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=== Infrastructure for packages using kconfig for configuration files

A popular way for a software package to handle user-specified
configuration is +kconfig+. Among others, it is used by the Linux
kernel, Busybox, and Buildroot itself. The presence of a .config file
and a +menuconfig+ target are two well-known symptoms of kconfig being
used.

Buildroot features an infrastructure for packages that use kconfig for
their configuration. This infrastructure provides the necessary logic to
expose the package's +menuconfig+ target as +foo-menuconfig+ in
Buildroot, and to handle the copying back and forth of the configuration
file in a correct way.

The main macro of the kconfig package infrastructure is
+kconfig-package+. It is similar to the +generic-package+ macro.

Just like the generic infrastructure, the kconfig infrastructure works
by defining a number of variables before calling the +kconfig-package+
macro.

All the package metadata information variables that exist in the
xref:generic-package-reference[generic package infrastructure] also
exist in the kconfig infrastructure.

In order to use the +kconfig-package+ infrastructure for a Buildroot
package, the minimally required lines in the +.mk+ file, in addition to
the variables required by the +generic-package+ infrastructure, are:

----
FOO_KCONFIG_FILE = reference-to-source-configuration-file

$(eval $(kconfig-package))
----

This snippet creates the following make targets:

* +foo-menuconfig+, which calls the package's +menuconfig+ target

* +foo-update-config+, which copies the configuration back to the
  source configuration file. It is not possible to use this target
  when fragment files are set.

* +foo-update-defconfig+, which copies the configuration back to the
  source configuration file. The configuration file will only list the
  options that differ from the default values. It is not possible to
  use this target when fragment files are set.

* +foo-diff-config+, which outputs the differences between the current
  configuration and the one defined in the Buildroot configuration for
  this kconfig package. The output is useful to identify the
  configuration changes that may have to be propagated to
  configuration fragments for example.

and ensures that the source configuration file is copied to the build
directory at the right moment.

There are two options to specify a configuration file to use, either
+FOO_KCONFIG_FILE+ (as in the example, above) or +FOO_KCONFIG_DEFCONFIG+.
It is mandatory to provide either, but not both:

* +FOO_KCONFIG_FILE+ specifies the path to a defconfig or full-config file
  to be used to configure the package.

* +FOO_KCONFIG_DEFCONFIG+ specifies the defconfig 'make' rule to call to
  configure the package.

In addition to these minimally required lines, several optional variables can
be set to suit the needs of the package under consideration:

* +FOO_KCONFIG_EDITORS+: a space-separated list of kconfig editors to
  support, for example 'menuconfig xconfig'. By default, 'menuconfig'.

* +FOO_KCONFIG_FRAGMENT_FILES+: a space-separated list of configuration
  fragment files that are merged to the main configuration file.
  Fragment files are typically used when there is a desire to stay in sync
  with an upstream (def)config file, with some minor modifications.

* +FOO_KCONFIG_OPTS+: extra options to pass when calling the kconfig
  editors. This may need to include '$(FOO_MAKE_OPTS)', for example. By
  default, empty.

* +FOO_KCONFIG_FIXUP_CMDS+: a list of shell commands needed to fixup the
  configuration file after copying it or running a kconfig editor. Such
  commands may be needed to ensure a configuration consistent with other
  configuration of Buildroot, for example. By default, empty.

* +FOO_KCONFIG_DOTCONFIG+: path (with filename) of the +.config+ file,
  relative to the package source tree. The default, +.config+, should
  be well suited for all packages that use the standard kconfig
  infrastructure as inherited from the Linux kernel; some packages use
  a derivative of kconfig that use a different location.

* +FOO_KCONFIG_DEPENDENCIES+: the list of packages (most probably, host
  packages) that need to be built before this package's kconfig is
  interpreted. Seldom used. By default, empty.

* +FOO_KCONFIG_SUPPORTS_DEFCONFIG+: whether the package's kconfig system
  supports using defconfig files; few packages do not. By default, 'YES'.
