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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 <deleted>
            --- 3.5 Library file goals - lib-y
            --- 3.6 Descending down in directories
            --- 3.7 Non-builtin vmlinux targets - extra-y
            --- 3.8 Always built goals - always-y
            --- 3.9 Compilation flags
            --- 3.10 Dependency tracking
            --- 3.11 Custom Rules
            --- 3.12 Command change detection
            --- 3.13 $(CC) support functions
            --- 3.14 $(LD) support functions
            --- 3.15 Script Invocation
 
         === 4 Host Program support
            --- 4.1 Simple Host Program
            --- 4.2 Composite Host Programs
            --- 4.3 Using C++ for host programs
            --- 4.4 Controlling compiler options for host programs
            --- 4.5 When host programs are actually built
 
         === 5 Userspace Program support
            --- 5.1 Simple Userspace Program
            --- 5.2 Composite Userspace Programs
            --- 5.3 Controlling compiler options for userspace programs
            --- 5.4 When userspace programs are actually built
 
         === 6 Kbuild clean infrastructure
 
         === 7 Architecture Makefiles
            --- 7.1 Set variables to tweak the build to the architecture
            --- 7.2 Add prerequisites to archheaders
            --- 7.3 Add prerequisites to archprepare
            --- 7.4 List directories to visit when descending
            --- 7.5 Architecture-specific boot images
            --- 7.6 Building non-kbuild targets
            --- 7.7 Commands useful for building a boot image
            --- 7.8 <deleted>
            --- 7.9 Preprocessing linker scripts
            --- 7.10 Generic header files
            --- 7.11 Post-link pass
 
         === 8 Kbuild syntax for exported headers
                 --- 8.1 no-export-headers
                 --- 8.2 generic-y
                 --- 8.3 generated-y
                 --- 8.4 mandatory-y
 
         === 9 Kbuild Variables
         === 10 Makefile language
         === 11 Credits
         === 12 TODO
 
 1 Overview
 ==========
 
 The Makefiles have five parts::
 
         Makefile                    the top Makefile.
         .config                     the kernel configuration file.
         arch/$(SRCARCH)/Makefile    the arch Makefile.
         scripts/Makefile.*          common rules etc. for all kbuild Makefiles.
         kbuild Makefiles            exist in every subdirectory
 
 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/$(SRCARCH)/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 $(obj-y) files.  It then calls
         "$(AR) rcSTP" to merge these files into one built-in.a file.
         This is a thin archive without a symbol table. It will be later
         linked into vmlinux by scripts/link-vmlinux.sh
 
         The order of files in $(obj-y) is significant.  Duplicates in
         the lists are allowed: the first instance will be linked into
         built-in.a 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-$(CONFIG_ISDN_I4L)         += isdn.o
                 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
 
 3.3 Loadable module goals - obj-m
 ---------------------------------
 
         $(obj-m) specifies 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; however,
         kbuild needs to know which object files you want to build your
         module from, so you have to tell it by setting a $(<module_name>-y)
         variable.
 
         Example::
 
                 #drivers/isdn/i4l/Makefile
                 obj-$(CONFIG_ISDN_I4L) += isdn.o
                 isdn-y := 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 $(isdn-y) and then run
         "$(LD) -r" on the list of these files to generate isdn.o.
 
         Due to kbuild recognizing $(<module_name>-y) for composite objects,
         you can use the value of a `CONFIG_` symbol to optionally include an
         object file as part of a composite object.
 
         Example::
 
                 #fs/ext2/Makefile
                 obj-$(CONFIG_EXT2_FS) += ext2.o
                 ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
                           namei.o super.o symlink.o
                 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
                                                 xattr_trusted.o
 
         In this example, xattr.o, xattr_user.o and xattr_trusted.o are 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.a, as you would expect.
 
 3.5 Library file goals - lib-y
 ------------------------------
 
         Objects listed with obj-* are used for modules, or
         combined in a built-in.a 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.a and a lib.a file.
 
         Example::
 
                 #arch/x86/lib/Makefile
                 lib-y    := delay.o
 
         This will create a library lib.a based on 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 "7.4 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 uses this information not only to decide that it needs to visit
         the directory, but also to decide whether or not to link objects from
         the directory into vmlinux.
 
