Linux内核精要指南：嵌入式系统开发宝典

linux

kernel

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Chapter 1

Introduction

Despite its large code base (over seven million lines of code), the Linux kernel is

the most flexible operating system that has ever been created. It can be tuned for a

wide range of different systems, running on everything from a radio-controlled

model helicoptor, to a cell phone, to the majority of the largest supercomputers in

the world. By customizing the kernel for your specific environment, it is possible

to create something that is both smaller and faster than the kernel provided by

most Linux distributions. This book will go into how to build and install a custom

kernel, and provide some hints on how to enable specific options that you will

probably wish to use for different situations.

No Linux distribution provides the exact kernel most of its users want. Modern

distributions have gotten very accommodating, compiling in support for every

known device, for sound, and even for power conservation. But you will likely

have a need that’s different from the majority of users (and every distribution has

to try to meet the needs of the majority). You may just have different hardware.

And when a new kernel comes out, you may want to start using it without waiting

for a distribution to be built around it.

For a host of reasons, you will want during your Linux career to sometimes build

a kernel, or to tweak the parameters of one you are running. This book gives you

the information you need to understand the kernel from a user’s point of view,

and to make the most common changes.

There are also good reasons to remove features from the kernel, particularly if you

are running it on an embedded system or one with a small form factor.

When tweaking, it’s helpful to understand the internals of kernel behavior. These

are beyond the scope of this book, except for brief summaries that appear with

certain options. Appendix B includes references to other books and material that

can give you more background.

,ch01.11032 Page 3 Friday, December 1, 2006 9:57 AM

Chapter 2: Requirements for Building and Using the Kernel

properly, so unless you wish to help debug compiler bugs, it is not recommended

that you try them out.

To determine which version of gcc you have on your system, run the following

command:

$ gcc --version

Linker

The C compiler, gcc, does not do all of the compiling on its own. It needs an addi-

tional set of tools known as binutils to do the linking and assembling of source

files. The binutils package also contains useful utilities that can manipulate object

files in lots of useful ways, such as to view the contents of a library.

binutils can usually be found in a distribution package called (not surprisingly)

binutils. If you wish to download and install the package yourself, you can find it

at http://www.gnu.org/software/binutils.

As of the 2.6.18 kernel release, the 2.12 release of binutils is the oldest that can

successfully link the kernel. To determine which version of binutils you have on

your system, run the following command:

$ ld -v

make

make is a tool that walks the kernel source tree to determine which files need to be

compiled, and then calls the compiler and other build tools to do the work in

building the kernel. The kernel requires the GNU version of make, which can

usually be found in a package called make for your distribution.

If you wish to download and install make youself, you can find it at http://www.

gnu.org/software/make.

As of the 2.6.18 kernel release, the 3.79.1 release of make is the oldest that can

properly build the kernel. It is recommended that you install the latest stable

version of make, because newer versions are known to work faster at processing

the build files.

To determine which version of make you have on your system, run the following

command:

$ make --version

Tools to Use the Kernel

While the version of the kernel that is running does not usually affect any user

application, there are a small number of program for which the kernel version is

important. This section describes a number of tools that are probably already

installed on your Linux system. If you upgrade your kernel to a version different

from the one that came with your distribution, some of these packages may also

need to be upgraded in order for the system to work properly.

,ch02.11211 Page 6 Friday, December 1, 2006 9:57 AM

Tools to Use the Kernel | 7

Requirements

util-linux

The util-linux package is a collection of small utilities that do a wide range of

different tasks. Most of these utilities handle the mounting and creation of disk

partitions and manipulation of the hardware clock in the system.

If you wish to download and install the util-linux package yourself, you can find it

at http://www.kernel.org/pub/linux/utils/util-linux.

As of the 2.6.18 kernel release, the 2.10 release of util-linux is the oldest that

works properly. It is recommended that you install the latest version of this

package, because new version support new features added to the kernel. Bind

mounts are one example of an option in newer kernels, and a newer version of

util-linux is needed in order to have them work properly.

To determine which version of the util-linux package you have on your system,

run the following command:

$ fdformat --version

module-init-tools

The module-init-tools package is needed if you wish to use Linux kernel modules.

A kernel module is a loadable chunk of code that can be added to or removed from

the kernel while the kernel is running. It is useful to compile device drivers as

modules and then load only the ones that correspond to the hardware present in

the system. All Linux distributions use modules in order to load only the needed

drivers and options for the system based on the hardware present, instead of being

forced to build all possible drivers and options in the kernel in one large chunk.

Modules save memory by loading just the code that is needed to control the

machine properly.

The kernel module loading process underwent a radical change in the 2.6 kernel

release. The linker for the module (the code that resolves all symbols and figures

out how to put the pieces together in memory) is now built into the kernel, which

makes the userspace tools quite small. Older distributions have a package called

modutils that does not work properly with the 2.6 kernel. The module-init-tools

package is what you need to get the 2.6 kernel to work properly with modules.

If you wish to download and install the module-init-tools package yourself, you

can find it at http://www.kernel.org/pub/linux/utils/kernel/module-init-tools.

As of the 2.6.18 kernel release, the 0.9.10 release of module-init-tools is the oldest

version that works properly. It is recommended that the latest version of this

package be installed, as new features added to the kernel can be used by newer

versions of this package. Blacklisting modules to prevent them from being auto-

matically loaded by the udev package is one such option that is present in newer

versions of module-init-tools, but not older ones.

To determine which version of the module-init-tools package you have on your

system, run the following command:

$ depmod -V

,ch02.11211 Page 7 Friday, December 1, 2006 9:57 AM

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