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Kconfig

System to configure how the kernel will be built.

Documented at:

Those files are used to generate .config with the configuration make targets.

There are around ~6k options!

Most options (TODO all) options generate preprocessor macros which control system through #ifdef checks.

How it works

For example, in arch/x86/Kconfig:

config SMP
	bool "Symmetric multi-processing support"
	---help---
	  This enables support for systems with more than one CPU. If you have
	  a system with only one CPU, say N. If you have a system with more
	  than one CPU, say Y.

leads to a line in .config of the type:

CONFIG_SMP=y

if set, and a line of type:

# CONFIG_SMP is not set

otherwise.

Then make reads the .config and generates:

include/generated/autoconf.h

TODO how is this included? Grepping the source shows a few -include on Makefiles, and very few #include.

Then throughout the kernel source ifdefs are used:

#ifdef CONFIG_SMP

#endif

help

The ---help--- is not necessary: most places use just help.

A patch to remove this inconsistency was WONTFIXed... https://lkml.org/lkml/2005/5/2/181

Important options

Quick selection of options that appear everywhere throughout the code and which every hacker should know.

  • arch/x86/Kconfig

    • SMP: symmetric multiprocessing
    • IA32_EMULATION

Cross compile the kernel

http://raspberrypi.stackexchange.com/questions/192/how-do-i-cross-compile-the-kernel-on-a-ubuntu-host