420 lines
14 KiB
Plaintext
420 lines
14 KiB
Plaintext
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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 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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=== 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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=== 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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=== 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
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=== 8. Tips & Tricks
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--- 8.1 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 modules and is used
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both during development and for modules that are not planned to be
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included in the kernel 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 modules 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 buld the module. A complete example will be present in
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chapter ¤. Creating a kbuild file for an external module".
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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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--- 2.1 Building external modules
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Use the following command to build an external module:
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make -C <path-to-kernel> M=`pwd`
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For the running kernel use:
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make -C /lib/modules/`uname -r`/build M=`pwd`
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For the above command to succeed the kernel must have been built with
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modules enabled.
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To install the modules that were just built:
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make -C <path-to-kernel> M=`pwd` modules_install
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More complex examples later, the above should get you going.
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--- 2.2 Available targets
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$KDIR refers to path to kernel source top-level directory
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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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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 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 chater.
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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 moddified.
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make -C $KDIR M=`pwd` help
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help will list the available target when building external
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modules.
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--- 2.3 Available options:
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$KDIR refer to path to kernel src
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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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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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make -C $KDIR SUBDIRS=`pwd`
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Same as M=. The SUBDIRS= syntax is kept for backwards
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compatibility.
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--- 2.4 Preparing the kernel tree for module build
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To make sure the kernel contains the information required to
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build external modules the target 'modules_prepare' must be used.
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'module_prepare' solely exists as a simple way to prepare
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a kernel for building external modules.
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Note: modules_prepare will not build Module.symvers even if
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CONFIG_MODULEVERSIONING is set.
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Therefore a full kernel build needs to be executed to make
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module versioning work.
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=== 3. Example commands
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This example shows the actual commands to be executed when building
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an external module for the currently running kernel.
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In the example below the distribution is supposed to use the
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facility to locate output files for a kernel compile in a different
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directory than the kernel source - but the examples will also work
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when the source and the output files are mixed in the same directory.
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# Kernel source
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/lib/modules/<kernel-version>/source -> /usr/src/linux-<version>
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# Output from kernel compile
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/lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up
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Change to the directory where the kbuild file is located and execute
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the following commands to build the module:
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cd /home/user/src/module
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make -C /usr/src/`uname -r`/source \
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O=/lib/modules/`uname-r`/build \
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M=`pwd`
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Then to install the module use the following command:
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make -C /usr/src/`uname -r`/source \
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O=/lib/modules/`uname-r`/build \
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M=`pwd` \
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modules_install
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If one looks closely you will see that this is the same commands as
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listed before - with the directories spelled out.
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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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=== 4. Creating a kbuild file for an external module
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kbuild is the build system for the kernel, and external modules
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must use kbuild to stay compatible with changes in the build system
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and to pick up the right flags to gcc etc.
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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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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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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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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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KERNELDIR := /lib/modules/`uname -r`/build
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all::
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$(MAKE) -C $KERNELDIR M=`pwd` $@
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# Module specific targets
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genbin:
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echo "X" > 8123_bini.o_shipped
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endif
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In example 1 the check for KERNELRELEASE is used to separate
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the two parts of the Makefile. kbuild will only see the two
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assignments whereas make will see everything except the two
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kbuild assignments.
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In recent versions of the kernel, kbuild will look for a file named
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Kbuild and as second option look for a file named Makefile.
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Utilising the Kbuild file makes us split up the Makefile in example 1
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into two files as shown in example 2:
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Example 2:
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--> filename: Kbuild
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obj-m := 8123.o
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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--> filename: Makefile
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KERNELDIR := /lib/modules/`uname -r`/build
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all::
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$(MAKE) -C $KERNELDIR M=`pwd` $@
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# Module specific targets
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genbin:
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echo "X" > 8123_bin_shipped
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In example 2 we are down to two fairly simple files and for simple
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files as used in this example the split is questionable. But some
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external modules use Makefiles of several hundred lines and here it
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really pays off to separate the kbuild part from the rest.
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Example 3 shows a backward compatible version.
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Example 3:
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--> filename: Kbuild
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obj-m := 8123.o
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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--> filename: Makefile
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ifneq ($(KERNELRELEASE),)
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include Kbuild
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else
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# Normal Makefile
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KERNELDIR := /lib/modules/`uname -r`/build
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all::
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$(MAKE) -C $KERNELDIR M=`pwd` $@
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# Module specific targets
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genbin:
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echo "X" > 8123_bin_shipped
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endif
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The trick here is to include the Kbuild file from Makefile so
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if an older version of kbuild picks up the Makefile the Kbuild
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file will be included.
