Documentation/kbuild/modules.txt - maze/linux - Git at Google


 In this document you will find information about:
 - how to build external modules
 - how to make your module use the kbuild infrastructure
 - how kbuild will install a kernel
 - how to install modules in a non-standard location

 === Table of Contents

 	=== 1 Introduction
 	=== 2 How to build external modules
 	   --- 2.1 Building external modules
 	   --- 2.2 Available targets
 	   --- 2.3 Available options
 	   --- 2.4 Preparing the kernel tree for module build
 	   --- 2.5 Building separate files for a module
 	=== 3. Example commands
 	=== 4. Creating a kbuild file for an external module
 	=== 5. Include files
 	   --- 5.1 How to include files from the kernel include dir
 	   --- 5.2 External modules using an include/ dir
 	   --- 5.3 External modules using several directories
 	=== 6. Module installation
 	   --- 6.1 INSTALL_MOD_PATH
 	   --- 6.2 INSTALL_MOD_DIR
 	=== 7. Module versioning & Module.symvers
 	   --- 7.1 Symbols fron the kernel (vmlinux + modules)
 	   --- 7.2 Symbols and external modules
 	   --- 7.3 Symbols from another external module
 	=== 8. Tips & Tricks
 	   --- 8.1 Testing for CONFIG_FOO_BAR


 === 1. Introduction

 kbuild includes functionality for building modules both
 within the kernel source tree and outside the kernel source tree.
 The latter is usually referred to as external or "out-of-tree"
 modules and is used both during development and for modules that
 are not planned to be included in the kernel tree.

 What is covered within this file is mainly information to authors
 of 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. A complete example will be present in
 chapter 4, "Creating a kbuild file for an external module".


 === 2. How to build external modules

 kbuild offers functionality to build external modules, with the
 prerequisite that there is a pre-built kernel available with full source.
 A subset of the targets available when building the kernel is available
 when building an external module.

 --- 2.1 Building external modules

 	Use the following command to build an external module:

 		make -C <path-to-kernel> M=`pwd`

 	For the running kernel use:
 		make -C /lib/modules/`uname -r`/build M=`pwd`

 	For the above command to succeed the kernel must have been built with
 	modules enabled.

 	To install the modules that were just built:

 		make -C <path-to-kernel> M=`pwd` modules_install

 	More complex examples later, the above should get you going.

 --- 2.2 Available targets

 	$KDIR refers to the path to the kernel source top-level directory

 	make -C $KDIR M=`pwd`
 		Will build the module(s) located in current directory.
 		All output files will be located in the same directory
 		as the module source.
 		No attempts are made to update the kernel source, and it is
 		a precondition that a successful make has been executed
 		for the kernel.

 	make -C $KDIR M=`pwd` modules
 		The modules target is implied when no target is given.
 		Same functionality as if no target was specified.
 		See description above.

 	make -C $KDIR M=$PWD modules_install
 		Install the external module(s).
 		Installation default is in /lib/modules/<kernel-version>/extra,
 		but may be prefixed with INSTALL_MOD_PATH - see separate
 		chapter.

 	make -C $KDIR M=$PWD clean
 		Remove all generated files for the module - the kernel
 		source directory is not modified.

 	make -C $KDIR M=`pwd` help
 		help will list the available target when building external
 		modules.

 --- 2.3 Available options:

 	$KDIR refers to the path to the kernel source top-level directory

 	make -C $KDIR
 		Used to specify where to find the kernel source.
 		'$KDIR' represent the directory where the kernel source is.
 		Make will actually change directory to the specified directory
 		when executed but change back when finished.

 	make -C $KDIR M=`pwd`
 		M= is used to tell kbuild that an external module is
 		being built.
 		The option given to M= is the directory where the external
 		module (kbuild file) is located.
 		When an external module is being built only a subset of the
 		usual targets are available.

 	make -C $KDIR SUBDIRS=`pwd`
 		Same as M=. The SUBDIRS= syntax is kept for backwards
 		compatibility.

