Initial import of modified Linux 2.6.28 tree

Original upstream URL:
git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git | branch linux-2.6.28.y

4 files changed, 2167 insertions, 0 deletions

diff --git a/Documentation/kbuild/00-INDEX b/Documentation/kbuild/00-INDEX
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+00-INDEX
+    	- this file: info on the kernel build process
+kconfig-language.txt
+	- specification of Config Language, the language in Kconfig files
+makefiles.txt
+	- developer information for linux kernel makefiles
+modules.txt
+	- how to build modules and to install them
diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt
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+Introduction
+------------
+
+The configuration database is a collection of configuration options
+organized in a tree structure:
+
+	+- Code maturity level options
+	|  +- Prompt for development and/or incomplete code/drivers
+	+- General setup
+	|  +- Networking support
+	|  +- System V IPC
+	|  +- BSD Process Accounting
+	|  +- Sysctl support
+	+- Loadable module support
+	|  +- Enable loadable module support
+	|     +- Set version information on all module symbols
+	|     +- Kernel module loader
+	+- ...
+
+Every entry has its own dependencies. These dependencies are used
+to determine the visibility of an entry. Any child entry is only
+visible if its parent entry is also visible.
+
+Menu entries
+------------
+
+Most entries define a config option; all other entries help to organize
+them. A single configuration option is defined like this:
+
+config MODVERSIONS
+	bool "Set version information on all module symbols"
+	depends on MODULES
+	help
+	  Usually, modules have to be recompiled whenever you switch to a new
+	  kernel.  ...
+
+Every line starts with a key word and can be followed by multiple
+arguments.  "config" starts a new config entry. The following lines
+define attributes for this config option. Attributes can be the type of
+the config option, input prompt, dependencies, help text and default
+values. A config option can be defined multiple times with the same
+name, but every definition can have only a single input prompt and the
+type must not conflict.
+
+Menu attributes
+---------------
+
+A menu entry can have a number of attributes. Not all of them are
+applicable everywhere (see syntax).
+
+- type definition: "bool"/"tristate"/"string"/"hex"/"int"
+  Every config option must have a type. There are only two basic types:
+  tristate and string; the other types are based on these two. The type
+  definition optionally accepts an input prompt, so these two examples
+  are equivalent:
+
+	bool "Networking support"
+  and
+	bool
+	prompt "Networking support"
+
+- input prompt: "prompt" <prompt> ["if" <expr>]
+  Every menu entry can have at most one prompt, which is used to display
+  to the user. Optionally dependencies only for this prompt can be added
+  with "if".
+
+- default value: "default" <expr> ["if" <expr>]
+  A config option can have any number of default values. If multiple
+  default values are visible, only the first defined one is active.
+  Default values are not limited to the menu entry where they are
+  defined. This means the default can be defined somewhere else or be
+  overridden by an earlier definition.
+  The default value is only assigned to the config symbol if no other
+  value was set by the user (via the input prompt above). If an input
+  prompt is visible the default value is presented to the user and can
+  be overridden by him.
+  Optionally, dependencies only for this default value can be added with
+  "if".
+
+- type definition + default value:
+	"def_bool"/"def_tristate" <expr> ["if" <expr>]
+  This is a shorthand notation for a type definition plus a value.
+  Optionally dependencies for this default value can be added with "if".
+
+- dependencies: "depends on" <expr>
+  This defines a dependency for this menu entry. If multiple
+  dependencies are defined, they are connected with '&&'. Dependencies
+  are applied to all other options within this menu entry (which also
+  accept an "if" expression), so these two examples are equivalent:
+
+	bool "foo" if BAR
+	default y if BAR
+  and
+	depends on BAR
+	bool "foo"
+	default y
+
+- reverse dependencies: "select" <symbol> ["if" <expr>]
+  While normal dependencies reduce the upper limit of a symbol (see
+  below), reverse dependencies can be used to force a lower limit of
+  another symbol. The value of the current menu symbol is used as the
+  minimal value <symbol> can be set to. If <symbol> is selected multiple
+  times, the limit is set to the largest selection.
+  Reverse dependencies can only be used with boolean or tristate
+  symbols.
+  Note:
+	select should be used with care. select will force
+	a symbol to a value without visiting the dependencies.
+	By abusing select you are able to select a symbol FOO even
+	if FOO depends on BAR that is not set.
+	In general use select only for non-visible symbols
+	(no prompts anywhere) and for symbols with no dependencies.
+	That will limit the usefulness but on the other hand avoid
+	the illegal configurations all over.
+	kconfig should one day warn about such things.
+
+- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
+  This allows to limit the range of possible input values for int
+  and hex symbols. The user can only input a value which is larger than
+  or equal to the first symbol and smaller than or equal to the second
+  symbol.
+
+- help text: "help" or "---help---"
+  This defines a help text. The end of the help text is determined by
+  the indentation level, this means it ends at the first line which has
+  a smaller indentation than the first line of the help text.
+  "---help---" and "help" do not differ in behaviour, "---help---" is
+  used to help visually separate configuration logic from help within
+  the file as an aid to developers.
+
+- misc options: "option" <symbol>[=<value>]
+  Various less common options can be defined via this option syntax,
+  which can modify the behaviour of the menu entry and its config
+  symbol. These options are currently possible:
+
+  - "defconfig_list"
+    This declares a list of default entries which can be used when
+    looking for the default configuration (which is used when the main
+    .config doesn't exists yet.)
+
+  - "modules"
+    This declares the symbol to be used as the MODULES symbol, which
+    enables the third modular state for all config symbols.
+
+  - "env"=<value>
+    This imports the environment variable into Kconfig. It behaves like
+    a default, except that the value comes from the environment, this
+    also means that the behaviour when mixing it with normal defaults is
+    undefined at this point. The symbol is currently not exported back
+    to the build environment (if this is desired, it can be done via
+    another symbol).
+
+Menu dependencies
+-----------------
+
+Dependencies define the visibility of a menu entry and can also reduce
+the input range of tristate symbols. The tristate logic used in the
+expressions uses one more state than normal boolean logic to express the
+module state. Dependency expressions have the following syntax:
+
+<expr> ::= <symbol>                             (1)
+           <symbol> '=' <symbol>                (2)
+           <symbol> '!=' <symbol>               (3)
+           '(' <expr> ')'                       (4)
+           '!' <expr>                           (5)
+           <expr> '&&' <expr>                   (6)
+           <expr> '||' <expr>                   (7)
+
+Expressions are listed in decreasing order of precedence. 
+
+(1) Convert the symbol into an expression. Boolean and tristate symbols
+    are simply converted into the respective expression values. All
+    other symbol types result in 'n'.
+(2) If the values of both symbols are equal, it returns 'y',
+    otherwise 'n'.
+(3) If the values of both symbols are equal, it returns 'n',
+    otherwise 'y'.
+(4) Returns the value of the expression. Used to override precedence.
+(5) Returns the result of (2-/expr/).
+(6) Returns the result of min(/expr/, /expr/).
+(7) Returns the result of max(/expr/, /expr/).
+
+An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
+respectively for calculations). A menu entry becomes visible when it's
+expression evaluates to 'm' or 'y'.
+
+There are two types of symbols: constant and non-constant symbols.
+Non-constant symbols are the most common ones and are defined with the
+'config' statement. Non-constant symbols consist entirely of alphanumeric
+characters or underscores.
+Constant symbols are only part of expressions. Constant symbols are
+always surrounded by single or double quotes. Within the quote, any
+other character is allowed and the quotes can be escaped using '\'.
+
+Menu structure
+--------------
+
+The position of a menu entry in the tree is determined in two ways. First
+it can be specified explicitly:
+
+menu "Network device support"
+	depends on NET
+
+config NETDEVICES
+	...
+
+endmenu
+
+All entries within the "menu" ... "endmenu" block become a submenu of
+"Network device support". All subentries inherit the dependencies from
+the menu entry, e.g. this means the dependency "NET" is added to the
+dependency list of the config option NETDEVICES.
+
+The other way to generate the menu structure is done by analyzing the
+dependencies. If a menu entry somehow depends on the previous entry, it
+can be made a submenu of it. First, the previous (parent) symbol must
+be part of the dependency list and then one of these two conditions
+must be true:
+- the child entry must become invisible, if the parent is set to 'n'
+- the child entry must only be visible, if the parent is visible
+
+config MODULES
+	bool "Enable loadable module support"
+
+config MODVERSIONS
+	bool "Set version information on all module symbols"
+	depends on MODULES
+
+comment "module support disabled"
+	depends on !MODULES
+
+MODVERSIONS directly depends on MODULES, this means it's only visible if
+MODULES is different from 'n'. The comment on the other hand is always
+visible when MODULES is visible (the (empty) dependency of MODULES is
+also part of the comment dependencies).
