Linux-2.6.12-rc2v2.6.12-rc2

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

Let it rip!

1 files changed, 1122 insertions, 0 deletions

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
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+Linux Kernel Makefiles
+
+This document describes the Linux kernel Makefiles.
+
+=== Table of Contents
+
+        === 1 Overview
+        === 2 Who does what
+        === 3 The kbuild files
+           --- 3.1 Goal definitions
+           --- 3.2 Built-in object goals - obj-y
+           --- 3.3 Loadable module goals - obj-m
+           --- 3.4 Objects which export symbols
+           --- 3.5 Library file goals - lib-y
+           --- 3.6 Descending down in directories
+           --- 3.7 Compilation flags
+           --- 3.8 Command line dependency
+           --- 3.9 Dependency tracking
+           --- 3.10 Special Rules
+
+        === 4 Host Program support
+           --- 4.1 Simple Host Program
+           --- 4.2 Composite Host Programs
+           --- 4.3 Defining shared libraries  
+           --- 4.4 Using C++ for host programs
+           --- 4.5 Controlling compiler options for host programs
+           --- 4.6 When host programs are actually built
+           --- 4.7 Using hostprogs-$(CONFIG_FOO)
+
+        === 5 Kbuild clean infrastructure
+
+        === 6 Architecture Makefiles
+           --- 6.1 Set variables to tweak the build to the architecture
+           --- 6.2 Add prerequisites to prepare:
+           --- 6.3 List directories to visit when descending
+           --- 6.4 Architecture specific boot images
+           --- 6.5 Building non-kbuild targets
+           --- 6.6 Commands useful for building a boot image
+           --- 6.7 Custom kbuild commands
+           --- 6.8 Preprocessing linker scripts
+           --- 6.9 $(CC) support functions
+
+        === 7 Kbuild Variables
+        === 8 Makefile language
+        === 9 Credits
+        === 10 TODO
+
+=== 1 Overview
+
+The Makefiles have five parts:
+
+        Makefile                the top Makefile.
+        .config                 the kernel configuration file.
+        arch/$(ARCH)/Makefile   the arch Makefile.
+        scripts/Makefile.*      common rules etc. for all kbuild Makefiles.
+        kbuild Makefiles        there are about 500 of these.
+
+The top Makefile reads the .config file, which comes from the kernel
+configuration process.
+
+The top Makefile is responsible for building two major products: vmlinux
+(the resident kernel image) and modules (any module files).
+It builds these goals by recursively descending into the subdirectories of
+the kernel source tree.
+The list of subdirectories which are visited depends upon the kernel
+configuration. The top Makefile textually includes an arch Makefile
+with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
+architecture-specific information to the top Makefile.
+
+Each subdirectory has a kbuild Makefile which carries out the commands
+passed down from above. The kbuild Makefile uses information from the
+.config file to construct various file lists used by kbuild to build 
+any built-in or modular targets.
+
+scripts/Makefile.* contains all the definitions/rules etc. that
+are used to build the kernel based on the kbuild makefiles.
+
+
+=== 2 Who does what
+
+People have four different relationships with the kernel Makefiles.
+
+*Users* are people who build kernels.  These people type commands such as
+"make menuconfig" or "make".  They usually do not read or edit
+any kernel Makefiles (or any other source files).
+
+*Normal developers* are people who work on features such as device
+drivers, file systems, and network protocols.  These people need to
+maintain the kbuild Makefiles for the subsystem that they are
+working on.  In order to do this effectively, they need some overall
+knowledge about the kernel Makefiles, plus detailed knowledge about the
+public interface for kbuild.
+
+*Arch developers* are people who work on an entire architecture, such
+as sparc or ia64.  Arch developers need to know about the arch Makefile
+as well as kbuild Makefiles.
+
+*Kbuild developers* are people who work on the kernel build system itself.
+These people need to know about all aspects of the kernel Makefiles.
+
+This document is aimed towards normal developers and arch developers.
+
+
+=== 3 The kbuild files
+
+Most Makefiles within the kernel are kbuild Makefiles that use the
+kbuild infrastructure. This chapter introduce the syntax used in the
+kbuild makefiles.
+The preferred name for the kbuild files is 'Kbuild' but 'Makefile' will
+continue to be supported. All new developmen is expected to use the
+Kbuild filename.
+
+Section 3.1 "Goal definitions" is a quick intro, further chapters provide
+more details, with real examples.
+
+--- 3.1 Goal definitions
+
+        Goal definitions are the main part (heart) of the kbuild Makefile.
+        These lines define the files to be built, any special compilation
+        options, and any subdirectories to be entered recursively.
+
+        The most simple kbuild makefile contains one line:
+
+        Example:
+                obj-y += foo.o
+
+        This tell kbuild that there is one object in that directory named
+        foo.o. foo.o will be built from foo.c or foo.S.
+
+        If foo.o shall be built as a module, the variable obj-m is used.
