2.6.18内核Makefile文档

henryscu

kernel-source/Documentation/kbuild/
Linux Kernel Makefiles
This document describes the Linux kernel Makefiles.
=== Table of Contents
    === 1 Overview
    === 2 Who does what
    === 3 The kbuild files
       --- 3.1 Goal definitions
       --- 3.2 Built-in object goals - obj-y
       --- 3.3 Loadable module goals - obj-m
       --- 3.4 Objects which export symbols
       --- 3.5 Library file goals - lib-y
       --- 3.6 Descending down in directories
       --- 3.7 Compilation flags
       --- 3.8 Command line dependency
       --- 3.9 Dependency tracking
       --- 3.10 Special Rules
       --- 3.11 $(CC) support functions
    === 4 Host Program support
       --- 4.1 Simple Host Program
       --- 4.2 Composite Host Programs
       --- 4.3 Defining shared libraries
       --- 4.4 Using C++ for host programs
       --- 4.5 Controlling compiler options for host programs
       --- 4.6 When host programs are actually built
       --- 4.7 Using hostprogs-$(CONFIG_FOO)
    === 5 Kbuild clean infrastructure
    === 6 Architecture Makefiles
       --- 6.1 Set variables to tweak the build to the architecture
       --- 6.2 Add prerequisites to archprepare:
       --- 6.3 List directories to visit when descending
       --- 6.4 Architecture-specific boot images
       --- 6.5 Building non-kbuild targets
       --- 6.6 Commands useful for building a boot image
       --- 6.7 Custom kbuild commands
       --- 6.8 Preprocessing linker scripts
    === 7 Kbuild Variables
    === 8 Makefile language
    === 9 Credits
    === 10 TODO
=== 1 Overview
The Makefiles have five parts:
    Makefile        the top Makefile.
    .config            the kernel configuration file.
    arch/$(ARCH)/Makefile    the arch Makefile.
    scripts/Makefile.*    common rules etc. for all kbuild Makefiles.
    kbuild Makefiles    there are about 500 of these.
The top Makefile reads the .config file, which comes from the kernel
configuration process.
The top Makefile is responsible for building two major products: vmlinux
(the resident kernel image) and modules (any module files).
It builds these goals by recursively descending into the subdirectories of
the kernel source tree.
The list of subdirectories which are visited depends upon the kernel
configuration. The top Makefile textually includes an arch Makefile
with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
architecture-specific information to the top Makefile.
Each subdirectory has a kbuild Makefile which carries out the commands
passed down from above. The kbuild Makefile uses information from the
.config file to construct various file lists used by kbuild to build
any built-in or modular targets.
scripts/Makefile.* contains all the definitions/rules etc. that
are used to build the kernel based on the kbuild makefiles.
=== 2 Who does what
People have four different relationships with the kernel Makefiles.
*Users* are people who build kernels.  These people type commands such as
"make menuconfig" or "make".  They usually do not read or edit
any kernel Makefiles (or any other source files).
*Normal developers* are people who work on features such as device
drivers, file systems, and network protocols.  These people need to
maintain the kbuild Makefiles for the subsystem they are
working on.  In order to do this effectively, they need some overall
knowledge about the kernel Makefiles, plus detailed knowledge about the
public interface for kbuild.
*Arch developers* are people who work on an entire architecture, such
as sparc or ia64.  Arch developers need to know about the arch Makefile
as well as kbuild Makefiles.
*Kbuild developers* are people who work on the kernel build system itself.
These people need to know about all aspects of the kernel Makefiles.
This document is aimed towards normal developers and arch developers.
=== 3 The kbuild files
Most Makefiles within the kernel are kbuild Makefiles that use the
kbuild infrastructure. This chapter introduces the syntax used in the
kbuild makefiles.
The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
file will be used.
Section 3.1 "Goal definitions" is a quick intro, further chapters provide
more details, with real examples.
--- 3.1 Goal definitions
    Goal definitions are the main part (heart) of the kbuild Makefile.
    These lines define the files to be built, any special compilation
    options, and any subdirectories to be entered recursively.
    The most simple kbuild makefile contains one line:
    Example:
        obj-y += foo.o
    This tells kbuild that there is one object in that directory, named
    foo.o. foo.o will be built from foo.c or foo.S.
    If foo.o shall be built as a module, the variable obj-m is used.
    Therefore the following pattern is often used:
    Example:
        obj-$(CONFIG_FOO) += foo.o
    $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
    If CONFIG_FOO is neither y nor m, then the file will not be compiled
    nor linked.
--- 3.2 Built-in object goals - obj-y
    The kbuild Makefile specifies object files for vmlinux
    in the $(obj-y) lists.  These lists depend on the kernel
    configuration.
    Kbuild compiles all the $(obj-y) files.  It then calls
    "$(LD) -r" to merge these files into one built-in.o file.
    built-in.o is later linked into vmlinux by the parent Makefile.
    The order of files in $(obj-y) is significant.  Duplicates in
    the lists are allowed: the first instance will be linked into
    built-in.o and succeeding instances will be ignored.
    Link order is significant, because certain functions
    (module_init() / __initcall) will be called during boot in the
    order they appear. So keep in mind that changing the link
    order may e.g. change the order in which your SCSI
    controllers are detected, and thus your disks are renumbered.
    Example:
        #drivers/isdn/i4l/Makefile
        # Makefile for the kernel ISDN subsystem and device drivers.
        # Each configuration option enables a list of files.
        obj-$(CONFIG_ISDN)             += isdn.o
        obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
--- 3.3 Loadable module goals - obj-m
    $(obj-m) specify object files which are built as loadable
    kernel modules.
    A module may be built from one source file or several source
    files. In the case of one source file, the kbuild makefile
    simply adds the file to $(obj-m).
    Example:
        #drivers/isdn/i4l/Makefile
        obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
    Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
    If a kernel module is built from several source files, you specify
    that you want to build a module in the same way as above.
    Kbuild needs to know which the parts that you want to build your
    module from, so you have to tell it by setting an
    $(-objs) variable.
    Example:
        #drivers/isdn/i4l/Makefile
        obj-$(CONFIG_ISDN) += isdn.o
        isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
    In this example, the module name will be isdn.o. Kbuild will
    compile the objects listed in $(isdn-objs) and then run
    "$(LD) -r" on the list of these files to generate isdn.o.
    Kbuild recognises objects used for composite objects by the suffix
    -objs, and the suffix -y. This allows the Makefiles to use
    the value of a CONFIG_ symbol to determine if an object is part
    of a composite object.
    Example:
        #fs/ext2/Makefile
            obj-$(CONFIG_EXT2_FS)        += ext2.o
        ext2-y                       := balloc.o bitmap.o
            ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
    In this example, xattr.o is only part of the composite object
    ext2.o if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
    Note: Of course, when you are building objects into the kernel,
    the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
    kbuild will build an ext2.o file for you out of the individual
    parts and then link this into built-in.o, as you would expect.
--- 3.4 Objects which export symbols
    No special notation is required in the makefiles for
    modules exporting symbols.
--- 3.5 Library file goals - lib-y
    Objects listed with obj-* are used for modules, or
    combined in a built-in.o for that specific directory.
    There is also the possibility to list objects that will
    be included in a library, lib.a.
    All objects listed with lib-y are combined in a single
    library for that directory.
    Objects that are listed in obj-y and additionally listed in
    lib-y will not be included in the library, since they will
    be accessible anyway.
    For consistency, objects listed in lib-m will be included in lib.a.
    Note that the same kbuild makefile may list files to be built-in
    and to be part of a library. Therefore the same directory
    may contain both a built-in.o and a lib.a file.