 --- 2.4 Preparing the kernel tree for module build

 	To make sure the kernel contains the information required to
 	build external modules the target 'modules_prepare' must be used.
 	'module_prepare' solely exists as a simple way to prepare
 	a kernel for building external modules.
 	Note: modules_prepare will not build Module.symvers even if
 	      CONFIG_MODULEVERSIONING is set.
 	      Therefore a full kernel build needs to be executed to make
 	      module versioning work.

 --- 2.5 Building separate files for a module
 	It is possible to build single files which are part of a module.
 	This works equal for the kernel, a module and even for external
 	modules.
 	Examples (module foo.ko, consist of bar.o, baz.o):
 		make -C $KDIR M=`pwd` bar.lst
 		make -C $KDIR M=`pwd` bar.o
 		make -C $KDIR M=`pwd` foo.ko
 		make -C $KDIR M=`pwd` /


 === 3. Example commands

 This example shows the actual commands to be executed when building
 an external module for the currently running kernel.
 In the example below, the distribution is supposed to use the
 facility to locate output files for a kernel compile in a different
 directory than the kernel source - but the examples will also work
 when the source and the output files are mixed in the same directory.

 # Kernel source
 /lib/modules/<kernel-version>/source -> /usr/src/linux-<version>

 # Output from kernel compile
 /lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up

 Change to the directory where the kbuild file is located and execute
 the following commands to build the module:

 	cd /home/user/src/module
 	make -C /usr/src/`uname -r`/source            \
 	        O=/lib/modules/`uname-r`/build        \
 	        M=`pwd`

 Then, to install the module use the following command:

 	make -C /usr/src/`uname -r`/source            \
 	        O=/lib/modules/`uname-r`/build        \
 	        M=`pwd`                               \
 		modules_install

 If one looks closely you will see that this is the same commands as
 listed before - with the directories spelled out.

 The above are rather long commands, and the following chapter
 lists a few tricks to make it all easier.


 === 4. Creating a kbuild file for an external module

 kbuild is the build system for the kernel, and external modules
 must use kbuild to stay compatible with changes in the build system
 and to pick up the right flags to gcc etc.

 The kbuild file used as input shall follow the syntax described
 in Documentation/kbuild/makefiles.txt. This chapter will introduce a few
 more tricks to be used when dealing with external modules.

 In the following a Makefile will be created for a module with the
 following files:
 	8123_if.c
 	8123_if.h
 	8123_pci.c
 	8123_bin.o_shipped	<= Binary blob

 --- 4.1 Shared Makefile for module and kernel

 	An external module always includes a wrapper Makefile supporting
 	building the module using 'make' with no arguments.
 	The Makefile provided will most likely include additional
 	functionality such as test targets etc. and this part shall
 	be filtered away from kbuild since it may impact kbuild if
 	name clashes occurs.

 	Example 1:
 		--> filename: Makefile
 		ifneq ($(KERNELRELEASE),)
 		# kbuild part of makefile
 		obj-m  := 8123.o
 		8123-y := 8123_if.o 8123_pci.o 8123_bin.o

 		else
 		# Normal Makefile

 		KERNELDIR := /lib/modules/`uname -r`/build
 		all::
 			$(MAKE) -C $(KERNELDIR) M=`pwd` $@

 		# Module specific targets
 		genbin:
 			echo "X" > 8123_bin.o_shipped

 		endif

 	In example 1, the check for KERNELRELEASE is used to separate
 	the two parts of the Makefile. kbuild will only see the two
 	assignments whereas make will see everything except the two
 	kbuild assignments.

 	In recent versions of the kernel, kbuild will look for a file named
 	Kbuild and as second option look for a file named Makefile.
 	Utilising the Kbuild file makes us split up the Makefile in example 1
 	into two files as shown in example 2:

 	Example 2:
 		--> filename: Kbuild
 		obj-m  := 8123.o
 		8123-y := 8123_if.o 8123_pci.o 8123_bin.o

 		--> filename: Makefile
 		KERNELDIR := /lib/modules/`uname -r`/build
 		all::
 			$(MAKE) -C $KERNELDIR M=`pwd` $@

 		# Module specific targets
 		genbin:
 			echo "X" > 8123_bin_shipped


 	In example 2, we are down to two fairly simple files and for simple
 	files as used in this example the split is questionable. But some
 	external modules use Makefiles of several hundred lines and here it
 	really pays off to separate the kbuild part from the rest.
 	Example 3 shows a backward compatible version.

 	Example 3:
 		--> filename: Kbuild
 		obj-m  := 8123.o
 		8123-y := 8123_if.o 8123_pci.o 8123_bin.o

 		--> filename: Makefile
 		ifneq ($(KERNELRELEASE),)
 		include Kbuild
 		else
 		# Normal Makefile

 		KERNELDIR := /lib/modules/`uname -r`/build
 		all::
 			$(MAKE) -C $KERNELDIR M=`pwd` $@

 		# Module specific targets
 		genbin:
 			echo "X" > 8123_bin_shipped

 		endif

 	The trick here is to include the Kbuild file from Makefile, so
 	if an older version of kbuild picks up the Makefile, the Kbuild
 	file will be included.

 --- 4.2 Binary blobs included in a module

 	Some external modules needs to include a .o as a blob. kbuild
 	has support for this, but requires the blob file to be named
 	<filename>_shipped. In our example the blob is named
 	8123_bin.o_shipped and when the kbuild rules kick in the file
 	8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file
 	with the _shipped part stripped of the filename.
 	This allows the 8123_bin.o filename to be used in the assignment to
 	the module.

 	Example 4:
 		obj-m  := 8123.o
 		8123-y := 8123_if.o 8123_pci.o 8123_bin.o

 	In example 4, there is no distinction between the ordinary .c/.h files
 	and the binary file. But kbuild will pick up different rules to create
 	the .o file.


 === 5. Include files

 Include files are a necessity when a .c file uses something from other .c
 files (not strictly in the sense of C, but if good programming practice is
 used). Any module that consists of more than one .c file will have a .h file
 for one of the .c files.

 - If the .h file only describes a module internal interface, then the .h file
   shall be placed in the same directory as the .c files.
 - If the .h files describe an interface used by other parts of the kernel
   located in different directories, the .h files shall be located in
   include/linux/ or other include/ directories as appropriate.

 One exception for this rule is larger subsystems that have their own directory
 under include/ such as include/scsi. Another exception is arch-specific
 .h files which are located under include/asm-$(ARCH)/*.

 External modules have a tendency to locate include files in a separate include/
 directory and therefore need to deal with this in their kbuild file.

 --- 5.1 How to include files from the kernel include dir

 	When a module needs to include a file from include/linux/, then one
 	just uses:

 		#include <linux/modules.h>

 	kbuild will make sure to add options to gcc so the relevant
 	directories are searched.
 	Likewise for .h files placed in the same directory as the .c file.

 		#include "8123_if.h"

 	will do the job.

 --- 5.2 External modules using an include/ dir

 	External modules often locate their .h files in a separate include/
 	directory although this is not usual kernel style. When an external
 	module uses an include/ dir then kbuild needs to be told so.
 	The trick here is to use either EXTRA_CFLAGS (take effect for all .c
 	files) or CFLAGS_$F.o (take effect only for a single file).

 	In our example, if we move 8123_if.h to a subdirectory named include/
 	the resulting Kbuild file would look like:

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

 		EXTRA_CFLAGS := -Iinclude
 		8123-y := 8123_if.o 8123_pci.o 8123_bin.o

 	Note that in the assignment there is no space between -I and the path.
 	This is a kbuild limitation:  there must be no space present.

 --- 5.3 External modules using several directories

 	If an external module does not follow the usual kernel style, but
 	decides to spread files over several directories, then kbuild can
 	handle this too.

 	Consider the following example:

 	|
 	+- src/complex_main.c
 	|   +- hal/hardwareif.c
 	|   +- hal/include/hardwareif.h
 	+- include/complex.h

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

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

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


 	kbuild knows how to handle .o files located in another directory -
 	although this is NOT recommended practice. The syntax is to specify
 	the directory relative to the directory where the Kbuild file is
 	located.

 	To find the .h files, we have to explicitly tell kbuild where to look
 	for the .h files. When kbuild executes, the current directory is always
 	the root of the kernel tree (argument to -C) and therefore we have to
 	tell kbuild how to find the .h files using absolute paths.
 	$(src) will specify the absolute path to the directory where the
 	Kbuild file are located when being build as an external module.
 	Therefore -I$(src)/ is used to point out the directory of the Kbuild
 	file and any additional path are just appended.

 === 6. Module installation

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

 	/lib/modules/$(KERNELRELEASE)/kernel

 External modules are installed in the directory:

 	/lib/modules/$(KERNELRELEASE)/extra

 --- 6.1 INSTALL_MOD_PATH

 	Above are the default directories, but as always, some level of
 	customization is possible. One can prefix the 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 in the
 	example above, can be specified on the command line when calling make.
 	INSTALL_MOD_PATH has effect both when installing modules included in
 	the kernel as well as when installing external modules.

 --- 6.2 INSTALL_MOD_DIR

 	When installing external modules they are by default installed to a
 	directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish
 	to locate modules for a specific functionality in a separate
 	directory. For this purpose, one can use INSTALL_MOD_DIR to specify an
 	alternative name to 'extra'.

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


 === 7. Module versioning & Module.symvers

 Module versioning is enabled by the CONFIG_MODVERSIONS tag.

 Module versioning is used as a simple ABI consistency check. The Module
 versioning creates a CRC value of the full prototype for an exported symbol and
 when a module is loaded/used then the CRC values contained in the kernel are
 compared with similar values in the module. If they are not equal, then the
 kernel refuses to load the module.

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

 --- 7.1 Symbols fron 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 symbols, the corresponding CRC value
 	is stored too.

 	The syntax of the Module.symvers file is:
 		<CRC>       <Symbol>           <module>
 	Sample:
 		0x2d036834  scsi_remove_host   drivers/scsi/scsi_mod

 	For a kernel build without CONFIG_MODVERSIONING enabled, the crc
 	would read: 0x00000000

 	Module.symvers serves two purposes:
 	1) It lists all exported symbols both from vmlinux and all modules
 	2) It lists the CRC if CONFIG_MODVERSION is enabled

 --- 7.2 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 and to obtain all
 	symbols, modpost reads Module.symvers from the kernel.
 	If a Module.symvers file is present in the directory where
 	the external module is being built, this file will be read too.
 	During the MODPOST step, a new Module.symvers file will be written
 	containing all exported symbols that were not defined in the kernel.

 --- 7.3 Symbols from another external module

 	Sometimes, an external module uses exported symbols from another
 	external module. Kbuild needs to have full knowledge on all symbols
 	to avoid spitting out warnings about undefined symbols.
 	Two solutions exist to let kbuild know all symbols of more than
 	one external module.
 	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' and 'bar', and 'foo' needs
 		symbols from 'bar', then one can use a common top-level kbuild
 		file so both modules are compiled in same build.

 		Consider following directory layout:
 		./foo/ <= contains the foo module
 		./bar/ <= contains the bar module
 		The top-level Kbuild file would then look like:

 		#./Kbuild: (this file may also be named Makefile)
 			obj-y := foo/ bar/

 		Executing:
 			make -C $KDIR M=`pwd`

 		will then do the expected and compile both modules with full
 		knowledge on symbols from both modules.

 	Use an extra Module.symvers file
 		When an external module is built, a Module.symvers file is
 		generated containing all exported symbols which are not
 		defined in the kernel.
 		To get access to symbols from module 'bar', one can copy the
 		Module.symvers file from the compilation of the 'bar' module
 		to the directory where the 'foo' module is built.
 		During the module build, kbuild will read the Module.symvers
 		file in the directory of the external module and when the
 		build is finished, a new Module.symvers file is created
 		containing the sum of all symbols defined and not part of the
 		kernel.

