Documentation/kbuild/modules.rst - linux - Git at Google

 =========================
 Building External Modules
 =========================

 This document describes how to build an out-of-tree kernel module.

 Introduction
 ============

 "kbuild" is the build system used by the Linux kernel. Modules must use
 kbuild to stay compatible with changes in the build infrastructure and
 to pick up the right flags to the compiler. Functionality for building modules
 both in-tree and out-of-tree is provided. The method for building
 either is similar, and all modules are initially developed and built
 out-of-tree.

 Covered in this document is information aimed at developers interested
 in building out-of-tree (or "external") modules. The author of an
 external module should supply a makefile that hides most of the
 complexity, so one only has to type "make" to build the module. This is
 easily accomplished, and a complete example will be presented in
 section `Creating a Kbuild File for an External Module`_.


 How to Build External Modules
 =============================

 To build external modules, you must have a prebuilt kernel available
 that contains the configuration and header files used in the build.
 Also, the kernel must have been built with modules enabled. If you are
 using a distribution kernel, there will be a package for the kernel you
 are running provided by your distribution.

 An alternative is to use the "make" target "modules_prepare." This will
 make sure the kernel contains the information required. The target
 exists solely as a simple way to prepare a kernel source tree for
 building external modules.

 NOTE: "modules_prepare" will not build Module.symvers even if
 CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
 executed to make module versioning work.

 Command Syntax
 --------------

 	The command to build an external module is::

 		$ make -C <path_to_kernel_dir> M=$PWD

 	The kbuild system knows that an external module is being built
 	due to the "M=<dir>" option given in the command.

 	To build against the running kernel use::

 		$ make -C /lib/modules/`uname -r`/build M=$PWD

 	Then to install the module(s) just built, add the target
 	"modules_install" to the command::

 		$ make -C /lib/modules/`uname -r`/build M=$PWD modules_install

 Options
 -------

 	($KDIR refers to the path of the kernel source directory, or the path
 	of the kernel output directory if the kernel was built in a separate
 	build directory.)

 	make -C $KDIR M=$PWD

 	-C $KDIR
 		The directory that contains the kernel and relevant build
 		artifacts used for building an external module.
 		"make" will actually change to the specified directory
 		when executing and will change back when finished.

 	M=$PWD
 		Informs kbuild that an external module is being built.
 		The value given to "M" is the absolute path of the
 		directory where the external module (kbuild file) is
 		located.

 Targets
 -------

 	When building an external module, only a subset of the "make"
 	targets are available.

 	make -C $KDIR M=$PWD [target]

 	The default will build the module(s) located in the current
 	directory, so a target does not need to be specified. All
 	output files will also be generated in this directory. No
 	attempts are made to update the kernel source, and it is a
 	precondition that a successful "make" has been executed for the
 	kernel.

 	modules
 		The default target for external modules. It has the
 		same functionality as if no target was specified. See
 		description above.

 	modules_install
 		Install the external module(s). The default location is
 		/lib/modules/<kernel_release>/updates/, but a prefix may
 		be added with INSTALL_MOD_PATH (discussed in section
 		`Module Installation`_).

 	clean
 		Remove all generated files in the module directory only.

 	help
 		List the available targets for external modules.

 Building Separate Files
 -----------------------

 	It is possible to build single files that are part of a module.
 	This works equally well for the kernel, a module, and even for
 	external modules.

 	Example (The module foo.ko, consist of bar.o and baz.o)::

 		make -C $KDIR M=$PWD bar.lst
 		make -C $KDIR M=$PWD baz.o
 		make -C $KDIR M=$PWD foo.ko
 		make -C $KDIR M=$PWD ./


 Creating a Kbuild File for an External Module
 =============================================

 In the last section we saw the command to build a module for the
 running kernel. The module is not actually built, however, because a
 build file is required. Contained in this file will be the name of
 the module(s) being built, along with the list of requisite source
 files. The file may be as simple as a single line::

 	obj-m := <module_name>.o

 The kbuild system will build <module_name>.o from <module_name>.c,
 and, after linking, will result in the kernel module <module_name>.ko.
 The above line can be put in either a "Kbuild" file or a "Makefile."
 When the module is built from multiple sources, an additional line is
 needed listing the files::

 	<module_name>-y := <src1>.o <src2>.o ...

 NOTE: Further documentation describing the syntax used by kbuild is
 located in Documentation/kbuild/makefiles.rst.

