package-infra: add helper to build kernel modules

The Linux kernel offers a nice and easy-to-use infra to build
out-of-tree kernel modules.

Currently, we have quite a few packages that build kernel modules, and
most duplicate (or rewrite) the same code over-and-over again.

Introduce a new infrastructure that provides helpers to build kernel
modules, so packages do not have to duplicate/rewrite that.

The infrastructure, unlike any other package infra, is not standalone.
It needs another package infra to be used. This is so that packages that
provide both userland and kernel modules can be built easily. So, this
infra only defines post-build and post-install hooks, that will build
the kernel modules after the rest of the package.

We need to override PWD, because some packages will use it to find their
own includes (and other helper files). PWD is inherited from the
environment, so it gets whatever value it had when make was launched,
which happens to be Buildroot's own top source tree. So, we just force
PWD to the proper value, rather than cd-ing first.

Also, no host version is provided, since it does not make sense to build
kernel modules for the host.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/Makefile.in

@@ -399,3 +399,4 @@ include package/pkg-virtual.mk
 include package/pkg-generic.mk
 include package/pkg-kconfig.mk
 include package/pkg-rebar.mk
+include package/pkg-kernel-module.mk

package/pkg-kernel-module.mk

@@ -0,0 +1,96 @@
+################################################################################
+# kernel module infrastructure for building Linux kernel modules
+#
+# This file implements an infrastructure that eases development of package
+# .mk files for out-of-tree Linux kernel modules. It should be used for all
+# packages that build a Linux kernel module using the kernel's out-of-tree
+# buildsystem, unless they use a complex custom buildsystem.
+#
+# The kernel-module infrastructure requires the packages that use it to also
+# use another package infrastructure. kernel-module only defines post-build
+# and post-install hooks. This allows the package to build both kernel
+# modules and/or user-space components (with any of the other *-package
+# infra).
+#
+# As such, it is to be used in conjunction with another *-package infra,
+# like so:
+#
+#   $(eval $(kernel-module))
+#   $(eval $(generic-package))
+#
+# Note: if the caller needs access to the kernel modules (either after they
+# are built or after they are installed), it will have to define its own
+# post-build/install hooks *after* calling kernel-module, but *before*
+# calling the other *-package infra, like so:
+#
+#   $(eval $(kernel-module))
+#   define FOO_MOD_TWEAK
+#   	# do something
+#   endef
+#   FOO_POST_BUILD_HOOKS += FOO_MOD_TWEAK
+#   $(eval $(generic-package))
+#
+# Note: this infra does not check that the kernel is enabled; it is expected
+# to be enforced at the Kconfig level with proper 'depends on'.
+################################################################################
+
+################################################################################
+# inner-kernel-module -- generates the make targets needed to support building
+# a kernel module
+#
+#  argument 1 is the lowercase package name
+#  argument 2 is the uppercase package name
+################################################################################
+
+define inner-kernel-module
+
+# The kernel must be built first.
+$(2)_DEPENDENCIES += linux
+
+# This is only defined in some infrastructures (e.g. autotools, cmake),
+# but not in others (e.g. generic). So define it here as well.
+$(2)_MAKE ?= $$(MAKE)
+
+# If not specified, consider the source of the kernel module to be at
+# the root of the package.
+$(2)_MODULE_SUBDIRS ?= .
+
+# Build the kernel module(s)
+# Force PWD for those packages that want to use it to find their
+# includes and other support files (Booo!)
+define $(2)_KERNEL_MODULES_BUILD
+	@$$(call MESSAGE,"Building kernel module(s)")
+	$$(foreach d,$$($(2)_MODULE_SUBDIRS), \
+		$$(LINUX_MAKE_ENV) $$($$(PKG)_MAKE) \
+			-C $$(LINUX_DIR) \
+			$$(LINUX_MAKE_FLAGS) \
+			$$($(2)_MODULE_MAKE_OPTS) \
+			PWD=$$(@D)/$$(d) \
+			M=$$(@D)/$$(d) \
+			modules$$(sep))
+endef
+$(2)_POST_BUILD_HOOKS += $(2)_KERNEL_MODULES_BUILD
+
+# Install the kernel module(s)
+# Force PWD for those packages that want to use it to find their
+# includes and other support files (Booo!)
+define $(2)_KERNEL_MODULES_INSTALL
+	@$$(call MESSAGE,"Installing kernel module(s)")
+	$$(foreach d,$$($(2)_MODULE_SUBDIRS), \
+		$$(LINUX_MAKE_ENV) $$($$(PKG)_MAKE) \
+			-C $$(LINUX_DIR) \
+			$$(LINUX_MAKE_FLAGS) \
+			$$($(2)_MODULE_MAKE_OPTS) \
+			PWD=$$(@D)/$$(d) \
+			M=$$(@D)/$$(d) \
+			modules_install$$(sep))
+endef
+$(2)_POST_INSTALL_TARGET_HOOKS += $(2)_KERNEL_MODULES_INSTALL
+
+endef
+
+################################################################################
+# kernel-module -- the target generator macro for kernel module packages
+################################################################################
+
+kernel-module = $(call inner-kernel-module,$(pkgname),$(call UPPERCASE,$(pkgname)))