Documentation/kbuild: major edit of modules.txt sections 1-4
Omit needless words and sentences; reorganize and tighten sentence structure; swap sections 2.2 and 2.3 for a more logical flow; remove section 3, therefore shifting 4->3; add to explanations; and add section on building multiple modules. Signed-off-by: matt mooney <mfm@muteddisk.com> Signed-off-by: Michal Marek <mmarek@suse.cz>
This commit is contained in:
matt mooney
Michal Marek
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@ -1,215 +1,185 @@
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Building External Modules
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In this document you will find information about:
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- how to build external modules
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- how to make your module use the kbuild infrastructure
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- how kbuild will install a kernel
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- how to install modules in a non-standard location
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This document describes how-to build an out-of-tree kernel module.
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=== Table of Contents
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=== 1 Introduction
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=== 2 How to build external modules
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--- 2.1 Building external modules
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--- 2.2 Available targets
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--- 2.3 Available options
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--- 2.4 Preparing the kernel tree for module build
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--- 2.5 Building separate files for a module
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=== 3. Example commands
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=== 4. Creating a kbuild file for an external module
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=== 5. Include files
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--- 5.1 How to include files from the kernel include dir
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--- 5.2 External modules using an include/ dir
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--- 5.3 External modules using several directories
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=== 6. Module installation
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--- 6.1 INSTALL_MOD_PATH
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--- 6.2 INSTALL_MOD_DIR
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=== 7. Module versioning & Module.symvers
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--- 7.1 Symbols from the kernel (vmlinux + modules)
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--- 7.2 Symbols and external modules
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--- 7.3 Symbols from another external module
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=== 8. Tips & Tricks
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--- 8.1 Testing for CONFIG_FOO_BAR
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=== 2 How-to Build External Modules
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--- 2.1 Command Syntax
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--- 2.2 Options
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--- 2.3 Targets
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--- 2.4 Building Separate Files
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=== 3. Creating a Kbuild File for an External Module
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--- 3.1 Shared Makefile
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--- 3.2 Separate Kbuild file and Makefile
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--- 3.3 Binary Blobs
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--- 3.4 Building Multiple Modules
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=== 4. Include files
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--- 4.1 How to include files from the kernel include dir
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--- 4.2 External modules using an include/ dir
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--- 4.3 External modules using several directories
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=== 5. Module installation
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--- 5.1 INSTALL_MOD_PATH
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--- 5.2 INSTALL_MOD_DIR
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=== 6. Module versioning & Module.symvers
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--- 6.1 Symbols from the kernel (vmlinux + modules)
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--- 6.2 Symbols and external modules
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--- 6.3 Symbols from another external module
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=== 7. Tips & Tricks
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--- 7.1 Testing for CONFIG_FOO_BAR
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=== 1. Introduction
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kbuild includes functionality for building modules both
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within the kernel source tree and outside the kernel source tree.
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The latter is usually referred to as external or "out-of-tree"
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modules and is used both during development and for modules that
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are not planned to be included in the kernel tree.
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"kbuild" is the build system used by the Linux kernel. Modules must use
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kbuild to stay compatible with changes in the build infrastructure and
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to pick up the right flags to "gcc." Functionality for building modules
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both in-tree and out-of-tree is provided. The method for building
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either is similar, and all modules are initially developed and built
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out-of-tree.
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What is covered within this file is mainly information to authors
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of modules. The author of an external module should supply
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a makefile that hides most of the complexity, so one only has to type
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'make' to build the module. A complete example will be presented in
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chapter 4, "Creating a kbuild file for an external module".
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Covered in this document is information aimed at developers interested
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in building out-of-tree (or "external") modules. The author of an
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external module should supply a makefile that hides most of the
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complexity, so one only has to type "make" to build the module. This is
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easily accomplished, and a complete example will be presented in
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section 3.
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=== 2. How to build external modules
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=== 2. How-to Build External Modules
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kbuild offers functionality to build external modules, with the
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prerequisite that there is a pre-built kernel available with full source.
