A follow-up to my edit of the first 4 sections. Shift sections down by one due to the deletion of section 3; grammar corrections along with some rewording; margin width cleanup; and change EXTRA_CFLAGS -> ccflags-y. Signed-off-by: matt mooney <mfm@xxxxxxxxxxxxx> --- Should I add a subsection to section 5. discussing MODLIB? Documentation/kbuild/modules.txt | 358 +++++++++++++++++++------------------- 1 files changed, 177 insertions(+), 181 deletions(-) diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt index 799b683..b572db3 100644 --- a/Documentation/kbuild/modules.txt +++ b/Documentation/kbuild/modules.txt @@ -15,17 +15,17 @@ This document describes how-to build an out-of-tree kernel module. --- 3.2 Separate Kbuild file and Makefile --- 3.3 Binary Blobs --- 3.4 Building Multiple Modules - === 4. Include files - --- 4.1 How to include files from the kernel include dir - --- 4.2 External modules using an include/ dir - --- 4.3 External modules using several directories - === 5. Module installation + === 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 & Module.symvers - --- 6.1 Symbols from the kernel (vmlinux + modules) - --- 6.2 Symbols and external modules - --- 6.3 Symbols from another external module + === 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 @@ -298,236 +298,232 @@ module 8123.ko, which is built from the following files: It is that simple! -=== 5. Include files +=== 4. Include Files -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. +Within the kernel, header files are kept in standard locations +according to the following rule: -- 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. + * 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/. -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-$(ARCH)/*. + NOTE: There are two notable exceptions to this rule: larger + subsystems have their own directory under include/, such as + include/scsi; and architecture specific headers are located + under arch/$(ARCH)/include/. -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. +--- 4.1 Kernel Includes ---- 5.1 How to include files from the kernel include dir - - When a module needs to include a file from include/linux/, then one - just uses: + To include a header file located under include/linux/, simply + use: #include <linux/modules.h> - 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. - - #include "8123_if.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_$F.o (take effect only for a single file). + External modules tend to place header files in a separate + include/ directory where their source is located, although this + is not the usual kernel style. To inform kbuild of the + directory use either ccflags-y or CFLAGS_<filename>.o. - In our example, if we move 8123_if.h to a subdirectory named include/ - the resulting Kbuild file would look like: + Using the example from section 3, if we moved 8123_if.h to a + subdirectory named include, the resulting kbuild file would + look like: --> filename: Kbuild - obj-m := 8123.o + obj-m := 8123.o - EXTRA_CFLAGS := -Iinclude + ccflags-y := -Iinclude 8123-y := 8123_if.o 8123_pci.o 8123_bin.o - Note that in the assignment there is no space between -I and the path. - This is a kbuild limitation: there must be no space present. - ---- 5.3 External modules using several directories + Note that in the assignment there is no space between -I and + the path. This is a limitation of kbuild: there must be no + space present. - If an external module does not follow the usual kernel style, but - decides to spread files over several directories, then kbuild can - handle this too. +--- 4.3 Several Subdirectories + kbuild can handle files that are spread over several directories. Consider the following example: - | - +- src/complex_main.c - | +- hal/hardwareif.c - | +- hal/include/hardwareif.h - +- include/complex.h - - To build a single module named complex.ko, we then need the following + . + |__ src + | |__ complex_main.c + | |__ hal + | |__ hardwareif.c + | |__ include + | |__ hardwareif.h + |__ include + |__ complex.h + + To build the module complex.ko, we then need the following kbuild file: - Kbuild: + --> filename: Kbuild obj-m := complex.o complex-y := src/complex_main.o complex-y += src/hal/hardwareif.o - EXTRA_CFLAGS := -I$(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/$(KERNELRELEASE)/kernel -External modules are installed in the directory: +And external modules are installed in: /lib/modules/$(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/$(KERNELRELEASE)/kernel - INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the - example above, can be specified on the command line when calling make. - INSTALL_MOD_PATH has effect both when installing modules included in - the kernel as well as when installing external modules. + INSTALL_MOD_PATH may be set as an ordinary shell variable or, + as shown above, can be specified on the command line when + calling "make." This has effect when installing both in-tree + and out-of-tree modules. ---- 6.2 INSTALL_MOD_DIR +--- 5.2 INSTALL_MOD_DIR - When installing external modules they are by default installed to a - directory under /lib/modules/$(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'. + 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 KERNELDIR \ - M=`pwd` modules_install + $ make INSTALL_MOD_DIR=gandalf -C $KDIR \ + M=$PWD modules_install => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf -=== 7. Module versioning & Module.symvers +=== 6. Module Versioning -Module versioning is enabled by the CONFIG_MODVERSIONS tag. +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 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.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. +--- 6.1 Symbols From the Kernel (vmlinux + modules) ---- 7.1 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, then you can use a + common top-level kbuild file so both modules are + compiled in the same build. Consider 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-$(CONFIG_EXT2_FS) += ext2.o @@ -535,9 +531,9 @@ Module.symvers contains a list of all exported symbols from a kernel build. ext2-y := balloc.o bitmap.o dir.o ext2-$(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. -- 1.7.2.1 -- To unsubscribe from this list: send the line "unsubscribe kernel-janitors" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
