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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 2010-09-19 23:06:36 -07:00 committed by Michal Marek
parent efdf02cf06
commit 9f02186c23
1 changed files with 167 additions and 171 deletions
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Documentation/kbuild/modules.txt
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@ -15,17 +15,17 @@ This document describes how-to build an out-of-tree kernel module.
--- 3.2 Separate Kbuild file and Makefile --- 3.2 Separate Kbuild file and Makefile
--- 3.3 Binary Blobs --- 3.3 Binary Blobs
--- 3.4 Building Multiple Modules --- 3.4 Building Multiple Modules
=== 4. Include files === 4. Include Files
--- 4.1 How to include files from the kernel include dir --- 4.1 Kernel Includes
--- 4.2 External modules using an include/ dir --- 4.2 Single Subdirectory
--- 4.3 External modules using several directories --- 4.3 Several Subdirectories
=== 5. Module installation === 5. Module Installation
--- 5.1 INSTALL_MOD_PATH --- 5.1 INSTALL_MOD_PATH
--- 5.2 INSTALL_MOD_DIR --- 5.2 INSTALL_MOD_DIR
=== 6. Module versioning & Module.symvers === 6. Module Versioning
--- 6.1 Symbols from the kernel (vmlinux + modules) --- 6.1 Symbols From the Kernel (vmlinux + modules)
--- 6.2 Symbols and external modules --- 6.2 Symbols and External Modules
--- 6.3 Symbols from another external module --- 6.3 Symbols From Another External Module
=== 7. Tips & Tricks === 7. Tips & Tricks
--- 7.1 Testing for CONFIG_FOO_BAR --- 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! It is that simple!
=== 5. Include files === 4. Include Files
Include files are a necessity when a .c file uses something from other .c Within the kernel, header files are kept in standard locations
files (not strictly in the sense of C, but if good programming practice is according to the following rule:
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 * If the header file only describes the internal interface of a
shall be placed in the same directory as the .c files. module, then the file is placed in the same directory as the
- If the .h files describe an interface used by other parts of the kernel source files.
located in different directories, the .h files shall be located in * If the header file describes an interface used by other parts
include/linux/ or other include/ directories as appropriate. 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 NOTE: There are two notable exceptions to this rule: larger
under include/ such as include/scsi. Another exception is arch-specific subsystems have their own directory under include/, such as
.h files which are located under include/asm-$(ARCH)/*. include/scsi; and architecture specific headers are located
under arch/$(ARCH)/include/.
External modules have a tendency to locate include files in a separate include/ --- 4.1 Kernel Includes
directory and therefore need to deal with this in their kbuild file.
--- 5.1 How to include files from the kernel include dir To include a header file located under include/linux/, simply
use:
When a module needs to include a file from include/linux/, then one
just uses:
#include <linux/modules.h> #include <linux/modules.h>
kbuild will make sure to add options to gcc so the relevant kbuild will add options to "gcc" so the relevant directories
directories are searched. are searched.
Likewise for .h files placed in the same directory as the .c file.
#include "8123_if.h" --- 4.2 Single Subdirectory
will do the job. 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.
--- 5.2 External modules using an include/ dir Using the example from section 3, if we moved 8123_if.h to a
subdirectory named include, the resulting kbuild file would
External modules often locate their .h files in a separate include/ look like:
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_$F.o (take effect only for a single file).
In our example, if we move 8123_if.h to a subdirectory named include/
the resulting Kbuild file would look like:
--> filename: Kbuild --> filename: Kbuild
obj-m := 8123.o obj-m := 8123.o
EXTRA_CFLAGS := -Iinclude ccflags-y := -Iinclude
8123-y := 8123_if.o 8123_pci.o 8123_bin.o 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
Note that in the assignment there is no space between -I and the path. Note that in the assignment there is no space between -I and
This is a kbuild limitation: there must be no space present. the path. This is a limitation of kbuild: there must be no
space present.
--- 5.3 External modules using several directories --- 4.3 Several Subdirectories
If an external module does not follow the usual kernel style, but
decides to spread files over several directories, then kbuild can
handle this too.
kbuild can handle files that are spread over several directories.
