572 lines
17 KiB
ReStructuredText
572 lines
17 KiB
ReStructuredText
=========================
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Building External Modules
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=========================
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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 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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===============
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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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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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================================
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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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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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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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2.1 Command Syntax
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==================
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The command to build an external module is::
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$ make -C <path_to_kernel_src> M=$PWD
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The kbuild system knows that an external module is being built
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due to the "M=<dir>" option given in the command.
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To build against the running kernel use::
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$ make -C /lib/modules/`uname -r`/build M=$PWD
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Then to install the module(s) just built, add the target
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"modules_install" to the command::
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$ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
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2.2 Options
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===========
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($KDIR refers to the path of the kernel source directory.)
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make -C $KDIR M=$PWD
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-C $KDIR
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The directory where the kernel source is located.
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"make" will actually change to the specified directory
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when executing and will change back when finished.
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M=$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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2.3 Targets
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===========
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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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make -C $KDIR M=$PWD [target]
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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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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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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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clean
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Remove all generated files in the module directory only.
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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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===========================
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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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3. Creating a Kbuild File for an External Module
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================================================
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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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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.rst.
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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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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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--- 3.1 Shared Makefile
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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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ifneq ($(KERNELRELEASE),)
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# kbuild part of makefile
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obj-m := 8123.o
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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else
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# normal makefile
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KDIR ?= /lib/modules/`uname -r`/build
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default:
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$(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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endif
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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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3.2 Separate Kbuild File and Makefile
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-------------------------------------
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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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obj-m := 8123.o
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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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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.
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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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obj-m := 8123.o
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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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ifneq ($(KERNELRELEASE),)
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# kbuild part of makefile
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include Kbuild
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else
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# normal makefile
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KDIR ?= /lib/modules/`uname -r`/build
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default:
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$(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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endif
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Here the "Kbuild" file is included from the makefile. This
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allows an older version of kbuild, which only knows of
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makefiles, to be used when the "make" and kbuild parts are
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split into separate files.
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3.3 Binary Blobs
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----------------
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Some external modules need to include an object file as a blob.
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kbuild has support for this, but requires the blob file to be
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named <filename>_shipped. When the kbuild rules kick in, a copy
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of <filename>_shipped is created with _shipped stripped off,
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giving us <filename>. This shortened filename can be used in
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the assignment to the module.
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Throughout this section, 8123_bin.o_shipped has been used to
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build the kernel module 8123.ko; it has been included as
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8123_bin.o::
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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Although there is no distinction between the ordinary source
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files and the binary file, kbuild will pick up different rules
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when creating the object file for the module.
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3.4 Building Multiple Modules
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=============================
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kbuild supports building multiple modules with a single build
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file. For example, if you wanted to build two modules, foo.ko
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and bar.ko, the kbuild lines would be::
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obj-m := foo.o bar.o
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foo-y := <foo_srcs>
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bar-y := <bar_srcs>
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It is that simple!
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4. Include Files
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================
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Within the kernel, header files are kept in standard locations
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according to the following rule:
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* If the header file only describes the internal interface of a
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module, then the file is placed in the same directory as the
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source files.
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* If the header file describes an interface used by other parts
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of the kernel that are located in different directories, then
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the file is placed in include/linux/.
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NOTE:
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There are two notable exceptions to this rule: larger
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subsystems have their own directory under include/, such as
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include/scsi; and architecture specific headers are located
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under arch/$(ARCH)/include/.
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4.1 Kernel Includes
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-------------------
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To include a header file located under include/linux/, simply
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use::
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#include <linux/module.h>
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kbuild will add options to "gcc" so the relevant directories
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are searched.
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4.2 Single Subdirectory
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-----------------------
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External modules tend to place header files in a separate
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include/ directory where their source is located, although this
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is not the usual kernel style. To inform kbuild of the
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directory, use either ccflags-y or CFLAGS_<filename>.o.
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Using the example from section 3, if we moved 8123_if.h to a
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subdirectory named include, the resulting kbuild file would
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look like::
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--> filename: Kbuild
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obj-m := 8123.o
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ccflags-y := -Iinclude
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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Note that in the assignment there is no space between -I and
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the path. This is a limitation of kbuild: there must be no
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space present.
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4.3 Several Subdirectories
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--------------------------
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kbuild can handle files that are spread over several directories.
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Consider the following example::
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.
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|__ src
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| |__ complex_main.c
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| |__ hal
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| |__ hardwareif.c
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| |__ include
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| |__ hardwareif.h
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|__ include
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|__ complex.h
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To build the module complex.ko, we then need the following
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kbuild file::
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--> filename: Kbuild
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obj-m := complex.o
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complex-y := src/complex_main.o
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complex-y += src/hal/hardwareif.o
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ccflags-y := -I$(src)/include
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ccflags-y += -I$(src)/src/hal/include
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As you can see, kbuild knows how to handle object files located
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in other directories. The trick is to specify the directory
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relative to the kbuild file's location. That being said, this
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is NOT recommended practice.
