Linux Kernel-3.10.72 Makefiles



Linux Kernel Makefiles

This document describes the Linux kernel Maefiles.

=== Table of Contents

 === 1 Overview
 === 2 Who does what
 === 3 The kbuild files
    --- 3.1 Goal definitions
    --- 3.2 Built-in object goals - obj-y
    --- 3.3 Loadable module goals - obj-m
    --- 3.4 Objects which export symbols
    --- 3.5 Library file goals - lib-y
    --- 3.6 Descending down in directories
    --- 3.7 Compilation flags
    --- 3.8 Command line dependency
    --- 3.9 Dependency tracking
    --- 3.10 Special Rules
    --- 3.11 $(CC) support functions
    --- 3.12 $(LD) support functions

 === 4 Host Program support
    --- 4.1 Simple Host Program
    --- 4.2 Composite Host Programs
    --- 4.3 Defining shared libraries
    --- 4.4 Using C++ for host programs
    --- 4.5 Controlling compiler options for host programs
    --- 4.6 When host programs are actually built
    --- 4.7 Using hostprogs-$(CONFIG_FOO)

 === 5 Kbuild clean infrastructure

 === 6 Architecture Makefiles
    --- 6.1 Set variables to tweak the build to the architecture
    --- 6.2 Add prerequisites to archheaders:
    --- 6.3 Add prerequisites to archprepare:
    --- 6.4 List directories to visit when descending
    --- 6.5 Architecture-specific boot images
    --- 6.6 Building non-kbuild targets
    --- 6.7 Commands useful for building a boot image
    --- 6.8 Custom kbuild commands
    --- 6.9 Preprocessing linker scripts
    --- 6.10 Generic header files

 === 7 Kbuild syntax for exported headers
  --- 7.1 header-y
  --- 7.2 genhdr-y
  --- 7.3 destination-y
  --- 7.4 generic-y

 === 8 Kbuild Variables
 === 9 Makefile language
 === 10 Credits
 === 11 TODO

=== 1 Overview

The Makefiles have five parts:

 Makefile  the top Makefile.
 .config   the kernel configuration file.
 arch/$(ARCH)/Makefile the arch Makefile.
 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
 kbuild Makefiles there are about 500 of these.

The top Makefile reads the .config file, which comes from the kernel
configuration process.

The top Makefile is responsible for building two major products: vmlinux
(the resident kernel image) and modules (any module files).
It builds these goals by recursively descending into the subdirectories of
the kernel source tree.
The list of subdirectories which are visited depends upon the kernel
configuration. The top Makefile textually includes an arch Makefile
with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
architecture-specific information to the top Makefile.

Each subdirectory has a kbuild Makefile which carries out the commands
passed down from above. The kbuild Makefile uses information from the
.config file to construct various file lists used by kbuild to build
any built-in or modular targets.

scripts/Makefile.* contains all the definitions/rules etc. that
are used to build the kernel based on the kbuild makefiles.

=== 2 Who does what

People have four different relationships with the kernel Makefiles.

*Users* are people who build kernels.  These people type commands such as
"make menuconfig" or "make".  They usually do not read or edit
any kernel Makefiles (or any other source files).

*Normal developers* are people who work on features such as device
drivers, file systems, and network protocols.  These people need to
maintain the kbuild Makefiles for the subsystem they are
working on.  In order to do this effectively, they need some overall
knowledge about the kernel Makefiles, plus detailed knowledge about the
public interface for kbuild.

*Arch developers* are people who work on an entire architecture, such
as sparc or ia64.  Arch developers need to know about the arch Makefile
as well as kbuild Makefiles.

*Kbuild developers* are people who work on the kernel build system itself.
These people need to know about all aspects of the kernel Makefiles.

This document is aimed towards normal developers and arch developers.

=== 3 The kbuild files

Most Makefiles within the kernel are kbuild Makefiles that use the
kbuild infrastructure. This chapter introduces the syntax used in the
kbuild makefiles.
The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
file will be used.

Section 3.1 "Goal definitions" is a quick intro, further chapters provide
more details, with real examples.

--- 3.1 Goal definitions

 Goal definitions are the main part (heart) of the kbuild Makefile.
 These lines define the files to be built, any special compilation
 options, and any subdirectories to be entered recursively.

 The most simple kbuild makefile contains one line:

 Example:
  obj-y += foo.o

 This tells kbuild that there is one object in that directory, named
 foo.o. foo.o will be built from foo.c or foo.S.

 If foo.o shall be built as a module, the variable obj-m is used.
 Therefore the following pattern is often used:

 Example:
  obj-$(CONFIG_FOO) += foo.o

 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
 If CONFIG_FOO is neither y nor m, then the file will not be compiled
 nor linked.

--- 3.2 Built-in object goals - obj-y

 The kbuild Makefile specifies object files for vmlinux
 in the $(obj-y) lists.  These lists depend on the kernel
 configuration.

 Kbuild compiles all the $(obj-y) files.  It then calls
 "$(LD) -r" to merge these files into one built-in.o file.
 built-in.o is later linked into vmlinux by the parent Makefile.

 The order of files in $(obj-y) is significant.  Duplicates in
 the lists are allowed: the first instance will be linked into
 built-in.o and succeeding instances will be ignored.

 Link order is significant, because certain functions
 (module_init() / __initcall) will be called during boot in the
 order they appear. So keep in mind that changing the link
 order may e.g. change the order in which your SCSI
 controllers are detected, and thus your disks are renumbered.

 Example:
  #drivers/isdn/i4l/Makefile
  # Makefile for the kernel ISDN subsystem and device drivers.
  # Each configuration option enables a list of files.
  obj-$(CONFIG_ISDN_I4L)         += isdn.o
  obj-$(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; 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.o, as you would expect.

--- 3.4 Objects which export symbols

 No special notation is required in the makefiles for
 modules exporting symbols.

--- 3.5 Library file goals - lib-y

 Objects listed with obj-* are used for modules, or
 combined in a built-in.o for that specific directory.
 There is also the possibility to list objects that will
 be included in a library, lib.a.
 All objects listed with lib-y are combined in a single
 library for that directory.
 Objects that are listed in obj-y and additionally listed in
 lib-y will not be included in the library, since they will
 be accessible anyway.
 For consistency, objects listed in lib-m will be included in lib.a.

 Note that the same kbuild makefile may list files to be built-in
 and to be part of a library. Therefore the same directory
 may contain both a built-in.o and a lib.a file.

 Example:
  #arch/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 "6.4 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 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

    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/Makefile
  ccflags-y := -Os
  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 options for assembling with $(AS).

 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

    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

 These two lines specify compilation flags for aha152x.o and gdth.o.

 $(AFLAGS_$@) is a similar feature for source files in assembly
 languages.

 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

--- 3.9 Dependency tracking

 Kbuild tracks dependencies on the following:
 1) All prerequisite files (both *.c and *.h)
 2) CONFIG_ options used in all prerequisite files
 3) Command-line used to compile target

 Thus, if you change an option to $(CC) all affected files will
 be re-compiled.

--- 3.10 Special Rules

 Special rules are used when the kbuild infrastructure does
 not provide the required support. A typical example is
 header files generated during the build process.
 Another example are the architecture-specific Makefiles which
 need special rules to prepare boot images etc.

 Special rules are written as normal Make rules.
 Kbuild is not executing in the directory where the Makefile is
 located, so all special rules shall provide a relative
 path to prerequisite files and target files.

 Two variables are used when defining special rules:

    $(src)
 $(src) is a relative path which points to the directory
 where the Makefile is located. Always use $(src) when
 referring to files located in the src tree.

    $(obj)
 $(obj) is a relative path which points to the directory
 where the target is saved. Always use $(obj) when
 referring to generated files.