         When Kbuild descends into the directory with 'y', all built-in objects
         from that directory are combined into the built-in.a, which will be
         eventually linked into vmlinux.
 
         When Kbuild descends into the directory with 'm', in contrast, nothing
         from that directory will be linked into vmlinux. If the Makefile in
         that directory specifies obj-y, those objects will be left orphan.
         It is very likely a bug of the Makefile or of dependencies in Kconfig.
 
         Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
         descending into subdirectories. It is a good fit when you know they
         do not contain kernel-space objects at all. A typical usage is to let
         Kbuild descend into subdirectories to build tools.
 
         Examples::
 
                 # scripts/Makefile
                 subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
                 subdir-$(CONFIG_MODVERSIONS) += genksyms
                 subdir-$(CONFIG_SECURITY_SELINUX) += selinux
 
         Unlike obj-y/m, subdir-y/m does not need the trailing slash since this
         syntax is always used for directories.
 
         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 Non-builtin vmlinux targets - extra-y
 -----------------------------------------
 
         extra-y specifies targets which are needed for building vmlinux,
         but not combined into built-in.a.
 
         Examples are:
 
         1) head objects
 
             Some objects must be placed at the head of vmlinux. They are
             directly linked to vmlinux without going through built-in.a
             A typical use-case is an object that contains the entry point.
 
             arch/$(SRCARCH)/Makefile should specify such objects as head-y.
 
             Discussion:
               Given that we can control the section order in the linker script,
               why do we need head-y?
 
         2) vmlinux linker script
 
             The linker script for vmlinux is located at
             arch/$(SRCARCH)/kernel/vmlinux.lds
 
         Example::
 
                 # arch/x86/kernel/Makefile
                 extra-y := head_$(BITS).o
                 extra-y += head$(BITS).o
                 extra-y += ebda.o
                 extra-y += platform-quirks.o
                 extra-y += vmlinux.lds
 
         $(extra-y) should only contain targets needed for vmlinux.
 
         Kbuild skips extra-y when vmlinux is apparently not a final goal.
         (e.g. 'make modules', or building external modules)
 
         If you intend to build targets unconditionally, always-y (explained
         in the next section) is the correct syntax to use.
 
 3.8 Always built goals - always-y
 ---------------------------------
 
         always-y specifies targets which are literally always built when
         Kbuild visits the Makefile.
 
         Example::
           # ./Kbuild
           offsets-file := include/generated/asm-offsets.h
           always-y += $(offsets-file)
 
 3.9 Compilation flags
 ---------------------
 
     ccflags-y, asflags-y and ldflags-y
         These three flags apply only to the kbuild makefile in which they
         are assigned. They are used for all the normal cc, as and ld
         invocations happening during a recursive build.
         Note: Flags with the same behaviour were previously named:
         EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
         They are still supported but their usage is deprecated.
 
         ccflags-y specifies options for compiling with $(CC).
 
         Example::
 
                 # drivers/acpi/acpica/Makefile
                 ccflags-y                       := -Os -D_LINUX -DBUILDING_ACPICA
                 ccflags-$(CONFIG_ACPI_DEBUG)    += -DACPI_DEBUG_OUTPUT
 
         This variable is necessary because the top Makefile owns the
         variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
         entire tree.
 
         asflags-y specifies assembler options.
 
         Example::
 
                 #arch/sparc/kernel/Makefile
                 asflags-y := -ansi
 
         ldflags-y specifies options for linking with $(LD).
 
         Example::
 
                 #arch/cris/boot/compressed/Makefile
                 ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
 
     subdir-ccflags-y, subdir-asflags-y
         The two flags listed above are similar to ccflags-y and asflags-y.
         The difference is that the subdir- variants have effect for the kbuild
         file where they are present and all subdirectories.
         Options specified using subdir-* are added to the commandline before
         the options specified using the non-subdir variants.
 
         Example::
 
                 subdir-ccflags-y := -Werror
 
     ccflags-remove-y, asflags-remove-y
         These flags are used to remove particular flags for the compiler,
         assembler invocations.
 