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A subset of the targets available when building the kernel is available
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when building an external module.
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To build external modules, you must have a pre-built kernel available
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that contains the configuration and header files used in the build.
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Also, the kernel must have been built with modules enabled. If you are
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using a distribution kernel, there will be a package for the kernel you
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are running provided by your distribution.
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--- 2.1 Building external modules
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An alternative is to use the "make" target "modules_prepare." This will
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make sure the kernel contains the information required. The target
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exists solely as a simple way to prepare a kernel source tree for
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building external modules.
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Use the following command to build an external module:
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NOTE: "modules_prepare" will not build Module.symvers even if
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CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
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executed to make module versioning work.
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make -C <path-to-kernel> M=`pwd`
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--- 2.1 Command Syntax
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For the running kernel use:
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The command to build an external module is:
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make -C /lib/modules/`uname -r`/build M=`pwd`
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make -C <path_to_kernel_src> M=$PWD
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For the above command to succeed, the kernel must have been
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built with modules enabled.
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The kbuild system knows that an external module is being built
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due to the "M=<dir>" option given in the command.
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To install the modules that were just built:
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To build against the running kernel use:
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make -C <path-to-kernel> M=`pwd` modules_install
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make -C /lib/modules/`uname -r`/build M=$PWD
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More complex examples will be shown later, the above should
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be enough to get you started.
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Then to install the module(s) just built, add the target
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"modules_install" to the command:
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--- 2.2 Available targets
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make -C /lib/modules/`uname -r`/build M=$PWD modules_install
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$KDIR refers to the path to the kernel source top-level directory
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--- 2.2 Options
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make -C $KDIR M=`pwd`
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Will build the module(s) located in current directory.
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All output files will be located in the same directory
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as the module source.
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No attempts are made to update the kernel source, and it is
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a precondition that a successful make has been executed
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for the kernel.
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($KDIR refers to the path of the kernel source directory.)
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make -C $KDIR M=`pwd` modules
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The modules target is implied when no target is given.
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Same functionality as if no target was specified.
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See description above.
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make -C $KDIR M=$PWD
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make -C $KDIR M=`pwd` modules_install
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Install the external module(s).
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Installation default is in /lib/modules/<kernel-version>/extra,
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but may be prefixed with INSTALL_MOD_PATH - see separate
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chapter.
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-C $KDIR
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The directory where the kernel source is located.
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"make" will actually change to the specified directory
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when executing and will change back when finished.
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make -C $KDIR M=`pwd` clean
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Remove all generated files for the module - the kernel
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source directory is not modified.
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M=$PWD
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Informs kbuild that an external module is being built.
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The value given to "M" is the absolute path of the
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directory where the external module (kbuild file) is
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located.
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make -C $KDIR M=`pwd` help
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help will list the available target when building external
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modules.
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--- 2.3 Targets
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--- 2.3 Available options:
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When building an external module, only a subset of the "make"
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targets are available.
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$KDIR refers to the path to the kernel source top-level directory
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make -C $KDIR M=$PWD [target]
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make -C $KDIR
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Used to specify where to find the kernel source.
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'$KDIR' represent the directory where the kernel source is.
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Make will actually change directory to the specified directory
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when executed but change back when finished.
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The default will build the module(s) located in the current
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directory, so a target does not need to be specified. All
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output files will also be generated in this directory. No
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attempts are made to update the kernel source, and it is a
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precondition that a successful "make" has been executed for the
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kernel.
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make -C $KDIR M=`pwd`
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M= is used to tell kbuild that an external module is
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being built.
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The option given to M= is the directory where the external
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module (kbuild file) is located.
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When an external module is being built only a subset of the
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usual targets are available.
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modules
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The default target for external modules. It has the
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same functionality as if no target was specified. See
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description above.
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make -C $KDIR SUBDIRS=`pwd`
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Same as M=. The SUBDIRS= syntax is kept for backwards
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compatibility.
