The Kconfig File Structure

Introduction

The Kconfig files describe: the configuration symbols supported in the build system, the logical organization and structure to group the symbols in menus and sub-menus, and the relationships between the different configuration symbols that govern the valid configuration combinations.

The Kconfig files are distributed across the build directory tree. The files are organized based on their common characteristics and on what new symbols they add to the configuration menus. For example:

• The Kconfig file at the root directory contains the general configuration options like ARCH and KERNEL VERSION. These symbols are defined for and apply to the entire build system.
• The Kconfig file at the kernel directory contains the general configuration related to the micro- and the nanokernel.
• The Kconfig file at the drivers directory organizes the inclusion of the various Kconfig files needed for each supported driver in the system.
• The Kconfig file at the misc directory contains the configuration symbols that affect different aspects of the build system. For example, the Custom Compiler Options and the Minimal Libc are general build options that apply to the build system. Debugging Options and System Monitoring Options are also examples of build options that apply to the entire system.
• The Kconfig file at the net directory contains the different symbols that define the configuration options for the communications stack code.
• The Kconfig file at the crypto directory contains the different symbols that define the configuration options for the cryptography algorithms supported.

Dependencies

Dependencies allow you to define the valid and invalid configuration combinations in the system. Each dependency is a rule that a particular symbol must follow to be either selected or allowed to have a specific value. For example, the configuration symbol B states a dependency on another one, A.

config B bool

config A depends on B

The symbol A does not exist as a configuration option in the system unless B is set to true.

The complete set of dependency rules defines the valid configuration combinations that the system can use.

Default Configurations

The default configuration files define the default configuration for a specific kernel on a specific platform. For example:

• arch/arm/configs,
• arch/x86/configs and
• arch/arc/configs.

All the default configuration files must end with the suffix _defconfig. For example, the galileo_defconfig file contains the configuration information for the galileo platform.

The Makefile.inc file uses defconfig files to provide a clean interface to developers using environment variables to define the kernel type and the platform of new projects. Developers can provide the build system with a target’s defconfig. The build system takes the specified defconfig file and sets it as the current .config file for the current project. For example:

$ make galileo_defconfig

The command takes the default configuration for the architecture and the galileo board configuration to compile the kernel.

Merging Configuration Fragments

Configuration file fragment can be merged with the current project configuration during the build.

Developers can provide a configuration file that defines a small subset of configuration options. The subset must contain the specific configuration options that differ from the default configuration.

The initconfig target pulls the default configuration file and merges it with the local configuration fragments. For example, the sample application hello world overrides the base configuration with the configuration fragment in prj.conf.