Consider the following example: Consider the following example:
| .
+- src/complex_main.c |__ src
| +- hal/hardwareif.c | |__ complex_main.c
| +- hal/include/hardwareif.h | |__ hal
+- include/complex.h | |__ hardwareif.c
| |__ include
| |__ hardwareif.h
|__ include
|__ complex.h
To build a single module named complex.ko, we then need the following To build the module complex.ko, we then need the following
kbuild file: kbuild file:
Kbuild: --> filename: Kbuild
obj-m := complex.o obj-m := complex.o
complex-y := src/complex_main.o complex-y := src/complex_main.o
complex-y += src/hal/hardwareif.o complex-y += src/hal/hardwareif.o
EXTRA_CFLAGS := -I$(src)/include ccflags-y := -I$(src)/include
EXTRA_CFLAGS += -I$(src)src/hal/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.
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.
kbuild knows how to handle .o files located in another directory - === 5. Module Installation
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 Modules which are included in the kernel are installed in the
for the .h files. When kbuild executes, the current directory is always directory:
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/$(KERNELRELEASE)/kernel /lib/modules/$(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 Above are the default directories but as always some level of
customization is possible. One can prefix the path using the variable customization is possible. A prefix can be added to the
INSTALL_MOD_PATH: installation path using the variable INSTALL_MOD_PATH:
$ make INSTALL_MOD_PATH=/frodo modules_install $ make INSTALL_MOD_PATH=/frodo modules_install
=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel
INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the INSTALL_MOD_PATH may be set as an ordinary shell variable or,
example above, can be specified on the command line when calling make. as shown above, can be specified on the command line when
INSTALL_MOD_PATH has effect both when installing modules included in calling "make." This has effect when installing both in-tree
the kernel as well as when installing external modules. 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 External modules are by default installed to a directory under
directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish /lib/modules/$(KERNELRELEASE)/extra, but you may wish to locate
to locate modules for a specific functionality in a separate modules for a specific functionality in a separate directory.
directory. For this purpose, one can use INSTALL_MOD_DIR to specify an For this purpose, use INSTALL_MOD_DIR to specify an alternative
alternative name to 'extra'. name to "extra."
$ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \ $ make INSTALL_MOD_DIR=gandalf -C $KDIR \
M=`pwd` modules_install M=$PWD modules_install
=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf => Install dir: /lib/modules/$(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 Module.symvers contains a list of all exported symbols from a kernel
versioning creates a CRC value of the full prototype for an exported symbol and build.
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. --- 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
During a kernel build, a file named Module.symvers will be generated. the kernel and compiled modules. For each symbol, the
Module.symvers contains all exported symbols from the kernel and corresponding CRC value is also stored.
compiled modules. For each symbols, the corresponding CRC value
is stored too.
The syntax of the Module.symvers file is: The syntax of the Module.symvers file is:
<CRC> <Symbol> <module> <CRC> <Symbol> <module>
Sample:
0x2d036834 scsi_remove_host drivers/scsi/scsi_mod 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod
For a kernel build without CONFIG_MODVERSIONS enabled, the crc For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
would read: 0x00000000 would read 0x00000000.
Module.symvers serves two purposes: Module.symvers serves two purposes:
1) It lists all exported symbols both from vmlinux and all modules 1) It lists all exported symbols from vmlinux and all modules.
2) It lists the CRC if CONFIG_MODVERSIONS is enabled 2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
--- 7.2 Symbols and external modules --- 6.2 Symbols and External Modules
When building an external module, the build system needs access to When building an external module, the build system needs access
the symbols from the kernel to check if all external symbols are to the symbols from the kernel to check if all external symbols
defined. This is done in the MODPOST step and to obtain all are defined. This is done in the MODPOST step. modpost obtains
symbols, modpost reads Module.symvers from the kernel. the symbols by reading Module.symvers from the kernel source
If a Module.symvers file is present in the directory where tree. If a Module.symvers file is present in the directory
the external module is being built, this file will be read too. where the external module is being built, this file will be
During the MODPOST step, a new Module.symvers file will be written read too. During the MODPOST step, a new Module.symvers file
containing all exported symbols that were not defined in the kernel. will be written containing all exported symbols that were not
defined in the kernel.