+        Therefore the following pattern is often used:
+
+        Example:
+                obj-$(CONFIG_FOO) += foo.o
+
+        $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
+        If CONFIG_FOO is neither y nor m, then the file will not be compiled
+        nor linked.
+
+--- 3.2 Built-in object goals - obj-y
+
+        The kbuild Makefile specifies object files for vmlinux
+        in the lists $(obj-y).  These lists depend on the kernel
+        configuration.
+
+        Kbuild compiles all the $(obj-y) files.  It then calls
+        "$(LD) -r" to merge these files into one built-in.o file.
+        built-in.o is later linked into vmlinux by the parent Makefile.
+
+        The order of files in $(obj-y) is significant.  Duplicates in
+        the lists are allowed: the first instance will be linked into
+        built-in.o and succeeding instances will be ignored.
+
+        Link order is significant, because certain functions
+        (module_init() / __initcall) will be called during boot in the
+        order they appear. So keep in mind that changing the link
+        order may e.g.  change the order in which your SCSI
+        controllers are detected, and thus you disks are renumbered.
+
+        Example:
+                #drivers/isdn/i4l/Makefile
+                # Makefile for the kernel ISDN subsystem and device drivers.
+                # Each configuration option enables a list of files.
+                obj-$(CONFIG_ISDN)             += isdn.o
+                obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
+
+--- 3.3 Loadable module goals - obj-m
+
+        $(obj-m) specify object files which are built as loadable
+        kernel modules.
+
+        A module may be built from one source file or several source
+        files. In the case of one source file, the kbuild makefile
+        simply adds the file to $(obj-m).
+
+        Example:
+                #drivers/isdn/i4l/Makefile
+                obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
+
+        Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
+
+        If a kernel module is built from several source files, you specify
+        that you want to build a module in the same way as above.
+
+        Kbuild needs to know which the parts that you want to build your
+        module from, so you have to tell it by setting an
+        $(<module_name>-objs) variable.
+
+        Example:
+                #drivers/isdn/i4l/Makefile
+                obj-$(CONFIG_ISDN) += isdn.o
+                isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
+
+        In this example, the module name will be isdn.o. Kbuild will
+        compile the objects listed in $(isdn-objs) and then run
+        "$(LD) -r" on the list of these files to generate isdn.o.
+
+        Kbuild recognises objects used for composite objects by the suffix
+        -objs, and the suffix -y. This allows the Makefiles to use
+        the value of a CONFIG_ symbol to determine if an object is part
+        of a composite object.
+
+        Example:
+                #fs/ext2/Makefile
+                obj-$(CONFIG_EXT2_FS)        += ext2.o
+                ext2-y                       := balloc.o bitmap.o
+                ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
+        
+        In this example xattr.o is only part of the composite object
+        ext2.o, if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
+
+        Note: Of course, when you are building objects into the kernel,
+        the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
+        kbuild will build an ext2.o file for you out of the individual
+        parts and then link this into built-in.o, as you would expect.
+
+--- 3.4 Objects which export symbols
+
+        No special notation is required in the makefiles for
+        modules exporting symbols.
+
+--- 3.5 Library file goals - lib-y
+
+        Objects listed with obj-* are used for modules or
+        combined in a built-in.o for that specific directory.
+        There is also the possibility to list objects that will
+        be included in a library, lib.a.
+        All objects listed with lib-y are combined in a single
+        library for that directory.
+        Objects that are listed in obj-y and additional listed in
+        lib-y will not be included in the library, since they will anyway
+        be accessible.
+        For consistency objects listed in lib-m will be included in lib.a. 
+
+        Note that the same kbuild makefile may list files to be built-in
+        and to be part of a library. Therefore the same directory
+        may contain both a built-in.o and a lib.a file.
+
+        Example:
+                #arch/i386/lib/Makefile
+                lib-y    := checksum.o delay.o
+
+        This will create a library lib.a based on checksum.o and delay.o.
+        For kbuild to actually recognize that there is a lib.a being build
+        the directory shall be listed in libs-y.
+        See also "6.3 List directories to visit when descending".
+ 
+        Usage of lib-y is normally restricted to lib/ and arch/*/lib.
+
+--- 3.6 Descending down in directories
+
+        A Makefile is only responsible for building objects in its own
+        directory. Files in subdirectories should be taken care of by
+        Makefiles in these subdirs. The build system will automatically
+        invoke make recursively in subdirectories, provided you let it know of
+        them.
+
+        To do so obj-y and obj-m are used.
+        ext2 lives in a separate directory, and the Makefile present in fs/
+        tells kbuild to descend down using the following assignment.
+
+        Example:
+                #fs/Makefile
+                obj-$(CONFIG_EXT2_FS) += ext2/
+
+        If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
+        the corresponding obj- variable will be set, and kbuild will descend
+        down in the ext2 directory.