 The examples below demonstrate how to create a build file for the
 module 8123.ko, which is built from the following files::

 	8123_if.c
 	8123_if.h
 	8123_pci.c

 Shared Makefile
 ---------------

 	An external module always includes a wrapper makefile that
 	supports building the module using "make" with no arguments.
 	This target is not used by kbuild; it is only for convenience.
 	Additional functionality, such as test targets, can be included
 	but should be filtered out from kbuild due to possible name
 	clashes.

 	Example 1::

 		--> filename: Makefile
 		ifneq ($(KERNELRELEASE),)
 		# kbuild part of makefile
 		obj-m  := 8123.o
 		8123-y := 8123_if.o 8123_pci.o

 		else
 		# normal makefile
 		KDIR ?= /lib/modules/`uname -r`/build

 		default:
 			$(MAKE) -C $(KDIR) M=$$PWD

 		endif

 	The check for KERNELRELEASE is used to separate the two parts
 	of the makefile. In the example, kbuild will only see the two
 	assignments, whereas "make" will see everything except these
 	two assignments. This is due to two passes made on the file:
 	the first pass is by the "make" instance run on the command
 	line; the second pass is by the kbuild system, which is
 	initiated by the parameterized "make" in the default target.

 Separate Kbuild File and Makefile
 ---------------------------------

 	Kbuild will first look for a file named "Kbuild", and if it is not
 	found, it will then look for "Makefile". Utilizing a "Kbuild" file
 	allows us to split up the "Makefile" from example 1 into two files:

 	Example 2::

 		--> filename: Kbuild
 		obj-m  := 8123.o
 		8123-y := 8123_if.o 8123_pci.o

 		--> filename: Makefile
 		KDIR ?= /lib/modules/`uname -r`/build

 		default:
 			$(MAKE) -C $(KDIR) M=$$PWD

 	The split in example 2 is questionable due to the simplicity of
 	each file; however, some external modules use makefiles
 	consisting of several hundred lines, and here it really pays
 	off to separate the kbuild part from the rest.

 Building Multiple Modules
 -------------------------

 	kbuild supports building multiple modules with a single build
 	file. For example, if you wanted to build two modules, foo.ko
 	and bar.ko, the kbuild lines would be::

 		obj-m := foo.o bar.o
 		foo-y := <foo_srcs>
 		bar-y := <bar_srcs>

 	It is that simple!


 Include Files
 =============

 Within the kernel, header files are kept in standard locations
 according to the following rule:

 	* If the header file only describes the internal interface of a
 	  module, then the file is placed in the same directory as the
 	  source files.
 	* If the header file describes an interface used by other parts
 	  of the kernel that are located in different directories, then
 	  the file is placed in include/linux/.

 	  NOTE:
 	      There are two notable exceptions to this rule: larger
 	      subsystems have their own directory under include/, such as
 	      include/scsi; and architecture specific headers are located
 	      under arch/$(SRCARCH)/include/.

 Kernel Includes
 ---------------

 	To include a header file located under include/linux/, simply
 	use::

 		#include <linux/module.h>

 	kbuild will add options to the compiler so the relevant directories
 	are searched.

 Single Subdirectory
 -------------------

 	External modules tend to place header files in a separate
 	include/ directory where their source is located, although this
 	is not the usual kernel style. To inform kbuild of the
 	directory, use either ccflags-y or CFLAGS_<filename>.o.

 	Using the example from section 3, if we moved 8123_if.h to a
 	subdirectory named include, the resulting kbuild file would
 	look like::

 		--> filename: Kbuild
 		obj-m := 8123.o

 		ccflags-y := -I $(src)/include
 		8123-y := 8123_if.o 8123_pci.o

 Several Subdirectories
 ----------------------

 	kbuild can handle files that are spread over several directories.
 	Consider the following example::

 		.
 		|__ src
 		|   |__ complex_main.c
 		|   |__ hal
 		|	|__ hardwareif.c
 		|	|__ include
 		|	    |__ hardwareif.h
 		|__ include
 		|__ complex.h

 	To build the module complex.ko, we then need the following
 	kbuild file::

 		--> filename: Kbuild
 		obj-m := complex.o
 		complex-y := src/complex_main.o
 		complex-y += src/hal/hardwareif.o

 		ccflags-y := -I$(src)/include
 		ccflags-y += -I$(src)/src/hal/include

 	As you can see, kbuild knows how to handle object files located
 	in other directories. The trick is to specify the directory
 	relative to the kbuild file's location. That being said, this
 	is NOT recommended practice.