+
+
+Kconfig syntax
+--------------
+
+The configuration file describes a series of menu entries, where every
+line starts with a keyword (except help texts). The following keywords
+end a menu entry:
+- config
+- menuconfig
+- choice/endchoice
+- comment
+- menu/endmenu
+- if/endif
+- source
+The first five also start the definition of a menu entry.
+
+config:
+
+	"config" <symbol>
+	<config options>
+
+This defines a config symbol <symbol> and accepts any of above
+attributes as options.
+
+menuconfig:
+	"menuconfig" <symbol>
+	<config options>
+
+This is similar to the simple config entry above, but it also gives a
+hint to front ends, that all suboptions should be displayed as a
+separate list of options.
+
+choices:
+
+	"choice"
+	<choice options>
+	<choice block>
+	"endchoice"
+
+This defines a choice group and accepts any of the above attributes as
+options. A choice can only be of type bool or tristate, while a boolean
+choice only allows a single config entry to be selected, a tristate
+choice also allows any number of config entries to be set to 'm'. This
+can be used if multiple drivers for a single hardware exists and only a
+single driver can be compiled/loaded into the kernel, but all drivers
+can be compiled as modules.
+A choice accepts another option "optional", which allows to set the
+choice to 'n' and no entry needs to be selected.
+
+comment:
+
+	"comment" <prompt>
+	<comment options>
+
+This defines a comment which is displayed to the user during the
+configuration process and is also echoed to the output files. The only
+possible options are dependencies.
+
+menu:
+
+	"menu" <prompt>
+	<menu options>
+	<menu block>
+	"endmenu"
+
+This defines a menu block, see "Menu structure" above for more
+information. The only possible options are dependencies.
+
+if:
+
+	"if" <expr>
+	<if block>
+	"endif"
+
+This defines an if block. The dependency expression <expr> is appended
+to all enclosed menu entries.
+
+source:
+
+	"source" <prompt>
+
+This reads the specified configuration file. This file is always parsed.
+
+mainmenu:
+
+	"mainmenu" <prompt>
+
+This sets the config program's title bar if the config program chooses
+to use it.
+
+
+Kconfig hints
+-------------
+This is a collection of Kconfig tips, most of which aren't obvious at
+first glance and most of which have become idioms in several Kconfig
+files.
+
+Adding common features and make the usage configurable
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It is a common idiom to implement a feature/functionality that are
+relevant for some architectures but not all.
+The recommended way to do so is to use a config variable named HAVE_*
+that is defined in a common Kconfig file and selected by the relevant
+architectures.
+An example is the generic IOMAP functionality.
+
+We would in lib/Kconfig see:
+
+# Generic IOMAP is used to ...
+config HAVE_GENERIC_IOMAP
+
+config GENERIC_IOMAP
+	depends on HAVE_GENERIC_IOMAP && FOO
+
+And in lib/Makefile we would see:
+obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
+
+For each architecture using the generic IOMAP functionality we would see:
+
+config X86
+	select ...
+	select HAVE_GENERIC_IOMAP
+	select ...
+
+Note: we use the existing config option and avoid creating a new
+config variable to select HAVE_GENERIC_IOMAP.
+
+Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
+introduced to overcome the limitation of select which will force a
+config option to 'y' no matter the dependencies.
+The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
+situation where select forces a symbol equals to 'y'.
+
+Build as module only
+~~~~~~~~~~~~~~~~~~~~
+To restrict a component build to module-only, qualify its config symbol
+with "depends on m".  E.g.:
+
+config FOO
+	depends on BAR && m
+
+limits FOO to module (=m) or disabled (=n).
+
diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
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+Linux Kernel Makefiles
+
+This document describes the Linux kernel Makefiles.
+
+=== Table of Contents
+
+	=== 1 Overview
+	=== 2 Who does what
+	=== 3 The kbuild files
+	   --- 3.1 Goal definitions
+	   --- 3.2 Built-in object goals - obj-y
+	   --- 3.3 Loadable module goals - obj-m
+	   --- 3.4 Objects which export symbols
+	   --- 3.5 Library file goals - lib-y
+	   --- 3.6 Descending down in directories
+	   --- 3.7 Compilation flags
+	   --- 3.8 Command line dependency
+	   --- 3.9 Dependency tracking
+	   --- 3.10 Special Rules
+	   --- 3.11 $(CC) support functions
+
+	=== 4 Host Program support
+	   --- 4.1 Simple Host Program
+	   --- 4.2 Composite Host Programs
+	   --- 4.3 Defining shared libraries
+	   --- 4.4 Using C++ for host programs
+	   --- 4.5 Controlling compiler options for host programs
+	   --- 4.6 When host programs are actually built
+	   --- 4.7 Using hostprogs-$(CONFIG_FOO)
+
+	=== 5 Kbuild clean infrastructure
+
+	=== 6 Architecture Makefiles
+	   --- 6.1 Set variables to tweak the build to the architecture
+	   --- 6.2 Add prerequisites to archprepare:
+	   --- 6.3 List directories to visit when descending
+	   --- 6.4 Architecture-specific boot images
+	   --- 6.5 Building non-kbuild targets
+	   --- 6.6 Commands useful for building a boot image
+	   --- 6.7 Custom kbuild commands
+	   --- 6.8 Preprocessing linker scripts
+
+	=== 7 Kbuild Variables
+	=== 8 Makefile language
+	=== 9 Credits
+	=== 10 TODO
+
+=== 1 Overview
+
+The Makefiles have five parts:
+
+	Makefile		the top Makefile.
+	.config			the kernel configuration file.
+	arch/$(ARCH)/Makefile	the arch Makefile.
+	scripts/Makefile.*	common rules etc. for all kbuild Makefiles.
+	kbuild Makefiles	there are about 500 of these.
+
+The top Makefile reads the .config file, which comes from the kernel
+configuration process.
+
+The top Makefile is responsible for building two major products: vmlinux
+(the resident kernel image) and modules (any module files).
+It builds these goals by recursively descending into the subdirectories of
+the kernel source tree.
+The list of subdirectories which are visited depends upon the kernel
+configuration. The top Makefile textually includes an arch Makefile
+with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
+architecture-specific information to the top Makefile.
+
+Each subdirectory has a kbuild Makefile which carries out the commands
+passed down from above. The kbuild Makefile uses information from the
+.config file to construct various file lists used by kbuild to build
+any built-in or modular targets.
+
+scripts/Makefile.* contains all the definitions/rules etc. that
+are used to build the kernel based on the kbuild makefiles.
+
+
+=== 2 Who does what
+
+People have four different relationships with the kernel Makefiles.
+
+*Users* are people who build kernels.  These people type commands such as
+"make menuconfig" or "make".  They usually do not read or edit
+any kernel Makefiles (or any other source files).
+
+*Normal developers* are people who work on features such as device
+drivers, file systems, and network protocols.  These people need to
+maintain the kbuild Makefiles for the subsystem they are
+working on.  In order to do this effectively, they need some overall
+knowledge about the kernel Makefiles, plus detailed knowledge about the
+public interface for kbuild.
+
+*Arch developers* are people who work on an entire architecture, such
+as sparc or ia64.  Arch developers need to know about the arch Makefile
+as well as kbuild Makefiles.
+
+*Kbuild developers* are people who work on the kernel build system itself.
+These people need to know about all aspects of the kernel Makefiles.
+
+This document is aimed towards normal developers and arch developers.
+
+
+=== 3 The kbuild files
+
+Most Makefiles within the kernel are kbuild Makefiles that use the
+kbuild infrastructure. This chapter introduces the syntax used in the
+kbuild makefiles.
+The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
+be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
+file will be used.
+
+Section 3.1 "Goal definitions" is a quick intro, further chapters provide
+more details, with real examples.
+
+--- 3.1 Goal definitions
+
+	Goal definitions are the main part (heart) of the kbuild Makefile.
+	These lines define the files to be built, any special compilation
+	options, and any subdirectories to be entered recursively.
+
+	The most simple kbuild makefile contains one line:
+
+	Example:
+		obj-y += foo.o
+
+	This tells kbuild that there is one object in that directory, named
+	foo.o. foo.o will be built from foo.c or foo.S.
+
+	If foo.o shall be built as a module, the variable obj-m is used.
+	Therefore the following pattern is often used:
+
+	Example:
+		obj-$(CONFIG_FOO) += foo.o
+
+	$(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
+	If CONFIG_FOO is neither y nor m, then the file will not be compiled
+	nor linked.
+
+--- 3.2 Built-in object goals - obj-y
+
+	The kbuild Makefile specifies object files for vmlinux
+	in the $(obj-y) lists.  These lists depend on the kernel
+	configuration.
+
+	Kbuild compiles all the $(obj-y) files.  It then calls
+	"$(LD) -r" to merge these files into one built-in.o file.
+	built-in.o is later linked into vmlinux by the parent Makefile.
+
+	The order of files in $(obj-y) is significant.  Duplicates in
+	the lists are allowed: the first instance will be linked into
+	built-in.o and succeeding instances will be ignored.
+
+	Link order is significant, because certain functions
+	(module_init() / __initcall) will be called during boot in the
+	order they appear. So keep in mind that changing the link
+	order may e.g. change the order in which your SCSI
+	controllers are detected, and thus your disks are renumbered.
+
+	Example:
+		#drivers/isdn/i4l/Makefile
+		# Makefile for the kernel ISDN subsystem and device drivers.
+		# Each configuration option enables a list of files.
+		obj-$(CONFIG_ISDN)             += isdn.o
+		obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
+
+--- 3.3 Loadable module goals - obj-m
+
+	$(obj-m) specify object files which are built as loadable
+	kernel modules.
+
+	A module may be built from one source file or several source
+	files. In the case of one source file, the kbuild makefile
+	simply adds the file to $(obj-m).
+
+	Example:
+		#drivers/isdn/i4l/Makefile
+		obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
+
+	Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
+
+	If a kernel module is built from several source files, you specify
+	that you want to build a module in the same way as above.
+
+	Kbuild needs to know which the parts that you want to build your
+	module from, so you have to tell it by setting an
+	$(<module_name>-objs) variable.
+
+	Example:
+		#drivers/isdn/i4l/Makefile
+		obj-$(CONFIG_ISDN) += isdn.o
+		isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
+
+	In this example, the module name will be isdn.o. Kbuild will
+	compile the objects listed in $(isdn-objs) and then run
+	"$(LD) -r" on the list of these files to generate isdn.o.
+
+	Kbuild recognises objects used for composite objects by the suffix
+	-objs, and the suffix -y. This allows the Makefiles to use
+	the value of a CONFIG_ symbol to determine if an object is part
+	of a composite object.
+
+	Example:
+		#fs/ext2/Makefile
+	        obj-$(CONFIG_EXT2_FS)        += ext2.o
+		ext2-y                       := balloc.o bitmap.o
+	        ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
+
+	In this example, xattr.o is only part of the composite object
+	ext2.o if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
+
+	Note: Of course, when you are building objects into the kernel,
+	the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
+	kbuild will build an ext2.o file for you out of the individual
+	parts and then link this into built-in.o, as you would expect.
+
+--- 3.4 Objects which export symbols
+
+	No special notation is required in the makefiles for
+	modules exporting symbols.
+
+--- 3.5 Library file goals - lib-y
+
+	Objects listed with obj-* are used for modules, or
+	combined in a built-in.o for that specific directory.
+	There is also the possibility to list objects that will
+	be included in a library, lib.a.
+	All objects listed with lib-y are combined in a single
+	library for that directory.
+	Objects that are listed in obj-y and additionally listed in
+	lib-y will not be included in the library, since they will
+	be accessible anyway.
+	For consistency, objects listed in lib-m will be included in lib.a.
+
+	Note that the same kbuild makefile may list files to be built-in
+	and to be part of a library. Therefore the same directory
+	may contain both a built-in.o and a lib.a file.
+
+	Example:
+		#arch/i386/lib/Makefile
+		lib-y    := checksum.o delay.o
+
+	This will create a library lib.a based on checksum.o and delay.o.
+	For kbuild to actually recognize that there is a lib.a being built,
+	the directory shall be listed in libs-y.
+	See also "6.3 List directories to visit when descending".
+
+	Use of lib-y is normally restricted to lib/ and arch/*/lib.
+
+--- 3.6 Descending down in directories
+
+	A Makefile is only responsible for building objects in its own
+	directory. Files in subdirectories should be taken care of by
+	Makefiles in these subdirs. The build system will automatically
+	invoke make recursively in subdirectories, provided you let it know of
+	them.
+
+	To do so, obj-y and obj-m are used.
+	ext2 lives in a separate directory, and the Makefile present in fs/
+	tells kbuild to descend down using the following assignment.
+
+	Example:
+		#fs/Makefile
+		obj-$(CONFIG_EXT2_FS) += ext2/
+
+	If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
+	the corresponding obj- variable will be set, and kbuild will descend
+	down in the ext2 directory.
+	Kbuild only uses this information to decide that it needs to visit
+	the directory, it is the Makefile in the subdirectory that
+	specifies what is modules and what is built-in.
+
+	It is good practice to use a CONFIG_ variable when assigning directory
+	names. This allows kbuild to totally skip the directory if the
+	corresponding CONFIG_ option is neither 'y' nor 'm'.
+
+--- 3.7 Compilation flags
+
+    ccflags-y, asflags-y and ldflags-y
+	The three flags listed above applies only to the kbuild makefile
+	where they are assigned. They are used for all the normal
+	cc, as and ld invocation happenign during a recursive build.
+	Note: Flags with the same behaviour were previously named:
+	EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
+	They are yet supported but their use are deprecated.
+
+	ccflags-y specifies options for compiling C files with $(CC).
+
+	Example:
+		# drivers/sound/emu10k1/Makefile
+		ccflags-y += -I$(obj)
+		ccflags-$(DEBUG) += -DEMU10K1_DEBUG
+
+
+	This variable is necessary because the top Makefile owns the
+	variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
+	entire tree.
+
+	asflags-y is a similar string for per-directory options
+	when compiling assembly language source.
+
+	Example:
+		#arch/x86_64/kernel/Makefile
+		asflags-y := -traditional
+
+
+	ldflags-y is a string for per-directory options to $(LD).
+
+	Example:
+		#arch/m68k/fpsp040/Makefile
+		ldflags-y := -x
+
+    CFLAGS_$@, AFLAGS_$@
+
+	CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
+	kbuild makefile.
+
+	$(CFLAGS_$@) specifies per-file options for $(CC).  The $@
+	part has a literal value which specifies the file that it is for.
+
+	Example:
+		# drivers/scsi/Makefile
+		CFLAGS_aha152x.o =   -DAHA152X_STAT -DAUTOCONF
+		CFLAGS_gdth.o    = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
+				     -DGDTH_STATISTICS
+		CFLAGS_seagate.o =   -DARBITRATE -DPARITY -DSEAGATE_USE_ASM
+
+	These three lines specify compilation flags for aha152x.o,
+	gdth.o, and seagate.o
+
+	$(AFLAGS_$@) is a similar feature for source files in assembly
+	languages.
+
+	Example:
+		# arch/arm/kernel/Makefile
+		AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional
+		AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional
+
+--- 3.9 Dependency tracking
+
+	Kbuild tracks dependencies on the following:
+	1) All prerequisite files (both *.c and *.h)
+	2) CONFIG_ options used in all prerequisite files
+	3) Command-line used to compile target
+
+	Thus, if you change an option to $(CC) all affected files will
+	be re-compiled.
+
+--- 3.10 Special Rules
+
+	Special rules are used when the kbuild infrastructure does
+	not provide the required support. A typical example is
+	header files generated during the build process.
+	Another example are the architecture-specific Makefiles which
+	need special rules to prepare boot images etc.
+
+	Special rules are written as normal Make rules.
+	Kbuild is not executing in the directory where the Makefile is
+	located, so all special rules shall provide a relative
+	path to prerequisite files and target files.
+
+	Two variables are used when defining special rules:
+
+    $(src)
+	$(src) is a relative path which points to the directory
+	where the Makefile is located. Always use $(src) when
+	referring to files located in the src tree.
+
+    $(obj)
+	$(obj) is a relative path which points to the directory
+	where the target is saved. Always use $(obj) when
+	referring to generated files.
+
+	Example:
+		#drivers/scsi/Makefile
+		$(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
+			$(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
+
+	This is a special rule, following the normal syntax
+	required by make.
+	The target file depends on two prerequisite files. References
+	to the target file are prefixed with $(obj), references
+	to prerequisites are referenced with $(src) (because they are not
+	generated files).
+
+--- 3.11 $(CC) support functions
+
+	The kernel may be built with several different versions of
+	$(CC), each supporting a unique set of features and options.
+	kbuild provide basic support to check for valid options for $(CC).
+	$(CC) is usually the gcc compiler, but other alternatives are
+	available.
+
+    as-option
+	as-option is used to check if $(CC) -- when used to compile
+	assembler (*.S) files -- supports the given option. An optional
+	second option may be specified if the first option is not supported.
+
+	Example:
+		#arch/sh/Makefile
+		cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
+
+	In the above example, cflags-y will be assigned the option
+	-Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
+	The second argument is optional, and if supplied will be used
+	if first argument is not supported.
+
+    ld-option
+	ld-option is used to check if $(CC) when used to link object files
+	supports the given option.  An optional second option may be
+	specified if first option are not supported.
+
+	Example:
+		#arch/i386/kernel/Makefile
+		vsyscall-flags += $(call ld-option, -Wl$(comma)--hash-style=sysv)
+
+	In the above example, vsyscall-flags will be assigned the option
+	-Wl$(comma)--hash-style=sysv if it is supported by $(CC).
+	The second argument is optional, and if supplied will be used
+	if first argument is not supported.
+
+    as-instr
+	as-instr checks if the assembler reports a specific instruction
+	and then outputs either option1 or option2
+	C escapes are supported in the test instruction
+	Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options
+
+    cc-option
+	cc-option is used to check if $(CC) supports a given option, and not
+	supported to use an optional second option.
+
+	Example:
+		#arch/i386/Makefile
+		cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
+
+	In the above example, cflags-y will be assigned the option
+	-march=pentium-mmx if supported by $(CC), otherwise -march=i586.
+	The second argument to cc-option is optional, and if omitted,
+	cflags-y will be assigned no value if first option is not supported.
+	Note: cc-option uses KBUILD_CFLAGS for $(CC) options
+
+   cc-option-yn
+	cc-option-yn is used to check if gcc supports a given option
+	and return 'y' if supported, otherwise 'n'.
+
+	Example:
+		#arch/ppc/Makefile
+		biarch := $(call cc-option-yn, -m32)
+		aflags-$(biarch) += -a32
+		cflags-$(biarch) += -m32
+
+	In the above example, $(biarch) is set to y if $(CC) supports the -m32
+	option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
+	and $(cflags-y) will be assigned the values -a32 and -m32,
+	respectively.
+	Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
+
+    cc-option-align
+	gcc versions >= 3.0 changed the type of options used to specify
+	alignment of functions, loops etc. $(cc-option-align), when used
+	as prefix to the align options, will select the right prefix:
+	gcc < 3.00
+		cc-option-align = -malign
+	gcc >= 3.00
+		cc-option-align = -falign
+
+	Example:
+		KBUILD_CFLAGS += $(cc-option-align)-functions=4
+
+	In the above example, the option -falign-functions=4 is used for
+	gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used.
+	Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options
+
+    cc-version
+	cc-version returns a numerical version of the $(CC) compiler version.
+	The format is <major><minor> where both are two digits. So for example
+	gcc 3.41 would return 0341.
+	cc-version is useful when a specific $(CC) version is faulty in one
+	area, for example -mregparm=3 was broken in some gcc versions
+	even though the option was accepted by gcc.
+
+	Example:
+		#arch/i386/Makefile
+		cflags-y += $(shell \
+		if [ $(call cc-version) -ge 0300 ] ; then \
+			echo "-mregparm=3"; fi ;)
+
+	In the above example, -mregparm=3 is only used for gcc version greater
+	than or equal to gcc 3.0.
+
+    cc-ifversion
+	cc-ifversion tests the version of $(CC) and equals last argument if
+	version expression is true.
+
+	Example:
+		#fs/reiserfs/Makefile
+		ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
+
+	In this example, ccflags-y will be assigned the value -O1 if the
+	$(CC) version is less than 4.2.
+	cc-ifversion takes all the shell operators:
+	-eq, -ne, -lt, -le, -gt, and -ge
+	The third parameter may be a text as in this example, but it may also
+	be an expanded variable or a macro.
+
+    cc-fullversion
+	cc-fullversion is useful when the exact version of gcc is needed.
+	One typical use-case is when a specific GCC version is broken.
+	cc-fullversion points out a more specific version than cc-version does.
+
+	Example:
+		#arch/powerpc/Makefile
+		$(Q)if test "$(call cc-fullversion)" = "040200" ; then \
+			echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \
+			false ; \
+		fi
+
+	In this example for a specific GCC version the build will error out explaining
+	to the user why it stops.
+
+    cc-cross-prefix
+	cc-cross-prefix is used to check if there exists a $(CC) in path with
+	one of the listed prefixes. The first prefix where there exist a
+	prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
+	then nothing is returned.
+	Additional prefixes are separated by a single space in the
+	call of cc-cross-prefix.
+	This functionality is useful for architecture Makefiles that try
+	to set CROSS_COMPILE to well-known values but may have several
+	values to select between.
+	It is recommended only to try to set CROSS_COMPILE if it is a cross
+	build (host arch is different from target arch). And if CROSS_COMPILE
+	is already set then leave it with the old value.
+
+	Example:
+		#arch/m68k/Makefile
+		ifneq ($(SUBARCH),$(ARCH))
+		        ifeq ($(CROSS_COMPILE),)
+		               CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
+			endif
+		endif
+
+=== 4 Host Program support
+
+Kbuild supports building executables on the host for use during the
+compilation stage.
+Two steps are required in order to use a host executable.
+
+The first step is to tell kbuild that a host program exists. This is
+done utilising the variable hostprogs-y.
+
+The second step is to add an explicit dependency to the executable.
+This can be done in two ways. Either add the dependency in a rule,
+or utilise the variable $(always).
+Both possibilities are described in the following.
+
+--- 4.1 Simple Host Program
+
+	In some cases there is a need to compile and run a program on the
+	computer where the build is running.
+	The following line tells kbuild that the program bin2hex shall be
+	built on the build host.
+
+	Example:
+		hostprogs-y := bin2hex
+
+	Kbuild assumes in the above example that bin2hex is made from a single
+	c-source file named bin2hex.c located in the same directory as
+	the Makefile.
+
+--- 4.2 Composite Host Programs
+
+	Host programs can be made up based on composite objects.
+	The syntax used to define composite objects for host programs is
+	similar to the syntax used for kernel objects.
+	$(<executable>-objs) lists all objects used to link the final
+	executable.
+
+	Example:
+		#scripts/lxdialog/Makefile
+		hostprogs-y   := lxdialog
+		lxdialog-objs := checklist.o lxdialog.o
+
+	Objects with extension .o are compiled from the corresponding .c
+	files. In the above example, checklist.c is compiled to checklist.o
+	and lxdialog.c is compiled to lxdialog.o.
+	Finally, the two .o files are linked to the executable, lxdialog.
+	Note: The syntax <executable>-y is not permitted for host-programs.
+
+--- 4.3 Defining shared libraries
+
+	Objects with extension .so are considered shared libraries, and
+	will be compiled as position independent objects.
+	Kbuild provides support for shared libraries, but the usage
+	shall be restricted.
+	In the following example the libkconfig.so shared library is used
+	to link the executable conf.
+
+	Example:
+		#scripts/kconfig/Makefile
+		hostprogs-y     := conf
+		conf-objs       := conf.o libkconfig.so
+		libkconfig-objs := expr.o type.o
+
+	Shared libraries always require a corresponding -objs line, and
+	in the example above the shared library libkconfig is composed by
+	the two objects expr.o and type.o.
+	expr.o and type.o will be built as position independent code and
+	linked as a shared library libkconfig.so. C++ is not supported for
+	shared libraries.
+
+--- 4.4 Using C++ for host programs
+
+	kbuild offers support for host programs written in C++. This was
+	introduced solely to support kconfig, and is not recommended
+	for general use.
+
+	Example:
+		#scripts/kconfig/Makefile
+		hostprogs-y   := qconf
+		qconf-cxxobjs := qconf.o
+
+	In the example above the executable is composed of the C++ file
+	qconf.cc - identified by $(qconf-cxxobjs).
+
+	If qconf is composed by a mixture of .c and .cc files, then an
+	additional line can be used to identify this.
+
+	Example:
+		#scripts/kconfig/Makefile
+		hostprogs-y   := qconf
+		qconf-cxxobjs := qconf.o
+		qconf-objs    := check.o
+
+--- 4.5 Controlling compiler options for host programs
+
+	When compiling host programs, it is possible to set specific flags.
+	The programs will always be compiled utilising $(HOSTCC) passed
+	the options specified in $(HOSTCFLAGS).
+	To set flags that will take effect for all host programs created
+	in that Makefile, use the variable HOST_EXTRACFLAGS.
+
+	Example:
+		#scripts/lxdialog/Makefile
+		HOST_EXTRACFLAGS += -I/usr/include/ncurses
+
+	To set specific flags for a single file the following construction
+	is used:
+
+	Example:
+		#arch/ppc64/boot/Makefile
+		HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
+
+	It is also possible to specify additional options to the linker.
+
+	Example:
+		#scripts/kconfig/Makefile
+		HOSTLOADLIBES_qconf := -L$(QTDIR)/lib
+
+	When linking qconf, it will be passed the extra option
+	"-L$(QTDIR)/lib".
+
+--- 4.6 When host programs are actually built
+
+	Kbuild will only build host-programs when they are referenced
+	as a prerequisite.
+	This is possible in two ways:
+
+	(1) List the prerequisite explicitly in a special rule.
+
+	Example:
+		#drivers/pci/Makefile
+		hostprogs-y := gen-devlist
+		$(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
+			( cd $(obj); ./gen-devlist ) < $<
+
+	The target $(obj)/devlist.h will not be built before
+	$(obj)/gen-devlist is updated. Note that references to
+	the host programs in special rules must be prefixed with $(obj).
+
+	(2) Use $(always)
+	When there is no suitable special rule, and the host program
+	shall be built when a makefile is entered, the $(always)
+	variable shall be used.
+
+	Example:
+		#scripts/lxdialog/Makefile
+		hostprogs-y   := lxdialog
+		always        := $(hostprogs-y)
+
+	This will tell kbuild to build lxdialog even if not referenced in
+	any rule.
+
+--- 4.7 Using hostprogs-$(CONFIG_FOO)
+
+	A typical pattern in a Kbuild file looks like this:
+
+	Example:
+		#scripts/Makefile
+		hostprogs-$(CONFIG_KALLSYMS) += kallsyms
+
+	Kbuild knows about both 'y' for built-in and 'm' for module.
+	So if a config symbol evaluate to 'm', kbuild will still build
+	the binary. In other words, Kbuild handles hostprogs-m exactly
+	like hostprogs-y. But only hostprogs-y is recommended to be used
+	when no CONFIG symbols are involved.
+
+=== 5 Kbuild clean infrastructure
+
+"make clean" deletes most generated files in the obj tree where the kernel
+is compiled. This includes generated files such as host programs.
+Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
+$(extra-y) and $(targets). They are all deleted during "make clean".
+Files matching the patterns "*.[oas]", "*.ko", plus some additional files
+generated by kbuild are deleted all over the kernel src tree when
+"make clean" is executed.
+
+Additional files can be specified in kbuild makefiles by use of $(clean-files).
+
+	Example:
+		#drivers/pci/Makefile
+		clean-files := devlist.h classlist.h
+
+When executing "make clean", the two files "devlist.h classlist.h" will
+be deleted. Kbuild will assume files to be in same relative directory as the
+Makefile except if an absolute path is specified (path starting with '/').
+
+To delete a directory hierarchy use:
+
+	Example:
+		#scripts/package/Makefile
+		clean-dirs := $(objtree)/debian/
+
+This will delete the directory debian, including all subdirectories.
+Kbuild will assume the directories to be in the same relative path as the
+Makefile if no absolute path is specified (path does not start with '/').
+
+Usually kbuild descends down in subdirectories due to "obj-* := dir/",
+but in the architecture makefiles where the kbuild infrastructure
+is not sufficient this sometimes needs to be explicit.
+
+	Example:
+		#arch/i386/boot/Makefile
+		subdir- := compressed/
+
+The above assignment instructs kbuild to descend down in the
+directory compressed/ when "make clean" is executed.
+
+To support the clean infrastructure in the Makefiles that builds the
+final bootimage there is an optional target named archclean:
+
+	Example:
+		#arch/i386/Makefile
+		archclean:
+			$(Q)$(MAKE) $(clean)=arch/i386/boot
+
+When "make clean" is executed, make will descend down in arch/i386/boot,
+and clean as usual. The Makefile located in arch/i386/boot/ may use
+the subdir- trick to descend further down.
+
+Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
+included in the top level makefile, and the kbuild infrastructure
+is not operational at that point.
+
+Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
+be visited during "make clean".
+
+=== 6 Architecture Makefiles
+
+The top level Makefile sets up the environment and does the preparation,
+before starting to descend down in the individual directories.
+The top level makefile contains the generic part, whereas
+arch/$(ARCH)/Makefile contains what is required to set up kbuild
+for said architecture.
+To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
+a few targets.
+
+When kbuild executes, the following steps are followed (roughly):
+1) Configuration of the kernel => produce .config
+2) Store kernel version in include/linux/version.h
+3) Symlink include/asm to include/asm-$(ARCH)
+4) Updating all other prerequisites to the target prepare:
+   - Additional prerequisites are specified in arch/$(ARCH)/Makefile
+5) Recursively descend down in all directories listed in
+   init-* core* drivers-* net-* libs-* and build all targets.
+   - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
+6) All object files are then linked and the resulting file vmlinux is
+   located at the root of the obj tree.
+   The very first objects linked are listed in head-y, assigned by
+   arch/$(ARCH)/Makefile.
+7) Finally, the architecture-specific part does any required post processing
+   and builds the final bootimage.
+   - This includes building boot records
+   - Preparing initrd images and the like
+
+
+--- 6.1 Set variables to tweak the build to the architecture
+
+    LDFLAGS		Generic $(LD) options
+
+	Flags used for all invocations of the linker.
+	Often specifying the emulation is sufficient.
+
+	Example:
+		#arch/s390/Makefile
+		LDFLAGS         := -m elf_s390
+	Note: ldflags-y can be used to further customise
+	the flags used. See chapter 3.7.
+
+    LDFLAGS_MODULE	Options for $(LD) when linking modules
+
+	LDFLAGS_MODULE is used to set specific flags for $(LD) when
+	linking the .ko files used for modules.
+	Default is "-r", for relocatable output.
+
+    LDFLAGS_vmlinux	Options for $(LD) when linking vmlinux
+
+	LDFLAGS_vmlinux is used to specify additional flags to pass to
+	the linker when linking the final vmlinux image.
+	LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
+
+	Example:
+		#arch/i386/Makefile
+		LDFLAGS_vmlinux := -e stext
+
+    OBJCOPYFLAGS	objcopy flags
+
+	When $(call if_changed,objcopy) is used to translate a .o file,
+	the flags specified in OBJCOPYFLAGS will be used.
+	$(call if_changed,objcopy) is often used to generate raw binaries on
+	vmlinux.
+
+	Example:
+		#arch/s390/Makefile
+		OBJCOPYFLAGS := -O binary
+
+		#arch/s390/boot/Makefile
+		$(obj)/image: vmlinux FORCE
+			$(call if_changed,objcopy)
+
+	In this example, the binary $(obj)/image is a binary version of
+	vmlinux. The usage of $(call if_changed,xxx) will be described later.
+
+    KBUILD_AFLAGS		$(AS) assembler flags
+
+	Default value - see top level Makefile
+	Append or modify as required per architecture.
+
+	Example:
+		#arch/sparc64/Makefile
+		KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
+
+    KBUILD_CFLAGS		$(CC) compiler flags
+
+	Default value - see top level Makefile
+	Append or modify as required per architecture.
+
+	Often, the KBUILD_CFLAGS variable depends on the configuration.
+
+	Example:
+		#arch/i386/Makefile
+		cflags-$(CONFIG_M386) += -march=i386
+		KBUILD_CFLAGS += $(cflags-y)
+
+	Many arch Makefiles dynamically run the target C compiler to
+	probe supported options:
+
+		#arch/i386/Makefile
+
+		...
+		cflags-$(CONFIG_MPENTIUMII)     += $(call cc-option,\
+						-march=pentium2,-march=i686)
+		...
+		# Disable unit-at-a-time mode ...
+		KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
+		...
+
+
+	The first example utilises the trick that a config option expands
+	to 'y' when selected.
+
+    CFLAGS_KERNEL	$(CC) options specific for built-in
+
+	$(CFLAGS_KERNEL) contains extra C compiler flags used to compile
+	resident kernel code.
+
+    CFLAGS_MODULE	$(CC) options specific for modules
+
+	$(CFLAGS_MODULE) contains extra C compiler flags used to compile code
+	for loadable kernel modules.
+
+
+--- 6.2 Add prerequisites to archprepare:
+
+	The archprepare: rule is used to list prerequisites that need to be
+	built before starting to descend down in the subdirectories.
+	This is usually used for header files containing assembler constants.
+
+		Example:
+		#arch/arm/Makefile
+		archprepare: maketools
+
+	In this example, the file target maketools will be processed
+	before descending down in the subdirectories.
+	See also chapter XXX-TODO that describe how kbuild supports
+	generating offset header files.