         Example::
 
                 ccflags-remove-$(CONFIG_MCOUNT) += -pg
 
     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.
 
         CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
         can re-add compiler flags that were removed by ccflags-remove-y.
 
         Example::
 
                 # drivers/scsi/Makefile
                 CFLAGS_aha152x.o =   -DAHA152X_STAT -DAUTOCONF
 
         This line specify compilation flags for aha152x.o.
 
         $(AFLAGS_$@) is a similar feature for source files in assembly
         languages.
 
         AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
         can re-add assembler flags that were removed by asflags-remove-y.
 
         Example::
 
                 # arch/arm/kernel/Makefile
                 AFLAGS_head.o        := -DTEXT_OFFSET=$(TEXT_OFFSET)
                 AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
                 AFLAGS_iwmmxt.o      := -Wa,-mcpu=iwmmxt
 
 
 3.10 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.
 
 3.11 Custom Rules
 -----------------
 
         Custom 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 custom rules to prepare boot images etc.
 
         Custom rules are written as normal Make rules.
         Kbuild is not executing in the directory where the Makefile is
         located, so all custom rules shall use a relative
         path to prerequisite files and target files.
 
         Two variables are used when defining custom 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 custom 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).
 
         $(kecho)
             echoing information to user in a rule is often a good practice
             but when execution "make -s" one does not expect to see any output
             except for warnings/errors.
             To support this kbuild defines $(kecho) which will echo out the
             text following $(kecho) to stdout except if "make -s" is used.
 
         Example::
 
                 # arch/arm/Makefile
                 $(BOOT_TARGETS): vmlinux
                         $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
                         @$(kecho) '  Kernel: $(boot)/$@ is ready'
 
         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::
 
                 # lib/Makefile
                 quiet_cmd_crc32 = GEN     $@
                       cmd_crc32 = $< > $@
 
                 $(obj)/crc32table.h: $(obj)/gen_crc32table
                         $(call cmd,crc32)
 
         When updating the $(obj)/crc32table.h target, the line:
 
                   GEN     lib/crc32table.h
 
         will be displayed with "make KBUILD_VERBOSE=0".
 
 3.12 Command change detection
 -----------------------------
 
         When the rule is evaluated, timestamps are compared between the target
         and its prerequisite files. GNU Make updates the target when any of the
         prerequisites is newer than that.
 
         The target should be rebuilt also when the command line has changed
         since the last invocation. This is not supported by Make itself, so
         Kbuild achieves this by a kind of meta-programming.
 
         if_changed is the macro used for this purpose, in the following form::
 
                 quiet_cmd_<command> = ...
                       cmd_<command> = ...
 
                 <target>: <source(s)> FORCE
                         $(call if_changed,<command>)
 
         Any target that utilizes if_changed must be listed in $(targets),
         otherwise the command line check will fail, and the target will
         always be built.
 
         If the target is already listed in the recognized syntax such as
         obj-y/m, lib-y/m, extra-y/m, always-y/m, hostprogs, userprogs, Kbuild
         automatically adds it to $(targets). Otherwise, the target must be
         explicitly added to $(targets).
 
         Assignments to $(targets) are without $(obj)/ prefix. if_changed may be
         used in conjunction with custom rules as defined in "3.11 Custom Rules".
 
         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, objcopy)
 
         Note:
                 if_changed should not be used more than once per target.
                 It stores the executed command in a corresponding .cmd
                 file and multiple calls would result in overwrites and
                 unwanted results when the target is up to date and only the
                 tests on changed commands trigger execution of commands.
 
 3.13 $(CC) support functions
 ----------------------------
 
         The kernel may be built with several different versions of
         $(CC), each supporting a unique set of features and options.
         kbuild provides basic support to check for valid options for $(CC).
         $(CC) is usually the gcc compiler, but other alternatives are
         available.
 
     as-option
         as-option is used to check if $(CC) -- when used to compile
         assembler (`*.S`) files -- supports the given option. An optional
         second option may be specified if the first option is not supported.
 
