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 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 $(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 additionaly 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 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

    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 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

    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.

   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.

    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:
		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 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
		EXTRA_CFLAGS := $(call cc-ifversion, -lt, 0402, -O1)

	In this example, EXTRA_CFLAGS 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.


=== 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) 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 thelike


--- 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 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.

    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 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 lists 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
	needs 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 assinged 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:
	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.


=== 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?