-
-In this document you will find information about:
-- how to build external modules
-- how to make your module use 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
- === 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
- === 6. Module installation
- --- 6.1 INSTALL_MOD_PATH
- --- 6.2 INSTALL_MOD_DIR
- === 7. Module versioning
- === 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 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 modules should supply
-a makefile that hides most of the complexity so one only has to type
-'make' to buld the module. A complete example will be present in
-chapter ยค. 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 path to 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 chater.
-
- make -C $KDIR M=$PWD clean
- Remove all generated files for the module - the kernel
- source directory is not moddified.
-
- make -C $KDIR M=`pwd` help
- help will list the available target when building external
- modules.
-
---- 2.3 Available options:
-
- $KDIR refer to path to kernel src
-
- 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.
-
-
-=== 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_bini.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` $@
-
- # Module specific targets
- genbin:
- echo "X" > 8123_bin_shipped
-
- endif
-
- The trick here is to include the Kbuild file from Makefile so
- if an older version of kbuild picks up the Makefile the Kbuild
- file will be included.
-
---- 4.2 Binary blobs included in a module
-
- Some external modules needs to include a .o as a blob. kbuild
- has support for this, but requires the blob file to be named
- <filename>_shipped. In our example the blob is named
- 8123_bin.o_shipped and when the kbuild rules kick in the file
- 8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file
- with the _shipped part stripped of the filename.
- This allows the 8123_bin.o filename to be used in the assignment to
- the module.
-
- Example 4:
- obj-m := 8123.o
- 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
-
- In example 4 there is no distinction between the ordinary .c/.h files
- and the binary file. But kbuild will pick up different rules to create
- the .o file.
-
-
-=== 5. Include files
-
-Include files are a necessity when a .c file uses something from another .c
-files (not strictly in the sense of .c but if good programming practice is
-used). Any module that consist of more than one .c file will have a .h file
-for one of the .c files.
-- If the .h file only describes a module internal interface then the .h file
- shall be placed in the same directory as the .c files.
-- If the .h files describe an interface used by other parts of the kernel
- located in different directories, the .h files shall be located in
- include/linux/ or other include/ directories as appropriate.
-
-One exception for this rule is larger subsystems that have their own directory
-under include/ such as include/scsi. Another exception is arch-specific
-.h files which are located under include/asm-$(ARCH)/*.
-
-External modules have a tendency to locate include files in a separate include/
-directory and therefore needs 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_$F.o (take effect only for a single file).
-
- In our example if we move 8123_if.h to a subdirectory named include/
- the resulting Kbuild file would look like:
-
- --> filename: Kbuild
- obj-m := 8123.o
-
- EXTRA_CFLAGS := -Iinclude
- 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
-
- Note that in the assingment there is no space between -I and the path.
- This is a kbuild limitation and no space must be present.
-
-
-=== 6. Module installation
-
-Modules which are included in the kernel is installed in the directory:
-
- /lib/modules/$(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/$(KERNELRELEASE)/kernel
-
- INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
- example above be specified on the commandline when calling make.
- INSTALL_MOD_PATH has effect both when installing modules included in
- the kernel as well as when installing external modules.
-
---- 6.2 INSTALL_MOD_DIR
-
- When installing external modules they are default installed in a
- directory under /lib/modules/$(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 than 'extra'.
-
- $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \
- M=`pwd` modules_install
- => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
-
-
-=== 7. Module versioning
-
-Module versioning are enabled by the CONFIG_MODVERSIONS tag.
-
-Module versioning is used as a simple ABI consistency check. The Module
-versioning creates a CRC value of the full prototype for an exported symbol and
-when a module is loaded/used then the CRC values contained in the kernel are
-compared with similar values in the module. If they are not equal then the
-kernel refuses to load the module.
-
-During a kernel build a file named Module.symvers will be generated. This
-file includes the symbol version of all symbols within the kernel. If the
-Module.symvers file is saved from the last full kernel compile one does not
-have to do a full kernel compile to build a module version's compatible module.
-
-=== 8. Tips & Tricks
-
---- 8.1 Testing for CONFIG_FOO_BAR
-
- Modules often needs to check for certain CONFIG_ options to decide if
- a specific feature shall be included in the module. When kbuild is used
- this is done by referencing the CONFIG_ variable directly.
-
- #fs/ext2/Makefile
- obj-$(CONFIG_EXT2_FS) += ext2.o
-
- ext2-y := balloc.o bitmap.o dir.o
- ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
-
- External modules have traditionally used grep to check for specific
- CONFIG_ settings directly in .config. This usage is broken.
- As introduced before external modules shall use kbuild when building
- and therefore can use the same methods as in-kernel modules when testing
- for CONFIG_ definitions.
-
+For now this is a raw copy from the old Documentation/kbuild/modules.txt,
+which was removed in 2.6.0-test5.
+The information herein is correct but not complete.
+
+Installing modules in a non-standard location
+---------------------------------------------
+When the modules needs to be installed under another directory
+the INSTALL_MOD_PATH can be used to prefix "/lib/modules" as seen
+in the following example:
+
+make INSTALL_MOD_PATH=/frodo modules_install
+
+This will install the modules in the directory /frodo/lib/modules.
+/frodo can be a NFS mounted filesystem on another machine, allowing
+out-of-the-box support for installation on remote machines.
+
+
+Compiling modules outside the official kernel
+---------------------------------------------
+
+Often modules are developed outside the official kernel. To keep up
+with changes in the build system the most portable way to compile a
+module outside the kernel is to use the kernel build system,
+kbuild. Use the following command-line:
+
+make -C path/to/kernel/src M=$PWD modules
+
+This requires that a makefile exits made in accordance to
+Documentation/kbuild/makefiles.txt. Read that file for more details on
+the build system.
+
+The following is a short summary of how to write your Makefile to get
+you up and running fast. Assuming your module will be called
+yourmodule.ko, your code should be in yourmodule.c and your Makefile
+should include
+
+obj-m := yourmodule.o
+
+If the code for your module is in multiple files that need to be
+linked, you need to tell the build system which files to compile. In
+the case of multiple files, none of these files can be named
+yourmodule.c because doing so would cause a problem with the linking
+step. Assuming your code exists in file1.c, file2.c, and file3.c and
+you want to build yourmodule.ko from them, your Makefile should
+include
+
+obj-m := yourmodule.o
+yourmodule-objs := file1.o file2.o file3.o
+
+Now for a final example to put it all together. Assuming the
+KERNEL_SOURCE environment variable is set to the directory where you
+compiled the kernel, a simple Makefile that builds yourmodule.ko as
+described above would look like
+
+# Tells the build system to build yourmodule.ko.
+obj-m := yourmodule.o
+
+# Tells the build system to build these object files and link them as
+# yourmodule.o, before building yourmodule.ko. This line can be left
+# out if all the code for your module is in one file, yourmodule.c. If
+# you are using multiple files, none of these files can be named
+# yourmodule.c.
+yourmodule-objs := file1.o file2.o file3.o
+
+# Invokes the kernel build system to come back to the current
+# directory and build yourmodule.ko.
+default:
+ make -C ${KERNEL_SOURCE} M=`pwd` modules
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