target machine

   developer@ldc:~$ ssh root@IP_ADDRESS
   

3. Extract the kernel modules archive and force the kernel to reload the modules. Make sure that the root flash is mounted read/write before extracting.

   root@emac-oe:~# cd /
   root@emac-oe:~# tar xzvf kernel-2.6.25.tar.gz
   root@emac-oe:~# depmod -a