 === 8. Tips & Tricks

 --- 8.1 Testing for CONFIG_FOO_BAR

 	Modules often need to check for certain CONFIG_ options to decide if
 	a specific feature shall be included in the module. When kbuild is used
 	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

 	External modules have traditionally used grep to check for specific
 	CONFIG_ settings directly in .config. This usage is broken.
 	As introduced before, external modules shall use kbuild when building
 	and therefore can use the same methods as in-kernel modules when
 	testing for CONFIG_ definitions.

	In this document you will find information about:
	- how to build external modules
	- how to make your module use the kbuild infrastructure
	- how kbuild will install a kernel
	- how to install modules in a non-standard location

	=== Table of Contents

	=== 1 Introduction
	=== 2 How to build external modules
	--- 2.1 Building external modules
	--- 2.2 Available targets
	--- 2.3 Available options
	--- 2.4 Preparing the kernel tree for module build
	--- 2.5 Building separate files for a module
	=== 3. Example commands
	=== 4. Creating a kbuild file for an external module
	=== 5. Include files
	--- 5.1 How to include files from the kernel include dir
	--- 5.2 External modules using an include/ dir
	--- 5.3 External modules using several directories
	=== 6. Module installation
	--- 6.1 INSTALL_MOD_PATH
	--- 6.2 INSTALL_MOD_DIR
	=== 7. Module versioning & Module.symvers
	--- 7.1 Symbols fron the kernel (vmlinux + modules)
	--- 7.2 Symbols and external modules
	--- 7.3 Symbols from another external module
	=== 8. Tips & Tricks
	--- 8.1 Testing for CONFIG_FOO_BAR



	=== 1. Introduction

	kbuild includes functionality for building modules both
	within the kernel source tree and outside the kernel source tree.
	The latter is usually referred to as external or "out-of-tree"
	modules and is used both during development and for modules that
	are not planned to be included in the kernel tree.

	What is covered within this file is mainly information to authors
	of 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. A complete example will be present in
	chapter 4, "Creating a kbuild file for an external module".


	=== 2. How to build external modules

	kbuild offers functionality to build external modules, with the
	prerequisite that there is a pre-built kernel available with full source.
	A subset of the targets available when building the kernel is available
	when building an external module.

	--- 2.1 Building external modules

	Use the following command to build an external module:

	make -C <path-to-kernel> M=`pwd`

	For the running kernel use:
	make -C /lib/modules/`uname -r`/build M=`pwd`

	For the above command to succeed the kernel must have been built with
	modules enabled.

	To install the modules that were just built:

	make -C <path-to-kernel> M=`pwd` modules_install

	More complex examples later, the above should get you going.

	--- 2.2 Available targets

	$KDIR refers to the path to the kernel source top-level directory

	make -C $KDIR M=`pwd`
	Will build the module(s) located in current directory.
	All output files will be located in the same directory
	as the module source.
	No attempts are made to update the kernel source, and it is
	a precondition that a successful make has been executed
	for the kernel.

	make -C $KDIR M=`pwd` modules
	The modules target is implied when no target is given.
	Same functionality as if no target was specified.
	See description above.

	make -C $KDIR M=$PWD modules_install
	Install the external module(s).
	Installation default is in /lib/modules/<kernel-version>/extra,
	but may be prefixed with INSTALL_MOD_PATH - see separate
	chapter.

	make -C $KDIR M=$PWD clean
	Remove all generated files for the module - the kernel
	source directory is not modified.

	make -C $KDIR M=`pwd` help
	help will list the available target when building external
	modules.

	--- 2.3 Available options:

	$KDIR refers to the path to the kernel source top-level directory

	make -C $KDIR
	Used to specify where to find the kernel source.
	'$KDIR' represent the directory where the kernel source is.
	Make will actually change directory to the specified directory
	when executed but change back when finished.

	make -C $KDIR M=`pwd`
	M= is used to tell kbuild that an external module is
	being built.
	The option given to M= is the directory where the external
	module (kbuild file) is located.
	When an external module is being built only a subset of the
	usual targets are available.

	make -C $KDIR SUBDIRS=`pwd`
	Same as M=. The SUBDIRS= syntax is kept for backwards
	compatibility.