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--- 2.4 Preparing the kernel tree for module build
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To make sure the kernel contains the information required to
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build external modules the target 'modules_prepare' must be used.
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'modules_prepare' exists solely as a simple way to prepare
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a kernel source tree for building external modules.
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Note: modules_prepare will not build Module.symvers even if
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CONFIG_MODVERSIONS is set. Therefore a full kernel build
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needs to be executed to make module versioning work.
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--- 2.5 Building separate files for a module
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It is possible to build single files which are part of a module.
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This works equally well for the kernel, a module and even for
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external modules.
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Examples (module foo.ko, consist of bar.o, baz.o):
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make -C $KDIR M=`pwd` bar.lst
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make -C $KDIR M=`pwd` bar.o
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make -C $KDIR M=`pwd` foo.ko
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make -C $KDIR M=`pwd` /
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=== 3. Example commands
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This example shows the actual commands to be executed when building
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an external module for the currently running kernel.
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In the example below, the distribution is supposed to use the
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facility to locate output files for a kernel compile in a different
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directory than the kernel source - but the examples will also work
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when the source and the output files are mixed in the same directory.
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# Kernel source
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/lib/modules/<kernel-version>/source -> /usr/src/linux-<version>
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# Output from kernel compile
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/lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up
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Change to the directory where the kbuild file is located and execute
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the following commands to build the module:
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cd /home/user/src/module
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make -C /usr/src/`uname -r`/source \
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O=/lib/modules/`uname-r`/build \
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M=`pwd`
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Then, to install the module use the following command:
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make -C /usr/src/`uname -r`/source \
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O=/lib/modules/`uname-r`/build \
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M=`pwd` \
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modules_install
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Install the external module(s). The default location is
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/lib/modules/<kernel_release>/extra, but a prefix may
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be added with INSTALL_MOD_PATH (discussed in section 5).
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If you look closely you will see that this is the same command as
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listed before - with the directories spelled out.
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clean
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Remove all generated files in the module directory only.
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The above are rather long commands, and the following chapter
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lists a few tricks to make it all easier.
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help
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List the available targets for external modules.
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--- 2.4 Building Separate Files
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It is possible to build single files that are part of a module.
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This works equally well for the kernel, a module, and even for
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external modules.
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Example (The module foo.ko, consist of bar.o and baz.o):
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make -C $KDIR M=$PWD bar.lst
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make -C $KDIR M=$PWD baz.o
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make -C $KDIR M=$PWD foo.ko
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make -C $KDIR M=$PWD /
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=== 4. Creating a kbuild file for an external module
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=== 3. Creating a Kbuild File for an External Module
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kbuild is the build system for the kernel, and external modules
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must use kbuild to stay compatible with changes in the build system
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and to pick up the right flags to gcc etc.
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In the last section we saw the command to build a module for the
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running kernel. The module is not actually built, however, because a
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build file is required. Contained in this file will be the name of
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the module(s) being built, along with the list of requisite source
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files. The file may be as simple as a single line:
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The kbuild file used as input shall follow the syntax described
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in Documentation/kbuild/makefiles.txt. This chapter will introduce a few
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more tricks to be used when dealing with external modules.
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obj-m := <module_name>.o
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The kbuild system will build <module_name>.o from <module_name>.c,
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and, after linking, will result in the kernel module <module_name>.ko.
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The above line can be put in either a "Kbuild" file or a "Makefile."
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When the module is built from multiple sources, an additional line is
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needed listing the files:
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<module_name>-y := <src1>.o <src2>.o ...
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NOTE: Further documentation describing the syntax used by kbuild is
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located in Documentation/kbuild/makefiles.txt.
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The examples below demonstrate how-to create a build file for the
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module 8123.ko, which is built from the following files:
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In the following a Makefile will be created for a module with the
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following files:
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8123_if.c
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8123_if.h
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8123_pci.c
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8123_bin.o_shipped <= Binary blob
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--- 4.1 Shared Makefile for module and kernel
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--- 3.1 Shared Makefile
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An external module always includes a wrapper Makefile supporting
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building the module using 'make' with no arguments.
