Linux Kernel Makefiles

This document describes the Linux kernel Makefiles.

=== Table of Contents

        === 1 Overview
        === 2 Who does what
        === 3 The kbuild Makefiles
           --- 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

        === 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

        === 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 Makefiles

Most Makefiles within the kernel are kbuild Makefiles that use the
kbuild infrastructure. This chapter introduce the syntax used in the
kbuild makefiles.

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 build 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.
        See also Documentation/modules.txt.

--- 3.5 Library file goals - lib-y

        Objects listed with obj-* is used for modules or
        are 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 track 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 host-prog.

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:
                host-progs := 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 objetcs 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
                host-progs    := 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
                host-progs      := 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
                host-progs    := 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
                host-progs    := 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
                host-progs := 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
                host-progs    := lxdialog
                always        := $(host-progs)

        This will tell kbuild to build lxdialog even if not referenced in
        any rule.

=== 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 $(host-progs), $(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 knows that files specified by $(clean-files) are
located in the same directory as the makefile.

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

    LDFLAGS_BLOB        Options for $(LD) when linking the initramfs blob

        The image used for initramfs is made during the build process.
        LDFLAGS_BLOB is used to specify additional flags to be used when
        creating the initramfs_data.o file.
        Example:
                #arch/i386/Makefile
                LDFLAGS_BLOB := --format binary --oformat elf32-i386

    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
                check_gcc = $(shell if $(CC) $(1) -S -o /dev/null -xc \
                            /dev/null\ > /dev/null 2>&1; then echo "$(1)"; \
                            else echo "$(2)"; fi)
                cflags-$(CONFIG_MCYRIXIII) += $(call check_gcc,\
                                                     -march=c3,-march=i486)

                CFLAGS += $(cflags-y)

        The above examples both utilise 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 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 provide 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 is 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".
        

=== 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?