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
|
|
@ -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.
|
||||
kbuild will add options to "gcc" so the relevant directories
|
||||
are searched.
|
||||
|
||||
#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
|
||||
|
||||
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_$F.o (take effect only for a single file).
|
||||
|
||||
In our example, if we move 8123_if.h to a subdirectory named include/
|
||||
the resulting Kbuild file would look like:
|
||||
Using the example from section 3, if we moved 8123_if.h to a
|
||||
subdirectory named include, the resulting kbuild file would
|
||||
look like:
|
||||
|
||||
--> filename: Kbuild
|
||||
obj-m := 8123.o
|
||||
obj-m := 8123.o
|
||||
|
||||
EXTRA_CFLAGS := -Iinclude
|
||||
ccflags-y := -Iinclude
|
||||
8123-y := 8123_if.o 8123_pci.o 8123_bin.o
|
||||
|
||||
Note that in the assignment there is no space between -I and the path.
|
||||
This is a kbuild limitation: there must be no space present.
|
||||
Note that in the assignment there is no space between -I and
|
||||
the path. This is a limitation of kbuild: there must be no
|
||||
space present.
|
||||
|
||||
--- 5.3 External modules using several directories
|
||||
|
||||
If an external module does not follow the usual kernel style, but
|
||||
decides to spread files over several directories, then kbuild can
|
||||
handle this too.
|
||||
--- 4.3 Several Subdirectories
|
||||
|
||||
kbuild can handle files that are spread over several directories.
|
||||
Consider the following example:
|
||||
|
||||
|
|
||||
+- src/complex_main.c
|
||||
| +- hal/hardwareif.c
|
||||
| +- hal/include/hardwareif.h
|
||||
+- include/complex.h
|
||||
.
|
||||
|__ src
|
||||
| |__ complex_main.c
|
||||
| |__ hal
|
||||
| |__ hardwareif.c
|
||||
| |__ include
|
||||
| |__ hardwareif.h
|
||||
|__ include
|
||||
|__ complex.h
|
||||
|
||||
To build a single module named complex.ko, we then need the following
|
||||
To build the module complex.ko, we then need the following
|
||||
kbuild file:
|
||||
|
||||
Kbuild:
|
||||
--> filename: Kbuild
|
||||
obj-m := complex.o
|
||||
complex-y := src/complex_main.o
|
||||
complex-y += src/hal/hardwareif.o
|
||||
|
||||
EXTRA_CFLAGS := -I$(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.
|
||||
|
||||
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 -
|
||||
although this is NOT recommended practice. The syntax is to specify
|
||||
the directory relative to the directory where the Kbuild file is
|
||||
located.
|
||||
=== 5. Module Installation
|
||||
|
||||
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:
|
||||
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
|
||||
1) It lists all exported symbols from vmlinux and all modules.
|
||||
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
|
||||
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.
|
||||
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.
|
||||
|
||||
--- 7.3 Symbols from another external module
|
||||
--- 6.3 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.
|
||||
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.
|
||||
|
||||
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.
|
||||
NOTE: The method with a top-level kbuild file is recommended
|
||||
but may be impractical in certain situations.
|
||||
|
||||
Consider following directory layout:
|
||||
./foo/ <= contains the foo module
|
||||
./bar/ <= contains the bar module
|
||||
The top-level Kbuild file would then look like:
|
||||
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:
|
||||
|
||||
#./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/
|
||||
|
||||
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.
|
||||
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 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.
|
||||
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.
|
||||
|
||||
=== 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
|
||||
a specific feature shall be included in the module. When kbuild is used
|
||||
this is done by referencing the CONFIG_ variable directly.
|
||||
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.
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue