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diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt new file mode 100644 index 0000000..1821c07 --- /dev/null +++ b/Documentation/kbuild/modules.txt @@ -0,0 +1,552 @@ + +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 + +=== 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 + + + +=== 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. + +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". + + +=== 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. + +--- 2.1 Building external modules + + Use the following command to build an external module: + + make -C <path-to-kernel> M=`pwd` + + For the running kernel use: + + make -C /lib/modules/`uname -r`/build M=`pwd` + + For the above command to succeed, the kernel must have been + built with modules enabled. + + To install the modules that were just built: + + make -C <path-to-kernel> M=`pwd` modules_install + + More complex examples will be shown later, the above should + be enough to get you started. + +--- 2.2 Available targets + + $KDIR refers to the path to the kernel source top-level directory + + 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. + + 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` 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. + + make -C $KDIR M=`pwd` clean + Remove all generated files for the module - the kernel + source directory is not modified. + + make -C $KDIR M=`pwd` help + help will list the available target when building external + modules. + +--- 2.3 Available options: + + $KDIR refers to the path to the kernel source top-level directory + + 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. + + 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. + + make -C $KDIR SUBDIRS=`pwd` + Same as M=. The SUBDIRS= syntax is kept for backwards + compatibility. + +--- 2.4 Preparing the kernel tree for module build + + 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. + +--- 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. + +# 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: + + cd /home/user/src/module + make -C /usr/src/`uname -r`/source \ + O=/lib/modules/`uname-r`/build \ + 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 + +If you look closely you will see that this is the same command as +listed before - with the directories spelled out. + +The above are rather long commands, and the following chapter +lists a few tricks to make it all easier. + + +=== 4. Creating a kbuild file for an external module + +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. + +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. + +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 + + 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. + + Example 1: + --> filename: Makefile + ifneq ($(KERNELRELEASE),) + # kbuild part of makefile + obj-m := 8123.o + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + else + # Normal Makefile + + KERNELDIR := /lib/modules/`uname -r`/build + all:: + $(MAKE) -C $(KERNELDIR) M=`pwd` $@ + + # Module specific targets + genbin: + echo "X" > 8123_bin.o_shipped + + 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. + + 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: + + Example 2: + --> filename: Kbuild + obj-m := 8123.o + 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` $@ + + # Module specific targets + genbin: + echo "X" > 8123_bin_shipped + + + In example 2, we are down to two fairly simple files and for simple + files as used in this example the split is questionable. But some + external modules use Makefiles of several hundred lines and here it + really pays off to separate the kbuild part from the rest. + Example 3 shows a backward compatible version. + + Example 3: + --> filename: Kbuild + obj-m := 8123.o + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + --> filename: Makefile + ifneq ($(KERNELRELEASE),) + include Kbuild + else + # Normal Makefile + + KERNELDIR := /lib/modules/`uname -r`/build + all:: + $(MAKE) -C $KERNELDIR M=`pwd` $@ + + # Module specific targets + genbin: + echo "X" > 8123_bin_shipped + + endif + + The trick here is to include the Kbuild file from Makefile, so + if an older version of kbuild picks up the Makefile, the Kbuild + file will be included. + +--- 4.2 Binary blobs included in a module + + Some external modules needs to include a .o as a blob. kbuild + has support for this, but requires the blob file to be named + <filename>_shipped. In our example the blob is named + 8123_bin.o_shipped and when the kbuild rules kick in the file + 8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file + with the _shipped part stripped of the filename. + This allows the 8123_bin.o filename to be used in the assignment to + the module. + + Example 4: + obj-m := 8123.o + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + In example 4, there is no distinction between the ordinary .c/.h files + and the binary file. But kbuild will pick up different rules to create + the .o file. + + +=== 5. Include files + +Include files are a necessity when a .c file uses something from other .c +files (not strictly in the sense of C, but if good programming practice is +used). Any module that consists of more than one .c file will have a .h file +for one of the .c files. + +- If the .h file only describes a module internal interface, then the .h file + shall be placed in the same directory as the .c files. +- If the .h files describe an interface used by other parts of the kernel + located in different directories, the .h files shall be located in + include/linux/ or other include/ directories as appropriate. + +One exception for this rule is larger subsystems that have their own directory +under include/ such as include/scsi. Another exception is arch-specific +.h files which are located under include/asm-$(ARCH)/*. + +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. + +--- 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: + + #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. + +--- 5.2 External modules using an include/ dir + + 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_$F.o (take effect only for a single file). + + In our example, if we move 8123_if.h to a subdirectory named include/ + the resulting Kbuild file would look like: + + --> filename: Kbuild + obj-m := 8123.o + + EXTRA_CFLAGS := -Iinclude + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + Note that in the assignment there is no space between -I and the path. + This is a kbuild limitation: there must be no space present. + +--- 5.3 External modules using several directories + + If an external module does not follow the usual kernel style, but + decides to spread files over several directories, then kbuild can + handle this too. + + Consider the following example: + + | + +- src/complex_main.c + | +- hal/hardwareif.c + | +- hal/include/hardwareif.h + +- include/complex.h + + To build a single module named complex.ko, we then need the following + kbuild file: + + Kbuild: + obj-m := complex.o + complex-y := src/complex_main.o + complex-y += src/hal/hardwareif.o + + EXTRA_CFLAGS := -I$(src)/include + EXTRA_CFLAGS += -I$(src)src/hal/include + + + 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. + + 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 + +Modules which are included in the kernel are installed in the directory: + + /lib/modules/$(KERNELRELEASE)/kernel + +External modules are installed in the directory: + + /lib/modules/$(KERNELRELEASE)/extra + +--- 6.1 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: + + $ make INSTALL_MOD_PATH=/frodo modules_install + => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel + + INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the + example above, can be specified on the command line when calling make. + INSTALL_MOD_PATH has effect both when installing modules included in + the kernel as well as when installing external modules. + +--- 6.2 INSTALL_MOD_DIR + + When installing external modules they are by default installed to a + directory under /lib/modules/$(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'. + + $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \ + M=`pwd` modules_install + => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf + + +=== 7. Module versioning & Module.symvers + +Module versioning is enabled by the CONFIG_MODVERSIONS tag. + +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. + +--- 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. + + The syntax of the Module.symvers file is: + <CRC> <Symbol> <module> + Sample: + 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod + + 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) + obj-y := foo/ bar/ + + Executing: + make -C $KDIR M=`pwd` + + will then do the expected and compile both modules with full + knowledge on symbols from both modules. + + Use an extra Module.symvers file + When an external module is built, a Module.symvers file is + generated containing all exported symbols which are not + defined in the kernel. + To get access to symbols from module 'bar', one can copy the + Module.symvers file from the compilation of the 'bar' module + to the directory where the 'foo' module is built. + During the module build, kbuild will read the Module.symvers + file in the directory of the external module and when the + build is finished, a new Module.symvers file is created + containing the sum of all symbols defined and not part of the + kernel. + + Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile + If it is impractical to copy Module.symvers from another + module, you can assign a space separated list of files to + KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be + loaded by modpost during the initialisation of its symbol + tables. + +=== 8. Tips & Tricks + +--- 8.1 Testing for CONFIG_FOO_BAR + + Modules often need to check for certain CONFIG_ options to decide if + a specific feature shall be included in the module. When kbuild is used + this is done by referencing the CONFIG_ variable directly. + + #fs/ext2/Makefile + obj-$(CONFIG_EXT2_FS) += ext2.o + + 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. + |