         Example::
 
                 #arch/sh/Makefile
                 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
 
         In the above example, cflags-y will be assigned the option
         -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
         The second argument is optional, and if supplied will be used
         if first argument is not supported.
 
     as-instr
         as-instr checks if the assembler reports a specific instruction
         and then outputs either option1 or option2
         C escapes are supported in the test instruction
         Note: as-instr-option uses KBUILD_AFLAGS for assembler options
 
     cc-option
         cc-option is used to check if $(CC) supports a given option, and if
         not supported to use an optional second option.
 
         Example::
 
                 #arch/x86/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.
         Note: cc-option uses KBUILD_CFLAGS for $(CC) options
 
    cc-option-yn
         cc-option-yn is used to check if gcc supports a given option
         and return 'y' if supported, otherwise 'n'.
 
         Example::
 
                 #arch/ppc/Makefile
                 biarch := $(call cc-option-yn, -m32)
                 aflags-$(biarch) += -a32
                 cflags-$(biarch) += -m32
 
         In the above example, $(biarch) is set to y if $(CC) supports the -m32
         option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
         and $(cflags-y) will be assigned the values -a32 and -m32,
         respectively.
         Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
 
     cc-disable-warning
         cc-disable-warning checks if gcc supports a given warning and returns
         the commandline switch to disable it. This special function is needed,
         because gcc 4.4 and later accept any unknown -Wno-* option and only
         warn about it if there is another warning in the source file.
 
         Example::
 
                 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 
         In the above example, -Wno-unused-but-set-variable will be added to
         KBUILD_CFLAGS only if gcc really accepts it.
 
     cc-ifversion
         cc-ifversion tests the version of $(CC) and equals the fourth parameter
         if version expression is true, or the fifth (if given) if the version
         expression is false.
 
         Example::
 
                 #fs/reiserfs/Makefile
                 ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
 
         In this example, ccflags-y 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.
 
     cc-cross-prefix
         cc-cross-prefix is used to check if there exists a $(CC) in path with
         one of the listed prefixes. The first prefix where there exist a
         prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
         then nothing is returned.
         Additional prefixes are separated by a single space in the
         call of cc-cross-prefix.
         This functionality is useful for architecture Makefiles that try
         to set CROSS_COMPILE to well-known values but may have several
         values to select between.
         It is recommended only to try to set CROSS_COMPILE if it is a cross
         build (host arch is different from target arch). And if CROSS_COMPILE
         is already set then leave it with the old value.
 
         Example::
 
                 #arch/m68k/Makefile
                 ifneq ($(SUBARCH),$(ARCH))
                         ifeq ($(CROSS_COMPILE),)
                                CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
                         endif
                 endif
 
 3.14 $(LD) support functions
 ----------------------------
 
     ld-option
         ld-option is used to check if $(LD) supports the supplied option.
         ld-option takes two options as arguments.
         The second argument is an optional option that can be used if the
         first option is not supported by $(LD).
 
         Example::
 
                 #Makefile
                 LDFLAGS_vmlinux += $(call ld-option, -X)
 
 3.15 Script invocation
 ----------------------
 
         Make rules may invoke scripts to build the kernel. The rules shall
         always provide the appropriate interpreter to execute the script. They
         shall not rely on the execute bits being set, and shall not invoke the
         script directly. For the convenience of manual script invocation, such
         as invoking ./scripts/checkpatch.pl, it is recommended to set execute
         bits on the scripts nonetheless.
 
         Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
         and $(PYTHON3) to refer to interpreters for the respective
         scripts.
 
         Example::
 
                 #Makefile
                 cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
                              $(KERNELRELEASE)
 
 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".
 
 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-y".
 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 := 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.
         $(<executable>-objs) lists all objects used to link the final
         executable.
 
         Example::
 
                 #scripts/lxdialog/Makefile
                 hostprogs     := 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 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     := 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 of a mixture of .c and .cc files, then an
         additional line can be used to identify this.
 
         Example::
 
                 #scripts/kconfig/Makefile
                 hostprogs     := qconf
                 qconf-cxxobjs := qconf.o
                 qconf-objs    := check.o
 
 4.4 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 $(KBUILD_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
                 HOSTLDLIBS_qconf := -L$(QTDIR)/lib
 
         When linking qconf, it will be passed the extra option
         "-L$(QTDIR)/lib".
 
