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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 Kernel Includes
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+ --- 4.2 Single Subdirectory
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+ --- 4.3 Several Subdirectories
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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
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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 prebuilt 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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+ 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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---- 2.4 Preparing the kernel tree for module build
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+ clean
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+ Remove all generated files in the module directory only.
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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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+ help
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+ List the available targets for external modules.
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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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-
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-
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-=== 3. Example commands
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-
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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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+--- 2.4 Building Separate Files
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-# Kernel source
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-/lib/modules/<kernel-version>/source -> /usr/src/linux-<version>
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-
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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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-
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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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+ 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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- 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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+ 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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-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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+=== 3. Creating a Kbuild File for an External Module
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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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+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 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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+ 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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-=== 4. Creating a kbuild file for an external module
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+ <module_name>-y := <src1>.o <src2>.o ...
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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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+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 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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+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 command
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+ 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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+
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+--- 3.2 Separate Kbuild File and Makefile
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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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+ 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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+
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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
|
|
|
|
|
|
+ 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:
|
|
@@ -283,260 +261,271 @@ 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 wanted to build two modules, foo.ko
|
|
|
+ and bar.ko, the kbuild lines would be:
|
|
|
|
|
|
-=== 5. Include files
|
|
|
+ obj-m := foo.o bar.o
|
|
|
+ foo-y := <foo_srcs>
|
|
|
+ bar-y := <bar_srcs>
|
|
|
|
|
|
-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.
|
|
|
+ It is that simple!
|
|
|
|
|
|
-- 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-$(ARCH)/*.
|
|
|
+=== 4. Include Files
|
|
|
|
|
|
-External modules have a tendency to locate include files in a separate include/
|
|
|
-directory and therefore need to deal with this in their kbuild file.
|
|
|
+Within the kernel, header files are kept in standard locations
|
|
|
+according to the following rule:
|
|
|
|
|
|
---- 5.1 How to include files from the kernel include dir
|
|
|
+ * If the header file only describes the internal interface of a
|
|
|
+ module, then the file is placed in the same directory as the
|
|
|
+ source files.
|
|
|
+ * If the header file describes an interface used by other parts
|
|
|
+ of the kernel that are located in different directories, then
|
|
|
+ the file is placed in include/linux/.
|
|
|
|
|
|
- When a module needs to include a file from include/linux/, then one
|
|
|
- just uses:
|
|
|
+ NOTE: There are two notable exceptions to this rule: larger
|
|
|
+ subsystems have their own directory under include/, such as
|
|
|
+ include/scsi; and architecture specific headers are located
|
|
|
+ under arch/$(ARCH)/include/.
|
|
|
|
|
|
- #include <linux/modules.h>
|
|
|
+--- 4.1 Kernel Includes
|
|
|
|
|
|
- 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.
|
|
|
+ To include a header file located under include/linux/, simply
|
|
|
+ use:
|
|
|
|
|
|
- #include "8123_if.h"
|
|
|
+ #include <linux/module.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
|
|
|
+ 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/$(KERNELRELEASE)/kernel
|
|
|
+ /lib/modules/$(KERNELRELEASE)/kernel/
|
|
|
|
|
|
-External modules are installed in the directory:
|
|
|
+And external modules are installed in:
|
|
|
|
|
|
- /lib/modules/$(KERNELRELEASE)/extra
|
|
|
+ /lib/modules/$(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/$(KERNELRELEASE)/kernel
|
|
|
-
|
|
|
- INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
|
|
|
- example above, can be specified on the command line when calling make.
|
|
|
- INSTALL_MOD_PATH has effect both when installing modules included in
|
|
|
- the kernel as well as when installing external modules.
|
|
|
+ => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/
|
|
|
|
|
|
---- 6.2 INSTALL_MOD_DIR
|
|
|
+ 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.
|
|
|
|
|
|
- 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'.
|
|
|
+--- 5.2 INSTALL_MOD_DIR
|
|
|
|
|
|
- $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \
|
|
|
- M=`pwd` modules_install
|
|
|
- => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
|
|
|
+ External modules are by default installed to a directory under
|
|
|
+ /lib/modules/$(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 $KDIR \
|
|
|
+ M=$PWD modules_install
|
|
|
+ => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/
|
|
|
|
|
|
-=== 7. Module versioning & Module.symvers
|
|
|
|
|
|
-Module versioning is enabled by the CONFIG_MODVERSIONS tag.
|
|
|
+=== 6. Module Versioning
|
|
|
|
|
|
-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 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.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.
|
|
|
|
|
|
---- 7.1 Symbols from the kernel (vmlinux + modules)
|
|
|
+--- 6.1 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:
|
|
|
+ <CRC> <Symbol> <module>
|
|
|
+
|
|
|
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.
|
|
|
+
|
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+--- 6.3 Symbols From Another External Module
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+
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+ Sometimes, an external module uses exported symbols from
|
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+ another external module. kbuild needs to have full knowledge of
|
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+ all symbols to avoid spitting out warnings about undefined
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|
+ symbols. Three solutions exist for this situation.
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+
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+ NOTE: The method with a top-level kbuild file is recommended
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+ but may be impractical in certain situations.
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+
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+ Use a top-level kbuild file
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+ If you have two modules, foo.ko and bar.ko, where
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|
+ foo.ko needs symbols from bar.ko, you can use a
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|
+ common top-level kbuild file so both modules are
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|
+ compiled in the same build. Consider the following
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+ directory layout:
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+
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+ ./foo/ <= contains foo.ko
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+ ./bar/ <= contains bar.ko
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+
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+ The top-level kbuild file would then look like:
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+
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+ #./Kbuild (or ./Makefile):
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obj-y := foo/ bar/
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|
|
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- Executing:
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- make -C $KDIR M=`pwd`
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+ And executing
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+
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+ $ make -C $KDIR M=$PWD
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|
|
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- will then do the expected and compile both modules with full
|
|
|
- knowledge on symbols from both modules.
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|
+ 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
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|
- 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.
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|
|
-
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|
|
- 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
|
|
|
-
|
|
|
---- 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.
|
|
|
+ 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.
|
|
|
+
|
|
|
+
|
|
|
+=== 7. Tips & Tricks
|
|
|
+
|
|
|
+--- 7.1 Testing for CONFIG_FOO_BAR
|
|
|
+
|
|
|
+ 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-$(CONFIG_EXT2_FS) += ext2.o
|
|
@@ -544,9 +533,9 @@ Module.symvers contains a list of all exported symbols from a kernel build.
|
|
|
ext2-y := balloc.o bitmap.o dir.o
|
|
|
ext2-$(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.
|
|
|
|