Linux-2.6.12-rc2v2.6.12-rc2

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!

1 files changed, 1122 insertions, 0 deletions

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
+
+	=== 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 prepare:
+	   --- 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
+	   --- 6.9 $(CC) support functions
+
+	=== 7 Kbuild Variables
+	=== 8 Makefile language
+	=== 9 Credits
+	=== 10 TODO
+
+=== 1 Overview
+
+The Makefiles have five parts:
+
+	Makefile		the top Makefile.
+	.config			the kernel configuration file.
+	arch/$(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 that 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 introduce the syntax used in the
+kbuild makefiles.
+The preferred name for the kbuild files is 'Kbuild' but 'Makefile' will
+continue to be supported. All new developmen is expected to use the
+Kbuild filename.
+
+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 tell 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 lists $(obj-y).  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 $(obj-y) is significant.  Duplicates in
+	the lists are allowed: the first instance will be linked into
+	built-in.o and succeeding instances will be ignored.
+
+	Link order is significant, because certain functions
+	(module_init() / __initcall) will be called during boot in the
+	order they appear. So keep in mind that changing the link
+	order may e.g.  change the order in which your SCSI
+	controllers are detected, and thus you disks are renumbered.
+
+	Example:
+		#drivers/isdn/i4l/Makefile
+		# Makefile for the kernel ISDN subsystem and device drivers.
+		# Each configuration option enables a list of files.
+		obj-$(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 additional listed in
+	lib-y will not be included in the library, since they will anyway
+	be accessible.
+	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 build
+	the directory shall be listed in libs-y.
+	See also "6.3 List directories to visit when descending".
+ 
+	Usage 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
+
+    EXTRA_CFLAGS, EXTRA_AFLAGS, EXTRA_LDFLAGS, EXTRA_ARFLAGS
+
+	All the EXTRA_ variables apply only to the kbuild makefile
+	where they are assigned. The EXTRA_ variables apply to all
+	commands executed in the kbuild makefile.
+
+	$(EXTRA_CFLAGS) specifies options for compiling C files with
+	$(CC).
+
+	Example:
+		# drivers/sound/emu10k1/Makefile
+		EXTRA_CFLAGS += -I$(obj)
+		ifdef DEBUG
+		    EXTRA_CFLAGS += -DEMU10K1_DEBUG
+		endif
+
+
+	This variable is necessary because the top Makefile owns the
+	variable $(CFLAGS) and uses it for compilation flags for the
+	entire tree.
+
+	$(EXTRA_AFLAGS) is a similar string for per-directory options
+	when compiling assembly language source.
+
+	Example:
+		#arch/x86_64/kernel/Makefile
+		EXTRA_AFLAGS := -traditional
+
+
+	$(EXTRA_LDFLAGS) and $(EXTRA_ARFLAGS) are similar strings for
+	per-directory options to $(LD) and $(AR).
+
+	Example:
+		#arch/m68k/fpsp040/Makefile
+		EXTRA_LDFLAGS := -x
+
+    CFLAGS_$@, 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 is the architecture specific Makefiles which
+	needs 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).
+
+
+=== 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.
+	$(<executeable>-objs) list 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 typcal pattern in a Kbuild file lok 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 handle hostprogs-m exactly
+	like hostprogs-y. But only hostprogs-y is recommend used
+	when no CONFIG symbol are involved.
+
+=== 5 Kbuild clean infrastructure
+
+"make clean" deletes most generated files in the src 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 hirachy 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 the
+arch/$(ARCH)/Makefile contains what is required to set-up kbuild
+to the said architecture.
+To do so arch/$(ARCH)/Makefile sets a number of variables, and defines
+a few targets.
+
+When kbuild executes the following steps are followed (roughly):
+1) Configuration of the kernel => produced .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 value of the above variables are extended in arch/$(ARCH)/Makefile.
+6) All object files are then linked and the resulting file vmlinux is 
+   located at the root of the src tree.
+   The very first objects linked are listed in head-y, assigned by
+   arch/$(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 $(LD) options
+
+	Flags used for all invocations of the linker.
+	Often specifying the emulation is sufficient.
+
+	Example:
+		#arch/s390/Makefile
+		LDFLAGS         := -m elf_s390
+	Note: EXTRA_LDFLAGS and LDFLAGS_$@ can be used to further customise