--- 7.3 Symbols from another external module --- 6.3 Symbols From Another External Module
Sometimes, an external module uses exported symbols from another Sometimes, an external module uses exported symbols from
external module. Kbuild needs to have full knowledge on all symbols another external module. kbuild needs to have full knowledge of
to avoid spitting out warnings about undefined symbols. all symbols to avoid spitting out warnings about undefined
Three solutions exist to let kbuild know all symbols of more than symbols. Three solutions exist for this situation.
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 NOTE: The method with a top-level kbuild file is recommended
If you have two modules: 'foo' and 'bar', and 'foo' needs but may be impractical in certain situations.
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: Use a top-level kbuild file
./foo/ <= contains the foo module If you have two modules, foo.ko and bar.ko, where
./bar/ <= contains the bar module foo.ko needs symbols from bar.ko, then you can use a
The top-level Kbuild file would then look like: common top-level kbuild file so both modules are
compiled in the same build. Consider following
directory layout:
#./Kbuild: (this file may also be named Makefile) ./foo/ <= contains foo.ko
./bar/ <= contains bar.ko
The top-level kbuild file would then look like:
#./Kbuild (or ./Makefile):
obj-y := foo/ bar/ obj-y := foo/ bar/
Executing: And executing:
make -C $KDIR M=`pwd` $ make -C $KDIR M=$PWD
will then do the expected and compile both modules with full Will then do the expected and compile both modules with
knowledge on symbols from both modules. full knowledge of symbols from either module.
Use an extra Module.symvers file Use an extra Module.symvers file
When an external module is built, a Module.symvers file is When an external module is built, a Module.symvers file
generated containing all exported symbols which are not is generated containing all exported symbols which are
defined in the kernel. not defined in the kernel. To get access to symbols
To get access to symbols from module 'bar', one can copy the from bar.ko, copy the Module.symvers file from the
Module.symvers file from the compilation of the 'bar' module compilation of bar.ko to the directory where foo.ko is
to the directory where the 'foo' module is built. built. During the module build, kbuild will read the
During the module build, kbuild will read the Module.symvers Module.symvers file in the directory of the external
file in the directory of the external module and when the module, and when the build is finished, a new
build is finished, a new Module.symvers file is created Module.symvers file is created containing the sum of
containing the sum of all symbols defined and not part of the all symbols defined and not part of the kernel.
kernel.
Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile Use "make" variable KBUILD_EXTRA_SYMBOLS
If it is impractical to copy Module.symvers from another If it is impractical to copy Module.symvers from
module, you can assign a space separated list of files to another module, you can assign a space separated list
KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be of files to KBUILD_EXTRA_SYMBOLS in your build
loaded by modpost during the initialisation of its symbol file. These files will be loaded by modpost during the
tables. initialization of its symbol tables.
=== 8. Tips & Tricks === 7. Tips & Tricks
--- 8.1 Testing for CONFIG_FOO_BAR --- 7.1 Testing for CONFIG_FOO_BAR
Modules often need to check for certain CONFIG_ options to decide if Modules often need to check for certain CONFIG_ options to
a specific feature shall be included in the module. When kbuild is used decide if a specific feature is included in the module. In
this is done by referencing the CONFIG_ variable directly. kbuild this is done by referencing the CONFIG_ variable
directly.
#fs/ext2/Makefile #fs/ext2/Makefile
obj-$(CONFIG_EXT2_FS) += ext2.o obj-$(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-y := balloc.o bitmap.o dir.o
ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
External modules have traditionally used grep to check for specific External modules have traditionally used "grep" to check for
CONFIG_ settings directly in .config. This usage is broken. specific CONFIG_ settings directly in .config. This usage is
As introduced before, external modules shall use kbuild when building broken. As introduced before, external modules should use
and therefore can use the same methods as in-kernel modules when kbuild for building and can therefore use the same methods as
testing for CONFIG_ definitions. in-tree modules when testing for CONFIG_ definitions.