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Documentation/kbuild: major edit of modules.txt sections 5-8
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@muteddisk.com> Signed-off-by: Michal Marek <mmarek@suse.cz>
This commit is contained in:
matt mooney
Michal Marek
parent
efdf02cf06
commit
9f02186c23
1 changed files with 167 additions and 171 deletions
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@ -15,17 +15,17 @@ This document describes how-to build an out-of-tree kernel module.
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--- 3.2 Separate Kbuild file and Makefile
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--- 3.3 Binary Blobs
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--- 3.4 Building Multiple Modules
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=== 4. Include files
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--- 4.1 How to include files from the kernel include dir
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--- 4.2 External modules using an include/ dir
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--- 4.3 External modules using several directories
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=== 5. Module installation
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=== 4. Include Files
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--- 4.1 Kernel Includes
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--- 4.2 Single Subdirectory
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--- 4.3 Several Subdirectories
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=== 5. Module Installation
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--- 5.1 INSTALL_MOD_PATH
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--- 5.2 INSTALL_MOD_DIR
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=== 6. Module versioning & Module.symvers
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--- 6.1 Symbols from the kernel (vmlinux + modules)
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--- 6.2 Symbols and external modules
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--- 6.3 Symbols from another external module
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=== 6. Module Versioning
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--- 6.1 Symbols From the Kernel (vmlinux + modules)
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--- 6.2 Symbols and External Modules
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--- 6.3 Symbols From Another External Module
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=== 7. Tips & Tricks
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--- 7.1 Testing for CONFIG_FOO_BAR
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@ -298,236 +298,232 @@ module 8123.ko, which is built from the following files:
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It is that simple!
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=== 5. Include files
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=== 4. Include Files
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Include files are a necessity when a .c file uses something from other .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 consists 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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Within the kernel, header files are kept in standard locations
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according to the following rule:
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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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* If the header file only describes the internal interface of a
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module, then the file is placed in the same directory as the
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source files.
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* If the header file describes an interface used by other parts
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of the kernel that are located in different directories, then
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the file is placed in include/linux/.
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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-$(ARCH)/*.
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NOTE: There are two notable exceptions to this rule: larger
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subsystems have their own directory under include/, such as
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include/scsi; and architecture specific headers are located
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under arch/$(ARCH)/include/.
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External modules have a tendency to locate include files in a separate include/
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directory and therefore need to deal with this in their kbuild file.
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--- 4.1 Kernel Includes
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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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To include a header file located under include/linux/, simply
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use:
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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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kbuild will add options to "gcc" so the relevant directories
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are searched.
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#include "8123_if.h"
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--- 4.2 Single Subdirectory
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will do the job.
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External modules tend to place header files in a separate
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include/ directory where their source is located, although this
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is not the usual kernel style. To inform kbuild of the
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directory use either ccflags-y or CFLAGS_<filename>.o.
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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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Using the example from section 3, if we moved 8123_if.h to a
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subdirectory named include, the resulting kbuild file would
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look like:
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--> filename: Kbuild
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obj-m := 8123.o
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obj-m := 8123.o
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EXTRA_CFLAGS := -Iinclude
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ccflags-y := -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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Note that in the assignment there is no space between -I and
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the path. This is a limitation of kbuild: there must be no
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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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decides to spread files over several directories, then kbuild can
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handle this too.
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--- 4.3 Several Subdirectories
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kbuild can handle files that are spread over several directories.
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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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.
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|__ src
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| |__ complex_main.c
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| |__ hal
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| |__ hardwareif.c
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| |__ include
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| |__ hardwareif.h
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|__ include
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|__ complex.h
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To build a single module named complex.ko, we then need the following
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To build the module complex.ko, we then need the following
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kbuild file:
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Kbuild:
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--> filename: 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$(src)src/hal/include
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ccflags-y := -I$(src)/include
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ccflags-y += -I$(src)/src/hal/include
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As you can see, kbuild knows how to handle object files located
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in other directories. The trick is to specify the directory
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relative to the kbuild file's location. That being said, this
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is NOT recommended practice.
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For the header files, kbuild must be explicitly told where to
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look. When kbuild executes, the current directory is always the
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root of the kernel tree (the argument to "-C") and therefore an
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absolute path is needed. $(src) provides the absolute path by
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pointing to the directory where the currently executing kbuild
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file is located.
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kbuild knows how to handle .o files located in another directory -
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although this is NOT recommended practice. The syntax is to specify
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the directory relative to the directory where the Kbuild file is
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located.
