Documentation/kbuild: major edit of modules.txt sections 5-8

	   --- 3.2 Separate Kbuild file and Makefile

	   --- 3.3 Binary Blobs

	   --- 3.4 Building Multiple Modules

	=== 4. Include files

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

	   --- 4.2 External modules using an include/ dir

	   --- 4.3 External modules using several directories

	=== 5. Module installation

	=== 4. Include Files

	   --- 4.1 Kernel Includes

	   --- 4.2 Single Subdirectory

	   --- 4.3 Several Subdirectories

	=== 5. Module Installation

	   --- 5.1 INSTALL_MOD_PATH

	   --- 5.2 INSTALL_MOD_DIR

	=== 6. Module versioning & Module.symvers

	   --- 6.1 Symbols from the kernel (vmlinux + modules)

	   --- 6.2 Symbols and external modules

	   --- 6.3 Symbols from another external module

	=== 6. Module Versioning

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

	   --- 6.2 Symbols and External Modules

	   --- 6.3 Symbols From Another External Module

	=== 7. Tips & Tricks

	   --- 7.1 Testing for CONFIG_FOO_BAR

	It is that simple!

=== 5. Include files

=== 4. 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.

Within the kernel, header files are kept in standard locations

according to the following rule:

- 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.

	* 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/.

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)/*.

	  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/$(ARCH)/include/.

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.

--- 4.1 Kernel Includes

--- 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:

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

	use:

		#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.

	kbuild will add options to "gcc" so the relevant directories

	are searched.

--- 5.2 External modules using an include/ dir

--- 4.2 Single Subdirectory

	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).

	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.

	In our example, if we move 8123_if.h to a subdirectory named include/

	the resulting Kbuild file would look like:

	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

		EXTRA_CFLAGS := -Iinclude

		ccflags-y := -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.

	Note that in the assignment there is no space between -I and

	the path. This is a limitation of kbuild: 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.

--- 4.3 Several Subdirectories

	kbuild can handle files that are spread over several directories.

	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

	.

	|__ 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:

	Kbuild:

		--> filename: 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

		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.

	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.

	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.

	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

=== 5. Module Installation

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

Modules which are included in the kernel are installed in the

directory:

	/lib/modules/$(KERNELRELEASE)/kernel

External modules are installed in the directory:

And external modules are installed in:

	/lib/modules/$(KERNELRELEASE)/extra

--- 6.1 INSTALL_MOD_PATH

--- 5.1 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:

	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 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.

	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.

--- 6.2 INSTALL_MOD_DIR

--- 5.2 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'.

	External modules are by default installed to a directory under

	/lib/modules/$(KERNELRELEASE)/extra, 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 "extra."

		$ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \

			M=`pwd` modules_install

		$ make INSTALL_MOD_DIR=gandalf -C $KDIR \

		       M=$PWD modules_install

		=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf

=== 7. Module versioning & Module.symvers

=== 6. Module Versioning

Module versioning is enabled by the CONFIG_MODVERSIONS tag.

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 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.

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

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

--- 7.1 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 symbols, the corresponding CRC value

	is stored too.

	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>

	Sample:

		0x2d036834  scsi_remove_host   drivers/scsi/scsi_mod

	For a kernel build without CONFIG_MODVERSIONS enabled, the crc

	would read: 0x00000000

	For a kernel build without CONFIG_MODVERSIONS 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_MODVERSIONS 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.

	Three 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)

	1) It lists all exported symbols from vmlinux and all modules.

	2) It lists the CRC if CONFIG_MODVERSIONS is enabled.

--- 6.2 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. modpost obtains

	the symbols by reading Module.symvers from the kernel source

	tree. 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.

--- 6.3 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. Three 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, then you can use a

		common top-level kbuild file so both modules are

		compiled in the same build. Consider following

		directory layout:

		./foo/ <= contains foo.ko

		./bar/ <= contains bar.ko

		The top-level kbuild file would then look like:

		#./Kbuild (or ./Makefile):

			obj-y := foo/ bar/

		Executing:

			make -C $KDIR M=`pwd`

		And executing:

			$ make -C $KDIR M=$PWD

		will then do the expected and compile both modules with full

		knowledge on symbols from both modules.

		Will then do the expected and compile both modules with

		full knowledge of symbols from either module.

	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.

		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 bar.ko, copy the Module.symvers file from the

		compilation of bar.ko to the directory where foo.ko 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.

	Use "make" variable KBUILD_EXTRA_SYMBOLS

		If it is impractical to copy Module.symvers from

		another module, 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.

	Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile

		If it is impractical to copy Module.symvers from another

		module, you can assign a space separated list of files to

		KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be

		loaded by modpost during the initialisation of its symbol

		tables.

=== 8. Tips & Tricks

=== 7. Tips & Tricks

--- 8.1 Testing for CONFIG_FOO_BAR

--- 7.1 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.

	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

	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.

	External modules have traditionally used "grep" to check for

	specific CONFIG_ settings directly in .config. This usage is

	broken. As introduced before, external modules should use

	kbuild for building and can therefore use the same methods as

	in-tree modules when testing for CONFIG_ definitions.