+        Kbuild only uses this information to decide that it needs to visit
+        the directory, it is the Makefile in the subdirectory that
+        specifies what is modules and what is built-in.
+
+        It is good practice to use a CONFIG_ variable when assigning directory
+        names. This allows kbuild to totally skip the directory if the
+        corresponding CONFIG_ option is neither 'y' nor 'm'.
+
+--- 3.7 Compilation flags
+
+    EXTRA_CFLAGS, EXTRA_AFLAGS, EXTRA_LDFLAGS, EXTRA_ARFLAGS
+
+        All the EXTRA_ variables apply only to the kbuild makefile
+        where they are assigned. The EXTRA_ variables apply to all
+        commands executed in the kbuild makefile.
+
+        $(EXTRA_CFLAGS) specifies options for compiling C files with
+        $(CC).
+
+        Example:
+                # drivers/sound/emu10k1/Makefile
+                EXTRA_CFLAGS += -I$(obj)
+                ifdef DEBUG
+                    EXTRA_CFLAGS += -DEMU10K1_DEBUG
+                endif
+
+
+        This variable is necessary because the top Makefile owns the
+        variable $(CFLAGS) and uses it for compilation flags for the
+        entire tree.
+
+        $(EXTRA_AFLAGS) is a similar string for per-directory options
+        when compiling assembly language source.
+
+        Example:
+                #arch/x86_64/kernel/Makefile
+                EXTRA_AFLAGS := -traditional
+
+
+        $(EXTRA_LDFLAGS) and $(EXTRA_ARFLAGS) are similar strings for
+        per-directory options to $(LD) and $(AR).
+
+        Example:
+                #arch/m68k/fpsp040/Makefile
+                EXTRA_LDFLAGS := -x
+
+    CFLAGS_$@, AFLAGS_$@
+
+        CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
+        kbuild makefile.
+
+        $(CFLAGS_$@) specifies per-file options for $(CC).  The $@
+        part has a literal value which specifies the file that it is for.
+
+        Example:
+                # drivers/scsi/Makefile
+                CFLAGS_aha152x.o =   -DAHA152X_STAT -DAUTOCONF
+                CFLAGS_gdth.o    = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
+                                     -DGDTH_STATISTICS
+                CFLAGS_seagate.o =   -DARBITRATE -DPARITY -DSEAGATE_USE_ASM
+
+        These three lines specify compilation flags for aha152x.o,
+        gdth.o, and seagate.o
+
+        $(AFLAGS_$@) is a similar feature for source files in assembly
+        languages.
+
+        Example:
+                # arch/arm/kernel/Makefile
+                AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional
+                AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional
+
+--- 3.9 Dependency tracking
+
+        Kbuild tracks dependencies on the following:
+        1) All prerequisite files (both *.c and *.h)
+        2) CONFIG_ options used in all prerequisite files
+        3) Command-line used to compile target
+
+        Thus, if you change an option to $(CC) all affected files will
+        be re-compiled.
+
+--- 3.10 Special Rules
+
+        Special rules are used when the kbuild infrastructure does
+        not provide the required support. A typical example is
+        header files generated during the build process.
+        Another example is the architecture specific Makefiles which
+        needs special rules to prepare boot images etc.
+
+        Special rules are written as normal Make rules.
+        Kbuild is not executing in the directory where the Makefile is
+        located, so all special rules shall provide a relative
+        path to prerequisite files and target files.
+
+        Two variables are used when defining special rules:
+
+    $(src)
+        $(src) is a relative path which points to the directory
+        where the Makefile is located. Always use $(src) when
+        referring to files located in the src tree.
+
+    $(obj)
+        $(obj) is a relative path which points to the directory
+        where the target is saved. Always use $(obj) when
+        referring to generated files.
+
+        Example:
+                #drivers/scsi/Makefile
+                $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
+                        $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
+
+        This is a special rule, following the normal syntax
+        required by make.
+        The target file depends on two prerequisite files. References
+        to the target file are prefixed with $(obj), references
+        to prerequisites are referenced with $(src) (because they are not
+        generated files).
+
+
+=== 4 Host Program support
+
+Kbuild supports building executables on the host for use during the
+compilation stage.
+Two steps are required in order to use a host executable.
+
+The first step is to tell kbuild that a host program exists. This is
+done utilising the variable hostprogs-y.
+
+The second step is to add an explicit dependency to the executable.
+This can be done in two ways. Either add the dependency in a rule, 
+or utilise the variable $(always).
+Both possibilities are described in the following.
+
+--- 4.1 Simple Host Program
+
+        In some cases there is a need to compile and run a program on the
+        computer where the build is running.
+        The following line tells kbuild that the program bin2hex shall be
+        built on the build host.
+
+        Example:
+                hostprogs-y := bin2hex
+
+        Kbuild assumes in the above example that bin2hex is made from a single
+        c-source file named bin2hex.c located in the same directory as
+        the Makefile.