 Example:
  #drivers/scsi/Makefile
  $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
   $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl

 This is a special rule, following the normal syntax
 required by make.
 The target file depends on two prerequisite files. References
 to the target file are prefixed with $(obj), references
 to prerequisites are referenced with $(src) (because they are not
 generated files).

    $(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 define $(kecho) which will echo out the
 text following $(kecho) to stdout except if "make -s" is used.

 Example:
  #arch/blackfin/boot/Makefile
  $(obj)/vmImage: $(obj)/vmlinux.gz
   $(call if_changed,uimage)
   @$(kecho) 'Kernel: $@ is ready'

--- 3.11 $(CC) support functions

 The kernel may be built with several different versions of
 $(CC), each supporting a unique set of features and options.
 kbuild provide basic support to check for valid options for $(CC).
 $(CC) is usually the gcc compiler, but other alternatives are
 available.

    as-option
 as-option is used to check if $(CC) -- when used to compile
 assembler (*.S) files -- supports the given option. An optional
 second option may be specified if the first option is not supported.

 Example:
  #arch/sh/Makefile
  cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)

 In the above example, cflags-y will be assigned the option
 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
 The second argument is optional, and if supplied will be used
 if first argument is not supported.

    cc-ldoption
 cc-ldoption is used to check if $(CC) when used to link object files
 supports the given option.  An optional second option may be
 specified if first option are not supported.

 Example:
  #arch/x86/kernel/Makefile
  vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv)

 In the above example, vsyscall-flags will be assigned the option
 -Wl$(comma)--hash-style=sysv if it is supported by $(CC).
 The second argument is optional, and if supplied will be used
 if first argument is not supported.

    as-instr
 as-instr checks if the assembler reports a specific instruction
 and then outputs either option1 or option2
 C escapes are supported in the test instruction
 Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options

    cc-option
 cc-option is used to check if $(CC) supports a given option, and not
 supported to use an optional second option.

 Example:
  #arch/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-option-align
 gcc versions >= 3.0 changed the type of options used to specify
 alignment of functions, loops etc. $(cc-option-align), when used
 as prefix to the align options, will select the right prefix:
 gcc < 3.00
  cc-option-align = -malign
 gcc >= 3.00
  cc-option-align = -falign

 Example:
  KBUILD_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.
 Note: cc-option-align 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-version
 cc-version returns a numerical version of the $(CC) compiler version.
 The format is <major><minor> where both are two digits. So for example
 gcc 3.41 would return 0341.
 cc-version is useful when a specific $(CC) version is faulty in one
 area, for example -mregparm=3 was broken in some gcc versions
 even though the option was accepted by gcc.

 Example:
  #arch/x86/Makefile
  cflags-y += $(shell \
  if [ $(call cc-version) -ge 0300 ] ; then \
   echo "-mregparm=3"; fi ;)

 In the above example, -mregparm=3 is only used for gcc version greater
 than or equal to gcc 3.0.

    cc-ifversion
 cc-ifversion tests the version of $(CC) and equals last argument if
 version expression is true.

 Example:
  #fs/reiserfs/Makefile
  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-fullversion
 cc-fullversion is useful when the exact version of gcc is needed.
 One typical use-case is when a specific GCC version is broken.
 cc-fullversion points out a more specific version than cc-version does.

 Example:
  #arch/powerpc/Makefile
  $(Q)if test "$(call cc-fullversion)" = "040200" ; then \
   echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \
   false ; \
  fi

 In this example for a specific GCC version the build will error out explaining
 to the user why it stops.

    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

--- 3.12 $(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)

=== 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.
 $(<executable>-objs) lists all objects used to link the final
 executable.

 Example:
  #scripts/lxdialog/Makefile
  hostprogs-y   := lxdialog
  lxdialog-objs := checklist.o lxdialog.o

 Objects with extension .o are compiled from the corresponding .c
 files. In the above example, checklist.c is compiled to checklist.o
 and lxdialog.c is compiled to lxdialog.o.
 Finally, the two .o files are linked to the executable, lxdialog.
 Note: The syntax <executable>-y is not permitted for host-programs.

--- 4.3 Defining shared libraries

 Objects with extension .so are considered shared libraries, and
 will be compiled as position independent objects.
 Kbuild provides support for shared libraries, but the usage
 shall be restricted.
 In the following example the libkconfig.so shared library is used
 to link the executable conf.

 Example:
  #scripts/kconfig/Makefile
  hostprogs-y     := conf
  conf-objs       := conf.o libkconfig.so
  libkconfig-objs := expr.o type.o

 Shared libraries always require a corresponding -objs line, and
 in the example above the shared library libkconfig is composed by
 the two objects expr.o and type.o.
 expr.o and type.o will be built as position independent code and
 linked as a shared library libkconfig.so. C++ is not supported for
 shared libraries.

--- 4.4 Using C++ for host programs

 kbuild offers support for host programs written in C++. This was
 introduced solely to support kconfig, and is not recommended
 for general use.

 Example:
  #scripts/kconfig/Makefile
  hostprogs-y   := qconf
  qconf-cxxobjs := qconf.o

 In the example above the executable is composed of the C++ file
 qconf.cc - identified by $(qconf-cxxobjs).

 If qconf is composed by a mixture of .c and .cc files, then an
 additional line can be used to identify this.

 Example:
  #scripts/kconfig/Makefile
  hostprogs-y   := qconf
  qconf-cxxobjs := qconf.o
  qconf-objs    := check.o

--- 4.5 Controlling compiler options for host programs

 When compiling host programs, it is possible to set specific flags.
 The programs will always be compiled utilising $(HOSTCC) passed
 the options specified in $(HOSTCFLAGS).
 To set flags that will take effect for all host programs created
 in that Makefile, use the variable HOST_EXTRACFLAGS.

 Example:
  #scripts/lxdialog/Makefile
  HOST_EXTRACFLAGS += -I/usr/include/ncurses

 To set specific flags for a single file the following construction
 is used:

 Example:
  #arch/ppc64/boot/Makefile
  HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)

 It is also possible to specify additional options to the linker.

 Example:
  #scripts/kconfig/Makefile
  HOSTLOADLIBES_qconf := -L$(QTDIR)/lib

 When linking qconf, it will be passed the extra option
 "-L$(QTDIR)/lib".

--- 4.6 When host programs are actually built

 Kbuild will only build host-programs when they are referenced
 as a prerequisite.
 This is possible in two ways:

 (1) List the prerequisite explicitly in a special rule.

 Example:
  #drivers/pci/Makefile
  hostprogs-y := gen-devlist
  $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
   ( cd $(obj); ./gen-devlist ) < $<

 The target $(obj)/devlist.h will not be built before
 $(obj)/gen-devlist is updated. Note that references to
 the host programs in special rules must be prefixed with $(obj).

 (2) Use $(always)
 When there is no suitable special rule, and the host program
 shall be built when a makefile is entered, the $(always)
 variable shall be used.

 Example:
  #scripts/lxdialog/Makefile
  hostprogs-y   := lxdialog
  always        := $(hostprogs-y)

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

--- 4.7 Using hostprogs-$(CONFIG_FOO)

 A typical pattern in a Kbuild file looks like this:

 Example:
  #scripts/Makefile
  hostprogs-$(CONFIG_KALLSYMS) += kallsyms

 Kbuild knows about both 'y' for built-in and 'm' for module.
 So if a config symbol evaluate to 'm', kbuild will still build
 the binary. In other words, Kbuild handles hostprogs-m exactly
 like hostprogs-y. But only hostprogs-y is recommended to be used
 when no CONFIG symbols are involved.

=== 5 Kbuild clean infrastructure

"make clean" deletes most generated files in the obj tree where the kernel
is compiled. This includes generated files such as host programs.
Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
$(extra-y) and $(targets). They are all deleted during "make clean".
Files matching the patterns "*.[oas]", "*.ko", plus some additional files
generated by kbuild are deleted all over the kernel src tree when
"make clean" is executed.

Additional files can be specified in kbuild makefiles by use of $(clean-files).

