Comprehensive Real-World Guidance for Every Embedded Developer and Engineer
This book brings together indispensable knowledge for building efficient, high-value, Linux-based embedded products: information that has never been assembled in one place before. Drawing on years of experience as an embedded Linux consultant and field application engineer, Christopher Hallinan offers solutions for the specific technical issues you're most likely to face, demonstrates how to build an effective embedded Linux environment, and shows how to use it as productively as possible.
Hallinan begins by touring a typical Linux-based embedded system, introducing key concepts and components, and calling attention to differences between Linux and traditional embedded environments. Writing from the embedded developer's viewpoint, he thoroughly addresses issues ranging from kernel building and initialization to bootloaders, device drivers to file systems.
Hallinan thoroughly covers the increasingly popular BusyBox utilities; presents a step-by-step walkthrough of porting Linux to custom boards; and introduces real-time configuration via CONFIG_RT--one of today's most exciting developments in embedded Linux. You'll find especially detailed coverage of using development tools to analyze and debug embedded systems--including the art of kernel debugging.
• Compare leading embedded Linux processors
• Understand the details of the Linux kernel initialization process
• Learn about the special role of bootloaders in embedded Linux systems, with specific emphasis on U-Boot
• Use embedded Linux file systems, including JFFS2--with detailed guidelines for building Flash-resident file system images
• Understand the Memory Technology Devices subsystem for flash (and other) memory devices
• Master gdb, KGDB, and hardware JTAG debugging
• Learn many tips and techniques for debugging within the Linux kernel
• Maximize your productivity in cross-development environments
• Prepare your entire development environment, including TFTP, DHCP, and NFS target servers
• Configure, build, and initialize BusyBox to support your unique requirements
8.2.5. depmod
8.2.5. depmod
How does modprobe know about the dependencies of a given module? The depmod utility plays a key role in this process. When modprobe is executed, it searches for a file called modules.dep in the same location where the modules are installed. The depmod utility creates this module-dependency file.
This file contains a list of all the modules that the kernel build system is configured for, along with dependency information for each. It is a simple file format: Each device driver module occupies one line in the file. If the module has dependencies, they are listed in order following the module name. For example, from Listing 8-7, we saw that the ext3 module had a dependency on the jbd module. The dependency line in modules.dep would look like this:
ext3.ko: jbd.ko
In actual practice, each module name is preceded by its absolute path in the file system, to avoid ambiguity. We have omitted the path information for readability. A more complicated dependency chain, such as sound drivers, might look like this:
snd-intel8x0.ko: snd-ac97-codec.ko snd-pcm.ko snd-timer.ko
snd.ko soundcore.ko snd-page-alloc.ko
Again, we have removed the leading path components for readability. Each module filename in the modules.dep file is an absolute filename, with complete path information, and exists on a single line. The previous example has been truncated to two lines, to fit in the space on this page.
Normally, depmod is run automatically during a kernel build. However, in a cross-development environment, you must have a cross-version of depmod that knows how to read the modules that are compiled in the native format of your target architecture. Alternatively, most embedded distributions have a method and init script entries to run depmod on each boot, to guarantee that the module dependencies are kept up-to-date.
