// -*- mode:doc; -*-
// vim: set syntax=asciidoc:

=== Infrastructure for packages building kernel modules

Buildroot offers a helper infrastructure to make it easy to write packages that
build and install Linux kernel modules. Some packages only contain a kernel
module, other packages contain programs and libraries in addition to kernel
modules. Buildroot's helper infrastructure supports either case.

[[kernel-module-tutorial]]
==== +kernel-module+ tutorial

Let's start with an example on how to prepare a simple package that only
builds a kernel module, and no other component:

----
01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06: 
07: FOO_VERSION = 1.2.3
08: FOO_SOURCE = foo-$(FOO_VERSION).tar.xz
09: FOO_SITE = http://www.foosoftware.org/download
10: FOO_LICENSE = GPLv2
11: FOO_LICENSE_FILES = COPYING
12: 
13: $(eval $(kernel-module))
14: $(eval $(generic-package))
----

Lines 7-11 define the usual meta-data to specify the version, archive name,
remote URI where to find the package source, licensing information.

On line 13, we invoke the +kernel-module+ helper infrastructure, that
generates all the appropriate Makefile rules and variables to build
that kernel module.

Finally, on line 14, we invoke the
xref:generic-package-tutorial[+generic-package+ infrastructure].

The dependency on +linux+ is automatically added, so it is not needed to
specify it in +FOO_DEPENDENCIES+.

What you may have noticed is that, unlike other package infrastructures,
we explicitly invoke a second infrastructure. This allows a package to
build a kernel module, but also, if needed, use any one of other package
infrastructures to build normal userland components (libraries,
executables...). Using the +kernel-module+ infrastructure on its own is
not sufficient; another package infrastructure *must* be used.

Let's look at a more complex example:

----
01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06: 
07: FOO_VERSION = 1.2.3
08: FOO_SOURCE = foo-$(FOO_VERSION).tar.xz
09: FOO_SITE = http://www.foosoftware.org/download
10: FOO_LICENSE = GPLv2
11: FOO_LICENSE_FILES = COPYING
12: 
13: FOO_MODULE_SUBDIRS = driver/base
14: FOO_MODULE_MAKE_OPTS = KVERSION=$(LINUX_VERSION_PROBED)
15: 
16: ifeq ($(BR2_PACKAGE_LIBBAR),y)
17: FOO_DEPENDENCIES = libbar
18: FOO_CONF_OPTS = --enable-bar
19: FOO_MODULE_SUBDIRS += driver/bar
20: else
21: FOO_CONF_OPTS = --disable-bar
22: endif
23: 
24: $(eval $(kernel-module))
26: $(eval $(autotools-package))
----

Here, we see that we have an autotools-based package, that also builds
the kernel module located in sub-directory +driver/base+ and, if libbar
is enabled, the kernel module located in sub-directory +driver/bar+, and
defines the variable +KVERSION+ to be passed to the Linux buildsystem
when building the module(s).


[[kernel-module-reference]]
==== +kernel-module+ reference

The main macro for the  kernel module infrastructure is +kernel-module+.
Unlike other package infrastructures, it is not stand-alone, and requires
any of the other +*-package+ macros be called after it.

The +kernel-module+ macro defines post-build and post-target-install
hooks to build the kernel modules. If the package's +.mk+ needs access
to the built kernel modules, it should do so in a post-build hook,
*registered after* the call to +kernel-module+. Similarly, if the
package's +.mk+ needs access to the kernel module after it has been
installed, it should do so in a post-install hook, *registered after*
the call to +kernel-module+. Here's an example:

----
$(eval $(kernel-module))

define FOO_DO_STUFF_WITH_KERNEL_MODULE
    # Do something with it...
endef
FOO_POST_BUILD_HOOKS += FOO_DO_STUFF_WITH_KERNEL_MODULE

$(eval $(generic-package))
----

Finally, unlike the other package infrastructures, there is no
+host-kernel-module+ variant to build a host kernel module.

The following additional variables can optionally be defined to further
configure the build of the kernel module:

* +FOO_MODULE_SUBDIRS+ may be set to one or more sub-directories (relative
  to the package source top-directory) where the kernel module sources are.
  If empty or not set, the sources for the kernel module(s) are considered
  to be located at the top of the package source tree.

* +FOO_MODULE_MAKE_OPTS+ may be set to contain extra variable definitions
  to pass to the Linux buildsystem.

[[kernel-variables]]
You may also reference (but you may *not* set!) those variables:

 * +LINUX_DIR+ contains the path to where the Linux kernel has been
   extracted and built.

 * +LINUX_VERSION+ contains the version string as configured by the user.

 * +LINUX_VERSION_PROBED+ contains the real version string of the kernel,
   retrieved with running `make -C $(LINUX_DIR) kernelrelease`

 * +KERNEL_ARCH+ contains the name of the current architecture, like `arm`,
   `mips`...