 Example:
  #drivers/pci/Makefile
  clean-files := devlist.h classlist.h

When executing "make clean", the two files "devlist.h classlist.h" will
be deleted. Kbuild will assume files to be in same relative directory as the
Makefile except if an absolute path is specified (path starting with '/').

To delete a directory hierarchy use:

 Example:
  #scripts/package/Makefile
  clean-dirs := $(objtree)/debian/

This will delete the directory debian, including all subdirectories.
Kbuild will assume the directories to be in the same relative path as the
Makefile if no absolute path is specified (path does not start with '/').

To exclude certain files from make clean, use the $(no-clean-files) variable.
This is only a special case used in the top level Kbuild file:

 Example:
  #Kbuild
  no-clean-files := $(bounds-file) $(offsets-file)

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.

To support the clean infrastructure in the Makefiles that builds the
final bootimage there is an optional target named archclean:

 Example:
  #arch/x86/Makefile
  archclean:
   $(Q)$(MAKE) $(clean)=arch/x86/boot

When "make clean" is executed, make will descend down in arch/x86/boot,
and clean as usual. The Makefile located in arch/x86/boot/ may use
the subdir- trick to descend further down.

Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
included in the top level makefile, and the kbuild infrastructure
is not operational at that point.

Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
be visited during "make clean".

=== 6 Architecture Makefiles

The top level Makefile sets up the environment and does the preparation,
before starting to descend down in the individual directories.
The top level makefile contains the generic part, whereas
arch/$(ARCH)/Makefile contains what is required to set up kbuild
for said architecture.
To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
a few targets.

When kbuild executes, the following steps are followed (roughly):
1) Configuration of the kernel => produce .config
2) Store kernel version in include/linux/version.h
3) Symlink include/asm to include/asm-$(ARCH)
4) Updating all other prerequisites to the target prepare:
   - Additional prerequisites are specified in arch/$(ARCH)/Makefile
5) Recursively descend down in all directories listed in
   init-* core* drivers-* net-* libs-* and build all targets.
   - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
6) All object files are then linked and the resulting file vmlinux is
   located at the root of the obj tree.
   The very first objects linked are listed in head-y, assigned by
   arch/$(ARCH)/Makefile.
7) Finally, the architecture-specific part does any required post processing
   and builds the final bootimage.
   - This includes building boot records
   - Preparing initrd images and 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: ldflags-y can be used to further customise
 the flags used. See chapter 3.7.

    LDFLAGS_MODULE Options for $(LD) when linking modules

 LDFLAGS_MODULE is used to set specific flags for $(LD) when
 linking the .ko files used for modules.
 Default is "-r", for relocatable output.

    LDFLAGS_vmlinux Options for $(LD) when linking vmlinux

 LDFLAGS_vmlinux is used to specify additional flags to pass to
 the linker when linking the final vmlinux image.
 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.

 Example:
  #arch/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  $(AS) 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 $(AS) options specific for built-in

 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
 resident kernel code.

    KBUILD_AFLAGS_MODULE   Options for $(AS) when building modules

 $(KBUILD_AFLAGS_MODULE) is used to add arch specific options that
 are used for $(AS).
 From commandline AFLAGS_MODULE shall be used (see kbuild.txt).

    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.txt).

    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.txt).

    KBUILD_ARFLAGS   Options for $(AR) when creating archives

 $(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic
 mode) if this option is supported by $(AR).

--- 6.2 Add prerequisites to archheaders:

 The archheaders: rule is used to generate header files that
 may be installed into user space by "make header_install" or
 "make headers_install_all".  In order to support
 "make headers_install_all", this target has to be able to run
 on an unconfigured tree, or a tree configured for another
 architecture.

 It is run before "make archprepare" when run on the
 architecture itself.

--- 6.3 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 describe how kbuild supports
 generating offset header files.

--- 6.4 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, init-y, core-y, libs-y, drivers-y, net-y

 $(head-y) lists objects to be linked first in vmlinux.
 $(libs-y) lists directories where a lib.a archive can be located.
 The rest list directories where a built-in.o object file can be
 located.