 4.5 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 custom rule.
 
         Example::
 
                 #drivers/pci/Makefile
                 hostprogs := 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 custom rules must be prefixed with $(obj).
 
         (2) Use always-y
 
         When there is no suitable custom rule, and the host program
         shall be built when a makefile is entered, the always-y
         variable shall be used.
 
         Example::
 
                 #scripts/lxdialog/Makefile
                 hostprogs     := lxdialog
                 always-y      := $(hostprogs)
 
         Kbuild provides the following shorthand for this:
 
                 hostprogs-always-y := lxdialog
 
         This will tell kbuild to build lxdialog even if not referenced in
         any rule.
 
 5 Userspace Program support
 ===========================
 
 Just like host programs, Kbuild also supports building userspace executables
 for the target architecture (i.e. the same architecture as you are building
 the kernel for).
 
 The syntax is quite similar. The difference is to use "userprogs" instead of
 "hostprogs".
 
 5.1 Simple Userspace Program
 ----------------------------
 
         The following line tells kbuild that the program bpf-direct shall be
         built for the target architecture.
 
         Example::
 
                 userprogs := bpf-direct
 
         Kbuild assumes in the above example that bpf-direct is made from a
         single C source file named bpf-direct.c located in the same directory
         as the Makefile.
 
 5.2 Composite Userspace Programs
 --------------------------------
 
         Userspace programs can be made up based on composite objects.
         The syntax used to define composite objects for userspace programs is
         similar to the syntax used for kernel objects.
         $(<executable>-objs) lists all objects used to link the final
         executable.
 
         Example::
 
                 #samples/seccomp/Makefile
                 userprogs      := bpf-fancy
                 bpf-fancy-objs := bpf-fancy.o bpf-helper.o
 
         Objects with extension .o are compiled from the corresponding .c
         files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
         and bpf-helper.c is compiled to bpf-helper.o.
 
         Finally, the two .o files are linked to the executable, bpf-fancy.
         Note: The syntax <executable>-y is not permitted for userspace programs.
 
 5.3 Controlling compiler options for userspace programs
 -------------------------------------------------------
 
         When compiling userspace programs, it is possible to set specific flags.
         The programs will always be compiled utilising $(CC) passed
         the options specified in $(KBUILD_USERCFLAGS).
         To set flags that will take effect for all userspace programs created
         in that Makefile, use the variable userccflags.
 
         Example::
 
                 # samples/seccomp/Makefile
                 userccflags += -I usr/include
 
         To set specific flags for a single file the following construction
         is used:
 
         Example::
 
                 bpf-helper-userccflags += -I user/include
 
         It is also possible to specify additional options to the linker.
 
         Example::
 
                 # net/bpfilter/Makefile
                 bpfilter_umh-userldflags += -static
 
         When linking bpfilter_umh, it will be passed the extra option -static.
 
         From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used.
 
 5.4 When userspace programs are actually built
 ----------------------------------------------
 
         Kbuild builds userspace programs only when told to do so.
         There are two ways to do this.
 
         (1) Add it as the prerequisite of another file
 
         Example::
 
                 #net/bpfilter/Makefile
                 userprogs := bpfilter_umh
                 $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
 
         $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o
 
         (2) Use always-y
 
         Example::
 
                 userprogs := binderfs_example
                 always-y := $(userprogs)
 
         Kbuild provides the following shorthand for this:
 
                 userprogs-always-y := binderfs_example
 
         This will tell Kbuild to build binderfs_example when it visits this
         Makefile.
 
 6 Kbuild clean infrastructure
 =============================
 
 "make clean" deletes most generated files in the obj tree where the kernel
 is compiled. This includes generated files such as host programs.
 Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m),
 $(always-), $(extra-y), $(extra-) 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
 source tree when "make clean" is executed.
 
 Additional files or directories can be specified in kbuild makefiles by use of
 $(clean-files).
 
         Example::
 
                 #lib/Makefile
                 clean-files := crc32table.h
 
 When executing "make clean", the file "crc32table.h" will be deleted.
 Kbuild will assume files to be in the same relative directory as the
 Makefile, except if prefixed with $(objtree).
 
