@@ -1,215 +1,185 @@
+Building External Modules
-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
+This document describes how-to build an out-of-tree kernel module.
=== 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 from 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
+ === 2 How-to Build External Modules
+ --- 2.1 Command Syntax
+ --- 2.2 Options
+ --- 2.3 Targets
+ --- 2.4 Building Separate Files
+ === 3. Creating a Kbuild File for an External Module
+ --- 3.1 Shared Makefile
+ --- 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
+ --- 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
+ === 7. Tips & Tricks
+ --- 7.1 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.
+"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 "gcc." 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.
-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 presented in
-chapter 4, "Creating a kbuild file for an external module".
+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 3.
-=== 2. How to build external modules
+=== 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.
+To build external modules, you must have a pre-built 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.
---- 2.1 Building external modules
+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.
- Use the following command to build an external module:
+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.
- make -C <path-to-kernel> M=`pwd`
+--- 2.1 Command Syntax
- For the running kernel use:
+ The command to build an external module is:
- make -C /lib/modules/`uname -r`/build M=`pwd`
+ make -C <path_to_kernel_src> M=$PWD
- For the above command to succeed, the kernel must have been
- built with modules enabled.
+ The kbuild system knows that an external module is being built
+ due to the "M=<dir>" option given in the command.
- To install the modules that were just built:
+ To build against the running kernel use:
- make -C <path-to-kernel> M=`pwd` modules_install
+ make -C /lib/modules/`uname -r`/build M=$PWD
- More complex examples will be shown later, the above should
- be enough to get you started.
+ Then to install the module(s) just built, add the target
+ "modules_install" to the command:
---- 2.2 Available targets
+ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
- $KDIR refers to the path to the kernel source top-level directory
+--- 2.2 Options
- make -C $KDIR M=`pwd`
- Will build the module(s) located in current directory.
- All output files will be located in the same directory
- as the module source.
- No attempts are made to update the kernel source, and it is
- a precondition that a successful make has been executed
- for the kernel.
+ ($KDIR refers to the path of the kernel source directory.)
- 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
- 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.
+ -C $KDIR
+ The directory where the kernel source is located.
+ "make" will actually change to the specified directory
+ when executing and will change back when finished.
- make -C $KDIR M=`pwd` clean
- Remove all generated files for the module - the kernel
- source directory is not modified.
+ M=$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.
- make -C $KDIR M=`pwd` help
- help will list the available target when building external
- modules.
+--- 2.3 Targets
---- 2.3 Available options:
+ When building an external module, only a subset of the "make"
+ targets are available.
- $KDIR refers to the path to the kernel source top-level directory
+ make -C $KDIR M=$PWD [target]
- 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.
+ 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.
- 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.
+ modules
+ The default target for external modules. It has the
+ same functionality as if no target was specified. See
+ description above.
- make -C $KDIR SUBDIRS=`pwd`
- Same as M=. The SUBDIRS= syntax is kept for backwards
- compatibility.
+ modules_install
+ Install the external module(s). The default location is
+ /lib/modules/<kernel_release>/extra, but a prefix may
+ be added with INSTALL_MOD_PATH (discussed in section 5).
---- 2.4 Preparing the kernel tree for module build
+ clean
+ Remove all generated files in the module directory only.
- To make sure the kernel contains the information required to
- build external modules the target 'modules_prepare' must be used.
- 'modules_prepare' 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.
+ help
+ List the available targets for external modules.
---- 2.5 Building separate files for a module
- It is possible to build single files which are part of a module.
- This works equally well 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>
+--- 2.4 Building Separate Files
-# 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:
+ 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.
- cd /home/user/src/module
- make -C /usr/src/`uname -r`/source \
- O=/lib/modules/`uname-r`/build \
- M=`pwd`
+ 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 /
-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
+=== 3. Creating a Kbuild File for an External Module
-If you look closely you will see that this is the same command as
-listed before - with the directories spelled out.
+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:
-The above are rather long commands, and the following chapter
-lists a few tricks to make it all easier.
+ 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:
-=== 4. Creating a kbuild file for an external module
+ <module_name>-y := <src1>.o <src2>.o ...
-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.
+NOTE: Further documentation describing the syntax used by kbuild is
+located in Documentation/kbuild/makefiles.txt.
-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.