 $(init-y) objects will be located after $(head-y).
 Then the rest follows in this order:
 $(core-y), $(libs-y), $(drivers-y) and $(net-y).

 The top level Makefile defines values for all generic directories,
 and arch/$(ARCH)/Makefile only adds architecture-specific directories.

 Example:
  #arch/sparc64/Makefile
  core-y += arch/sparc64/kernel/
  libs-y += arch/sparc64/prom/ arch/sparc64/lib/
  drivers-$(CONFIG_OPROFILE)  += arch/sparc64/oprofile/

--- 6.5 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/$(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/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      - Image (arch/$(ARCH)/boot/bzImage)'
  endif

 When make is executed without arguments, the first goal encountered
 will be built. In the top level Makefile the first goal present
 is all:.
 An architecture shall always, per default, build a bootable image.
 In "make help", the default goal is highlighted with a '*'.
 Add a new prerequisite to all: to select a default goal different
 from vmlinux.

 Example:
  #arch/x86/Makefile
  all: bzImage

 When "make" is executed without arguments, bzImage will be built.

--- 6.6 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/x86/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.

--- 6.7 Commands useful for building a boot image

 Kbuild provides a few macros that are useful when building a
 boot image.

    if_changed

 if_changed is the infrastructure used for the following commands.

 Usage:
  target: source(s) FORCE
   $(call if_changed,ld/objcopy/gzip)

 When the rule is evaluated, it is checked to see if any files
 need an update, or the command line has changed since the last
 invocation. The latter will force a rebuild if any options
 to the executable have changed.
 Any target that utilises if_changed must be listed in $(targets),
 otherwise the command line check will fail, and the target will
 always be built.
 Assignments to $(targets) are without $(obj)/ prefix.
 if_changed may be used in conjunction with custom commands as
 defined in 6.8 "Custom kbuild commands".

 Note: It is a typical mistake to forget the FORCE prerequisite.
 Another common pitfall is that whitespace is sometimes
 significant; for instance, the below will fail (note the extra space
 after the comma):
  target: source(s) FORCE
 #WRONG!# $(call if_changed, ld/objcopy/gzip)

    ld
 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.

    objcopy
 Copy binary. Uses OBJCOPYFLAGS usually specified in
 arch/$(ARCH)/Makefile.
 OBJCOPYFLAGS_$@ may be used to set additional options.

    gzip
 Compress target. Use maximum compression to compress target.

 Example:
  #arch/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
 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.

    dtc
 Create flattend 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)
  clean-files += *.dtb
  DTC_FLAGS ?= -p 1024

    dtc_cpp
 This is just like dtc as describe above, except that the C pre-
 processor is invoked upon the .dtsp file before compiling the result
 with dtc.

 In order for build dependencies to work, all files compiled using
 dtc_cpp must use the C pre-processor's #include functionality and not
 dtc's /include/ functionality.

 Using the C pre-processor allows use of #define to create named
 constants. In turn, the #defines will typically appear in a header
 file, which may be shared with regular C code. Since the dtc language
 represents a data structure rather than code in C syntax, similar
 restrictions are placed on a header file included by a device tree
 file as for a header file included by an assembly language file.
 In particular, the C pre-processor is passed -x assembler-with-cpp,
 which sets macro __ASSEMBLY__. __DTS__ is also set. These allow header
 files to restrict their content to that compatible with device tree
 source.

 A central rule exists to create $(obj)/%.dtb from $(src)/%.dtsp;
 architecture Makefiles do no need to explicitly write out that rule.

--- 6.8 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/x86/boot/bzImage

 will be displayed with "make KBUILD_VERBOSE=0".

--- 6.9 Preprocessing linker scripts

 When the vmlinux image is built, the linker script
 arch/$(ARCH)/kernel/vmlinux.lds is used.
 The script is a preprocessed variant of the file vmlinux.lds.S
 located in the same directory.
 kbuild knows .lds files and includes a rule *lds.S -> *lds.

 Example:
  #arch/x86/kernel/Makefile
  always := vmlinux.lds

  #Makefile
  export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)

 The assignment to $(always) is used to tell kbuild to build the
 target vmlinux.lds.
 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
 specified options when building the target vmlinux.lds.

 When building the *.lds target, kbuild uses the variables:
 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 file are used in several
 architecture-specific files.

--- 6.10 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 "7.4 generic-y" for further info on syntax etc.

=== 7 Kbuild syntax for exported headers

The kernel include 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__)

Each relevant directory contains a file name "Kbuild" which specifies the
headers to be exported.
See subsequent chapter for the syntax of the Kbuild file.

 --- 7.1 header-y

 header-y specify header files to be exported.

  Example:
   #include/linux/Kbuild
   header-y += usb/
   header-y += aio_abi.h

 The convention is to list one file per line and
 preferably in alphabetic order.

 header-y also specify which subdirectories to visit.
 A subdirectory is identified by a trailing '/' which
 can be seen in the example above for the usb subdirectory.

 Subdirectories are visited before their parent directories.

 --- 7.2 genhdr-y

 genhdr-y specifies generated files to be exported.
 Generated files are special as they need to be looked
 up in another directory when doing 'make O=...' builds.

  Example:
   #include/linux/Kbuild
   genhdr-y += version.h

 --- 7.3 destination-y

 When an architecture have a set of exported headers that needs to be
 exported to a different directory destination-y is used.
 destination-y specify the destination directory for all exported
 headers in the file where it is present.

  Example:
   #arch/xtensa/platforms/s6105/include/platform/Kbuild
   destination-y := include/linux

 In the example above all exported headers in the Kbuild file
 will be located in the directory "include/linux" when exported.

 --- 7.4 generic-y

 If an architecture uses a verbatim copy of a header from
 include/asm-generic then this is listed in the file
 arch/$(ARCH)/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/$(ARCH)/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 Kbuild Variables

The top Makefile exports the following variables:

    VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION

 These variables define the current kernel version.  A few arch
 Makefiles actually use these values directly; they should use
 $(KERNELRELEASE) instead.

 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
 three-part version number, such as "2", "4", and "0".  These three
 values are always numeric.

 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
 or additional patches. It is usually some non-numeric string
 such as "-pre4", and is often blank.

    KERNELRELEASE

 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
 for constructing installation directory names or showing in
 version strings.  Some arch Makefiles use it for this purpose.

    ARCH

 This variable defines the target architecture, such as "i386",
 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
 determine which files to compile.

 By default, the top Makefile sets $(ARCH) to be the same as the
 host system architecture.  For a cross build, a user may
 override the value of $(ARCH) on the command line:

     make ARCH=m68k ...

    INSTALL_PATH

 This variable defines a place for the arch Makefiles to install
 the resident kernel image and System.map file.
 Use this for architecture-specific install targets.

    INSTALL_MOD_PATH, MODLIB

 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
 installation.  This variable is not defined in the Makefile but
 may be passed in by the user if desired.

 $(MODLIB) specifies the directory for module installation.
 The top Makefile defines $(MODLIB) to
 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE).  The user may
 override this value on the command line if desired.

    INSTALL_MOD_STRIP

 If this variable is specified, will cause modules to be stripped
 after they are installed.  If INSTALL_MOD_STRIP is '1', then the
 default option --strip-debug will be used.  Otherwise,
 INSTALL_MOD_STRIP value will be used as the option(s) to the strip
 command.

=== 9 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.

=== 10 Credits

Original version made by Michael Elizabeth Chastain, <mailto:[e-mail]>
Updates by Kai Germaschewski <[e-mail]>
Updates by Sam Ravnborg <[e-mail]>
Language QA by Jan Engelhardt <[e-mail]>

=== 11 TODO

- Describe how kbuild supports shipped files with _shipped.
- Generating offset header files.
- Add more variables to section 7?