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=== 5. Module Installation
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To find the .h files, we have to explicitly tell kbuild where to look
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for the .h files. When kbuild executes, the current directory is always
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the root of the kernel tree (argument to -C) and therefore we have to
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tell kbuild how to find the .h files using absolute paths.
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$(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$(src)/ is used to point out the directory of the Kbuild
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file and any additional path are just appended.
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=== 6. Module installation
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Modules which are included in the kernel are installed in the directory:
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Modules which are included in the kernel are installed in the
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directory:
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/lib/modules/$(KERNELRELEASE)/kernel
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External modules are installed in the directory:
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And external modules are installed in:
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/lib/modules/$(KERNELRELEASE)/extra
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--- 6.1 INSTALL_MOD_PATH
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--- 5.1 INSTALL_MOD_PATH
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Above are the default directories, but as always, some level of
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customization is possible. One can prefix the path using the variable
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INSTALL_MOD_PATH:
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Above are the default directories but as always some level of
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customization is possible. A prefix can be added to the
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installation path using the variable INSTALL_MOD_PATH:
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$ make INSTALL_MOD_PATH=/frodo modules_install
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=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel
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INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
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example above, can be specified on the command line when calling make.
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INSTALL_MOD_PATH has effect both when installing modules included in
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the kernel as well as when installing external modules.
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INSTALL_MOD_PATH may be set as an ordinary shell variable or,
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as shown above, can be specified on the command line when
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calling "make." This has effect when installing both in-tree
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and out-of-tree modules.
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--- 6.2 INSTALL_MOD_DIR
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--- 5.2 INSTALL_MOD_DIR
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When installing external modules they are by default installed to a
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directory under /lib/modules/$(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 to 'extra'.
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External modules are by default installed to a directory under
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/lib/modules/$(KERNELRELEASE)/extra, but you may wish to locate
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modules for a specific functionality in a separate directory.
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For this purpose, use INSTALL_MOD_DIR to specify an alternative
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name to "extra."
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$ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \
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M=`pwd` modules_install
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$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
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M=$PWD modules_install
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=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
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=== 7. Module versioning & Module.symvers
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=== 6. Module Versioning
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Module versioning is enabled by the CONFIG_MODVERSIONS tag.
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Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
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as a simple ABI consistency check. A CRC value of the full prototype
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for an exported symbol is created. When a module is loaded/used, the
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CRC values contained in the kernel are compared with similar values in
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the module; if they are not equal, the kernel refuses to load the
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module.
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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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Module.symvers contains a list of all exported symbols from a kernel
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build.
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Module.symvers contains a list of all exported symbols from a kernel build.
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--- 6.1 Symbols From the Kernel (vmlinux + modules)
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--- 7.1 Symbols from the kernel (vmlinux + modules)
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During a kernel build, a file named Module.symvers will be generated.
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Module.symvers contains all exported symbols from the kernel and
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compiled modules. For each symbols, the corresponding CRC value
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is stored too.
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During a kernel build, a file named Module.symvers will be
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generated. Module.symvers contains all exported symbols from
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the kernel and compiled modules. For each symbol, the
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corresponding CRC value is also stored.
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The syntax of the Module.symvers file is:
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<CRC> <Symbol> <module>
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Sample:
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<CRC> <Symbol> <module>
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0x2d036834 scsi_remove_host drivers/scsi/scsi_mod
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For a kernel build without CONFIG_MODVERSIONS enabled, the crc
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would read: 0x00000000
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For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
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would read 0x00000000.
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Module.symvers serves two purposes:
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1) It lists all exported symbols both from vmlinux and all modules
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2) It lists the CRC if CONFIG_MODVERSIONS is enabled
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1) It lists all exported symbols from vmlinux and all modules.
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2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
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--- 7.2 Symbols and external modules
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--- 6.2 Symbols and External Modules
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When building an external module, the build system needs access to
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the symbols from the kernel to check if all external symbols are
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defined. This is done in the MODPOST step and to obtain all
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symbols, modpost reads Module.symvers from the kernel.
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If a Module.symvers file is present in the directory where
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the external module is being built, this file will be read too.
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During the MODPOST step, a new Module.symvers file will be written
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containing all exported symbols that were not defined in the kernel.