	--- 2.4 Preparing the kernel tree for module build

	To make sure the kernel contains the information required to
	build external modules the target 'modules_prepare' must be used.
	'module_prepare' solely exists as a simple way to prepare
	a kernel for building external modules.
	Note: modules_prepare will not build Module.symvers even if
	CONFIG_MODULEVERSIONING is set.
	Therefore a full kernel build needs to be executed to make
	module versioning work.

	--- 2.5 Building separate files for a module
	It is possible to build single files which are part of a module.
	This works equal for the kernel, a module and even for external
	modules.
	Examples (module foo.ko, consist of bar.o, baz.o):
	make -C $KDIR M=`pwd` bar.lst
	make -C $KDIR M=`pwd` bar.o
	make -C $KDIR M=`pwd` foo.ko
	make -C $KDIR M=`pwd` /


	=== 3. Example commands

	This example shows the actual commands to be executed when building
	an external module for the currently running kernel.
	In the example below, the distribution is supposed to use the
	facility to locate output files for a kernel compile in a different
	directory than the kernel source - but the examples will also work
	when the source and the output files are mixed in the same directory.

	# Kernel source
	/lib/modules/<kernel-version>/source -> /usr/src/linux-<version>

	# Output from kernel compile
	/lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up

	Change to the directory where the kbuild file is located and execute
	the following commands to build the module:

	cd /home/user/src/module
	make -C /usr/src/`uname -r`/source \
	O=/lib/modules/`uname-r`/build \
	M=`pwd`

	Then, to install the module use the following command:

	make -C /usr/src/`uname -r`/source \
	O=/lib/modules/`uname-r`/build \
	M=`pwd` \
	modules_install

	If one looks closely you will see that this is the same commands as
	listed before - with the directories spelled out.

	The above are rather long commands, and the following chapter
	lists a few tricks to make it all easier.


	=== 4. Creating a kbuild file for an external module

	kbuild is the build system for the kernel, and external modules
	must use kbuild to stay compatible with changes in the build system
	and to pick up the right flags to gcc etc.

	The kbuild file used as input shall follow the syntax described
	in Documentation/kbuild/makefiles.txt. This chapter will introduce a few
	more tricks to be used when dealing with external modules.

	In the following a Makefile will be created for a module with the
	following files:
	8123_if.c
	8123_if.h
	8123_pci.c
	8123_bin.o_shipped <= Binary blob

	--- 4.1 Shared Makefile for module and kernel

	An external module always includes a wrapper Makefile supporting
	building the module using 'make' with no arguments.
	The Makefile provided will most likely include additional
	functionality such as test targets etc. and this part shall
	be filtered away from kbuild since it may impact kbuild if
	name clashes occurs.

	Example 1:
	--> filename: Makefile
	ifneq ($(KERNELRELEASE),)
	# kbuild part of makefile
	obj-m := 8123.o
	8123-y := 8123_if.o 8123_pci.o 8123_bin.o

	else
	# Normal Makefile

	KERNELDIR := /lib/modules/`uname -r`/build
	all::
	$(MAKE) -C $(KERNELDIR) M=`pwd` $@

	# Module specific targets
	genbin:
	echo "X" > 8123_bin.o_shipped

	endif

	In example 1, the check for KERNELRELEASE is used to separate
	the two parts of the Makefile. kbuild will only see the two
	assignments whereas make will see everything except the two
	kbuild assignments.

	In recent versions of the kernel, kbuild will look for a file named
	Kbuild and as second option look for a file named Makefile.
	Utilising the Kbuild file makes us split up the Makefile in example 1
	into two files as shown in example 2:

	Example 2:
	--> filename: Kbuild
	obj-m := 8123.o
	8123-y := 8123_if.o 8123_pci.o 8123_bin.o

	--> filename: Makefile
	KERNELDIR := /lib/modules/`uname -r`/build
	all::
	$(MAKE) -C $KERNELDIR M=`pwd` $@

	# Module specific targets
	genbin:
	echo "X" > 8123_bin_shipped


	In example 2, we are down to two fairly simple files and for simple
	files as used in this example the split is questionable. But some
	external modules use Makefiles of several hundred lines and here it
	really pays off to separate the kbuild part from the rest.
	Example 3 shows a backward compatible version.