 To exclude certain files or directories from make clean, use the
 $(no-clean-files) variable.
 
 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/x86/boot/Makefile
                 subdir- := compressed
 
 The above assignment instructs kbuild to descend down in the
 directory compressed/ when "make clean" is executed.
 
 Note 1: arch/$(SRCARCH)/Makefile cannot use "subdir-", because that file is
 included in the top level makefile. Instead, arch/$(SRCARCH)/Kbuild can use
 "subdir-".
 
 Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
 be visited during "make clean".
 
 7 Architecture Makefiles
 ========================
 
 The top level Makefile sets up the environment and does the preparation,
 before starting to descend down in the individual directories.
 The top level makefile contains the generic part, whereas
 arch/$(SRCARCH)/Makefile contains what is required to set up kbuild
 for said architecture.
 To do so, arch/$(SRCARCH)/Makefile sets up a number of variables and defines
 a few targets.
 
 When kbuild executes, the following steps are followed (roughly):
 
 1) Configuration of the kernel => produce .config
 2) Store kernel version in include/linux/version.h
 3) Updating all other prerequisites to the target prepare:
    - Additional prerequisites are specified in arch/$(SRCARCH)/Makefile
 4) Recursively descend down in all directories listed in
    init-* core* drivers-* net-* libs-* and build all targets.
    - The values of the above variables are expanded in arch/$(SRCARCH)/Makefile.
 5) 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/$(SRCARCH)/Makefile.
 6) 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
 
 
 7.1 Set variables to tweak the build to the architecture
 --------------------------------------------------------
 
     KBUILD_LDFLAGS
         Generic $(LD) options
 
         Flags used for all invocations of the linker.
         Often specifying the emulation is sufficient.
 
         Example::
 
                 #arch/s390/Makefile
                 KBUILD_LDFLAGS         := -m elf_s390
 
         Note: ldflags-y can be used to further customise
         the flags used. See section 3.7.
 
     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/x86/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.
 
     KBUILD_AFLAGS
         Assembler flags
 
         Default value - see top level Makefile
         Append or modify as required per architecture.
 
         Example::
 
                 #arch/sparc64/Makefile
                 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
 
     KBUILD_CFLAGS
         $(CC) compiler flags
 
         Default value - see top level Makefile
         Append or modify as required per architecture.
 
         Often, the KBUILD_CFLAGS variable depends on the configuration.
 
         Example::
 
                 #arch/x86/boot/compressed/Makefile
                 cflags-$(CONFIG_X86_32) := -march=i386
                 cflags-$(CONFIG_X86_64) := -mcmodel=small
                 KBUILD_CFLAGS += $(cflags-y)
 
         Many arch Makefiles dynamically run the target C compiler to
         probe supported options::
 
                 #arch/x86/Makefile
 
                 ...
                 cflags-$(CONFIG_MPENTIUMII)     += $(call cc-option,\
                                                 -march=pentium2,-march=i686)
                 ...
                 # Disable unit-at-a-time mode ...
                 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
                 ...
 
 
         The first example utilises the trick that a config option expands
         to 'y' when selected.
 
     KBUILD_AFLAGS_KERNEL
         Assembler options specific for built-in
 
         $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
         resident kernel code.
 
     KBUILD_AFLAGS_MODULE
         Assembler options specific for modules
 
         $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
         are used for assembler.
 
         From commandline AFLAGS_MODULE shall be used (see kbuild.rst).
 
     KBUILD_CFLAGS_KERNEL
         $(CC) options specific for built-in
 
         $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
         resident kernel code.
 
     KBUILD_CFLAGS_MODULE
         Options for $(CC) when building modules
 
         $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
         are used for $(CC).
         From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
 
     KBUILD_LDFLAGS_MODULE
         Options for $(LD) when linking modules
 
         $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
         used when linking modules. This is often a linker script.
 
         From commandline LDFLAGS_MODULE shall be used (see kbuild.rst).
 
     KBUILD_LDS
 
         The linker script with full path. Assigned by the top-level Makefile.
 
     KBUILD_LDS_MODULE
 
         The module linker script with full path. Assigned by the top-level
         Makefile and additionally by the arch Makefile.
 