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For the header files, kbuild must be explicitly told where to
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look. When kbuild executes, the current directory is always the
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root of the kernel tree (the argument to "-C") and therefore an
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absolute path is needed. $(src) provides the absolute path by
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pointing to the directory where the currently executing kbuild
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file is located.
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5. Module Installation
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======================
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Modules which are included in the kernel are installed in the
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directory:
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/lib/modules/$(KERNELRELEASE)/kernel/
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And external modules are installed in:
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/lib/modules/$(KERNELRELEASE)/extra/
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5.1 INSTALL_MOD_PATH
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--------------------
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Above are the default directories but as always some level of
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customization is possible. A prefix can be added to the
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installation path using the variable INSTALL_MOD_PATH::
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$ make INSTALL_MOD_PATH=/frodo modules_install
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=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/
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INSTALL_MOD_PATH may be set as an ordinary shell variable or,
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as shown above, can be specified on the command line when
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calling "make." This has effect when installing both in-tree
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and out-of-tree modules.
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5.2 INSTALL_MOD_DIR
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-------------------
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External modules are by default installed to a directory under
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/lib/modules/$(KERNELRELEASE)/extra/, but you may wish to
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locate modules for a specific functionality in a separate
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directory. For this purpose, use INSTALL_MOD_DIR to specify an
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alternative name to "extra."::
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$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
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M=$PWD modules_install
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=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/
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6. Module Versioning
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====================
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Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
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as a simple ABI consistency check. A CRC value of the full prototype
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for an exported symbol is created. When a module is loaded/used, the
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CRC values contained in the kernel are compared with similar values in
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the module; if they are not equal, the kernel refuses to load the
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module.
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Module.symvers contains a list of all exported symbols from a kernel
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build.
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6.1 Symbols From the Kernel (vmlinux + modules)
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-----------------------------------------------
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During a kernel build, a file named Module.symvers will be
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generated. Module.symvers contains all exported symbols from
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the kernel and compiled modules. For each symbol, the
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corresponding CRC value is also stored.
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The syntax of the Module.symvers file is::
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<CRC> <Symbol> <module>
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0x2d036834 scsi_remove_host drivers/scsi/scsi_mod
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For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
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would read 0x00000000.
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Module.symvers serves two purposes:
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1) It lists all exported symbols from vmlinux and all modules.
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2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
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6.2 Symbols and External Modules
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--------------------------------
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When building an external module, the build system needs access
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to the symbols from the kernel to check if all external symbols
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are defined. This is done in the MODPOST step. modpost obtains
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the symbols by reading Module.symvers from the kernel source
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tree. If a Module.symvers file is present in the directory
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where the external module is being built, this file will be
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read too. During the MODPOST step, a new Module.symvers file
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will be written containing all exported symbols that were not
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defined in the kernel.
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--- 6.3 Symbols From Another External Module
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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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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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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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./foo/ <= contains foo.ko
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./bar/ <= contains bar.ko
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The top-level kbuild file would then look like::
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#./Kbuild (or ./Makefile):
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obj-y := foo/ bar/
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And executing::
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$ make -C $KDIR M=$PWD
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will then do the expected and compile both modules with
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full knowledge of symbols from either module.
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Use an extra Module.symvers file
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When an external module is built, a Module.symvers file
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is generated containing all exported symbols which are
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not defined in the kernel. To get access to symbols
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from bar.ko, copy the Module.symvers file from the
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compilation of bar.ko to the directory where foo.ko is
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built. During the module build, kbuild will read the
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Module.symvers file in the directory of the external
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module, and when the build is finished, a new
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Module.symvers file is created containing the sum of
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all symbols defined and not part of the kernel.
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Use "make" variable KBUILD_EXTRA_SYMBOLS
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If it is impractical to copy Module.symvers from
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another module, you can assign a space separated list
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of files to KBUILD_EXTRA_SYMBOLS in your build file.
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These files will be loaded by modpost during the
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initialization of its symbol tables.
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7. Tips & Tricks
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================
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|
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7.1 Testing for CONFIG_FOO_BAR
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------------------------------
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|
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Modules often need to check for certain `CONFIG_` options to
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decide if a specific feature is included in the module. In
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kbuild this is done by referencing the `CONFIG_` variable
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|
directly::
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|
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|
#fs/ext2/Makefile
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|
obj-$(CONFIG_EXT2_FS) += ext2.o
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|
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ext2-y := balloc.o bitmap.o dir.o
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|
ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
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|
|
|
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.
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