First, preparation work
a) Start by ensuring you have a PC with Linux installed. This is a fundamental step, and while it may seem obvious, it's essential to get this right before proceeding.
b) Install the necessary development tools such as GCC (the host compiler used for compiling programs on your PC), make, and ncurses. These tools are crucial for building and managing kernel components.
c) Download a clean Linux kernel source package and extract it. If you're compiling the kernel for your current PC, using the corresponding distribution's source is recommended. However, even if you use a different version, like the standard kernel linux-2.6.32.65.tar.xz on a Fedora 13 system, it can still compile successfully, provided you use the correct .config file from your running kernel.
d) For embedded systems, download and install a cross-compilation toolchain. For example, if your target architecture is ARM or MIPS, install the appropriate toolchain and add its path to your environment variables. Verify the installation by running a command like arm-linux-gcc --version. The output should confirm that the toolchain is correctly set up.
Note: You can find ARM toolchains online; just search for "ARM" to access them.
Second, set the compilation target
Before configuring or compiling the kernel, determine the target CPU architecture and the toolchain to be used. This is the most basic information you need to ensure a successful build.
If you're compiling for your own PC, no special settings are needed. However, for other architectures like ARM, you must explicitly set these parameters.
To set the target, you can either modify the Makefile directly or pass the values through the make command line. For example:
Make ARCH=arm CROSS_COMPILE=arm-linux- menuconfig
This tells the build system to use the ARM architecture and the specified cross-compiler.
The kernel’s top-level Makefile automatically determines the ARCH value based on your machine type using uname -m. If not set, it defaults to the host architecture, which is typically what you want for PC kernels.
Third, configure the kernel
The kernel has thousands of configuration options, and deciding which features to include (as built-in modules or loadable modules) is critical. Before compiling, generate a .config file in the kernel source directory.
For ARM targets, copy an existing configuration file (like s3c2410_defconfig) from the arch/arm/configs directory to the root of the kernel source and rename it to .config.
If you're working on your PC, use the .config file from your currently running kernel located at /lib/modules/`uname -r`/build/.config. This ensures compatibility and reduces unnecessary configuration steps.
Run make menuconfig to adjust the configuration. Even if you don't change anything, running it ensures the configuration is updated and compatible with the current kernel version.
Fourth, compile the kernel
Compiling the kernel is straightforward. Use the command make to start the process. The build system reads the .config file and generates the necessary code.
During compilation, the system creates an auto.conf file in include/config/, which contains the same configuration data but in a format suitable for the build process.
Each subsystem or module has its own Makefile that defines how to compile its code. The top-level Makefile passes variables like CONFIG_HELLO to these subdirectory Makefiles, allowing them to decide whether to build a feature as a module or into the kernel.
A generated header file, include/generated/autoconf.h, allows kernel code to check if a feature is enabled using #ifdef CONFIG_XXX directives.
Fifth, install the kernel
a) For a PC, run make modules_install and make install to install the kernel and modules. Update the bootloader configuration (e.g., grub.conf) to include the new kernel entry and set a timeout of 5 seconds for selection.
b) For an embedded system, the process is more involved. Connect via serial or network, use TFTP to transfer the kernel image (zImage), and load it into the board's flash using U-Boot commands. Modules can be installed using make -C /path/to/kernel_src_dir modules_install INSTALL_MOD_PATH=/path/to/rootfs_dir.
Finally, package the root filesystem into a format like CramFS or JFFS2 and burn it to flash for use on the target hardware.
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