+  
+--- 4.2 Composite Host Programs
+
+        Host programs can be made up based on composite objects.
+        The syntax used to define composite objects for host programs is
+        similar to the syntax used for kernel objects.
+        $(<executeable>-objs) list all objects used to link the final
+        executable.
+
+        Example:
+                #scripts/lxdialog/Makefile
+                hostprogs-y   := lxdialog  
+                lxdialog-objs := checklist.o lxdialog.o
+
+        Objects with extension .o are compiled from the corresponding .c
+        files. In the above example checklist.c is compiled to checklist.o
+        and lxdialog.c is compiled to lxdialog.o.
+        Finally the two .o files are linked to the executable, lxdialog.
+        Note: The syntax <executable>-y is not permitted for host-programs.
+
+--- 4.3 Defining shared libraries  
+  
+        Objects with extension .so are considered shared libraries, and
+        will be compiled as position independent objects.
+        Kbuild provides support for shared libraries, but the usage
+        shall be restricted.
+        In the following example the libkconfig.so shared library is used
+        to link the executable conf.
+
+        Example:
+                #scripts/kconfig/Makefile
+                hostprogs-y     := conf
+                conf-objs       := conf.o libkconfig.so
+                libkconfig-objs := expr.o type.o
+  
+        Shared libraries always require a corresponding -objs line, and
+        in the example above the shared library libkconfig is composed by
+        the two objects expr.o and type.o.
+        expr.o and type.o will be built as position independent code and
+        linked as a shared library libkconfig.so. C++ is not supported for
+        shared libraries.
+
+--- 4.4 Using C++ for host programs
+
+        kbuild offers support for host programs written in C++. This was
+        introduced solely to support kconfig, and is not recommended
+        for general use.
+
+        Example:
+                #scripts/kconfig/Makefile
+                hostprogs-y   := qconf
+                qconf-cxxobjs := qconf.o
+
+        In the example above the executable is composed of the C++ file
+        qconf.cc - identified by $(qconf-cxxobjs).
+        
+        If qconf is composed by a mixture of .c and .cc files, then an
+        additional line can be used to identify this.
+
+        Example:
+                #scripts/kconfig/Makefile
+                hostprogs-y   := qconf
+                qconf-cxxobjs := qconf.o
+                qconf-objs    := check.o
+        
+--- 4.5 Controlling compiler options for host programs
+
+        When compiling host programs, it is possible to set specific flags.
+        The programs will always be compiled utilising $(HOSTCC) passed
+        the options specified in $(HOSTCFLAGS).
+        To set flags that will take effect for all host programs created
+        in that Makefile use the variable HOST_EXTRACFLAGS.
+
+        Example:
+                #scripts/lxdialog/Makefile
+                HOST_EXTRACFLAGS += -I/usr/include/ncurses
+  
+        To set specific flags for a single file the following construction
+        is used:
+
+        Example:
+                #arch/ppc64/boot/Makefile
+                HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
+  
+        It is also possible to specify additional options to the linker.
+  
+        Example:
+                #scripts/kconfig/Makefile
+                HOSTLOADLIBES_qconf := -L$(QTDIR)/lib
+
+        When linking qconf it will be passed the extra option "-L$(QTDIR)/lib".
+ 
+--- 4.6 When host programs are actually built
+
+        Kbuild will only build host-programs when they are referenced
+        as a prerequisite.
+        This is possible in two ways:
+
+        (1) List the prerequisite explicitly in a special rule.
+
+        Example:
+                #drivers/pci/Makefile
+                hostprogs-y := gen-devlist
+                $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
+                        ( cd $(obj); ./gen-devlist ) < $<
+
+        The target $(obj)/devlist.h will not be built before 
+        $(obj)/gen-devlist is updated. Note that references to
+        the host programs in special rules must be prefixed with $(obj).
+
+        (2) Use $(always)
+        When there is no suitable special rule, and the host program
+        shall be built when a makefile is entered, the $(always)
+        variable shall be used.
+
+        Example:
+                #scripts/lxdialog/Makefile
+                hostprogs-y   := lxdialog
+                always        := $(hostprogs-y)
+
+        This will tell kbuild to build lxdialog even if not referenced in
+        any rule.
+
+--- 4.7 Using hostprogs-$(CONFIG_FOO)
+
+        A typcal pattern in a Kbuild file lok like this:
+
+        Example:
+                #scripts/Makefile
+                hostprogs-$(CONFIG_KALLSYMS) += kallsyms
+
+        Kbuild knows about both 'y' for built-in and 'm' for module.
+        So if a config symbol evaluate to 'm', kbuild will still build
+        the binary. In other words Kbuild handle hostprogs-m exactly
+        like hostprogs-y. But only hostprogs-y is recommend used
+        when no CONFIG symbol are involved.