     KBUILD_VMLINUX_OBJS
 
         All object files for vmlinux. They are linked to vmlinux in the same
         order as listed in KBUILD_VMLINUX_OBJS.
 
     KBUILD_VMLINUX_LIBS
 
         All .a "lib" files for vmlinux. KBUILD_VMLINUX_OBJS and
         KBUILD_VMLINUX_LIBS together specify all the object files used to
         link vmlinux.
 
 7.2 Add prerequisites to archheaders
 ------------------------------------
 
         The archheaders: rule is used to generate header files that
         may be installed into user space by "make header_install".
 
         It is run before "make archprepare" when run on the
         architecture itself.
 
 
 7.3 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 describes how kbuild supports
         generating offset header files.
 
 
 7.4 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, core-y, libs-y, drivers-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.a object file can be
             located.
 
             Then the rest follows in this order:
 
                 $(core-y), $(libs-y), $(drivers-y)
 
             The top level Makefile defines values for all generic directories,
             and arch/$(SRCARCH)/Makefile only adds architecture-specific
             directories.
 
             Example::
 
                 # arch/sparc/Makefile
                 core-y                 += arch/sparc/
 
                 libs-y                 += arch/sparc/prom/
                 libs-y                 += arch/sparc/lib/
 
                 drivers-$(CONFIG_PM) += arch/sparc/power/
 
 7.5 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/$(SRCARCH)/.
 
         Kbuild does not provide any smart way to support building a
         target specified in boot/. Therefore arch/$(SRCARCH)/Makefile shall
         call make manually to build a target in boot/.
 
         The recommended approach is to include shortcuts in 
         arch/$(SRCARCH)/Makefile, and use the full path when calling down
         into the arch/$(SRCARCH)/boot/Makefile.
 
         Example::
 
                 #arch/x86/Makefile
                 boot := arch/x86/boot
                 bzImage: vmlinux
                         $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 
         "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
         make in a subdirectory.
 
         There are no rules for naming architecture-specific targets,
         but executing "make help" will list all relevant targets.
         To support this, $(archhelp) must be defined.
 
         Example::
 
                 #arch/x86/Makefile
                 define archhelp
                   echo  '* bzImage      - Compressed kernel image (arch/x86/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/x86/Makefile
                 all: bzImage
 
         When "make" is executed without arguments, bzImage will be built.
 
 7.7 Commands useful for building a boot image
 ---------------------------------------------
 
     Kbuild provides a few macros that are useful when building a
     boot image.
 
     ld
         Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
 
         Example::
 
                 #arch/x86/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
         frees us from listing the setup.o and bootsect.o files.
 
         Note:
               It is a common mistake to forget the "targets :=" assignment,
               resulting in the target file being recompiled for no
               obvious reason.
 
     objcopy
         Copy binary. Uses OBJCOPYFLAGS usually specified in
         arch/$(SRCARCH)/Makefile.
         OBJCOPYFLAGS_$@ may be used to set additional options.
 
     gzip
         Compress target. Use maximum compression to compress target.
 
         Example::
 
                 #arch/x86/boot/compressed/Makefile
                 $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
                         $(call if_changed,gzip)
 
     dtc
         Create flattened device tree blob object suitable for linking
         into vmlinux. Device tree blobs linked into vmlinux are placed
         in an init section in the image. Platform code *must* copy the
         blob to non-init memory prior to calling unflatten_device_tree().
 
         To use this command, simply add `*.dtb` into obj-y or targets, or make
         some other target depend on `%.dtb`
 
         A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
         architecture Makefiles do no need to explicitly write out that rule.
 
         Example::
 
                 targets += $(dtb-y)
                 DTC_FLAGS ?= -p 1024
 
 7.9 Preprocessing linker scripts
 --------------------------------
 
         When the vmlinux image is built, the linker script
         arch/$(SRCARCH)/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/x86/kernel/Makefile
                 extra-y := vmlinux.lds
 
         The assignment to extra-y 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::
 
                 KBUILD_CPPFLAGS : Set in top-level Makefile
                 cppflags-y      : 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` files is used in several
         architecture-specific files.
 