+
+
+--- 6.3 List directories to visit when descending
+
+	An arch Makefile cooperates with the top Makefile to define variables
+	which specify how to build the vmlinux file.  Note that there is no
+	corresponding arch-specific section for modules; the module-building
+	machinery is all architecture-independent.
+
+
+    head-y, init-y, core-y, libs-y, drivers-y, net-y
+
+	$(head-y) lists objects to be linked first in vmlinux.
+	$(libs-y) lists directories where a lib.a archive can be located.
+	The rest list directories where a built-in.o object file can be
+	located.
+
+	$(init-y) objects will be located after $(head-y).
+	Then the rest follows in this order:
+	$(core-y), $(libs-y), $(drivers-y) and $(net-y).
+
+	The top level Makefile defines values for all generic directories,
+	and arch/$(ARCH)/Makefile only adds architecture-specific directories.
+
+	Example:
+		#arch/sparc64/Makefile
+		core-y += arch/sparc64/kernel/
+		libs-y += arch/sparc64/prom/ arch/sparc64/lib/
+		drivers-$(CONFIG_OPROFILE)  += arch/sparc64/oprofile/
+
+
+--- 6.4 Architecture-specific boot images
+
+	An arch Makefile specifies goals that take the vmlinux file, compress
+	it, wrap it in bootstrapping code, and copy the resulting files
+	somewhere. This includes various kinds of installation commands.
+	The actual goals are not standardized across architectures.
+
+	It is common to locate any additional processing in a boot/
+	directory below arch/$(ARCH)/.
+
+	Kbuild does not provide any smart way to support building a
+	target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
+	call make manually to build a target in boot/.
+
+	The recommended approach is to include shortcuts in
+	arch/$(ARCH)/Makefile, and use the full path when calling down
+	into the arch/$(ARCH)/boot/Makefile.
+
+	Example:
+		#arch/i386/Makefile
+		boot := arch/i386/boot
+		bzImage: vmlinux
+			$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+
+	"$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
+	make in a subdirectory.
+
+	There are no rules for naming architecture-specific targets,
+	but executing "make help" will list all relevant targets.
+	To support this, $(archhelp) must be defined.
+
+	Example:
+		#arch/i386/Makefile
+		define archhelp
+		  echo  '* bzImage      - Image (arch/$(ARCH)/boot/bzImage)'
+		endif
+
+	When make is executed without arguments, the first goal encountered
+	will be built. In the top level Makefile the first goal present
+	is all:.
+	An architecture shall always, per default, build a bootable image.
+	In "make help", the default goal is highlighted with a '*'.
+	Add a new prerequisite to all: to select a default goal different
+	from vmlinux.
+
+	Example:
+		#arch/i386/Makefile
+		all: bzImage
+
+	When "make" is executed without arguments, bzImage will be built.
+
+--- 6.5 Building non-kbuild targets
+
+    extra-y
+
+	extra-y specify additional targets created in the current
+	directory, in addition to any targets specified by obj-*.
+
+	Listing all targets in extra-y is required for two purposes:
+	1) Enable kbuild to check changes in command lines
+	   - When $(call if_changed,xxx) is used
+	2) kbuild knows what files to delete during "make clean"
+
+	Example:
+		#arch/i386/kernel/Makefile
+		extra-y := head.o init_task.o
+
+	In this example, extra-y is used to list object files that
+	shall be built, but shall not be linked as part of built-in.o.
+
+
+--- 6.6 Commands useful for building a boot image
+
+	Kbuild provides a few macros that are useful when building a
+	boot image.
+
+    if_changed
+
+	if_changed is the infrastructure used for the following commands.
+
+	Usage:
+		target: source(s) FORCE
+			$(call if_changed,ld/objcopy/gzip)
+
+	When the rule is evaluated, it is checked to see if any files
+	need an update, or the command line has changed since the last
+	invocation. The latter will force a rebuild if any options
+	to the executable have changed.
+	Any target that utilises if_changed must be listed in $(targets),
+	otherwise the command line check will fail, and the target will
+	always be built.
+	Assignments to $(targets) are without $(obj)/ prefix.
+	if_changed may be used in conjunction with custom commands as
+	defined in 6.7 "Custom kbuild commands".
+
+	Note: It is a typical mistake to forget the FORCE prerequisite.
+	Another common pitfall is that whitespace is sometimes
+	significant; for instance, the below will fail (note the extra space
+	after the comma):
+		target: source(s) FORCE
+	#WRONG!#	$(call if_changed, ld/objcopy/gzip)
+
+    ld
+	Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
+
+    objcopy
+	Copy binary. Uses OBJCOPYFLAGS usually specified in
+	arch/$(ARCH)/Makefile.
+	OBJCOPYFLAGS_$@ may be used to set additional options.
+
+    gzip
+	Compress target. Use maximum compression to compress target.
+
+	Example:
+		#arch/i386/boot/Makefile
+		LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
+		LDFLAGS_setup    := -Ttext 0x0 -s --oformat binary -e begtext
+
+		targets += setup setup.o bootsect bootsect.o
+		$(obj)/setup $(obj)/bootsect: %: %.o FORCE
+			$(call if_changed,ld)
+
+	In this example, there are two possible targets, requiring different
+	options to the linker. The linker options are specified using the
+	LDFLAGS_$@ syntax - one for each potential target.
+	$(targets) are assigned all potential targets, by which kbuild knows
+	the targets and will:
+		1) check for commandline changes
+		2) delete target during make clean
+
+	The ": %: %.o" part of the prerequisite is a shorthand that
+	free us from listing the setup.o and bootsect.o files.
+	Note: It is a common mistake to forget the "target :=" assignment,
+	      resulting in the target file being recompiled for no
+	      obvious reason.
+
+
+--- 6.7 Custom kbuild commands
+
+	When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
+	of a command is normally displayed.
+	To enable this behaviour for custom commands kbuild requires
+	two variables to be set:
+	quiet_cmd_<command>	- what shall be echoed
+	      cmd_<command>	- the command to execute
+
+	Example:
+		#
+		quiet_cmd_image = BUILD   $@
+		      cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
+		                                     $(obj)/vmlinux.bin > $@
+
+		targets += bzImage
+		$(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
+			$(call if_changed,image)
+			@echo 'Kernel: $@ is ready'
+
+	When updating the $(obj)/bzImage target, the line
+
+	BUILD    arch/i386/boot/bzImage
+
+	will be displayed with "make KBUILD_VERBOSE=0".
+
+
+--- 6.8 Preprocessing linker scripts
+
+	When the vmlinux image is built, the linker script
+	arch/$(ARCH)/kernel/vmlinux.lds is used.
+	The script is a preprocessed variant of the file vmlinux.lds.S
+	located in the same directory.
+	kbuild knows .lds files and includes a rule *lds.S -> *lds.
+
+	Example:
+		#arch/i386/kernel/Makefile
+		always := vmlinux.lds
+
+		#Makefile
+		export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
+
+	The assignment to $(always) is used to tell kbuild to build the
+	target vmlinux.lds.
+	The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
+	specified options when building the target vmlinux.lds.
+
+	When building the *.lds target, kbuild uses the variables:
+	KBUILD_CPPFLAGS	: Set in top-level Makefile
+	cppflags-y	: May be set in the kbuild makefile
+	CPPFLAGS_$(@F)  : Target specific flags.
+	                  Note that the full filename is used in this
+	                  assignment.
+
+	The kbuild infrastructure for *lds file are used in several
+	architecture-specific files.
+
+
+=== 7 Kbuild Variables
+
+The top Makefile exports the following variables:
+
+    VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
+
+	These variables define the current kernel version.  A few arch
+	Makefiles actually use these values directly; they should use
+	$(KERNELRELEASE) instead.
+
+	$(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
+	three-part version number, such as "2", "4", and "0".  These three
+	values are always numeric.
+
+	$(EXTRAVERSION) defines an even tinier sublevel for pre-patches
+	or additional patches.	It is usually some non-numeric string
+	such as "-pre4", and is often blank.
+
+    KERNELRELEASE
+
+	$(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
+	for constructing installation directory names or showing in
+	version strings.  Some arch Makefiles use it for this purpose.
+
+    ARCH
+
+	This variable defines the target architecture, such as "i386",
+	"arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
+	determine which files to compile.
+
+	By default, the top Makefile sets $(ARCH) to be the same as the
+	host system architecture.  For a cross build, a user may
+	override the value of $(ARCH) on the command line:
+
+	    make ARCH=m68k ...
+
+
+    INSTALL_PATH
+
+	This variable defines a place for the arch Makefiles to install
+	the resident kernel image and System.map file.
+	Use this for architecture-specific install targets.
+
+    INSTALL_MOD_PATH, MODLIB
+
+	$(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
+	installation.  This variable is not defined in the Makefile but
+	may be passed in by the user if desired.
+
+	$(MODLIB) specifies the directory for module installation.
+	The top Makefile defines $(MODLIB) to
+	$(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE).  The user may
+	override this value on the command line if desired.
+
+    INSTALL_MOD_STRIP
+
+	If this variable is specified, will cause modules to be stripped
+	after they are installed.  If INSTALL_MOD_STRIP is '1', then the
+	default option --strip-debug will be used.  Otherwise,
+	INSTALL_MOD_STRIP will used as the option(s) to the strip command.
+
+
+=== 8 Makefile language
+
+The kernel Makefiles are designed to be run with GNU Make.  The Makefiles
+use only the documented features of GNU Make, but they do use many
+GNU extensions.
+
+GNU Make supports elementary list-processing functions.  The kernel
+Makefiles use a novel style of list building and manipulation with few
+"if" statements.
+
+GNU Make has two assignment operators, ":=" and "=".  ":=" performs
+immediate evaluation of the right-hand side and stores an actual string
+into the left-hand side.  "=" is like a formula definition; it stores the
+right-hand side in an unevaluated form and then evaluates this form each
+time the left-hand side is used.
+
+There are some cases where "=" is appropriate.  Usually, though, ":="
+is the right choice.
+
+=== 9 Credits
+
+Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
+Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
+Updates by Sam Ravnborg <sam@ravnborg.org>
+Language QA by Jan Engelhardt <jengelh@gmx.de>
+
+=== 10 TODO
+
+- Describe how kbuild supports shipped files with _shipped.
+- Generating offset header files.
+- Add more variables to section 7?
+
+
+
diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt
new file mode 100644
index 0000000..1821c07
--- /dev/null
+++ b/Documentation/kbuild/modules.txt
@@ -0,0 +1,552 @@
+
+In this document you will find information about:
+- how to build external modules
+- how to make your module use the kbuild infrastructure
+- how kbuild will install a kernel
+- how to install modules in a non-standard location
+
+=== Table of Contents
+
+	=== 1 Introduction
+	=== 2 How to build external modules
+	   --- 2.1 Building external modules
+	   --- 2.2 Available targets
+	   --- 2.3 Available options
+	   --- 2.4 Preparing the kernel tree for module build
+	   --- 2.5 Building separate files for a module
+	=== 3. Example commands
+	=== 4. Creating a kbuild file for an external module
+	=== 5. Include files
+	   --- 5.1 How to include files from the kernel include dir
+	   --- 5.2 External modules using an include/ dir
+	   --- 5.3 External modules using several directories
+	=== 6. Module installation
+	   --- 6.1 INSTALL_MOD_PATH
+	   --- 6.2 INSTALL_MOD_DIR
+	=== 7. Module versioning & Module.symvers
+	   --- 7.1 Symbols from the kernel (vmlinux + modules)
+	   --- 7.2 Symbols and external modules
+	   --- 7.3 Symbols from another external module
+	=== 8. Tips & Tricks
+	   --- 8.1 Testing for CONFIG_FOO_BAR
+
+
+
+=== 1. Introduction
+
+kbuild includes functionality for building modules both
+within the kernel source tree and outside the kernel source tree.
+The latter is usually referred to as external or "out-of-tree"
+modules and is used both during development and for modules that
+are not planned to be included in the kernel tree.
+
+What is covered within this file is mainly information to authors
+of modules. The author of an external module should supply
+a makefile that hides most of the complexity, so one only has to type
+'make' to build the module. A complete example will be presented in
+chapter 4, "Creating a kbuild file for an external module".
+
+
+=== 2. How to build external modules
+
+kbuild offers functionality to build external modules, with the
+prerequisite that there is a pre-built kernel available with full source.
+A subset of the targets available when building the kernel is available
+when building an external module.
+
+--- 2.1 Building external modules
+
+	Use the following command to build an external module:
+
+		make -C <path-to-kernel> M=`pwd`
+
+	For the running kernel use:
+
+		make -C /lib/modules/`uname -r`/build M=`pwd`
+
+	For the above command to succeed, the kernel must have been
+	built with modules enabled.
+
+	To install the modules that were just built:
+
+		make -C <path-to-kernel> M=`pwd` modules_install
+
+	More complex examples will be shown later, the above should
+	be enough to get you started.
+
+--- 2.2 Available targets
+
+	$KDIR refers to the path to the kernel source top-level directory
+
+	make -C $KDIR M=`pwd`
+		Will build the module(s) located in current directory.
+		All output files will be located in the same directory
+		as the module source.
+		No attempts are made to update the kernel source, and it is
+		a precondition that a successful make has been executed
+		for the kernel.
+
+	make -C $KDIR M=`pwd` modules
+		The modules target is implied when no target is given.
+		Same functionality as if no target was specified.
+		See description above.
+
+	make -C $KDIR M=`pwd` modules_install
+		Install the external module(s).
+		Installation default is in /lib/modules/<kernel-version>/extra,
+		but may be prefixed with INSTALL_MOD_PATH - see separate
+		chapter.
+
+	make -C $KDIR M=`pwd` clean
+		Remove all generated files for the module - the kernel
+		source directory is not modified.
+
+	make -C $KDIR M=`pwd` help
+		help will list the available target when building external
+		modules.
+
+--- 2.3 Available options:
+
+	$KDIR refers to the path to the kernel source top-level directory
+
+	make -C $KDIR
+		Used to specify where to find the kernel source.
+		'$KDIR' represent the directory where the kernel source is.
+		Make will actually change directory to the specified directory
+		when executed but change back when finished.
+
+	make -C $KDIR M=`pwd`
+		M= is used to tell kbuild that an external module is
+		being built.
+		The option given to M= is the directory where the external
+		module (kbuild file) is located.
+		When an external module is being built only a subset of the
+		usual targets are available.
+
+	make -C $KDIR SUBDIRS=`pwd`
+		Same as M=. The SUBDIRS= syntax is kept for backwards
+		compatibility.
+
+--- 2.4 Preparing the kernel tree for module build
+
+	To make sure the kernel contains the information required to
+	build external modules the target 'modules_prepare' must be used.
+	'modules_prepare' exists solely as a simple way to prepare
+	a kernel source tree for building external modules.
+	Note: modules_prepare will not build Module.symvers even if
+	CONFIG_MODVERSIONS is set. Therefore a full kernel build
+	needs to be executed to make module versioning work.
+
+--- 2.5 Building separate files for a module
+	It is possible to build single files which are part of a module.
+	This works equally well for the kernel, a module and even for
+	external modules.
+	Examples (module foo.ko, consist of bar.o, baz.o):
+		make -C $KDIR M=`pwd` bar.lst
+		make -C $KDIR M=`pwd` bar.o
+		make -C $KDIR M=`pwd` foo.ko
+		make -C $KDIR M=`pwd` /
+
+
+=== 3. Example commands
+
+This example shows the actual commands to be executed when building
+an external module for the currently running kernel.
+In the example below, the distribution is supposed to use the
+facility to locate output files for a kernel compile in a different
+directory than the kernel source - but the examples will also work
+when the source and the output files are mixed in the same directory.
+
+# Kernel source
+/lib/modules/<kernel-version>/source -> /usr/src/linux-<version>
+
+# Output from kernel compile
+/lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up
+
+Change to the directory where the kbuild file is located and execute
+the following commands to build the module:
+
+	cd /home/user/src/module
+	make -C /usr/src/`uname -r`/source            \
+	        O=/lib/modules/`uname-r`/build        \
+	        M=`pwd`
+
+Then, to install the module use the following command:
+
+	make -C /usr/src/`uname -r`/source            \
+	        O=/lib/modules/`uname-r`/build        \
+	        M=`pwd`                               \
+		modules_install
+
+If you look closely you will see that this is the same command as
+listed before - with the directories spelled out.
+
+The above are rather long commands, and the following chapter
+lists a few tricks to make it all easier.
+
+
+=== 4. Creating a kbuild file for an external module
+
+kbuild is the build system for the kernel, and external modules
+must use kbuild to stay compatible with changes in the build system
+and to pick up the right flags to gcc etc.
+
+The kbuild file used as input shall follow the syntax described
+in Documentation/kbuild/makefiles.txt. This chapter will introduce a few
+more tricks to be used when dealing with external modules.
+
+In the following a Makefile will be created for a module with the
+following files:
+	8123_if.c
+	8123_if.h
+	8123_pci.c
+	8123_bin.o_shipped	<= Binary blob
+
+--- 4.1 Shared Makefile for module and kernel
+
+	An external module always includes a wrapper Makefile supporting
+	building the module using 'make' with no arguments.