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When building an external module, the build system needs access
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to the symbols from the kernel to check if all external symbols
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are defined. This is done in the MODPOST step. modpost obtains
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the symbols by reading Module.symvers from the kernel source
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tree. If a Module.symvers file is present in the directory
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where the external module is being built, this file will be
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read too. During the MODPOST step, a new Module.symvers file
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will be written containing all exported symbols that were not
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defined in the kernel.
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--- 7.3 Symbols from another external module
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--- 6.3 Symbols From Another External Module
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Sometimes, an external module uses exported symbols from another
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external module. Kbuild needs to have full knowledge on all symbols
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to avoid spitting out warnings about undefined symbols.
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Three solutions exist to let kbuild know all symbols of more than
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one external module.
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The method with a top-level kbuild file is recommended but may be
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impractical in certain situations.
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Sometimes, an external module uses exported symbols from
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another external module. kbuild needs to have full knowledge of
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all symbols to avoid spitting out warnings about undefined
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symbols. Three solutions exist for this situation.
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Use a top-level Kbuild file
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If you have two modules: 'foo' and 'bar', and 'foo' needs
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symbols from 'bar', then one can use a common top-level kbuild
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file so both modules are compiled in same build.
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NOTE: The method with a top-level kbuild file is recommended
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but may be impractical in certain situations.
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Consider following directory layout:
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./foo/ <= contains the foo module
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./bar/ <= contains the bar module
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The top-level Kbuild file would then look like:
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Use a top-level kbuild file
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If you have two modules, foo.ko and bar.ko, where
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foo.ko needs symbols from bar.ko, then you can use a
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common top-level kbuild file so both modules are
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compiled in the same build. Consider following
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directory layout:
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#./Kbuild: (this file may also be named Makefile)
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./foo/ <= contains foo.ko
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./bar/ <= contains bar.ko
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The top-level kbuild file would then look like:
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#./Kbuild (or ./Makefile):
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obj-y := foo/ bar/
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Executing:
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make -C $KDIR M=`pwd`
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And executing:
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$ make -C $KDIR M=$PWD
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will then do the expected and compile both modules with full
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knowledge on symbols from both modules.
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Will then do the expected and compile both modules with
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full knowledge of symbols from either module.
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Use an extra Module.symvers file
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When an external module is built, a Module.symvers file is
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generated containing all exported symbols which are not
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defined in the kernel.
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To get access to symbols from module 'bar', one can copy the
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Module.symvers file from the compilation of the 'bar' module
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to the directory where the 'foo' module is built.
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During the module build, kbuild will read the Module.symvers
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file in the directory of the external module and when the
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build is finished, a new Module.symvers file is created
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containing the sum of all symbols defined and not part of the
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kernel.
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When an external module is built, a Module.symvers file
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is generated containing all exported symbols which are
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not defined in the kernel. To get access to symbols
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from bar.ko, copy the Module.symvers file from the
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compilation of bar.ko to the directory where foo.ko is
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built. During the module build, kbuild will read the
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Module.symvers file in the directory of the external
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module, and when the build is finished, a new
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Module.symvers file is created containing the sum of
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all symbols defined and not part of the kernel.
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Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile
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If it is impractical to copy Module.symvers from another
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module, you can assign a space separated list of files to
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KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be
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loaded by modpost during the initialisation of its symbol
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tables.
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Use "make" variable KBUILD_EXTRA_SYMBOLS
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If it is impractical to copy Module.symvers from
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another module, you can assign a space separated list
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of files to KBUILD_EXTRA_SYMBOLS in your build
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file. These files will be loaded by modpost during the
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initialization of its symbol tables.
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=== 8. Tips & Tricks
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=== 7. Tips & Tricks
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--- 8.1 Testing for CONFIG_FOO_BAR
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--- 7.1 Testing for CONFIG_FOO_BAR
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Modules often need 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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Modules often need to check for certain CONFIG_ options to
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decide if a specific feature is included in the module. In
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kbuild this is done by referencing the CONFIG_ variable
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directly.
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#fs/ext2/Makefile
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obj-$(CONFIG_EXT2_FS) += ext2.o
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@ -535,9 +531,9 @@ Module.symvers contains a list of all exported symbols from a kernel build.
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ext2-y := balloc.o bitmap.o dir.o
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ext2-$(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
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testing for CONFIG_ definitions.
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External modules have traditionally used "grep" to check for
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specific CONFIG_ settings directly in .config. This usage is
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broken. As introduced before, external modules should use
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kbuild for building and can therefore use the same methods as
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in-tree modules when testing for CONFIG_ definitions.
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