 	For the header files, kbuild must be explicitly told where to
 	look. When kbuild executes, the current directory is always the
 	root of the kernel tree (the argument to "-C") and therefore an
 	absolute path is needed. $(src) provides the absolute path by
 	pointing to the directory where the currently executing kbuild
 	file is located.


 Module Installation
 ===================

 Modules which are included in the kernel are installed in the
 directory:

 	/lib/modules/$(KERNELRELEASE)/kernel/

 And external modules are installed in:

 	/lib/modules/$(KERNELRELEASE)/updates/

 INSTALL_MOD_PATH
 ----------------

 	Above are the default directories but as always some level of
 	customization is possible. A prefix can be added to the
 	installation path using the variable INSTALL_MOD_PATH::

 		$ make INSTALL_MOD_PATH=/frodo modules_install
 		=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/

 	INSTALL_MOD_PATH may be set as an ordinary shell variable or,
 	as shown above, can be specified on the command line when
 	calling "make." This has effect when installing both in-tree
 	and out-of-tree modules.

 INSTALL_MOD_DIR
 ---------------

 	External modules are by default installed to a directory under
 	/lib/modules/$(KERNELRELEASE)/updates/, but you may wish to
 	locate modules for a specific functionality in a separate
 	directory. For this purpose, use INSTALL_MOD_DIR to specify an
 	alternative name to "updates."::

 		$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
 		       M=$PWD modules_install
 		=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/


 Module Versioning
 =================

 Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
 as a simple ABI consistency check. A CRC value of the full prototype
 for an exported symbol is created. When a module is loaded/used, the
 CRC values contained in the kernel are compared with similar values in
 the module; if they are not equal, the kernel refuses to load the
 module.

 Module.symvers contains a list of all exported symbols from a kernel
 build.

 Symbols From the Kernel (vmlinux + modules)
 -------------------------------------------

 	During a kernel build, a file named Module.symvers will be
 	generated. Module.symvers contains all exported symbols from
 	the kernel and compiled modules. For each symbol, the
 	corresponding CRC value is also stored.

 	The syntax of the Module.symvers file is::

 		<CRC>       <Symbol>         <Module>                         <Export Type>     <Namespace>

 		0xe1cc2a05  usb_stor_suspend drivers/usb/storage/usb-storage  EXPORT_SYMBOL_GPL USB_STORAGE

 	The fields are separated by tabs and values may be empty (e.g.
 	if no namespace is defined for an exported symbol).

 	For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
 	would read 0x00000000.

 	Module.symvers serves two purposes:

 	1) It lists all exported symbols from vmlinux and all modules.
 	2) It lists the CRC if CONFIG_MODVERSIONS is enabled.

 Symbols and External Modules
 ----------------------------

 	When building an external module, the build system needs access
 	to the symbols from the kernel to check if all external symbols
 	are defined. This is done in the MODPOST step. modpost obtains
 	the symbols by reading Module.symvers from the kernel source
 	tree. During the MODPOST step, a new Module.symvers file will be
 	written containing all exported symbols from that external module.

 Symbols From Another External Module
 ------------------------------------

 	Sometimes, an external module uses exported symbols from
 	another external module. Kbuild needs to have full knowledge of
 	all symbols to avoid spitting out warnings about undefined
 	symbols. Two solutions exist for this situation.

 	NOTE: The method with a top-level kbuild file is recommended
 	but may be impractical in certain situations.

 	Use a top-level kbuild file
 		If you have two modules, foo.ko and bar.ko, where
 		foo.ko needs symbols from bar.ko, you can use a
 		common top-level kbuild file so both modules are
 		compiled in the same build. Consider the following
 		directory layout::

 			./foo/ <= contains foo.ko
 			./bar/ <= contains bar.ko

 		The top-level kbuild file would then look like::

 			#./Kbuild (or ./Makefile):
 				obj-m := foo/ bar/

 		And executing::

 			$ make -C $KDIR M=$PWD

 		will then do the expected and compile both modules with
 		full knowledge of symbols from either module.

 	Use "make" variable KBUILD_EXTRA_SYMBOLS
 		If it is impractical to add a top-level kbuild file,
 		you can assign a space separated list
 		of files to KBUILD_EXTRA_SYMBOLS in your build file.
 		These files will be loaded by modpost during the
 		initialization of its symbol tables.