    Example:
        #arch/i386/lib/Makefile
        lib-y    := checksum.o delay.o
    This will create a library lib.a based on checksum.o and delay.o.
    For kbuild to actually recognize that there is a lib.a being built,
    the directory shall be listed in libs-y.
    See also "6.3 List directories to visit when descending".
    Use of lib-y is normally restricted to lib/ and arch/*/lib.
--- 3.6 Descending down in directories
    A Makefile is only responsible for building objects in its own
    directory. Files in subdirectories should be taken care of by
    Makefiles in these subdirs. The build system will automatically
    invoke make recursively in subdirectories, provided you let it know of
    them.
    To do so, obj-y and obj-m are used.
    ext2 lives in a separate directory, and the Makefile present in fs/
    tells kbuild to descend down using the following assignment.
    Example:
        #fs/Makefile
        obj-$(CONFIG_EXT2_FS) += ext2/
    If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
    the corresponding obj- variable will be set, and kbuild will descend
    down in the ext2 directory.
    Kbuild only uses this information to decide that it needs to visit
    the directory, it is the Makefile in the subdirectory that
    specifies what is modules and what is built-in.
    It is good practice to use a CONFIG_ variable when assigning directory
    names. This allows kbuild to totally skip the directory if the
    corresponding CONFIG_ option is neither 'y' nor 'm'.
--- 3.7 Compilation flags
    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 - = 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 = -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  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.
    $(-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 -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 $(obj); ./gen-devlist )  produce .config
2) Store kernel version in include/linux/version.h
3) Symlink include/asm to include/asm-$(ARCH)
4) Updating all other prerequisites to the target prepare:
   - Additional prerequisites are specified in arch/$(ARCH)/Makefile
5) Recursively descend down in all directories listed in
   init-* core* drivers-* net-* libs-* and build all targets.
   - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
6) All object files are then linked and the resulting file vmlinux is
   located at the root of the obj tree.
   The very first objects linked are listed in head-y, assigned by
   arch/$(ARCH)/Makefile.
7) Finally, the architecture-specific part does any required post processing
   and builds the final bootimage.
   - This includes building boot records
   - Preparing initrd images and the like
--- 6.1 Set variables to tweak the build to the architecture
    LDFLAGS        Generic $(LD) options
    Flags used for all invocations of the linker.
    Often specifying the emulation is sufficient.
    Example:
        #arch/s390/Makefile
        LDFLAGS         := -m elf_s390
    Note: EXTRA_LDFLAGS and LDFLAGS_$@ 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 list directories where a built-in.o object file can be
    located.
    $(init-y) objects will be located after $(head-y).
    Then the rest follows in this order:
    $(core-y), $(libs-y), $(drivers-y) and $(net-y).
    The top level Makefile defines values for all generic directories,
    and arch/$(ARCH)/Makefile only adds architecture-specific directories.
    Example:
        #arch/sparc64/Makefile
        core-y += arch/sparc64/kernel/
        libs-y += arch/sparc64/prom/ arch/sparc64/lib/
        drivers-$(CONFIG_OPROFILE)  += arch/sparc64/oprofile/
--- 6.4 Architecture-specific boot images
    An arch Makefile specifies goals that take the vmlinux file, compress
    it, wrap it in bootstrapping code, and copy the resulting files
    somewhere. This includes various kinds of installation commands.
    The actual goals are not standardized across architectures.
    It is common to locate any additional processing in a boot/
    directory below arch/$(ARCH)/.
    Kbuild does not provide any smart way to support building a
    target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
    call make manually to build a target in boot/.
    The recommended approach is to include shortcuts in
    arch/$(ARCH)/Makefile, and use the full path when calling down
    into the arch/$(ARCH)/boot/Makefile.
    Example:
        #arch/i386/Makefile
        boot := arch/i386/boot
        bzImage: vmlinux
            $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
    "$(Q)$(MAKE) $(build)=" is the recommended way to invoke
    make in a subdirectory.
