DESCRIPTION

This document describes the build system of the kernel. The build system is the software used to generate the configuration, to build the kernel, its documentation, and install generated files.

USAGE

The build system is meant to be used directly from the root directory.

Online help

In order to obtain online help from the build system, use the following command :

$ make help

The output of this command is a list of make targets to use to trigger specific actions, such as generating the kernel configuration, building the kernel, or the documentation.

Configuration

Before building the kernel, its configuration must be generated. The current configuration is stored into a file named .config inside the root directory. Most configuration targets generate this .config file. Some such as oldconfig reuse an existing .config file placed by the user before invoking the build system.

Default configurations

A default configuration can be generated with the defconfig target. It selects a default configuration depending on the target architecture. In addition, each architecture may provide more predefined configurations, all reported by the online help.

You may create your own default configuration with the savedefconfig target, which creates a file named defconfig inside the root directory. This file may then be renamed and added to the list of architecture-specific default configurations.

Cleaning

The following targets may be used to clean the working tree :

clean

This target removes most generated files, such as the kernel, any object file used to build the kernel, as well as the generated documentation files. It ignores the current configuration.

distclean

This target removes all generated files, including the configuration.

Building

If no target is passed, or if the target is all or x15, the kernel is built. There are several ways to influence a build in addition to the configuration. Builds are incremental, meaning that, when a source file is modified, the build system only rebuilds files that depend on the modified source. The build system itself as well as the configuration are considered global dependencies, so that any modification to either will make all generated files obsolete.

Parallel builds

In order to speed up builds, it is possible to make them parallel. This only has a real effect if your machine has multiple processors. For example :

make -j4

If unsure about the value, use the number of available processors.

Verbosity

By default, or if the V variable is set to 0, the build is quiet, only reporting a short name of actions along with their target file. If the V variable is set to 1, the build is verbose, showing all commands with their arguments.

Compiler and flags

In order to make the kernel easy to integrate in meta builders such as distribution packaging systems, the compiler and its flags may be given at configuration time through the traditional CC and CFLAGS variables. For example :

make i386_defconfig CC=clang CFLAGS="-Os -march=geode"

Out-of-tree builds

The kernel may be built outside the working tree. In order to perform such a build, simply use the -f make option with a path to the root Makefile. For example :

mkdir build
cd build
make -f /path/to/x15/Makefile defconfig
make -f /path/to/x15/Makefile -j4 x15

Installation

The various install targets are used to install the kernel and its documentation. The installation process can be altered with the DESTDIR and PREFIX variables. The DESTDIR variable is normally empty, but can be set to the root of a staging directory tree. This is particularly useful when building images for embedded systems. The PREFIX variable has the value "/usr/local" by default, and may be changed to another insallation path relative to DESTDIR.

DEVELOPMENT

Configuration

The configuration part of the build system is known as kconfig, originally imported from the Linux kernel. Description of the format used in Kconfig files can be found as part of the Linux kernel documentation. Module options are however not supported.

Makefiles

When adding or removing source files to/from the project, the x15_SOURCES-y variable should be updated accordingly. The reason for such a name is that it makes conditional compilation very easy to achieve. When adding a source file that should be built only when an option is enabled, use the following syntax :

x15_SOURCES-$(CONFIG_OPTION) += path/to/source.c

SEE

intro(9)

https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt