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diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt index 0767cf6..3fb39e0 100644 --- a/Documentation/kbuild/modules.txt +++ b/Documentation/kbuild/modules.txt @@ -1,215 +1,185 @@ +Building External Modules -In this document you will find information about: -- how to build external modules -- how to make your module use the kbuild infrastructure -- how kbuild will install a kernel -- how to install modules in a non-standard location +This document describes how to build an out-of-tree kernel module. === Table of Contents === 1 Introduction - === 2 How to build external modules - --- 2.1 Building external modules - --- 2.2 Available targets - --- 2.3 Available options - --- 2.4 Preparing the kernel tree for module build - --- 2.5 Building separate files for a module - === 3. Example commands - === 4. Creating a kbuild file for an external module - === 5. Include files - --- 5.1 How to include files from the kernel include dir - --- 5.2 External modules using an include/ dir - --- 5.3 External modules using several directories - === 6. Module installation - --- 6.1 INSTALL_MOD_PATH - --- 6.2 INSTALL_MOD_DIR - === 7. Module versioning & Module.symvers - --- 7.1 Symbols from the kernel (vmlinux + modules) - --- 7.2 Symbols and external modules - --- 7.3 Symbols from another external module - === 8. Tips & Tricks - --- 8.1 Testing for CONFIG_FOO_BAR + === 2 How to Build External Modules + --- 2.1 Command Syntax + --- 2.2 Options + --- 2.3 Targets + --- 2.4 Building Separate Files + === 3. Creating a Kbuild File for an External Module + --- 3.1 Shared Makefile + --- 3.2 Separate Kbuild file and Makefile + --- 3.3 Binary Blobs + --- 3.4 Building Multiple Modules + === 4. Include Files + --- 4.1 Kernel Includes + --- 4.2 Single Subdirectory + --- 4.3 Several Subdirectories + === 5. Module Installation + --- 5.1 INSTALL_MOD_PATH + --- 5.2 INSTALL_MOD_DIR + === 6. Module Versioning + --- 6.1 Symbols From the Kernel (vmlinux + modules) + --- 6.2 Symbols and External Modules + --- 6.3 Symbols From Another External Module + === 7. Tips & Tricks + --- 7.1 Testing for CONFIG_FOO_BAR === 1. Introduction -kbuild includes functionality for building modules both -within the kernel source tree and outside the kernel source tree. -The latter is usually referred to as external or "out-of-tree" -modules and is used both during development and for modules that -are not planned to be included in the kernel tree. +"kbuild" is the build system used by the Linux kernel. Modules must use +kbuild to stay compatible with changes in the build infrastructure and +to pick up the right flags to "gcc." Functionality for building modules +both in-tree and out-of-tree is provided. The method for building +either is similar, and all modules are initially developed and built +out-of-tree. -What is covered within this file is mainly information to authors -of modules. The author of an external module should supply -a makefile that hides most of the complexity, so one only has to type -'make' to build the module. A complete example will be presented in -chapter 4, "Creating a kbuild file for an external module". +Covered in this document is information aimed at developers interested +in building out-of-tree (or "external") modules. The author of an +external module should supply a makefile that hides most of the +complexity, so one only has to type "make" to build the module. This is +easily accomplished, and a complete example will be presented in +section 3. -=== 2. How to build external modules +=== 2. How to Build External Modules -kbuild offers functionality to build external modules, with the -prerequisite that there is a pre-built kernel available with full source. -A subset of the targets available when building the kernel is available -when building an external module. +To build external modules, you must have a prebuilt kernel available +that contains the configuration and header files used in the build. +Also, the kernel must have been built with modules enabled. If you are +using a distribution kernel, there will be a package for the kernel you +are running provided by your distribution. ---- 2.1 Building external modules +An alternative is to use the "make" target "modules_prepare." This will +make sure the kernel contains the information required. The target +exists solely as a simple way to prepare a kernel source tree for +building external modules. - Use the following command to build an external module: +NOTE: "modules_prepare" will not build Module.symvers even if +CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be +executed to make module versioning work. - make -C <path-to-kernel> M=`pwd` +--- 2.1 Command Syntax - For the running kernel use: + The command to build an external module is: - make -C /lib/modules/`uname -r`/build M=`pwd` + $ make -C <path_to_kernel_src> M=$PWD - For the above command to succeed, the kernel must have been - built with modules enabled. + The kbuild system knows that an external module is being built + due to the "M=<dir>" option given in the command. - To install the modules that were just built: + To build against the running kernel use: - make -C <path-to-kernel> M=`pwd` modules_install + $ make -C /lib/modules/`uname -r`/build M=$PWD - More complex examples will be shown later, the above should - be enough to get you started. + Then to install the module(s) just built, add the target + "modules_install" to the command: ---- 2.2 Available targets + $ make -C /lib/modules/`uname -r`/build M=$PWD modules_install - $KDIR refers to the path to the kernel source top-level directory +--- 2.2 Options - make -C $KDIR M=`pwd` - Will build the module(s) located in current directory. - All output files will be located in the same directory - as the module source. - No attempts are made to update the kernel source, and it is - a precondition that a successful make has been executed - for the kernel. + ($KDIR refers to the path of the kernel source directory.) - make -C $KDIR M=`pwd` modules - The modules target is implied when no target is given. - Same functionality as if no target was specified. - See description above. + make -C $KDIR M=$PWD - make -C $KDIR M=`pwd` modules_install - Install the external module(s). - Installation default is in /lib/modules/<kernel-version>/extra, - but may be prefixed with INSTALL_MOD_PATH - see separate - chapter. + -C $KDIR + The directory where the kernel source is located. + "make" will actually change to the specified directory + when executing and will change back when finished. - make -C $KDIR M=`pwd` clean - Remove all generated files for the module - the kernel - source directory is not modified. + M=$PWD + Informs kbuild that an external module is being built. + The value given to "M" is the absolute path of the + directory where the external module (kbuild file) is + located. - make -C $KDIR M=`pwd` help - help will list the available target when building external - modules. +--- 2.3 Targets ---- 2.3 Available options: + When building an external module, only a subset of the "make" + targets are available. - $KDIR refers to the path to the kernel source top-level directory + make -C $KDIR M=$PWD [target] - make -C $KDIR - Used to specify where to find the kernel source. - '$KDIR' represent the directory where the kernel source is. - Make will actually change directory to the specified directory - when executed but change back when finished. + The default will build the module(s) located in the current + directory, so a target does not need to be specified. All + output files will also be generated in this directory. No + attempts are made to update the kernel source, and it is a + precondition that a successful "make" has been executed for the + kernel. - make -C $KDIR M=`pwd` - M= is used to tell kbuild that an external module is - being built. - The option given to M= is the directory where the external - module (kbuild file) is located. - When an external module is being built only a subset of the - usual targets are available. + modules + The default target for external modules. It has the + same functionality as if no target was specified. See + description above. - make -C $KDIR SUBDIRS=`pwd` - Same as M=. The SUBDIRS= syntax is kept for backwards - compatibility. + modules_install + Install the external module(s). The default location is + /lib/modules/<kernel_release>/extra/, but a prefix may + be added with INSTALL_MOD_PATH (discussed in section 5). ---- 2.4 Preparing the kernel tree for module build + clean + Remove all generated files in the module directory only. - To make sure the kernel contains the information required to - build external modules the target 'modules_prepare' must be used. - 'modules_prepare' exists solely as a simple way to prepare - a kernel source tree for building external modules. - Note: modules_prepare will not build Module.symvers even if - CONFIG_MODVERSIONS is set. Therefore a full kernel build - needs to be executed to make module versioning work. + help + List the available targets for external modules. ---- 2.5 Building separate files for a module - It is possible to build single files which are part of a module. - This works equally well for the kernel, a module and even for - external modules. - Examples (module foo.ko, consist of bar.o, baz.o): - make -C $KDIR M=`pwd` bar.lst - make -C $KDIR M=`pwd` bar.o - make -C $KDIR M=`pwd` foo.ko - make -C $KDIR M=`pwd` / - - -=== 3. Example commands - -This example shows the actual commands to be executed when building -an external module for the currently running kernel. -In the example below, the distribution is supposed to use the -facility to locate output files for a kernel compile in a different -directory than the kernel source - but the examples will also work -when the source and the output files are mixed in the same directory. +--- 2.4 Building Separate Files -# Kernel source -/lib/modules/<kernel-version>/source -> /usr/src/linux-<version> - -# Output from kernel compile -/lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up - -Change to the directory where the kbuild file is located and execute -the following commands to build the module: + It is possible to build single files that are part of a module. + This works equally well for the kernel, a module, and even for + external modules. - cd /home/user/src/module - make -C /usr/src/`uname -r`/source \ - O=/lib/modules/`uname-r`/build \ - M=`pwd` + Example (The module foo.ko, consist of bar.o and baz.o): + make -C $KDIR M=$PWD bar.lst + make -C $KDIR M=$PWD baz.o + make -C $KDIR M=$PWD foo.ko + make -C $KDIR M=$PWD / -Then, to install the module use the following command: - make -C /usr/src/`uname -r`/source \ - O=/lib/modules/`uname-r`/build \ - M=`pwd` \ - modules_install +=== 3. Creating a Kbuild File for an External Module -If you look closely you will see that this is the same command as -listed before - with the directories spelled out. +In the last section we saw the command to build a module for the +running kernel. The module is not actually built, however, because a +build file is required. Contained in this file will be the name of +the module(s) being built, along with the list of requisite source +files. The file may be as simple as a single line: -The above are rather long commands, and the following chapter -lists a few tricks to make it all easier. + obj-m := <module_name>.o +The kbuild system will build <module_name>.o from <module_name>.c, +and, after linking, will result in the kernel module <module_name>.ko. +The above line can be put in either a "Kbuild" file or a "Makefile." +When the module is built from multiple sources, an additional line is +needed listing the files: -=== 4. Creating a kbuild file for an external module + <module_name>-y := <src1>.o <src2>.o ... -kbuild is the build system for the kernel, and external modules -must use kbuild to stay compatible with changes in the build system -and to pick up the right flags to gcc etc. +NOTE: Further documentation describing the syntax used by kbuild is +located in Documentation/kbuild/makefiles.txt. -The kbuild file used as input shall follow the syntax described -in Documentation/kbuild/makefiles.txt. This chapter will introduce a few -more tricks to be used when dealing with external modules. +The examples below demonstrate how to create a build file for the +module 8123.ko, which is built from the following files: -In the following a Makefile will be created for a module with the -following files: 8123_if.c 8123_if.h 8123_pci.c 8123_bin.o_shipped <= Binary blob ---- 4.1 Shared Makefile for module and kernel +--- 3.1 Shared Makefile - An external module always includes a wrapper Makefile supporting - building the module using 'make' with no arguments. - The Makefile provided will most likely include additional - functionality such as test targets etc. and this part shall - be filtered away from kbuild since it may impact kbuild if - name clashes occurs. + An external module always includes a wrapper makefile that + supports building the module using "make" with no arguments. + This target is not used by kbuild; it is only for convenience. + Additional functionality, such as test targets, can be included + but should be filtered out from kbuild due to possible name + clashes. Example 1: --> filename: Makefile @@ -219,11 +189,11 @@ following files: 8123-y := 8123_if.o 8123_pci.o 8123_bin.o else - # Normal Makefile + # normal makefile + KDIR ?= /lib/modules/`uname -r`/build - KERNELDIR := /lib/modules/`uname -r`/build - all:: - $(MAKE) -C $(KERNELDIR) M=`pwd` $@ + default: + $(MAKE) -C $(KDIR) M=$$PWD # Module specific targets genbin: @@ -231,15 +201,20 @@ following files: endif - In example 1, the check for KERNELRELEASE is used to separate - the two parts of the Makefile. kbuild will only see the two - assignments whereas make will see everything except the two - kbuild assignments. + The check for KERNELRELEASE is used to separate the two parts + of the makefile. In the example, kbuild will only see the two + assignments, whereas "make" will see everything except these + two assignments. This is due to two passes made on the file: + the first pass is by the "make" instance run on the command + line; the second pass is by the kbuild system, which is + initiated by the parameterized "make" in the default target. + +--- 3.2 Separate Kbuild File and Makefile - In recent versions of the kernel, kbuild will look for a file named - Kbuild and as second option look for a file named Makefile. - Utilising the Kbuild file makes us split up the Makefile in example 1 - into two files as shown in example 2: + In newer versions of the kernel, kbuild will first look for a + file named "Kbuild," and only if that is not found, will it + then look for a makefile. Utilizing a "Kbuild" file allows us + to split up the makefile from example 1 into two files: Example 2: --> filename: Kbuild @@ -247,20 +222,21 @@ following files: 8123-y := 8123_if.o 8123_pci.o 8123_bin.o --> filename: Makefile - KERNELDIR := /lib/modules/`uname -r`/build - all:: - $(MAKE) -C $(KERNELDIR) M=`pwd` $@ + KDIR ?= /lib/modules/`uname -r`/build + + default: + $(MAKE) -C $(KDIR) M=$$PWD # Module specific targets genbin: echo "X" > 8123_bin.o_shipped + The split in example 2 is questionable due to the simplicity of + each file; however, some external modules use makefiles + consisting of several hundred lines, and here it really pays + off to separate the kbuild part from the rest. - In example 2, we are down to two fairly simple files and for simple - files as used in this example the split is questionable. But some - external modules use Makefiles of several hundred lines and here it - really pays off to separate the kbuild part from the rest. - Example 3 shows a backward compatible version. + The next example shows a backward compatible version. Example 3: --> filename: Kbuild @@ -269,13 +245,15 @@ following files: --> filename: Makefile ifneq ($(KERNELRELEASE),) + # kbuild part of makefile include Kbuild + else - # Normal Makefile + # normal makefile + KDIR ?= /lib/modules/`uname -r`/build - KERNELDIR := /lib/modules/`uname -r`/build - all:: - $(MAKE) -C $(KERNELDIR) M=`pwd` $@ + default: + $(MAKE) -C $(KDIR) M=$$PWD # Module specific targets genbin: @@ -283,260 +261,271 @@ following files: endif - The trick here is to include the Kbuild file from Makefile, so - if an older version of kbuild picks up the Makefile, the Kbuild - file will be included. + Here the "Kbuild" file is included from the makefile. This + allows an older version of kbuild, which only knows of + makefiles, to be used when the "make" and kbuild parts are + split into separate files. ---- 4.2 Binary blobs included in a module +--- 3.3 Binary Blobs - Some external modules needs to include a .o as a blob. kbuild - has support for this, but requires the blob file to be named - <filename>_shipped. In our example the blob is named - 8123_bin.o_shipped and when the kbuild rules kick in the file - 8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file - with the _shipped part stripped of the filename. - This allows the 8123_bin.o filename to be used in the assignment to - the module. + Some external modules need to include an object file as a blob. + kbuild has support for this, but requires the blob file to be + named <filename>_shipped. When the kbuild rules kick in, a copy + of <filename>_shipped is created with _shipped stripped off, + giving us <filename>. This shortened filename can be used in + the assignment to the module. + + Throughout this section, 8123_bin.o_shipped has been used to + build the kernel module 8123.ko; it has been included as + 8123_bin.o. - Example 4: - obj-m := 8123.o 8123-y := 8123_if.o 8123_pci.o 8123_bin.o - In example 4, there is no distinction between the ordinary .c/.h files - and the binary file. But kbuild will pick up different rules to create - the .o file. + Although there is no distinction between the ordinary source + files and the binary file, kbuild will pick up different rules + when creating the object file for the module. + +--- 3.4 Building Multiple Modules + kbuild supports building multiple modules with a single build + file. For example, if you wanted to build two modules, foo.ko + and bar.ko, the kbuild lines would be: -=== 5. Include files + obj-m := foo.o bar.o + foo-y := <foo_srcs> + bar-y := <bar_srcs> -Include files are a necessity when a .c file uses something from other .c -files (not strictly in the sense of C, but if good programming practice is -used). Any module that consists of more than one .c file will have a .h file -for one of the .c files. + It is that simple! -- If the .h file only describes a module internal interface, then the .h file - shall be placed in the same directory as the .c files. -- If the .h files describe an interface used by other parts of the kernel - located in different directories, the .h files shall be located in - include/linux/ or other include/ directories as appropriate. -One exception for this rule is larger subsystems that have their own directory -under include/ such as include/scsi. Another exception is arch-specific -.h files which are located under include/asm-$(ARCH)/*. +=== 4. Include Files -External modules have a tendency to locate include files in a separate include/ -directory and therefore need to deal with this in their kbuild file. +Within the kernel, header files are kept in standard locations +according to the following rule: ---- 5.1 How to include files from the kernel include dir + * If the header file only describes the internal interface of a + module, then the file is placed in the same directory as the + source files. + * If the header file describes an interface used by other parts + of the kernel that are located in different directories, then + the file is placed in include/linux/. - When a module needs to include a file from include/linux/, then one - just uses: + NOTE: There are two notable exceptions to this rule: larger + subsystems have their own directory under include/, such as + include/scsi; and architecture specific headers are located + under arch/$(ARCH)/include/. - #include <linux/modules.h> +--- 4.1 Kernel Includes - kbuild will make sure to add options to gcc so the relevant - directories are searched. - Likewise for .h files placed in the same directory as the .c file. + To include a header file located under include/linux/, simply + use: - #include "8123_if.h" + #include <linux/module.h> - will do the job. + kbuild will add options to "gcc" so the relevant directories + are searched. ---- 5.2 External modules using an include/ dir +--- 4.2 Single Subdirectory - External modules often locate their .h files in a separate include/ - directory although this is not usual kernel style. When an external - module uses an include/ dir then kbuild needs to be told so. - The trick here is to use either EXTRA_CFLAGS (take effect for all .c - files) or CFLAGS_$F.o (take effect only for a single file). + External modules tend to place header files in a separate + include/ directory where their source is located, although this + is not the usual kernel style. To inform kbuild of the + directory, use either ccflags-y or CFLAGS_<filename>.o. - In our example, if we move 8123_if.h to a subdirectory named include/ - the resulting Kbuild file would look like: + Using the example from section 3, if we moved 8123_if.h to a + subdirectory named include, the resulting kbuild file would + look like: --> filename: Kbuild - obj-m := 8123.o + obj-m := 8123.o - EXTRA_CFLAGS := -Iinclude + ccflags-y := -Iinclude 8123-y := 8123_if.o 8123_pci.o 8123_bin.o - Note that in the assignment there is no space between -I and the path. - This is a kbuild limitation: there must be no space present. + Note that in the assignment there is no space between -I and + the path. This is a limitation of kbuild: there must be no + space present. ---- 5.3 External modules using several directories - - If an external module does not follow the usual kernel style, but - decides to spread files over several directories, then kbuild can - handle this too. +--- 4.3 Several Subdirectories + kbuild can handle files that are spread over several directories. Consider the following example: - | - +- src/complex_main.c - | +- hal/hardwareif.c - | +- hal/include/hardwareif.h - +- include/complex.h - - To build a single module named complex.ko, we then need the following + . + |__ src + | |__ complex_main.c + | |__ hal + | |__ hardwareif.c + | |__ include + | |__ hardwareif.h + |__ include + |__ complex.h + + To build the module complex.ko, we then need the following kbuild file: - Kbuild: + --> filename: Kbuild obj-m := complex.o complex-y := src/complex_main.o complex-y += src/hal/hardwareif.o - EXTRA_CFLAGS := -I$(src)/include - EXTRA_CFLAGS += -I$(src)src/hal/include + ccflags-y := -I$(src)/include + ccflags-y += -I$(src)/src/hal/include + As you can see, kbuild knows how to handle object files located + in other directories. The trick is to specify the directory + relative to the kbuild file's location. That being said, this + is NOT recommended practice. - kbuild knows how to handle .o files located in another directory - - although this is NOT recommended practice. The syntax is to specify - the directory relative to the directory where the Kbuild file is - located. + For the header files, kbuild must be explicitly told where to + look. When kbuild executes, the current directory is always the + root of the kernel tree (the argument to "-C") and therefore an + absolute path is needed. $(src) provides the absolute path by + pointing to the directory where the currently executing kbuild + file is located. - To find the .h files, we have to explicitly tell kbuild where to look - for the .h files. When kbuild executes, the current directory is always - the root of the kernel tree (argument to -C) and therefore we have to - tell kbuild how to find the .h files using absolute paths. - $(src) will specify the absolute path to the directory where the - Kbuild file are located when being build as an external module. - Therefore -I$(src)/ is used to point out the directory of the Kbuild - file and any additional path are just appended. -=== 6. Module installation +=== 5. Module Installation -Modules which are included in the kernel are installed in the directory: +Modules which are included in the kernel are installed in the +directory: - /lib/modules/$(KERNELRELEASE)/kernel + /lib/modules/$(KERNELRELEASE)/kernel/ -External modules are installed in the directory: +And external modules are installed in: - /lib/modules/$(KERNELRELEASE)/extra + /lib/modules/$(KERNELRELEASE)/extra/ ---- 6.1 INSTALL_MOD_PATH +--- 5.1 INSTALL_MOD_PATH - Above are the default directories, but as always, some level of - customization is possible. One can prefix the path using the variable - INSTALL_MOD_PATH: + Above are the default directories but as always some level of + customization is possible. A prefix can be added to the + installation path using the variable INSTALL_MOD_PATH: $ make INSTALL_MOD_PATH=/frodo modules_install - => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel - - INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the - example above, can be specified on the command line when calling make. - INSTALL_MOD_PATH has effect both when installing modules included in - the kernel as well as when installing external modules. + => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/ ---- 6.2 INSTALL_MOD_DIR + INSTALL_MOD_PATH may be set as an ordinary shell variable or, + as shown above, can be specified on the command line when + calling "make." This has effect when installing both in-tree + and out-of-tree modules. - When installing external modules they are by default installed to a - directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish - to locate modules for a specific functionality in a separate - directory. For this purpose, one can use INSTALL_MOD_DIR to specify an - alternative name to 'extra'. +--- 5.2 INSTALL_MOD_DIR - $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \ - M=`pwd` modules_install - => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf + External modules are by default installed to a directory under + /lib/modules/$(KERNELRELEASE)/extra/, but you may wish to + locate modules for a specific functionality in a separate + directory. For this purpose, use INSTALL_MOD_DIR to specify an + alternative name to "extra." + $ make INSTALL_MOD_DIR=gandalf -C $KDIR \ + M=$PWD modules_install + => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/ -=== 7. Module versioning & Module.symvers -Module versioning is enabled by the CONFIG_MODVERSIONS tag. +=== 6. Module Versioning -Module versioning is used as a simple ABI consistency check. The Module -versioning creates a CRC value of the full prototype for an exported symbol and -when a module is loaded/used then the CRC values contained in the kernel are -compared with similar values in the module. If they are not equal, then the -kernel refuses to load the module. +Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used +as a simple ABI consistency check. A CRC value of the full prototype +for an exported symbol is created. When a module is loaded/used, the +CRC values contained in the kernel are compared with similar values in +the module; if they are not equal, the kernel refuses to load the +module. -Module.symvers contains a list of all exported symbols from a kernel build. +Module.symvers contains a list of all exported symbols from a kernel +build. ---- 7.1 Symbols from the kernel (vmlinux + modules) +--- 6.1 Symbols From the Kernel (vmlinux + modules) - During a kernel build, a file named Module.symvers will be generated. - Module.symvers contains all exported symbols from the kernel and - compiled modules. For each symbols, the corresponding CRC value - is stored too. + During a kernel build, a file named Module.symvers will be + generated. Module.symvers contains all exported symbols from + the kernel and compiled modules. For each symbol, the + corresponding CRC value is also stored. The syntax of the Module.symvers file is: - <CRC> <Symbol> <module> - Sample: + <CRC> <Symbol> <module> + 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod - For a kernel build without CONFIG_MODVERSIONS enabled, the crc - would read: 0x00000000 + For a kernel build without CONFIG_MODVERSIONS enabled, the CRC + would read 0x00000000. Module.symvers serves two purposes: - 1) It lists all exported symbols both from vmlinux and all modules - 2) It lists the CRC if CONFIG_MODVERSIONS is enabled - ---- 7.2 Symbols and external modules - - When building an external module, the build system needs access to - the symbols from the kernel to check if all external symbols are - defined. This is done in the MODPOST step and to obtain all - symbols, modpost reads Module.symvers from the kernel. - If a Module.symvers file is present in the directory where - the external module is being built, this file will be read too. - During the MODPOST step, a new Module.symvers file will be written - containing all exported symbols that were not defined in the kernel. - ---- 7.3 Symbols from another external module - - Sometimes, an external module uses exported symbols from another - external module. Kbuild needs to have full knowledge on all symbols - to avoid spitting out warnings about undefined symbols. - Three solutions exist to let kbuild know all symbols of more than - one external module. - The method with a top-level kbuild file is recommended but may be - impractical in certain situations. - - Use a top-level Kbuild file - If you have two modules: 'foo' and 'bar', and 'foo' needs - symbols from 'bar', then one can use a common top-level kbuild - file so both modules are compiled in same build. - - Consider following directory layout: - ./foo/ <= contains the foo module - ./bar/ <= contains the bar module - The top-level Kbuild file would then look like: - - #./Kbuild: (this file may also be named Makefile) + 1) It lists all exported symbols from vmlinux and all modules. + 2) It lists the CRC if CONFIG_MODVERSIONS is enabled. + +--- 6.2 Symbols and External Modules + + When building an external module, the build system needs access + to the symbols from the kernel to check if all external symbols + are defined. This is done in the MODPOST step. modpost obtains + the symbols by reading Module.symvers from the kernel source + tree. If a Module.symvers file is present in the directory + where the external module is being built, this file will be + read too. During the MODPOST step, a new Module.symvers file + will be written containing all exported symbols that were not + defined in the kernel. + +--- 6.3 Symbols From Another External Module + + Sometimes, an external module uses exported symbols from + another external module. kbuild needs to have full knowledge of + all symbols to avoid spitting out warnings about undefined + symbols. Three solutions exist for this situation. + + NOTE: The method with a top-level kbuild file is recommended + but may be impractical in certain situations. + + Use a top-level kbuild file + If you have two modules, foo.ko and bar.ko, where + foo.ko needs symbols from bar.ko, you can use a + common top-level kbuild file so both modules are + compiled in the same build. Consider the following + directory layout: + + ./foo/ <= contains foo.ko + ./bar/ <= contains bar.ko + + The top-level kbuild file would then look like: + + #./Kbuild (or ./Makefile): obj-y := foo/ bar/ - Executing: - make -C $KDIR M=`pwd` + And executing + + $ make -C $KDIR M=$PWD - will then do the expected and compile both modules with full - knowledge on symbols from both modules. + will then do the expected and compile both modules with + full knowledge of symbols from either module. Use an extra Module.symvers file - When an external module is built, a Module.symvers file is - generated containing all exported symbols which are not - defined in the kernel. - To get access to symbols from module 'bar', one can copy the - Module.symvers file from the compilation of the 'bar' module - to the directory where the 'foo' module is built. - During the module build, kbuild will read the Module.symvers - file in the directory of the external module and when the - build is finished, a new Module.symvers file is created - containing the sum of all symbols defined and not part of the - kernel. - - Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile - If it is impractical to copy Module.symvers from another - module, you can assign a space separated list of files to - KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be - loaded by modpost during the initialisation of its symbol - tables. - -=== 8. Tips & Tricks - ---- 8.1 Testing for CONFIG_FOO_BAR - - Modules often need to check for certain CONFIG_ options to decide if - a specific feature shall be included in the module. When kbuild is used - this is done by referencing the CONFIG_ variable directly. + When an external module is built, a Module.symvers file + is generated containing all exported symbols which are + not defined in the kernel. To get access to symbols + from bar.ko, copy the Module.symvers file from the + compilation of bar.ko to the directory where foo.ko is + built. During the module build, kbuild will read the + Module.symvers file in the directory of the external + module, and when the build is finished, a new + Module.symvers file is created containing the sum of + all symbols defined and not part of the kernel. + + Use "make" variable KBUILD_EXTRA_SYMBOLS + If it is impractical to copy Module.symvers from + another module, you can assign a space separated list + of files to KBUILD_EXTRA_SYMBOLS in your build file. + These files will be loaded by modpost during the + initialization of its symbol tables. + + +=== 7. Tips & Tricks + +--- 7.1 Testing for CONFIG_FOO_BAR + + Modules often need to check for certain CONFIG_ options to + decide if a specific feature is included in the module. In + kbuild this is done by referencing the CONFIG_ variable + directly. #fs/ext2/Makefile obj-$(CONFIG_EXT2_FS) += ext2.o @@ -544,9 +533,9 @@ Module.symvers contains a list of all exported symbols from a kernel build. ext2-y := balloc.o bitmap.o dir.o ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o - External modules have traditionally used grep to check for specific - CONFIG_ settings directly in .config. This usage is broken. - As introduced before, external modules shall use kbuild when building - and therefore can use the same methods as in-kernel modules when - testing for CONFIG_ definitions. + External modules have traditionally used "grep" to check for + specific CONFIG_ settings directly in .config. This usage is + broken. As introduced before, external modules should use + kbuild for building and can therefore use the same methods as + in-tree modules when testing for CONFIG_ definitions. |