+
+=== 5 Kbuild clean infrastructure
+
+"make clean" deletes most generated files in the src tree where the kernel
+is compiled. This includes generated files such as host programs.
+Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
+$(extra-y) and $(targets). They are all deleted during "make clean".
+Files matching the patterns "*.[oas]", "*.ko", plus some additional files
+generated by kbuild are deleted all over the kernel src tree when
+"make clean" is executed.
+
+Additional files can be specified in kbuild makefiles by use of $(clean-files).
+
+        Example:
+                #drivers/pci/Makefile
+                clean-files := devlist.h classlist.h
+
+When executing "make clean", the two files "devlist.h classlist.h" will
+be deleted. Kbuild will assume files to be in same relative directory as the
+Makefile except if an absolute path is specified (path starting with '/').
+
+To delete a directory hirachy use:
+        Example:
+                #scripts/package/Makefile
+                clean-dirs := $(objtree)/debian/
+
+This will delete the directory debian, including all subdirectories.
+Kbuild will assume the directories to be in the same relative path as the
+Makefile if no absolute path is specified (path does not start with '/').
+
+Usually kbuild descends down in subdirectories due to "obj-* := dir/",
+but in the architecture makefiles where the kbuild infrastructure
+is not sufficient this sometimes needs to be explicit.
+
+        Example:
+                #arch/i386/boot/Makefile
+                subdir- := compressed/
+
+The above assignment instructs kbuild to descend down in the
+directory compressed/ when "make clean" is executed.
+
+To support the clean infrastructure in the Makefiles that builds the
+final bootimage there is an optional target named archclean:
+
+        Example:
+                #arch/i386/Makefile
+                archclean:
+                        $(Q)$(MAKE) $(clean)=arch/i386/boot
+
+When "make clean" is executed, make will descend down in arch/i386/boot,
+and clean as usual. The Makefile located in arch/i386/boot/ may use
+the subdir- trick to descend further down.
+
+Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
+included in the top level makefile, and the kbuild infrastructure
+is not operational at that point.
+
+Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
+be visited during "make clean".
+
+=== 6 Architecture Makefiles
+
+The top level Makefile sets up the environment and does the preparation,
+before starting to descend down in the individual directories.
+The top level makefile contains the generic part, whereas the
+arch/$(ARCH)/Makefile contains what is required to set-up kbuild
+to the said architecture.
+To do so arch/$(ARCH)/Makefile sets a number of variables, and defines
+a few targets.
+
+When kbuild executes the following steps are followed (roughly):
+1) Configuration of the kernel => produced .config
+2) Store kernel version in include/linux/version.h
+3) Symlink include/asm to include/asm-$(ARCH)
+4) Updating all other prerequisites to the target prepare:
+   - Additional prerequisites are specified in arch/$(ARCH)/Makefile
+5) Recursively descend down in all directories listed in
+   init-* core* drivers-* net-* libs-* and build all targets.
+   - The value of the above variables are extended in arch/$(ARCH)/Makefile.
+6) All object files are then linked and the resulting file vmlinux is 
+   located at the root of the src tree.
+   The very first objects linked are listed in head-y, assigned by
+   arch/$(ARCH)/Makefile.
+7) Finally the architecture specific part does any required post processing
+   and builds the final bootimage.
+   - This includes building boot records
+   - Preparing initrd images and the like
+
+
+--- 6.1 Set variables to tweak the build to the architecture
+
+    LDFLAGS             Generic $(LD) options
+
+        Flags used for all invocations of the linker.
+        Often specifying the emulation is sufficient.
+
+        Example:
+                #arch/s390/Makefile
+                LDFLAGS         := -m elf_s390
+        Note: EXTRA_LDFLAGS and LDFLAGS_$@ can be used to further customise
+        the flags used. See chapter 7.
+        
+    LDFLAGS_MODULE      Options for $(LD) when linking modules
+
+        LDFLAGS_MODULE is used to set specific flags for $(LD) when
+        linking the .ko files used for modules.
+        Default is "-r", for relocatable output.
+
+    LDFLAGS_vmlinux     Options for $(LD) when linking vmlinux
+
+        LDFLAGS_vmlinux is used to specify additional flags to pass to
+        the linker when linking the final vmlinux.
+        LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
+
+        Example:
+                #arch/i386/Makefile
+                LDFLAGS_vmlinux := -e stext
+
+    OBJCOPYFLAGS        objcopy flags
+
+        When $(call if_changed,objcopy) is used to translate a .o file,
+        then the flags specified in OBJCOPYFLAGS will be used.
+        $(call if_changed,objcopy) is often used to generate raw binaries on
+        vmlinux.
+
+        Example:
+                #arch/s390/Makefile
+                OBJCOPYFLAGS := -O binary
+
+                #arch/s390/boot/Makefile
+                $(obj)/image: vmlinux FORCE
+                        $(call if_changed,objcopy)
+
+        In this example the binary $(obj)/image is a binary version of
+        vmlinux. The usage of $(call if_changed,xxx) will be described later.
+
+    AFLAGS              $(AS) assembler flags
+
+        Default value - see top level Makefile
+        Append or modify as required per architecture.
+
+        Example:
+                #arch/sparc64/Makefile
+                AFLAGS += -m64 -mcpu=ultrasparc
+
+    CFLAGS              $(CC) compiler flags
+
+        Default value - see top level Makefile
+        Append or modify as required per architecture.
+
+        Often the CFLAGS variable depends on the configuration.
+
+        Example:
+                #arch/i386/Makefile
+                cflags-$(CONFIG_M386) += -march=i386
+                CFLAGS += $(cflags-y)
+
+        Many arch Makefiles dynamically run the target C compiler to
+        probe supported options:
+
+                #arch/i386/Makefile
+
+                ...
+                cflags-$(CONFIG_MPENTIUMII)     += $(call cc-option,\
+                                                -march=pentium2,-march=i686)
+                ...
+                # Disable unit-at-a-time mode ...
+                CFLAGS += $(call cc-option,-fno-unit-at-a-time)
+                ...
+
+
+        The first examples utilises the trick that a config option expands
+        to 'y' when selected.
+
+    CFLAGS_KERNEL       $(CC) options specific for built-in
+
+        $(CFLAGS_KERNEL) contains extra C compiler flags used to compile
+        resident kernel code.
+
+    CFLAGS_MODULE       $(CC) options specific for modules
+
+        $(CFLAGS_MODULE) contains extra C compiler flags used to compile code
+        for loadable kernel modules.
+
+ 
+--- 6.2 Add prerequisites to prepare:
+
+        The prepare: rule is used to list prerequisites that needs to be
+        built before starting to descend down in the subdirectories.
+        This is usual header files containing assembler constants.
+
+                Example:
+                #arch/s390/Makefile
+                prepare: include/asm-$(ARCH)/offsets.h
+
+        In this example the file include/asm-$(ARCH)/offsets.h will
+        be built before descending down in the subdirectories.
+        See also chapter XXX-TODO that describe how kbuild supports
+        generating offset header files.
+
+
+--- 6.3 List directories to visit when descending
+
+        An arch Makefile cooperates with the top Makefile to define variables
+        which specify how to build the vmlinux file.  Note that there is no
+        corresponding arch-specific section for modules; the module-building
+        machinery is all architecture-independent.
+
+        
+    head-y, init-y, core-y, libs-y, drivers-y, net-y
+
+        $(head-y) list objects to be linked first in vmlinux.
+        $(libs-y) list directories where a lib.a archive can be located.
+        The rest list directories where a built-in.o object file can be located.
+
+        $(init-y) objects will be located after $(head-y).
+        Then the rest follows in this order:
+        $(core-y), $(libs-y), $(drivers-y) and $(net-y).
+
+        The top level Makefile define values for all generic directories,
+        and arch/$(ARCH)/Makefile only adds architecture specific directories.
+
+        Example:
+                #arch/sparc64/Makefile
+                core-y += arch/sparc64/kernel/
+                libs-y += arch/sparc64/prom/ arch/sparc64/lib/
+                drivers-$(CONFIG_OPROFILE)  += arch/sparc64/oprofile/
+
+
+--- 6.4 Architecture specific boot images
+
+        An arch Makefile specifies goals that take the vmlinux file, compress
+        it, wrap it in bootstrapping code, and copy the resulting files
+        somewhere. This includes various kinds of installation commands.
+        The actual goals are not standardized across architectures.
+
+        It is common to locate any additional processing in a boot/
+        directory below arch/$(ARCH)/.
+
+        Kbuild does not provide any smart way to support building a
+        target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
+        call make manually to build a target in boot/.
+
+        The recommended approach is to include shortcuts in
+        arch/$(ARCH)/Makefile, and use the full path when calling down
+        into the arch/$(ARCH)/boot/Makefile.
+
+        Example:
+                #arch/i386/Makefile
+                boot := arch/i386/boot
+                bzImage: vmlinux
+                        $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+
+        "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
+        make in a subdirectory.
+
+        There are no rules for naming of the architecture specific targets,
+        but executing "make help" will list all relevant targets.
+        To support this $(archhelp) must be defined.
+
+        Example:
+                #arch/i386/Makefile
+                define archhelp
+                  echo  '* bzImage      - Image (arch/$(ARCH)/boot/bzImage)'
+                endef
+
+        When make is executed without arguments, the first goal encountered
+        will be built. In the top level Makefile the first goal present
+        is all:.
+        An architecture shall always per default build a bootable image.
+        In "make help" the default goal is highlighted with a '*'.
+        Add a new prerequisite to all: to select a default goal different
+        from vmlinux.
+
+        Example:
+                #arch/i386/Makefile
+                all: bzImage 
+
+        When "make" is executed without arguments, bzImage will be built.