 Tips & Tricks
 =============

 Testing for CONFIG_FOO_BAR
 --------------------------

 	Modules often need to check for certain `CONFIG_` options to
 	decide if a specific feature is included in the module. In
 	kbuild this is done by referencing the `CONFIG_` variable
 	directly::

 		#fs/ext2/Makefile
 		obj-$(CONFIG_EXT2_FS) += ext2.o

 		ext2-y := balloc.o bitmap.o dir.o
 		ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
	=========================
	Building External Modules
	=========================

	This document describes how to build an out-of-tree kernel module.

	Introduction
	============

	"kbuild" is the build system used by the Linux kernel. Modules must use
	kbuild to stay compatible with changes in the build infrastructure and
	to pick up the right flags to the compiler. Functionality for building modules
	both in-tree and out-of-tree is provided. The method for building
	either is similar, and all modules are initially developed and built
	out-of-tree.

	Covered in this document is information aimed at developers interested
	in building out-of-tree (or "external") modules. The author of an
	external module should supply a makefile that hides most of the
	complexity, so one only has to type "make" to build the module. This is
	easily accomplished, and a complete example will be presented in
	section `Creating a Kbuild File for an External Module`_.


	How to Build External Modules
	=============================

	To build external modules, you must have a prebuilt kernel available
	that contains the configuration and header files used in the build.
	Also, the kernel must have been built with modules enabled. If you are
	using a distribution kernel, there will be a package for the kernel you
	are running provided by your distribution.

	An alternative is to use the "make" target "modules_prepare." This will
	make sure the kernel contains the information required. The target
	exists solely as a simple way to prepare a kernel source tree for
	building external modules.

	NOTE: "modules_prepare" will not build Module.symvers even if
	CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
	executed to make module versioning work.

	Command Syntax
	--------------

	The command to build an external module is::

	$ make -C <path_to_kernel_dir> M=$PWD

	The kbuild system knows that an external module is being built
	due to the "M=<dir>" option given in the command.

	To build against the running kernel use::

	$ make -C /lib/modules/`uname -r`/build M=$PWD

	Then to install the module(s) just built, add the target
	"modules_install" to the command::

	$ make -C /lib/modules/`uname -r`/build M=$PWD modules_install

	Options
	-------

	($KDIR refers to the path of the kernel source directory, or the path
	of the kernel output directory if the kernel was built in a separate
	build directory.)

	make -C $KDIR M=$PWD

	-C $KDIR
	The directory that contains the kernel and relevant build
	artifacts used for building an external module.
	"make" will actually change to the specified directory
	when executing and will change back when finished.

	M=$PWD
	Informs kbuild that an external module is being built.
	The value given to "M" is the absolute path of the
	directory where the external module (kbuild file) is
	located.

	Targets
	-------

	When building an external module, only a subset of the "make"
	targets are available.

	make -C $KDIR M=$PWD [target]

	The default will build the module(s) located in the current
	directory, so a target does not need to be specified. All
	output files will also be generated in this directory. No
	attempts are made to update the kernel source, and it is a
	precondition that a successful "make" has been executed for the
	kernel.

	modules
	The default target for external modules. It has the
	same functionality as if no target was specified. See
	description above.

	modules_install
	Install the external module(s). The default location is
	/lib/modules/<kernel_release>/updates/, but a prefix may
	be added with INSTALL_MOD_PATH (discussed in section
	`Module Installation`_).

	clean
	Remove all generated files in the module directory only.

	help
	List the available targets for external modules.

	Building Separate Files
	-----------------------

	It is possible to build single files that are part of a module.
	This works equally well for the kernel, a module, and even for
	external modules.

	Example (The module foo.ko, consist of bar.o and baz.o)::

	make -C $KDIR M=$PWD bar.lst
	make -C $KDIR M=$PWD baz.o
	make -C $KDIR M=$PWD foo.ko
	make -C $KDIR M=$PWD ./


	Creating a Kbuild File for an External Module
	=============================================

	In the last section we saw the command to build a module for the
	running kernel. The module is not actually built, however, because a
	build file is required. Contained in this file will be the name of
	the module(s) being built, along with the list of requisite source
	files. The file may be as simple as a single line::

	obj-m := <module_name>.o

	The kbuild system will build <module_name>.o from <module_name>.c,
	and, after linking, will result in the kernel module <module_name>.ko.
	The above line can be put in either a "Kbuild" file or a "Makefile."
	When the module is built from multiple sources, an additional line is
	needed listing the files::

	<module_name>-y := <src1>.o <src2>.o ...