+	The Makefile provided will most likely include additional
+	functionality such as test targets etc. and this part shall
+	be filtered away from kbuild since it may impact kbuild if
+	name clashes occurs.
+
+	Example 1:
+		--> filename: Makefile
+		ifneq ($(KERNELRELEASE),)
+		# kbuild part of makefile
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		else
+		# Normal Makefile
+
+		KERNELDIR := /lib/modules/`uname -r`/build
+		all::
+			$(MAKE) -C $(KERNELDIR) M=`pwd` $@
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin.o_shipped
+
+		endif
+
+	In example 1, the check for KERNELRELEASE is used to separate
+	the two parts of the Makefile. kbuild will only see the two
+	assignments whereas make will see everything except the two
+	kbuild assignments.
+
+	In recent versions of the kernel, kbuild will look for a file named
+	Kbuild and as second option look for a file named Makefile.
+	Utilising the Kbuild file makes us split up the Makefile in example 1
+	into two files as shown in example 2:
+
+	Example 2:
+		--> filename: Kbuild
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		--> filename: Makefile
+		KERNELDIR := /lib/modules/`uname -r`/build
+		all::
+			$(MAKE) -C $(KERNELDIR) M=`pwd` $@
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin_shipped
+
+
+	In example 2, we are down to two fairly simple files and for simple
+	files as used in this example the split is questionable. But some
+	external modules use Makefiles of several hundred lines and here it
+	really pays off to separate the kbuild part from the rest.
+	Example 3 shows a backward compatible version.
+
+	Example 3:
+		--> filename: Kbuild
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		--> filename: Makefile
+		ifneq ($(KERNELRELEASE),)
+		include Kbuild
+		else
+		# Normal Makefile
+
+		KERNELDIR := /lib/modules/`uname -r`/build
+		all::
+			$(MAKE) -C $KERNELDIR M=`pwd` $@
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin_shipped
+
+		endif
+
+	The trick here is to include the Kbuild file from Makefile, so
+	if an older version of kbuild picks up the Makefile, the Kbuild
+	file will be included.
+
+--- 4.2 Binary blobs included in a module
+
+	Some external modules needs to include a .o as a blob. kbuild
+	has support for this, but requires the blob file to be named
+	<filename>_shipped. In our example the blob is named
+	8123_bin.o_shipped and when the kbuild rules kick in the file
+	8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file
+	with the _shipped part stripped of the filename.
+	This allows the 8123_bin.o filename to be used in the assignment to
+	the module.
+
+	Example 4:
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+	In example 4, there is no distinction between the ordinary .c/.h files
+	and the binary file. But kbuild will pick up different rules to create
+	the .o file.
+
+
+=== 5. Include files
+
+Include files are a necessity when a .c file uses something from other .c
+files (not strictly in the sense of C, but if good programming practice is
+used). Any module that consists of more than one .c file will have a .h file
+for one of the .c files.
+
+- If the .h file only describes a module internal interface, then the .h file
+  shall be placed in the same directory as the .c files.
+- If the .h files describe an interface used by other parts of the kernel
+  located in different directories, the .h files shall be located in
+  include/linux/ or other include/ directories as appropriate.
+
+One exception for this rule is larger subsystems that have their own directory
+under include/ such as include/scsi. Another exception is arch-specific
+.h files which are located under include/asm-$(ARCH)/*.
+
+External modules have a tendency to locate include files in a separate include/
+directory and therefore need to deal with this in their kbuild file.
+
+--- 5.1 How to include files from the kernel include dir
+
+	When a module needs to include a file from include/linux/, then one
+	just uses:
+
+		#include <linux/modules.h>
+
+	kbuild will make sure to add options to gcc so the relevant
+	directories are searched.
+	Likewise for .h files placed in the same directory as the .c file.
+
+		#include "8123_if.h"
+
+	will do the job.
+
+--- 5.2 External modules using an include/ dir
+
+	External modules often locate their .h files in a separate include/
+	directory although this is not usual kernel style. When an external
+	module uses an include/ dir then kbuild needs to be told so.
+	The trick here is to use either EXTRA_CFLAGS (take effect for all .c
+	files) or CFLAGS_$F.o (take effect only for a single file).
+
+	In our example, if we move 8123_if.h to a subdirectory named include/
+	the resulting Kbuild file would look like:
+
+		--> filename: Kbuild
+		obj-m  := 8123.o
+
+		EXTRA_CFLAGS := -Iinclude
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+	Note that in the assignment there is no space between -I and the path.
+	This is a kbuild limitation:  there must be no space present.
+
+--- 5.3 External modules using several directories
+
+	If an external module does not follow the usual kernel style, but
+	decides to spread files over several directories, then kbuild can
+	handle this too.
+
+	Consider the following example:
+
+	|
+	+- src/complex_main.c
+	|   +- hal/hardwareif.c
+	|   +- hal/include/hardwareif.h
+	+- include/complex.h
+
+	To build a single module named complex.ko, we then need the following
+	kbuild file:
+
+	Kbuild:
+		obj-m := complex.o
+		complex-y := src/complex_main.o
+		complex-y += src/hal/hardwareif.o
+
+		EXTRA_CFLAGS := -I$(src)/include
+		EXTRA_CFLAGS += -I$(src)src/hal/include
+
+
+	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.
+
+	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:
+
+	/lib/modules/$(KERNELRELEASE)/kernel
+
+External modules are installed in the directory:
+
+	/lib/modules/$(KERNELRELEASE)/extra
+
+--- 6.1 INSTALL_MOD_PATH
+
+	Above are the default directories, but as always, some level of
+	customization is possible. One can prefix the path using the variable
+	INSTALL_MOD_PATH:
+
+		$ make INSTALL_MOD_PATH=/frodo modules_install
+		=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel
+
+	INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
+	example above, can be specified on the command line when calling make.
+	INSTALL_MOD_PATH has effect both when installing modules included in
+	the kernel as well as when installing external modules.
+
+--- 6.2 INSTALL_MOD_DIR
+
+	When installing external modules they are by default installed to a
+	directory under /lib/modules/$(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'.
+
+		$ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \
+			M=`pwd` modules_install
+		=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
+
+
+=== 7. Module versioning & Module.symvers
+
+Module versioning is enabled by the CONFIG_MODVERSIONS tag.
+
+Module versioning is used as a simple ABI consistency check. The Module
+versioning creates a CRC value of the full prototype for an exported symbol and
+when a module is loaded/used then the CRC values contained in the kernel are
+compared with similar values in the module. If they are not equal, then the
+kernel refuses to load the module.
+
+Module.symvers contains a list of all exported symbols from a kernel build.
+
+--- 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.
+
+	The syntax of the Module.symvers file is:
+		<CRC>       <Symbol>           <module>
+	Sample:
+		0x2d036834  scsi_remove_host   drivers/scsi/scsi_mod
+
+	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)
+			obj-y := foo/ bar/
+
+		Executing:
+			make -C $KDIR M=`pwd`
+
+		will then do the expected and compile both modules with full
+		knowledge on symbols from both modules.
+
+	Use an extra Module.symvers file
+		When an external module is built, a Module.symvers file is
+		generated containing all exported symbols which are not
+		defined in the kernel.
+		To get access to symbols from module 'bar', one can copy the
+		Module.symvers file from the compilation of the 'bar' module
+		to the directory where the 'foo' module is built.
+		During the module build, kbuild will read the Module.symvers
+		file in the directory of the external module and when the
+		build is finished, a new Module.symvers file is created
+		containing the sum of all symbols defined and not part of the
+		kernel.
+
+	Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile
+		If it is impractical to copy Module.symvers from another
+		module, you can assign a space separated list of files to
+		KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be
+		loaded by modpost during the initialisation of its symbol
+		tables.
+
+=== 8. Tips & Tricks
+
+--- 8.1 Testing for CONFIG_FOO_BAR
+
+	Modules often need to check for certain CONFIG_ options to decide if
+	a specific feature shall be included in the module. When kbuild is used
+	this is done by referencing the CONFIG_ variable directly.
+
+		#fs/ext2/Makefile
+		obj-$(CONFIG_EXT2_FS) += ext2.o
+
+		ext2-y := balloc.o bitmap.o dir.o
+		ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
+
+	External modules have traditionally used grep to check for specific
+	CONFIG_ settings directly in .config. This usage is broken.
+	As introduced before, external modules shall use kbuild when building
+	and therefore can use the same methods as in-kernel modules when
+	testing for CONFIG_ definitions.
+