+
+--- 6.5 Building non-kbuild targets
+
+    extra-y
+
+        extra-y specify additional targets created in the current
+        directory, in addition to any targets specified by obj-*.
+
+        Listing all targets in extra-y is required for two purposes:
+        1) Enable kbuild to check changes in command lines
+           - When $(call if_changed,xxx) is used
+        2) kbuild knows what files to delete during "make clean"
+
+        Example:
+                #arch/i386/kernel/Makefile
+                extra-y := head.o init_task.o
+
+        In this example extra-y is used to list object files that
+        shall be built, but shall not be linked as part of built-in.o.
+
+        
+--- 6.6 Commands useful for building a boot image
+
+        Kbuild provides a few macros that are useful when building a
+        boot image.
+
+    if_changed
+
+        if_changed is the infrastructure used for the following commands.
+
+        Usage:
+                target: source(s) FORCE
+                        $(call if_changed,ld/objcopy/gzip)
+
+        When the rule is evaluated it is checked to see if any files
+        needs an update, or the commandline has changed since last
+        invocation. The latter will force a rebuild if any options
+        to the executable have changed.
+        Any target that utilises if_changed must be listed in $(targets),
+        otherwise the command line check will fail, and the target will
+        always be built.
+        Assignments to $(targets) are without $(obj)/ prefix.
+        if_changed may be used in conjunction with custom commands as
+        defined in 6.7 "Custom kbuild commands".
+        Note: It is a typical mistake to forget the FORCE prerequisite.
+
+    ld
+        Link target. Often LDFLAGS_$@ is used to set specific options to ld.
+        
+    objcopy
+        Copy binary. Uses OBJCOPYFLAGS usually specified in
+        arch/$(ARCH)/Makefile.
+        OBJCOPYFLAGS_$@ may be used to set additional options.
+
+    gzip
+        Compress target. Use maximum compression to compress target.
+
+        Example:
+                #arch/i386/boot/Makefile
+                LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
+                LDFLAGS_setup    := -Ttext 0x0 -s --oformat binary -e begtext
+
+                targets += setup setup.o bootsect bootsect.o
+                $(obj)/setup $(obj)/bootsect: %: %.o FORCE
+                        $(call if_changed,ld)
+
+        In this example there are two possible targets, requiring different
+        options to the linker. the linker options are specified using the
+        LDFLAGS_$@ syntax - one for each potential target.
+        $(targets) are assinged all potential targets, herby kbuild knows
+        the targets and will:
+                1) check for commandline changes
+                2) delete target during make clean
+
+        The ": %: %.o" part of the prerequisite is a shorthand that
+        free us from listing the setup.o and bootsect.o files.
+        Note: It is a common mistake to forget the "target :=" assignment,
+              resulting in the target file being recompiled for no
+              obvious reason.
+
+
+--- 6.7 Custom kbuild commands
+
+        When kbuild is executing with KBUILD_VERBOSE=0 then only a shorthand
+        of a command is normally displayed.
+        To enable this behaviour for custom commands kbuild requires
+        two variables to be set:
+        quiet_cmd_<command>     - what shall be echoed
+              cmd_<command>     - the command to execute
+
+        Example:
+                #
+                quiet_cmd_image = BUILD   $@
+                      cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
+                                                     $(obj)/vmlinux.bin > $@
+
+                targets += bzImage
+                $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
+                        $(call if_changed,image)
+                        @echo 'Kernel: $@ is ready'
+
+        When updating the $(obj)/bzImage target the line:
+
+        BUILD    arch/i386/boot/bzImage
+
+        will be displayed with "make KBUILD_VERBOSE=0".
+        
+
+--- 6.8 Preprocessing linker scripts
+
+        When the vmlinux image is build the linker script:
+        arch/$(ARCH)/kernel/vmlinux.lds is used.
+        The script is a preprocessed variant of the file vmlinux.lds.S
+        located in the same directory.
+        kbuild knows .lds file and includes a rule *lds.S -> *lds.
+        
+        Example:
+                #arch/i386/kernel/Makefile
+                always := vmlinux.lds
+        
+                #Makefile
+                export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
+                
+        The assigment to $(always) is used to tell kbuild to build the
+        target: vmlinux.lds.
+        The assignment to $(CPPFLAGS_vmlinux.lds) tell kbuild to use the
+        specified options when building the target vmlinux.lds.
+        
+        When building the *.lds target kbuild used the variakles:
+        CPPFLAGS        : Set in top-level Makefile
+        EXTRA_CPPFLAGS  : May be set in the kbuild makefile
+        CPPFLAGS_$(@F)  : Target specific flags.
+                          Note that the full filename is used in this
+                          assignment.
+
+        The kbuild infrastructure for *lds file are used in several
+        architecture specific files.
+
+
+--- 6.9 $(CC) support functions
+
+        The kernel may be build with several different versions of
+        $(CC), each supporting a unique set of features and options.