+	the flags used. See chapter 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 $(LD) when linking vmlinux
+
+	LDFLAGS_vmlinux is used to specify additional flags to pass to
+	the linker when linking the final vmlinux.
+	LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
+
+	Example:
+		#arch/i386/Makefile
+		LDFLAGS_vmlinux := -e stext
+
+    OBJCOPYFLAGS	objcopy flags
+
+	When $(call if_changed,objcopy) is used to translate a .o file,
+	then 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.
+
+    AFLAGS		$(AS) assembler flags
+
+	Default value - see top level Makefile
+	Append or modify as required per architecture.
+
+	Example:
+		#arch/sparc64/Makefile
+		AFLAGS += -m64 -mcpu=ultrasparc
+
+    CFLAGS		$(CC) compiler flags
+
+	Default value - see top level Makefile
+	Append or modify as required per architecture.
+
+	Often the CFLAGS variable depends on the configuration.
+
+	Example:
+		#arch/i386/Makefile
+		cflags-$(CONFIG_M386) += -march=i386
+		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 ...
+		CFLAGS += $(call cc-option,-fno-unit-at-a-time)
+		...
+
+
+	The first examples 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
+
+	$(CFLAGS_MODULE) contains extra C compiler flags used to compile code
+	for loadable kernel modules.
+
+ 
+--- 6.2 Add prerequisites to prepare:
+
+	The prepare: rule is used to list prerequisites that needs to be
+	built before starting to descend down in the subdirectories.
+	This is usual header files containing assembler constants.
+
+		Example:
+		#arch/s390/Makefile
+		prepare: include/asm-$(ARCH)/offsets.h
+
+	In this example the file include/asm-$(ARCH)/offsets.h will
+	be built 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) list objects to be linked first in vmlinux.
+	$(libs-y) list 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 $(net-y).
+
+	The top level Makefile define values for all generic directories,
+	and arch/$(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 of the 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)'
+		endef
+
+	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
+	needs an update, or the commandline has changed since 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 $(obj)/ prefix.
+	if_changed may be used in conjunction with custom commands as
+	defined in 6.7 "Custom kbuild commands".
+	Note: It is a typical mistake to forget the FORCE prerequisite.
+
+    ld
+	Link target. Often LDFLAGS_$@ 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 assinged all potential targets, herby 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 $(obj)/bzImage target the line:
+
+	BUILD    arch/i386/boot/bzImage
+
+	will be displayed with "make KBUILD_VERBOSE=0".
+	
+
+--- 6.8 Preprocessing linker scripts
+
+	When the vmlinux image is build the linker script:
+	arch/$(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 file 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 assigment to $(always) is used to tell kbuild to build the
+	target: vmlinux.lds.
+	The assignment to $(CPPFLAGS_vmlinux.lds) tell kbuild to use the
+	specified options when building the target vmlinux.lds.
+	
+	When building the *.lds target kbuild used the variakles:
+	CPPFLAGS	: Set in top-level Makefile
+	EXTRA_CPPFLAGS	: May be set in the kbuild makefile
+	CPPFLAGS_$(@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.
+
+
+--- 6.9 $(CC) support functions
+
+	The kernel may be build 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 useally the gcc compiler, but other alternatives are
+	available.
+
+    cc-option
+	cc-option is used to check if $(CC) support 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.
+
+   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 to y the expanded variables $(aflags-y)
+	and $(cflags-y) will be assigned the values -a32 and -m32.
+
+    cc-option-align
+	gcc version >= 3.0 shifted type of options used to speify
+	alignment of functions, loops etc. $(cc-option-align) whrn 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:
+		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.
+	
+    cc-version
+	cc-version return 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 the -mregparm=3 were broken in some gcc version
+	even though the option was accepted by gcc.
+
+	Example:
+		#arch/i386/Makefile
+		GCC_VERSION := $(call cc-version)
+		cflags-y += $(shell \
+		if [ $(GCC_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.
+	
+
+=== 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.
+
+=== 8 Makefile language
+
+The kernel Makefiles are designed to 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>
+
+=== 10 TODO
+
+- Describe how kbuild support shipped files with _shipped.
+- Generating offset header files.
+- Add more variables to section 7?
+