When building the kernel, the Makefiles scan the configuration and decide what subdirectories to descend into and what source files to compile for a given configuration. To complete the example of adding support for two custom hardware platforms, Vega and Constellation, let's look at the makefile that would read this configuration and take some action based on customizations.
Because you're dealing with hardware specific options in this example, assume that the customizations are represented by two hardware-setup modules called vega_setup.c and constellation_setup.c. We've placed these C source files in the .../arch/arm/mach-ixp4xx subdirectory of the kernel source tree. Listing 4-12 contains the complete makefile for this directory from a recent Linux release.
Listing 4-12. Makefile from …/arch/arm/mach-ixp4xx Kernel Subdirectory
#
# 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]
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.
4.3.7. Kernel Documentation
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in reading this material: Given the rapid pace of kernel development and release, this documentation tends to become outdated quickly.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] 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,
4.4. Obtaining a Linux Kernel
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.
4.4.1. What Else Do I Need?
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.
4.5. Chapter Summary
• 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