+        kbuild provide basic support to check for valid options for $(CC).
+        $(CC) is useally the gcc compiler, but other alternatives are
+        available.
+
+    cc-option
+        cc-option is used to check if $(CC) support a given option, and not
+        supported to use an optional second option.
+
+        Example:
+                #arch/i386/Makefile
+                cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
+
+        In the above example cflags-y will be assigned the option
+        -march=pentium-mmx if supported by $(CC), otherwise -march-i586.
+        The second argument to cc-option is optional, and if omitted
+        cflags-y will be assigned no value if first option is not supported.
+
+   cc-option-yn
+        cc-option-yn is used to check if gcc supports a given option
+        and return 'y' if supported, otherwise 'n'.
+
+        Example:
+                #arch/ppc/Makefile
+                biarch := $(call cc-option-yn, -m32)
+                aflags-$(biarch) += -a32
+                cflags-$(biarch) += -m32
+        
+        In the above example $(biarch) is set to y if $(CC) supports the -m32
+        option. When $(biarch) equals to y the expanded variables $(aflags-y)
+        and $(cflags-y) will be assigned the values -a32 and -m32.
+
+    cc-option-align
+        gcc version >= 3.0 shifted type of options used to speify
+        alignment of functions, loops etc. $(cc-option-align) whrn used
+        as prefix to the align options will select the right prefix:
+        gcc < 3.00
+                cc-option-align = -malign
+        gcc >= 3.00
+                cc-option-align = -falign
+        
+        Example:
+                CFLAGS += $(cc-option-align)-functions=4
+
+        In the above example the option -falign-functions=4 is used for
+        gcc >= 3.00. For gcc < 3.00 -malign-functions=4 is used.
+        
+    cc-version
+        cc-version return a numerical version of the $(CC) compiler version.
+        The format is <major><minor> where both are two digits. So for example
+        gcc 3.41 would return 0341.
+        cc-version is useful when a specific $(CC) version is faulty in one
+        area, for example the -mregparm=3 were broken in some gcc version
+        even though the option was accepted by gcc.
+
+        Example:
+                #arch/i386/Makefile
+                GCC_VERSION := $(call cc-version)
+                cflags-y += $(shell \
+                if [ $(GCC_VERSION) -ge 0300 ] ; then echo "-mregparm=3"; fi ;)
+
+        In the above example -mregparm=3 is only used for gcc version greater
+        than or equal to gcc 3.0.
+        
+
+=== 7 Kbuild Variables
+
+The top Makefile exports the following variables:
+
+    VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
+
+        These variables define the current kernel version.  A few arch
+        Makefiles actually use these values directly; they should use
+        $(KERNELRELEASE) instead.
+
+        $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
+        three-part version number, such as "2", "4", and "0".  These three
+        values are always numeric.
+
+        $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
+        or additional patches.  It is usually some non-numeric string
+        such as "-pre4", and is often blank.
+
+    KERNELRELEASE
+
+        $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
+        for constructing installation directory names or showing in
+        version strings.  Some arch Makefiles use it for this purpose.
+
+    ARCH
+
+        This variable defines the target architecture, such as "i386",
+        "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
+        determine which files to compile.
+
+        By default, the top Makefile sets $(ARCH) to be the same as the
+        host system architecture.  For a cross build, a user may
+        override the value of $(ARCH) on the command line:
+
+            make ARCH=m68k ...
+
+
+    INSTALL_PATH
+
+        This variable defines a place for the arch Makefiles to install
+        the resident kernel image and System.map file.
+        Use this for architecture specific install targets.
+
+    INSTALL_MOD_PATH, MODLIB
+
+        $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
+        installation.  This variable is not defined in the Makefile but
+        may be passed in by the user if desired.
+
+        $(MODLIB) specifies the directory for module installation.
+        The top Makefile defines $(MODLIB) to
+        $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE).  The user may
+        override this value on the command line if desired.
+
+=== 8 Makefile language
+
+The kernel Makefiles are designed to run with GNU Make.  The Makefiles
+use only the documented features of GNU Make, but they do use many
+GNU extensions.
+
+GNU Make supports elementary list-processing functions.  The kernel
+Makefiles use a novel style of list building and manipulation with few
+"if" statements.
+
+GNU Make has two assignment operators, ":=" and "=".  ":=" performs
+immediate evaluation of the right-hand side and stores an actual string
+into the left-hand side.  "=" is like a formula definition; it stores the
+right-hand side in an unevaluated form and then evaluates this form each
+time the left-hand side is used.
+
+There are some cases where "=" is appropriate.  Usually, though, ":="
+is the right choice.
+
+=== 9 Credits
+
+Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
+Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
+Updates by Sam Ravnborg <sam@ravnborg.org>
+
+=== 10 TODO
+
+- Describe how kbuild support shipped files with _shipped.
+- Generating offset header files.
+- Add more variables to section 7?
+