Christopher Hallinan - Embedded Linux Primer - A Practical, Real-World Approach

#

# Makefile for the linux kernel.

#

obj-y += common.o common-pci.o

obj-$(CONFIG_ARCH_IXDP4XX) += ixdp425-pci.o ixdp425-setup.o

obj-$(CONFIG_MACH_IXDPG425) += ixdpg425-pci.o coyote-setup.o

obj-$(CONFIG_ARCH_ADI_COYOTE) += coyote-pci.o coyote-setup.o

obj-$(CONFIG_MACH_GTWX5715) += gtwx5715-pci.o gtwx5715-setup.o

You might be surprised by the simplicity of this makefile. Much work has gone into the development of the kernel build system for just this reason. For the average developer who simply needs to add support for his custom hardware, the design of the kernel build system makes these kinds of customizations very straightforward. [34] In actuality, the kernel build system is very complicated, but most of the complexity is cleverly hidden from the average developer. As a result, it is relatively easy to add, modify, or delete configurations without having to be an expert.

Looking at this makefile, it might be obvious what must be done to introduce new hardware setup routines conditionally based on your configuration options. Simply add the following two lines at the bottom of the makefile, and you're done:

obj-$(CONFIG_ARCH_VEGA) += vega_setup.o

obj-$(CONFIG_ARCH_CONSTELLATION) += costellation_setup.o

These steps complete the simple addition of setup modules specific to the hypothetical example custom hardware. Using similar logic, you should now be able to make your own modifications to the kernel configuration/build system.

A wealth of information is available in the Linux source tree itself. It would be difficult indeed to read it all because there are nearly 650 documentation files in 42 subdirectories in the .../Documentation directory. Be cautious in reading this material: Given the rapid pace of kernel development and release, this documentation tends to become outdated quickly. Nonetheless, it often provides a great starting point from which you can form a foundation on a particular kernel subsystem or concept.

Do not neglect the Linux Documentation Project, found at www.tldp.org, where you might find the most up-to-date version of a particular document or man page. [35] Always assume that features advance faster than the corresponding documentation, so treat the docs as a guide rather than indisputable facts. The list of suggested reading at the end of this chapter duplicates the URL for the Linux Documentation Project, for easy reference. Of particular interest to the previous discussion is the Kbuild documentation found in the kernel .../Documentation/kbuild subdirectory.

No discussion of Kernel documentation would be complete without mentioning Google. One day soon, Googling will appear in Merriam Webster's as a verb! Chances are, many problems and questions you might ask have already been asked and answered before. Spend some time to become proficient in searching the Internet for answers to questions. You will discover numerous mailing lists and other information repositories full of useful information related to your specific project or problem. Appendix E contains a useful list of open-source resources.

In general, you can obtain an embedded Linux kernel for your hardware platform in three ways: You can purchase a suitable commercial embedded Linux distribution; you can download a free embedded distribution, if you can find one suitable for your particular architecture and processor; or you can find the closest open-source Linux kernel to your application and port it yourself. We discuss Linux porting in Chapter 16, "Porting Linux."

Although porting an open source kernel to your custom board is not necessarily difficult, it represents a significant investment in engineering/development resources. This approach gives you access to free software, but deploying Linux in your development project is far from free, as we discussed in Chapter 1, "Introduction." Even for a small system with minimal application requirements, you need many more components than just a Linux kernel.

This chapter has focused on the layout and construction of the Linux kernel itself. As you might have already discovered, Linux is only a small component of an embedded system based on Linux. In addition to the Linux kernel, you need the following components to develop, test, and launch your embedded Linux widget:

• Bootloader ported to and configured for your specific hardware platform

• Cross-compiler and associated toolchain for your chosen architecture

• File system containing many packagesbinary executables and libraries compiled for your native hardware architecture/processor

• Device drivers for any custom devices on your board

• Development environment, including host tools and utilities

• Linux kernel source tree enabled for your particular processor and board

These are the components of an embedded Linux distribution.

• The Linux kernel is more than 10 years old and has become a mainstream, well-supported operating system for many architectures.

• The Linux open source home is found at www.kernel.org. Virtually every release version of the kernel is available there, going all the way back to Linux 1.0.

• We leave it to other great books to describe the theory and operation of the Linux kernel. Here we discussed how it is built and identified the components that make up the image. Breaking up the kernel into understandable pieces is the key to learning how to navigate this large software project.

• This chapter covered the kernel build system and the process of modifying the build system to facilitate modifications.

• Several kernel configuration editors exist. We chose one and examined how it is driven and how to modify the menus and menu items within. These concepts apply to all the graphical front ends.

• The kernel itself comes with an entire directory structure full of useful kernel documentation. This is a helpful resource for understanding and navigating the kernel and its operation.

• This chapter concluded with a brief introduction to the options available for obtaining an embedded Linux distribution.

Linux Kernel HOWTO:

www.tldp.org/HOWTO/Kernel-HOWTO

Kernel Kbuild documentation:

http://sourceforge.net/projects/kbuild/

The Linux Documentation Project:

www.tldp.org/

Tool Interface Standard (TIS) Executable and Linking Format (ELF) Specification,

Version 1.2

TIS Committee, May 1995

Linux kernel source tree:

…/Documentation/kbuild/makefiles.txt

Linux kernel source tree:

…/Documentation/kbuild/kconfig-language.txt

Linux Kernel Development,

2nd Edition Rovert Love

Novell Press, 2005

When the power is applied to an embedded Linux system, a complex sequence of events is started. After a few seconds, the Linux kernel is operational and has spawned a series of application programs as specified by the system init scripts . A significant portion of these activities are governed by system configuration and are under the control of the embedded developer.

This chapter examines the initial sequence of events in the Linux kernel. We take a detailed look at the mechanisms and processes used during kernel initialization. We describe the Linux kernel command line and its use to customize the Linux environment on startup. With this knowledge, you will be able to customize and control the initialization sequence to meet the requirements of your particular embedded system.