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Update modules.txt with info how to build external modules with files in several directories. The question popped up on lkml often enough to warrant this, let's see if people read this stuff - or google hits it. Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
460 lines
15 KiB
Plaintext
460 lines
15 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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--- 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
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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 build 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 the path to the 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 chapter.
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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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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 refers to the path to the kernel source top-level directory
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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` $@
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# Module specific targets
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genbin:
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echo "X" > 8123_bin.o_shipped
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endif
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In example 1 the check for KERNELRELEASE is used to separate
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the two parts of the Makefile. kbuild will only see the two
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assignments whereas make will see everything except the two
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kbuild assignments.
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In recent versions of the kernel, kbuild will look for a file named
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Kbuild and as second option look for a file named Makefile.
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Utilising the Kbuild file makes us split up the Makefile in example 1
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into two files as shown in example 2:
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Example 2:
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--> filename: Kbuild
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obj-m := 8123.o
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8123-y := 8123_if.o 8123_pci.o 8123_bin.o
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--> filename: Makefile
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KERNELDIR := /lib/modules/`uname -r`/build
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all::
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$(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 assignment there is no space between -I and the path.
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This is a kbuild limitation: there must be no space present.
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--- 5.3 External modules using several directories
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If an external module does not follow the usual kernel style but
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decide to spread files over several directories then kbuild can
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support this too.
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Consider the following example:
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+- src/complex_main.c
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+- include/complex.h
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To build a single module named complex.ko we then need the following
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kbuild file:
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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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EXTRA_CFLAGS := -I$(src)/include
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EXTRA_CFLAGS += -I$(src)src/hal/include
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kbuild knows how to handle .o files located in another directory -
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although this is NOT reccommended practice. The syntax is to specify
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the directory relative to the directory where the Kbuild file is
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located.
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To find the .h files we have to explicitly tell kbuild where to look
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for the .h files. When kbuild executes current directory is always
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the root of the kernel tree (argument to -C) and therefore we have to
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tell kbuild how to find the .h files using absolute paths.
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$(src) will specify the absolute path to the directory where the
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Kbuild file are located when being build as an external module.
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Therefore -I$(src)/ is used to point out the directory of the Kbuild
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file and any additional path are just appended.
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=== 6. Module installation
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Modules which are included in the kernel are installed in the directory:
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/lib/modules/$(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/$(KERNELRELEASE)/kernel
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INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
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example above be specified on the command line when calling make.
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INSTALL_MOD_PATH has effect both when installing modules included in
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the kernel as well as when installing external modules.
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--- 6.2 INSTALL_MOD_DIR
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When installing external modules they are default installed in a
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directory under /lib/modules/$(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/$(KERNELRELEASE)/gandalf
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=== 7. Module versioning
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Module versioning is enabled by the CONFIG_MODVERSIONS tag.
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Module versioning is used as a simple ABI consistency check. The Module
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versioning creates a CRC value of the full prototype for an exported symbol and
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when a module is loaded/used then the CRC values contained in the kernel are
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compared with similar values in the module. If they are not equal then the
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kernel refuses to load the module.
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During a kernel build a file named Module.symvers will be generated. This
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file includes the symbol version of all symbols within the kernel. If the
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Module.symvers file is saved from the last full kernel compile one does not
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have to do a full kernel compile to build a module version's compatible module.
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=== 8. Tips & Tricks
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--- 8.1 Testing for CONFIG_FOO_BAR
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Modules often needs to check for certain CONFIG_ options to decide if
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a specific feature shall be included in the module. When kbuild is used
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this is done by referencing the CONFIG_ variable directly.
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#fs/ext2/Makefile
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obj-$(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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