	NOTE: Further documentation describing the syntax used by kbuild is
	located in Documentation/kbuild/makefiles.rst.

	The examples below demonstrate how to create a build file for the
	module 8123.ko, which is built from the following files::

	8123_if.c
	8123_if.h
	8123_pci.c

	Shared Makefile
	---------------

	An external module always includes a wrapper makefile that
	supports building the module using "make" with no arguments.
	This target is not used by kbuild; it is only for convenience.
	Additional functionality, such as test targets, can be included
	but should be filtered out from kbuild due to possible name
	clashes.

	Example 1::

	--> filename: Makefile
	ifneq ($(KERNELRELEASE),)
	# kbuild part of makefile
	obj-m := 8123.o
	8123-y := 8123_if.o 8123_pci.o

	else
	# normal makefile
	KDIR ?= /lib/modules/`uname -r`/build

	default:
	$(MAKE) -C $(KDIR) M=$$PWD

	endif

	The check for KERNELRELEASE is used to separate the two parts
	of the makefile. In the example, kbuild will only see the two
	assignments, whereas "make" will see everything except these
	two assignments. This is due to two passes made on the file:
	the first pass is by the "make" instance run on the command
	line; the second pass is by the kbuild system, which is
	initiated by the parameterized "make" in the default target.

	Separate Kbuild File and Makefile
	---------------------------------

	Kbuild will first look for a file named "Kbuild", and if it is not
	found, it will then look for "Makefile". Utilizing a "Kbuild" file
	allows us to split up the "Makefile" from example 1 into two files:

	Example 2::

	--> filename: Kbuild
	obj-m := 8123.o
	8123-y := 8123_if.o 8123_pci.o

	--> filename: Makefile
	KDIR ?= /lib/modules/`uname -r`/build

	default:
	$(MAKE) -C $(KDIR) M=$$PWD

	The split in example 2 is questionable due to the simplicity of
	each file; however, some external modules use makefiles
	consisting of several hundred lines, and here it really pays
	off to separate the kbuild part from the rest.

	Building Multiple Modules
	-------------------------

	kbuild supports building multiple modules with a single build
	file. For example, if you wanted to build two modules, foo.ko
	and bar.ko, the kbuild lines would be::

	obj-m := foo.o bar.o
	foo-y := <foo_srcs>
	bar-y := <bar_srcs>

	It is that simple!


	Include Files
	=============

	Within the kernel, header files are kept in standard locations
	according to the following rule:

	* If the header file only describes the internal interface of a
	module, then the file is placed in the same directory as the
	source files.
	* If the header file describes an interface used by other parts
	of the kernel that are located in different directories, then
	the file is placed in include/linux/.

	NOTE:
	There are two notable exceptions to this rule: larger
	subsystems have their own directory under include/, such as
	include/scsi; and architecture specific headers are located
	under arch/$(SRCARCH)/include/.

	Kernel Includes
	---------------

	To include a header file located under include/linux/, simply
	use::

	#include <linux/module.h>

	kbuild will add options to the compiler so the relevant directories
	are searched.

	Single Subdirectory
	-------------------

	External modules tend to place header files in a separate
	include/ directory where their source is located, although this
	is not the usual kernel style. To inform kbuild of the
	directory, use either ccflags-y or CFLAGS_<filename>.o.

	Using the example from section 3, if we moved 8123_if.h to a
	subdirectory named include, the resulting kbuild file would
	look like::

	--> filename: Kbuild
	obj-m := 8123.o

	ccflags-y := -I $(src)/include
	8123-y := 8123_if.o 8123_pci.o

	Several Subdirectories
	----------------------

	kbuild can handle files that are spread over several directories.
	Consider the following example::

	.
	\|__ src
	\| \|__ complex_main.c
	\| \|__ hal
	\| \|__ hardwareif.c
	\| \|__ include
	\| \|__ hardwareif.h
	\|__ include
	\|__ complex.h

	To build the module complex.ko, we then need the following
	kbuild file::

	--> filename: Kbuild
	obj-m := complex.o
	complex-y := src/complex_main.o
	complex-y += src/hal/hardwareif.o

	ccflags-y := -I$(src)/include
	ccflags-y += -I$(src)/src/hal/include

	As you can see, kbuild knows how to handle object files located
	in other directories. The trick is to specify the directory
	relative to the kbuild file's location. That being said, this
	is NOT recommended practice.