	Example 3:
	--> filename: Kbuild
	obj-m := 8123.o
	8123-y := 8123_if.o 8123_pci.o 8123_bin.o

	--> filename: Makefile
	ifneq ($(KERNELRELEASE),)
	include Kbuild
	else
	# Normal Makefile

	KERNELDIR := /lib/modules/`uname -r`/build
	all::
	$(MAKE) -C $KERNELDIR M=`pwd` $@

	# Module specific targets
	genbin:
	echo "X" > 8123_bin_shipped

	endif

	The trick here is to include the Kbuild file from Makefile, so
	if an older version of kbuild picks up the Makefile, the Kbuild
	file will be included.

	--- 4.2 Binary blobs included in a module

	Some external modules needs to include a .o as a blob. kbuild
	has support for this, but requires the blob file to be named
	<filename>_shipped. In our example the blob is named
	8123_bin.o_shipped and when the kbuild rules kick in the file
	8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file
	with the _shipped part stripped of the filename.
	This allows the 8123_bin.o filename to be used in the assignment to
	the module.

	Example 4:
	obj-m := 8123.o
	8123-y := 8123_if.o 8123_pci.o 8123_bin.o

	In example 4, there is no distinction between the ordinary .c/.h files
	and the binary file. But kbuild will pick up different rules to create
	the .o file.


	=== 5. Include files

	Include files are a necessity when a .c file uses something from other .c
	files (not strictly in the sense of C, but if good programming practice is
	used). Any module that consists of more than one .c file will have a .h file
	for one of the .c files.

	- If the .h file only describes a module internal interface, then the .h file
	shall be placed in the same directory as the .c files.
	- If the .h files describe an interface used by other parts of the kernel
	located in different directories, the .h files shall be located in
	include/linux/ or other include/ directories as appropriate.

	One exception for this rule is larger subsystems that have their own directory
	under include/ such as include/scsi. Another exception is arch-specific
	.h files which are located under include/asm-$(ARCH)/*.

	External modules have a tendency to locate include files in a separate include/
	directory and therefore need to deal with this in their kbuild file.

	--- 5.1 How to include files from the kernel include dir

	When a module needs to include a file from include/linux/, then one
	just uses:

	#include <linux/modules.h>

	kbuild will make sure to add options to gcc so the relevant
	directories are searched.
	Likewise for .h files placed in the same directory as the .c file.

	#include "8123_if.h"

	will do the job.

	--- 5.2 External modules using an include/ dir

	External modules often locate their .h files in a separate include/
	directory although this is not usual kernel style. When an external
	module uses an include/ dir then kbuild needs to be told so.
	The trick here is to use either EXTRA_CFLAGS (take effect for all .c
	files) or CFLAGS_$F.o (take effect only for a single file).

	In our example, if we move 8123_if.h to a subdirectory named include/
	the resulting Kbuild file would look like:

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

	EXTRA_CFLAGS := -Iinclude
	8123-y := 8123_if.o 8123_pci.o 8123_bin.o

	Note that in the assignment there is no space between -I and the path.
	This is a kbuild limitation: there must be no space present.

	--- 5.3 External modules using several directories

	If an external module does not follow the usual kernel style, but
	decides to spread files over several directories, then kbuild can
	handle this too.

	Consider the following example:

	\|
	+- src/complex_main.c
	\| +- hal/hardwareif.c
	\| +- hal/include/hardwareif.h
	+- include/complex.h

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

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

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


	kbuild knows how to handle .o files located in another directory -
	although this is NOT recommended practice. The syntax is to specify
	the directory relative to the directory where the Kbuild file is
	located.

