kumquat-buildroot/docs/manual/adding-packages-kernel-module.txt
Rahul Bedarkar 5286dec396 docs/manual: refer to SPDX License List
We want to use SPDX short identifier for license string as much as
possible. So refer to SPDX license list and use SPDX short identifier
for license strings in documentation.

Also remove License abbreviations that we have.

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
[Thomas: take into account suggestions from Arnout.]
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
2017-04-01 15:11:18 +02:00

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// -*- 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 = GPL-2.0
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 = GPL-2.0
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`...