+The examples below demonstrate how-to create a build file for the
+module 8123.ko, which is built from the following files:
-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
+--- 3.1 Shared Makefile
- 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.
+ 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
@@ -219,11 +189,11 @@ following files:
8123-y := 8123_if.o 8123_pci.o 8123_bin.o
else
- # Normal Makefile
+ # normal makefile
+ KDIR ?= /lib/modules/`uname -r`/build
- KERNELDIR := /lib/modules/`uname -r`/build
- all::
- $(MAKE) -C $(KERNELDIR) M=`pwd` $@
+ default:
+ $(MAKE) -C $(KDIR) M=$$PWD
# Module specific targets
genbin:
@@ -231,15 +201,20 @@ following files:
endif
- In example 1, the check for KERNELRELEASE is used to separate
- the two parts of the Makefile. kbuild will only see the two
- assignments whereas make will see everything except the two
- kbuild assignments.
+ The check for KERNELRELEASE is used to separate the two parts
+ of the makefile. In the example, kbuild will only see the two
+ assignments, whereas "make" will see everything except these
+ two assignments. This is due to two passes made on the file:
+ the first pass is by the "make" instance run on the
+ command line; the second pass is by the kbuild system, which is
+ initiated by the parameterized "make" in the default target.
- In recent versions of the kernel, kbuild will look for a file named
- Kbuild and as second option look for a file named Makefile.
- Utilising the Kbuild file makes us split up the Makefile in example 1
- into two files as shown in example 2:
+--- 3.2 Separate Kbuild File and Makefile
+
+ In newer versions of the kernel, kbuild will first look for a
+ file named "Kbuild", and only if that is not found, will it
+ then look for a makefile. Utilizing a "Kbuild" file allows us
+ to split up the makefile from example 1 into two files:
Example 2:
--> filename: Kbuild
@@ -247,20 +222,21 @@ following files:
8123-y := 8123_if.o 8123_pci.o 8123_bin.o
--> filename: Makefile
- KERNELDIR := /lib/modules/`uname -r`/build
- all::
- $(MAKE) -C $(KERNELDIR) M=`pwd` $@
+ KDIR ?= /lib/modules/`uname -r`/build
+
+ default:
+ $(MAKE) -C $(KDIR) M=$$PWD
# Module specific targets
genbin:
echo "X" > 8123_bin.o_shipped
+ 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.
- 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.
+ The next example shows a backward compatible version.
Example 3:
--> filename: Kbuild
@@ -269,13 +245,15 @@ following files:
--> filename: Makefile
ifneq ($(KERNELRELEASE),)
+ # kbuild part of makefile
include Kbuild
+
else
- # Normal Makefile
+ # normal makefile
+ KDIR ?= /lib/modules/`uname -r`/build
- KERNELDIR := /lib/modules/`uname -r`/build
- all::
- $(MAKE) -C $(KERNELDIR) M=`pwd` $@
+ default:
+ $(MAKE) -C $(KDIR) M=$$PWD
# Module specific targets
genbin:
@@ -283,28 +261,41 @@ following files:
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.
+ Here the "Kbuild" file is included from the makefile. This
+ allows an older version of kbuild, which only knows of
+ makefiles, to be used when the "make" and kbuild parts are
+ split into separate files.
---- 4.2 Binary blobs included in a module
+--- 3.3 Binary Blobs
- 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.
+ Some external modules need to include an object file as a blob.
+ kbuild has support for this, but requires the blob file to be
+ named <filename>_shipped. When the kbuild rules kick in, a copy
+ of <filename>_shipped is created with _shipped stripped off,
+ giving us <filename>. This shortened filename can be used in
+ the assignment to the module.
+
+ Throughout this section, 8123_bin.o_shipped has been used to
+ build the kernel module 8123.ko; it has been included as
+ 8123_bin.o.
- 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.
+ Although there is no distinction between the ordinary source
+ files and the binary file, kbuild will pick up different rules
+ when creating the object file for the module.
+
+--- 3.4 Building Multiple Modules
+
+ kbuild supports building multiple modules with a single build
+ file. For example, if you want to build two modules, foo and
+ bar, the kbuild lines would be:
+
+ obj-m := foo.o bar.o
+ foo-y := <foo_srcs>
+ bar-y := <bar_srcs>
+
+ It is that simple!
=== 5. Include files