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The Makefile provided will most likely include additional
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functionality such as test targets etc. and this part shall
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be filtered away from kbuild since it may impact kbuild if
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name clashes occurs.
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An external module always includes a wrapper makefile that
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supports building the module using "make" with no arguments.
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This target is not used by kbuild; it is only for convenience.
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Additional functionality, such as test targets, can be included
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but should be filtered out from kbuild due to possible name
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clashes.
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Example 1:
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--> filename: Makefile
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@ -219,11 +189,11 @@ following files:
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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else
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# Normal Makefile
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# normal makefile
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KDIR ?= /lib/modules/`uname -r`/build
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KERNELDIR := /lib/modules/`uname -r`/build
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all::
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$(MAKE) -C $(KERNELDIR) M=`pwd` $@
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default:
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$(MAKE) -C $(KDIR) M=$$PWD
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# Module specific targets
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genbin:
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@ -231,15 +201,20 @@ following files:
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endif
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In example 1, the check for KERNELRELEASE is used to separate
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the two parts of the Makefile. kbuild will only see the two
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assignments whereas make will see everything except the two
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kbuild assignments.
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The check for KERNELRELEASE is used to separate the two parts
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of the makefile. In the example, kbuild will only see the two
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assignments, whereas "make" will see everything except these
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two assignments. This is due to two passes made on the file:
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the first pass is by the "make" instance run on the
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command line; the second pass is by the kbuild system, which is
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initiated by the parameterized "make" in the default target.
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In recent versions of the kernel, kbuild will look for a file named
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Kbuild and as second option look for a file named Makefile.
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Utilising the Kbuild file makes us split up the Makefile in example 1
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into two files as shown in example 2:
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--- 3.2 Separate Kbuild File and Makefile
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In newer versions of the kernel, kbuild will first look for a
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file named "Kbuild", and only if that is not found, will it
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then look for a makefile. Utilizing a "Kbuild" file allows us
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to split up the makefile from example 1 into two files:
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Example 2:
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--> filename: Kbuild
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@ -247,20 +222,21 @@ following files:
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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--> filename: Makefile
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KERNELDIR := /lib/modules/`uname -r`/build
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all::
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$(MAKE) -C $(KERNELDIR) M=`pwd` $@
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KDIR ?= /lib/modules/`uname -r`/build
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default:
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$(MAKE) -C $(KDIR) M=$$PWD
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# Module specific targets
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genbin:
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echo "X" > 8123_bin.o_shipped
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The split in example 2 is questionable due to the simplicity of
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each file; however, some external modules use makefiles
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consisting of several hundred lines, and here it really pays
|
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off to separate the kbuild part from the rest.
|
||||
|
||||
In example 2, we are down to two fairly simple files and for simple
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files as used in this example the split is questionable. But some
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external modules use Makefiles of several hundred lines and here it
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really pays off to separate the kbuild part from the rest.
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Example 3 shows a backward compatible version.
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The next example shows a backward compatible version.
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Example 3:
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--> filename: Kbuild
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@ -269,13 +245,15 @@ following files:
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--> filename: Makefile
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ifneq ($(KERNELRELEASE),)
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# kbuild part of makefile
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include Kbuild
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else
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# Normal Makefile
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KERNELDIR := /lib/modules/`uname -r`/build
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all::
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$(MAKE) -C $(KERNELDIR) M=`pwd` $@
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else
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# normal makefile
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KDIR ?= /lib/modules/`uname -r`/build
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||||
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||||
default:
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$(MAKE) -C $(KDIR) M=$$PWD
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||||
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||||
# Module specific targets
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||||
genbin:
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@ -283,28 +261,41 @@ following files:
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||||
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||||
endif
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||||
|
||||
The trick here is to include the Kbuild file from Makefile, so
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||||
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.
|
||||
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||||
--- 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
|
||||
|
||||
Loading…
Reference in New Issue
Block a user