First, preparation work
a) Begin by ensuring you have a PC running Linux. This is essential and not something to be ignored.
b) Install necessary tools such as GCC (host compiler for compiling programs on the PC), make, and ncurses. These are fundamental for kernel development.
c) Download a clean Linux kernel source package and extract it. If you're compiling for your current PC, it's best to use the version that matches your distribution. However, even if you use a different version, it can still work. For example, I used a kernel version 2.6.32.65 on Fedora 13, which had a kernel version of 2.6.33.3, and it compiled successfully. The configuration file used was from the system’s /lib/modules/`uname -r`/build/.config.
d) If you're porting Linux to an embedded system, you'll need a cross-compilation toolchain. For example, if your target CPU is ARM or MIPS, install the corresponding toolchain and add its path to your PATH environment variable. You can verify the installation with a command like arm-linux-gcc --version. Note that you can download ARM toolchains by replying "ARM" in the discussion.
Second, set the compilation target
Before configuring or compiling the kernel, determine the target CPU architecture and the toolchain to use. This is crucial for correct compilation. If you're building for your own PC, no additional settings are needed. Otherwise, you must configure it clearly. For example, when working with ARM, you can either modify the Makefile directly or pass parameters through the command line. Modifying the Makefile involves setting ARCH and CROSS_COMPILE variables. Alternatively, you can specify these during the make command using flags like ARCH=arm and CROSS_COMPILE=arm-linux-. The default values are automatically determined based on the host system, but for cross-compilation, explicit settings are required.
Third, configure the kernel
The kernel has thousands of configuration options, and you need to decide which features to include, whether as built-in modules or loadable modules. Before compilation, generate a .config file in the kernel source directory. To simplify this process, copy a suitable configuration file from the arch/arm/configs directory, such as s3c2410_defconfig. For PCs, use the .config file from /lib/modules/`uname -r`/build/. After copying, run make menuconfig to adjust settings. Even if you don’t change anything, running this step ensures the configuration is up-to-date, as new kernel versions may introduce dependencies that require adjustments.
Fourth, compile the kernel
Compiling the kernel is straightforward. Run make in the kernel source directory. The build system uses the .config file to generate a list of variables defining how each part of the kernel is compiled. Additionally, a generated file called include/config/auto.conf is created, containing similar settings. Each subsystem or module has its own Makefile, which defines what needs to be compiled. These Makefiles use variables like CONFIG_XXX to determine whether a feature is built into the kernel or as a module. During compilation, the top-level Makefile passes these variables down, allowing the build system to compile the correct parts of the kernel.
Fifth, install the kernel
a) For a PC, after compilation, run make modules_install and make install to install the kernel and modules. Then update the bootloader configuration, such as grub.conf, to include the new kernel entry. Set the timeout to 5 seconds so you can choose the kernel at boot. Restart your system and select the new kernel from the bootloader menu.
b) For embedded systems, the process is more involved. Connect via serial, use TFTP to transfer the kernel image, and flash it 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 the board for final use.
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