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--- 4.2 Binary blobs included in a module
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Some external modules needs to include a .o as a blob. kbuild
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has support for this, but requires the blob file to be named
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<filename>_shipped. In our example the blob is named
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8123_bin.o_shipped and when the kbuild rules kick in the file
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8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file
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with the _shipped part stripped of the filename.
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This allows the 8123_bin.o filename to be used in the assignment to
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the module.
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Example 4:
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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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In example 4 there is no distinction between the ordinary .c/.h files
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and the binary file. But kbuild will pick up different rules to create
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the .o file.
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=== 5. Include files
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Include files are a necessity when a .c file uses something from another .c
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files (not strictly in the sense of .c but if good programming practice is
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used). Any module that consist of more than one .c file will have a .h file
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for one of the .c files.
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- If the .h file only describes a module internal interface then the .h file
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shall be placed in the same directory as the .c files.
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- If the .h files describe an interface used by other parts of the kernel
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located in different directories, the .h files shall be located in
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include/linux/ or other include/ directories as appropriate.
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One exception for this rule is larger subsystems that have their own directory
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under include/ such as include/scsi. Another exception is arch-specific
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.h files which are located under include/asm-$(ARCH)/*.
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External modules have a tendency to locate include files in a separate include/
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directory and therefore needs to deal with this in their kbuild file.
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--- 5.1 How to include files from the kernel include dir
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When a module needs to include a file from include/linux/ then one
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just uses:
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#include <linux/modules.h>
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kbuild will make sure to add options to gcc so the relevant
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directories are searched.
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Likewise for .h files placed in the same directory as the .c file.
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#include "8123_if.h"
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will do the job.
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--- 5.2 External modules using an include/ dir
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External modules often locate their .h files in a separate include/
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directory although this is not usual kernel style. When an external
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module uses an include/ dir then kbuild needs to be told so.
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The trick here is to use either EXTRA_CFLAGS (take effect for all .c
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files) or CFLAGS_$F.o (take effect only for a single file).
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In our example if we move 8123_if.h to a subdirectory named include/
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the resulting Kbuild file would look like:
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--> filename: Kbuild
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obj-m := 8123.o
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EXTRA_CFLAGS := -Iinclude
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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Note that in the assingment there is no space between -I and the path.
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This is a kbuild limitation and no space must be present.
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=== 6. Module installation
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Modules which are included in the kernel is installed in the directory:
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/lib/modules/$(KERNELRELEASE)/kernel
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External modules are installed in the directory:
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/lib/modules/$(KERNELRELEASE)/extra
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--- 6.1 INSTALL_MOD_PATH
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Above are the default directories, but as always some level of
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customization is possible. One can prefix the path using the variable
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INSTALL_MOD_PATH:
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$ make INSTALL_MOD_PATH=/frodo modules_install
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=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel
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INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
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example above be specified on the commandline when calling make.
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INSTALL_MOD_PATH has effect both when installing modules included in
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the kernel as well as when installing external modules.
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--- 6.2 INSTALL_MOD_DIR
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When installing external modules they are default installed in a
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directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish
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to locate modules for a specific functionality in a separate
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directory. For this purpose one can use INSTALL_MOD_DIR to specify an
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alternative name than 'extra'.
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$ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \
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M=`pwd` modules_install
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=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
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=== 7. Module versioning
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Module versioning are enabled by the CONFIG_MODVERSIONS tag.
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Module versioning is used as a simple ABI consistency check. The Module
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versioning creates a CRC value of the full prototype for an exported symbol and
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when a module is loaded/used then the CRC values contained in the kernel are
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compared with similar values in the module. If they are not equal then the
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kernel refuses to load the module.
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During a kernel build a file named Module.symvers will be generated. This
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file includes the symbol version of all symbols within the kernel. If the
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Module.symvers file is saved from the last full kernel compile one does not
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have to do a full kernel compile to build a module version's compatible module.
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=== 8. Tips & Tricks
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--- 8.1 Testing for CONFIG_FOO_BAR
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Modules often needs to check for certain CONFIG_ options to decide if
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a specific feature shall be included in the module. When kbuild is used
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this is done by referencing the CONFIG_ variable directly.
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#fs/ext2/Makefile
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obj-$(CONFIG_EXT2_FS) += ext2.o
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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
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CONFIG_ settings directly in .config. This usage is broken.
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As introduced before external modules shall use kbuild when building
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and therefore can use the same methods as in-kernel modules when testing
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for CONFIG_ definitions.
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