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diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt new file mode 100644 index 0000000..c91caf7 --- /dev/null +++ b/Documentation/kbuild/modules.txt @@ -0,0 +1,419 @@ + +In this document you will find information about: +- how to build external modules +- how to make your module use 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 + === 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 + === 6. Module installation + --- 6.1 INSTALL_MOD_PATH + --- 6.2 INSTALL_MOD_DIR + === 7. Module versioning + === 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 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 modules should supply +a makefile that hides most of the complexity so one only has to type +'make' to buld the module. A complete example will be present in +chapter ¤. 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 later, the above should get you going. + +--- 2.2 Available targets + + $KDIR refers to path to 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 chater. + + make -C $KDIR M=$PWD clean + Remove all generated files for the module - the kernel + source directory is not moddified. + + make -C $KDIR M=`pwd` help + help will list the available target when building external + modules. + +--- 2.3 Available options: + + $KDIR refer to path to kernel src + + 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. + 'module_prepare' solely exists as a simple way to prepare + a kernel for building external modules. + Note: modules_prepare will not build Module.symvers even if + CONFIG_MODULEVERSIONING is set. + Therefore a full kernel build needs to be executed to make + module versioning work. + + +=== 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 one looks closely you will see that this is the same commands 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_bini.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` $@ + + # Module specific targets + genbin: + echo "X" > 8123_bin_shipped + + endif + + The trick here is to include the Kbuild file from Makefile so + if an older version of kbuild picks up the Makefile the Kbuild + file will be included. + +--- 4.2 Binary blobs included in a module + + Some external modules needs to include a .o as a blob. kbuild + has support for this, but requires the blob file to be named + <filename>_shipped. In our example the blob is named + 8123_bin.o_shipped and when the kbuild rules kick in the file + 8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file + with the _shipped part stripped of the filename. + This allows the 8123_bin.o filename to be used in the assignment to + the module. + + Example 4: + obj-m := 8123.o + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + In example 4 there is no distinction between the ordinary .c/.h files + and the binary file. But kbuild will pick up different rules to create + the .o file. + + +=== 5. Include files + +Include files are a necessity when a .c file uses something from another .c +files (not strictly in the sense of .c but if good programming practice is +used). Any module that consist of more than one .c file will have a .h file +for one of the .c files. +- If the .h file only describes a module internal interface then the .h file + shall be placed in the same directory as the .c files. +- If the .h files describe an interface used by other parts of the kernel + located in different directories, the .h files shall be located in + include/linux/ or other include/ directories as appropriate. + +One exception for this rule is larger subsystems that have their own directory +under include/ such as include/scsi. Another exception is arch-specific +.h files which are located under include/asm-$(ARCH)/*. + +External modules have a tendency to locate include files in a separate include/ +directory and therefore needs 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_$F.o (take effect only for a single file). + + In our example if we move 8123_if.h to a subdirectory named include/ + the resulting Kbuild file would look like: + + --> filename: Kbuild + obj-m := 8123.o + + EXTRA_CFLAGS := -Iinclude + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + Note that in the assingment there is no space between -I and the path. + This is a kbuild limitation and no space must be present. + + +=== 6. Module installation + +Modules which are included in the kernel is installed in the directory: + + /lib/modules/$(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/$(KERNELRELEASE)/kernel + + INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the + example above be specified on the commandline when calling make. + INSTALL_MOD_PATH has effect both when installing modules included in + the kernel as well as when installing external modules. + +--- 6.2 INSTALL_MOD_DIR + + When installing external modules they are default installed in a + directory under /lib/modules/$(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 than 'extra'. + + $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \ + M=`pwd` modules_install + => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf + + +=== 7. Module versioning + +Module versioning are enabled by the CONFIG_MODVERSIONS tag. + +Module versioning is used as a simple ABI consistency check. The Module +versioning creates a CRC value of the full prototype for an exported symbol and +when a module is loaded/used then the CRC values contained in the kernel are +compared with similar values in the module. If they are not equal then the +kernel refuses to load the module. + +During a kernel build a file named Module.symvers will be generated. This +file includes the symbol version of all symbols within the kernel. If the +Module.symvers file is saved from the last full kernel compile one does not +have to do a full kernel compile to build a module version's compatible module. + +=== 8. Tips & Tricks + +--- 8.1 Testing for CONFIG_FOO_BAR + + Modules often needs to check for certain CONFIG_ options to decide if + a specific feature shall be included in the module. When kbuild is used + this is done by referencing the CONFIG_ variable directly. + + #fs/ext2/Makefile + obj-$(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. + |