	To find the .h files, we have to explicitly tell kbuild where to look
	for the .h files. When kbuild executes, the current directory is always
	the root of the kernel tree (argument to -C) and therefore we have to
	tell kbuild how to find the .h files using absolute paths.
	$(src) will specify the absolute path to the directory where the
	Kbuild file are located when being build as an external module.
	Therefore -I$(src)/ is used to point out the directory of the Kbuild
	file and any additional path are just appended.

	=== 6. Module installation

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

	/lib/modules/$(KERNELRELEASE)/kernel

	External modules are installed in the directory:

	/lib/modules/$(KERNELRELEASE)/extra

	--- 6.1 INSTALL_MOD_PATH

	Above are the default directories, but as always, some level of
	customization is possible. One can prefix the 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 in the
	example above, can be specified on the command line when calling make.
	INSTALL_MOD_PATH has effect both when installing modules included in
	the kernel as well as when installing external modules.

	--- 6.2 INSTALL_MOD_DIR

	When installing external modules they are by default installed to a
	directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish
	to locate modules for a specific functionality in a separate
	directory. For this purpose, one can use INSTALL_MOD_DIR to specify an
	alternative name to 'extra'.

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


	=== 7. Module versioning & Module.symvers

	Module versioning is enabled by the CONFIG_MODVERSIONS tag.

	Module versioning is used as a simple ABI consistency check. The Module
	versioning creates a CRC value of the full prototype for an exported symbol and
	when a module is loaded/used then the CRC values contained in the kernel are
	compared with similar values in the module. If they are not equal, then the
	kernel refuses to load the module.

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

	--- 7.1 Symbols fron 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 symbols, the corresponding CRC value
	is stored too.

	The syntax of the Module.symvers file is:
	<CRC> <Symbol> <module>
	Sample:
	0x2d036834 scsi_remove_host drivers/scsi/scsi_mod

	For a kernel build without CONFIG_MODVERSIONING enabled, the crc
	would read: 0x00000000

	Module.symvers serves two purposes:
	1) It lists all exported symbols both from vmlinux and all modules
	2) It lists the CRC if CONFIG_MODVERSION is enabled

	--- 7.2 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 and to obtain all
	symbols, modpost reads Module.symvers from the kernel.
	If a Module.symvers file is present in the directory where
	the external module is being built, this file will be read too.
	During the MODPOST step, a new Module.symvers file will be written
	containing all exported symbols that were not defined in the kernel.

	--- 7.3 Symbols from another external module

	Sometimes, an external module uses exported symbols from another
	external module. Kbuild needs to have full knowledge on all symbols
	to avoid spitting out warnings about undefined symbols.
	Two solutions exist to let kbuild know all symbols of more than
	one external module.
	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' and 'bar', and 'foo' needs
	symbols from 'bar', then one can use a common top-level kbuild
	file so both modules are compiled in same build.

	Consider following directory layout:
	./foo/ <= contains the foo module
	./bar/ <= contains the bar module
	The top-level Kbuild file would then look like:

	#./Kbuild: (this file may also be named Makefile)
	obj-y := foo/ bar/

	Executing:
	make -C $KDIR M=`pwd`

	will then do the expected and compile both modules with full
	knowledge on symbols from both modules.

	Use an extra Module.symvers file
	When an external module is built, a Module.symvers file is
	generated containing all exported symbols which are not
	defined in the kernel.
	To get access to symbols from module 'bar', one can copy the
	Module.symvers file from the compilation of the 'bar' module
	to the directory where the 'foo' module is built.
	During the module build, kbuild will read the Module.symvers
	file in the directory of the external module and when the
	build is finished, a new Module.symvers file is created
	containing the sum of all symbols defined and not part of the
	kernel.

	=== 8. Tips & Tricks

	--- 8.1 Testing for CONFIG_FOO_BAR

	Modules often need to check for certain CONFIG_ options to decide if
	a specific feature shall be included in the module. When kbuild is used
	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

	External modules have traditionally used grep to check for specific
	CONFIG_ settings directly in .config. This usage is broken.
	As introduced before, external modules shall use kbuild when building
	and therefore can use the same methods as in-kernel modules when
	testing for CONFIG_ definitions.