	For the header files, kbuild must be explicitly told where to
	look. When kbuild executes, the current directory is always the
	root of the kernel tree (the argument to "-C") and therefore an
	absolute path is needed. $(src) provides the absolute path by
	pointing to the directory where the currently executing kbuild
	file is located.


	Module Installation
	===================

	Modules which are included in the kernel are installed in the
	directory:

	/lib/modules/$(KERNELRELEASE)/kernel/

	And external modules are installed in:

	/lib/modules/$(KERNELRELEASE)/updates/

	INSTALL_MOD_PATH
	----------------

	Above are the default directories but as always some level of
	customization is possible. A prefix can be added to the
	installation path using the variable INSTALL_MOD_PATH::

	$ make INSTALL_MOD_PATH=/frodo modules_install
	=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/

	INSTALL_MOD_PATH may be set as an ordinary shell variable or,
	as shown above, can be specified on the command line when
	calling "make." This has effect when installing both in-tree
	and out-of-tree modules.

	INSTALL_MOD_DIR
	---------------

	External modules are by default installed to a directory under
	/lib/modules/$(KERNELRELEASE)/updates/, but you may wish to
	locate modules for a specific functionality in a separate
	directory. For this purpose, use INSTALL_MOD_DIR to specify an
	alternative name to "updates."::

	$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
	M=$PWD modules_install
	=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/


	Module Versioning
	=================

	Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
	as a simple ABI consistency check. A CRC value of the full prototype
	for an exported symbol is created. When a module is loaded/used, the
	CRC values contained in the kernel are compared with similar values in
	the module; if they are not equal, the kernel refuses to load the
	module.

	Module.symvers contains a list of all exported symbols from a kernel
	build.

	Symbols From the Kernel (vmlinux + modules)
	-------------------------------------------

	During a kernel build, a file named Module.symvers will be
	generated. Module.symvers contains all exported symbols from
	the kernel and compiled modules. For each symbol, the
	corresponding CRC value is also stored.

	The syntax of the Module.symvers file is::

	<CRC> <Symbol> <Module> <Export Type> <Namespace>

	0xe1cc2a05 usb_stor_suspend drivers/usb/storage/usb-storage EXPORT_SYMBOL_GPL USB_STORAGE

	The fields are separated by tabs and values may be empty (e.g.
	if no namespace is defined for an exported symbol).

	For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
	would read 0x00000000.

	Module.symvers serves two purposes:

	1) It lists all exported symbols from vmlinux and all modules.
	2) It lists the CRC if CONFIG_MODVERSIONS is enabled.

	Symbols and External Modules
	----------------------------

	When building an external module, the build system needs access
	to the symbols from the kernel to check if all external symbols
	are defined. This is done in the MODPOST step. modpost obtains
	the symbols by reading Module.symvers from the kernel source
	tree. During the MODPOST step, a new Module.symvers file will be
	written containing all exported symbols from that external module.

	Symbols From Another External Module
	------------------------------------

	Sometimes, an external module uses exported symbols from
	another external module. Kbuild needs to have full knowledge of
	all symbols to avoid spitting out warnings about undefined
	symbols. Two solutions exist for this situation.

	NOTE: The method with a top-level kbuild file is recommended
	but may be impractical in certain situations.

	Use a top-level kbuild file
	If you have two modules, foo.ko and bar.ko, where
	foo.ko needs symbols from bar.ko, you can use a
	common top-level kbuild file so both modules are
	compiled in the same build. Consider the following
	directory layout::

	./foo/ <= contains foo.ko
	./bar/ <= contains bar.ko

	The top-level kbuild file would then look like::

	#./Kbuild (or ./Makefile):
	obj-m := foo/ bar/

	And executing::

	$ make -C $KDIR M=$PWD

	will then do the expected and compile both modules with
	full knowledge of symbols from either module.

	Use "make" variable KBUILD_EXTRA_SYMBOLS
	If it is impractical to add a top-level kbuild file,
	you can assign a space separated list
	of files to KBUILD_EXTRA_SYMBOLS in your build file.
	These files will be loaded by modpost during the
	initialization of its symbol tables.


	Tips & Tricks
	=============

	Testing for CONFIG_FOO_BAR
	--------------------------

	Modules often need to check for certain `CONFIG_` options to
	decide if a specific feature is included in the module. In
	kbuild this is done by referencing the `CONFIG_` variable
	directly::

	#fs/ext2/Makefile
	obj-$(CONFIG_EXT2_FS) += ext2.o

	ext2-y := balloc.o bitmap.o dir.o
	ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o