    There are no rules for naming architecture-specific targets,
    but executing "make help" will list all relevant targets.
    To support this, $(archhelp) must be defined.
    Example:
        #arch/i386/Makefile
        define archhelp
          echo  '* bzImage      - Image (arch/$(ARCH)/boot/bzImage)'
        endif
    When make is executed without arguments, the first goal encountered
    will be built. In the top level Makefile the first goal present
    is all:.
    An architecture shall always, per default, build a bootable image.
    In "make help", the default goal is highlighted with a '*'.
    Add a new prerequisite to all: to select a default goal different
    from vmlinux.
    Example:
        #arch/i386/Makefile
        all: bzImage
    When "make" is executed without arguments, bzImage will be built.
--- 6.5 Building non-kbuild targets
    extra-y
    extra-y specify additional targets created in the current
    directory, in addition to any targets specified by obj-*.
    Listing all targets in extra-y is required for two purposes:
    1) Enable kbuild to check changes in command lines
       - When $(call if_changed,xxx) is used
    2) kbuild knows what files to delete during "make clean"
    Example:
        #arch/i386/kernel/Makefile
        extra-y := head.o init_task.o
    In this example, extra-y is used to list object files that
    shall be built, but shall not be linked as part of built-in.o.
--- 6.6 Commands useful for building a boot image
    Kbuild provides a few macros that are useful when building a
    boot image.
    if_changed
    if_changed is the infrastructure used for the following commands.
    Usage:
        target: source(s) FORCE
            $(call if_changed,ld/objcopy/gzip)
    When the rule is evaluated, it is checked to see if any files
    need an update, or the command line has changed since the last
    invocation. The latter will force a rebuild if any options
    to the executable have changed.
    Any target that utilises if_changed must be listed in $(targets),
    otherwise the command line check will fail, and the target will
    always be built.
    Assignments to $(targets) are without $(obj)/ prefix.
    if_changed may be used in conjunction with custom commands as
    defined in 6.7 "Custom kbuild commands".
    Note: It is a typical mistake to forget the FORCE prerequisite.
    Another common pitfall is that whitespace is sometimes
    significant; for instance, the below will fail (note the extra space
    after the comma):
        target: source(s) FORCE
    #WRONG!#    $(call if_changed, ld/objcopy/gzip)
    ld
    Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
    objcopy
    Copy binary. Uses OBJCOPYFLAGS usually specified in
    arch/$(ARCH)/Makefile.
    OBJCOPYFLAGS_$@ may be used to set additional options.
    gzip
    Compress target. Use maximum compression to compress target.
    Example:
        #arch/i386/boot/Makefile
        LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
        LDFLAGS_setup    := -Ttext 0x0 -s --oformat binary -e begtext
        targets += setup setup.o bootsect bootsect.o
        $(obj)/setup $(obj)/bootsect: %: %.o FORCE
            $(call if_changed,ld)
    In this example, there are two possible targets, requiring different
    options to the linker. The linker options are specified using the
    LDFLAGS_$@ syntax - one for each potential target.
    $(targets) are assigned all potential targets, by which kbuild knows
    the targets and will:
        1) check for commandline changes
        2) delete target during make clean
    The ": %: %.o" part of the prerequisite is a shorthand that
    free us from listing the setup.o and bootsect.o files.
    Note: It is a common mistake to forget the "target :=" assignment,
          resulting in the target file being recompiled for no
          obvious reason.
--- 6.7 Custom kbuild commands
    When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
    of a command is normally displayed.
    To enable this behaviour for custom commands kbuild requires
    two variables to be set:
    quiet_cmd_    - what shall be echoed
          cmd_    - 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,
Updates by Kai Germaschewski
Updates by Sam Ravnborg
Language QA by Jan Engelhardt
=== 10 TODO
- Describe how kbuild supports shipped files with _shipped.
- Generating offset header files.
- Add more variables to section 7?
               
               
               

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