Kernel Development

1.Kernel Source Code

The Linux kernel code is designed to be portable. All code outside arch/ should be portable.

Endianness: cpu_to_be32, cpu_to_le32, be32_to_cpu, le32_to_cpu

Never use floating point numbers in kernel code.

Internal kernel API to implement kernel code can undergo changes between two 2.6.x releases.

External API must not change (system calls, /proc, /sys), as it could break existing programs.

New features can be added, but kernel developers try to keep backward compatibility with earlier versions, at least for 1 or several years.

arch/<arch>	Architecture specific code
arch/<arch>/<mach>	Machine / board specific code
block/	Block layer core
drivers/	All device drivers except sound ones (usb, pci...)
fs/	Filesystems (fs/ext3/, etc.)
include/	Kernel headers
include/asm-<arch>	Architecture and machine dependent headers
include/linux	Linux kernel core headers
init/	Linux initialization (including main.c)
ipc/	Code used for process communication
kernel/	Linux kernel core (very small!)
mm/	Memory management code (small too!)
net/	Network support code (not drivers)
sound/	Sound support code and drivers
lib/	Misc library routines (zlib, crc32...)
security/	Security model implementations (SELinux...)
Kbuild	Part of the kernel build system
Makefile	Top Linux makefile (sets arch and version)
scripts/	Scripts for internal or external use
MAINTAINERS	Maintainers of each kernel part. Very useful!
README	Overview and building instructions

Kernel development sources are now managed with git: http://kernel.org/pub/software/scm/git/

Linus' git tree: http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=tree

Proxy: export http_proxy="proxy.server.com:8080"; export ftp_proxy="proxy.server.com:8080";

git-clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git

2.Kernel Configuration

The configuration is stored in the .config file at the root of kernel sources. As options have dependencies, typically never edited by hand, but through graphical interfaces:

make [xconfig|gconfig|menuconfig|oldconfig]

To modify a kernel in a GNU/Linux distribution: the configuration files are usually released in /boot/, together with kernel images: /boot/config-2.6.17-11-generic.

3.Kernel Compilation

1 make -j 4

Vmlinux: the raw uncompressed kernel image, at the ELF format, useful for debugging purposes, but cannot be booted.

arch/<arch>/boot/*Image: the final, usually compressed, kernel image that can be booted (bzImage for x86, zImage for ARM, vmImage.gz for Blackfin, etc.).

*.ko: All kernel modules, spread over the kernel source tree, as .ko files.

2 make install

Do the installation for the host system by default, so needs to be run as root.

/boot/vmlinuz-<version>: Compressed kernel image. Same as the one in arch/<arch>/boot.

/boot/System.map-<version>: Stores kernel symbol addresses.

/boot/config-<version>: Kernel configuration for this version.

3 make modules_install

Do the installation for the host system by default, so needs to be run as root.

Installs all modules in /lib/modules/<version>/

kernel/: Module .ko (Kernel Object) files, in the same directory structure as in the sources.

modules.alias: Module aliases for module loading utilities. Example line: alias sound-service-?-0 snd_mixer_oss.

modules.dep: Module dependencies.

modules.symbols: Tells which module a given symbol belongs to.

Kernel cleanup targets

make clean: clean-up generated files(to force recompiling drivers)

make mrproper: remove all generated files, include .config file

make distclean: also remove editor backup and patch reject files

Device file creation

Device files are not created when a driver is loaded. They have to be created in advance:

sudo mknod /dev/<device> [c|b] <major> <minor>

Specifying cross-compilation

ARCH ?= arm CROSS_COMPILE ?= arm-linux-

The Makefile defines CC = $(CROSS_COMPILE)gcc

Pass ARCH and CROSS_COMPILE on the make command line.

Define ARCH and CROSS_COMPILE as environment variables.

Default configuration in boards/machines! Check if one exists in arch/<arch>/configs/ for your target.

1.1 make <machine>_defconfig                                 #in toplevel source directory

1.2 make

1.3 copy arch/<arch>/boot/zImage to target.       #autocopy with arch/<arch>/boot/install.sh

1.4 make INSTALL_MOD_PATH=<dir>/ modules_install

1.5 copy <dir>/lib/modules/ to /lib/modules/ on the target storage