Most users find that the precompiled Fedora kernel suits their needs. At some point, you might need to recompile the kernel to support a specific piece of hardware or add a new feature to the operating system. If you have heard horror stories about the difficulties of recompiling the Linux kernel, you can relax; this chapter gives you all the information you need to understand when recompiling is necessary and how to painlessly work through the process.
The Linux Kernel
The Linux kernel is the management part of the operating system that many people call "Linux." Although many think of the entire distribution as Linux, the only piece that can correctly be called Linux is the kernel. Fedora, like many Linux distributions, includes a kernel packaged with add-on software that interacts with the kernel so that the user can interface with the system in a meaningful manner.
The system utilities and user programs enable computers to become valuable tools to a user.
In 1991, Linus Torvalds released version .99 of the Linux kernel as the result of his desire for a powerful, Unix-like operating system for his Intel 80386 personal computer. Linus wrote the initial code necessary to create what is now known as the Linux kernel and combined it with Richard Stallman's GNU tools. Indeed, because many of the Linux basic system tools come from the GNU Project, many people refer to the operating system as GNU/Linux. Since then, Linux has benefited as thousands of contributors have added their talents and time to the Linux project. Linus still maintains the kernel, deciding on what will and will not make it into the kernel as official releases, known to many as the vanilla or Linus Linux kernel.
The Linux Source Tree
The source code for the Linux kernel is kept in a group of directories called the kernel source tree. The structure of the kernel source tree is important because the process of compiling (building) the kernel is automated; it is controlled by scripts interpreted by the make application. These scripts, known as Makefiles, expect to find the pieces of the kernel code in specific places or they do not work. You will learn how to use make to compile a kernel later in this chapter.
It is not necessary for the Linux kernel source code to be installed on your system for the system to run or for you to accomplish typical tasks such as email, web browsing, or word processing. It is necessary that the kernel sources be installed, however, if you want to compile a new kernel. In the next section, we show you how to install the kernel source files and how to set up the special symbolic link required. That link is /usr/src/kernels/<yourkernelversion>, and it is how we will refer to the directory of the kernel source tree as we examine the contents of the kernel source tree.
The /usr/src/kernels/<yourkernelversion> directory contains the .config and the Makefile files among others. The .config file is the configuration of your Linux kernel as it was compiled. There is no .config file by default; you must select one from the /configs subdirectory. There you will find configuration files for each flavor of the kernel Fedora provides; simply copy the one appropriate for your system to the default directory and rename it .config.
We have already discussed the contents of the /configs subdirectory, so now you can examine the other directories found under /usr/src/kernels/<yourkernelversion>. The most useful is the Documentation directory. In it and its subdirectories, you will find almost all the documentation concerning every part of the kernel. The 00-INDEX file (each Documentation subdirectory also contains a 00-INDEX file as well) contains a list of the files in the main directory and a brief explanation of what they are. Many files are written solely for kernel programmers and application writers, but a few are useful to the intermediate or advanced Linux user attempting to learn about kernel and device driver issues. Some of the more interesting and useful documents are
► devices.txt — A list of all possible Linux devices that are represented in the /dev directory, giving major and minor numbers and a short description. If you have ever gotten an error message that mentions char-major-xxx, this file is where that list is kept. Devices are mentioned in Chapter 35, "Managing the File System."
► ide.txt — If your system uses IDE hard drives, this file discusses how the kernel interacts with them and lists the various kernel commands that can be used to solve IDE-related hardware problems, manually set data transfer modes, and otherwise manually manage your IDE drives. Most of this management is automatic, but if you want to understand how the kernel interacts with IDE devices, this file explains it.
► initrd.txt — Chapter 34 also discusses the initial RAM disk. This file provides much more in-depth knowledge of initial RAM disks, giving details on the loopback file system used to create and mount them and explaining how the kernel interacts with them.
► kernel-parameters.txt — This file is a list of most of the arguments that you can pass at boot time to configure kernel or hardware settings, but it does not appear too useful at first glance because it is just a list. However, knowing that a parameter exists and might relate to something you are looking for can assist you in tracking down more information because now you have terms to enter into an Internet search engine such ashttp://www.google.com/linux.
► sysrq.txt — If you have ever wondered what that SysRq key on your keyboard is used for, this file has the answer. Briefly, it is a key combination hardwired into the kernel that can help you recover from a system lockup. Fedora disables this function by default for security reasons. You can re-enable it by entering the command # echo "1" > /proc/sys/kernel/sysrq and disable it by echoing a value of 0 instead of 1.
In the other directories found in Documentation, you will find similar text files that deal with the kernel modules for CD-ROM drivers, file system drivers, gameport and joystick drivers, video drivers (not graphics card drivers — those belong to X11R6 and not to the kernel), network drivers, and all the other drivers and systems found in the Linux operating system. Again, these documents are usually written for programmers, but they can provide useful information to the intermediate and advanced Linux user as well.
The directory named scripts contains many of the scripts that make uses. It really does not contain anything of interest to anyone who is not a programmer or a kernel developer (also known as a kernel hacker).
After a kernel is built, all the compiled files wind up in the arch directory and its subdirectories. Although you can manually move them to their final location, we will show you later in this chapter how the make scripts will do it for you. In the early days of Linux, this post-compilation file relocation was all done by hand; you should be grateful for make.