 7.10 Generic header files
 -------------------------
 
         The directory include/asm-generic contains the header files
         that may be shared between individual architectures.
         The recommended approach how to use a generic header file is
         to list the file in the Kbuild file.
         See "8.2 generic-y" for further info on syntax etc.
 
 7.11 Post-link pass
 -------------------
 
         If the file arch/xxx/Makefile.postlink exists, this makefile
         will be invoked for post-link objects (vmlinux and modules.ko)
         for architectures to run post-link passes on. Must also handle
         the clean target.
 
         This pass runs after kallsyms generation. If the architecture
         needs to modify symbol locations, rather than manipulate the
         kallsyms, it may be easier to add another postlink target for
         .tmp_vmlinux? targets to be called from link-vmlinux.sh.
 
         For example, powerpc uses this to check relocation sanity of
         the linked vmlinux file.
 
 8 Kbuild syntax for exported headers
 ------------------------------------
 
 The kernel includes a set of headers that is exported to userspace.
 Many headers can be exported as-is but other headers require a
 minimal pre-processing before they are ready for user-space.
 The pre-processing does:
 
 - drop kernel-specific annotations
 - drop include of compiler.h 
 - drop all sections that are kernel internal (guarded by `ifdef __KERNEL__`)
 
 All headers under include/uapi/, include/generated/uapi/,
 arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
 are exported.
 
 A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
 arch/<arch>/include/asm/ to list asm files coming from asm-generic.
 See subsequent chapter for the syntax of the Kbuild file.
 
 8.1 no-export-headers
 ---------------------
 
         no-export-headers is essentially used by include/uapi/linux/Kbuild to
         avoid exporting specific headers (e.g. kvm.h) on architectures that do
         not support it. It should be avoided as much as possible.
 
 8.2 generic-y
 -------------
 
         If an architecture uses a verbatim copy of a header from
         include/asm-generic then this is listed in the file
         arch/$(SRCARCH)/include/asm/Kbuild like this:
 
                 Example::
 
                         #arch/x86/include/asm/Kbuild
                         generic-y += termios.h
                         generic-y += rtc.h
 
         During the prepare phase of the build a wrapper include
         file is generated in the directory:: 
 
                 arch/$(SRCARCH)/include/generated/asm
 
         When a header is exported where the architecture uses
         the generic header a similar wrapper is generated as part
         of the set of exported headers in the directory::
 
                 usr/include/asm
 
         The generated wrapper will in both cases look like the following:
 
                 Example: termios.h::
 
                         #include <asm-generic/termios.h>
 
 8.3 generated-y
 ---------------
 
         If an architecture generates other header files alongside generic-y
         wrappers, generated-y specifies them.
 
         This prevents them being treated as stale asm-generic wrappers and
         removed.
 
                 Example::
 
                         #arch/x86/include/asm/Kbuild
                         generated-y += syscalls_32.h
 
 8.4 mandatory-y
 ---------------
 
         mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
         to define the minimum set of ASM headers that all architectures must have.
 
         This works like optional generic-y. If a mandatory header is missing
         in arch/$(SRCARCH)/include/(uapi/)/asm, Kbuild will automatically
         generate a wrapper of the asm-generic one.
 
 9 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 ...
 
     SRCARCH
         This variable specifies the directory in arch/ to build.
 
         ARCH and SRCARCH may not necessarily match. A couple of arch
         directories are biarch, that is, a single `arch/*/` directory supports
         both 32-bit and 64-bit.
 
         For example, you can pass in ARCH=i386, ARCH=x86_64, or ARCH=x86.
         For all of them, SRCARCH=x86 because arch/x86/ supports both i386 and
         x86_64.
 
     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, it 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, the
         INSTALL_MOD_STRIP value will be used as the option(s) to the strip
         command.
 
 
 10 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.
 
 11 Credits
 ==========
 
 - Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
 - Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
 - Updates by Sam Ravnborg <sam@ravnborg.org>
 - Language QA by Jan Engelhardt <jengelh@gmx.de>
 
 12 TODO
 =======
 
 - Describe how kbuild supports shipped files with _shipped.
 - Generating offset header files.
 - Add more variables to chapters 7 or 9?

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