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>

1 files changed, 177 insertions, 181 deletions

diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt
index 799b6835993f..b572db3c5fee 100644
--- a/Documentation/kbuild/modules.txt
+++ b/Documentation/kbuild/modules.txt
@@ -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.
-
-                #include "8123_if.h"
-
-        will do the job.
+        kbuild will add options to "gcc" so the relevant directories
+        are searched.
 
---- 5.2 External modules using an include/ dir
+--- 4.2 Single Subdirectory
 
-        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).
+        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.
 
-        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.
-
---- 5.3 External modules using several directories
+        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.
 
-        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
-
-        To build a single module named complex.ko, we then need the following
+        .
+        |__ src
+        |   |__ complex_main.c
+        |   |__ hal
+        |       |__ hardwareif.c
+        |       |__ include
+        |           |__ hardwareif.h
+        |__ include
+            |__ complex.h
+
+        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.
 
-        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.
+        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.
 
-        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
+=== 5. 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
-
---- 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)
+        1) It lists all exported symbols from vmlinux and all modules.
+        2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
+
+--- 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. 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.
+
+--- 6.3 Symbols From Another External Module
+
+        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.
+
+        NOTE: 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.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:
+
+                ./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.
-
-        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.
+                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
+                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.
+
+=== 7. Tips & Tricks
+
+--- 7.1 Testing for CONFIG_FOO_BAR
+
+        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.