makefiles.rst

   1======================
   2Linux Kernel Makefiles
   3======================
   4
   5This document describes the Linux kernel Makefiles.
   6
   7Overview
   8========
   9
  10The Makefiles have five parts::
  11
  12	Makefile                    the top Makefile.
  13	.config                     the kernel configuration file.
  14	arch/$(SRCARCH)/Makefile    the arch Makefile.
  15	scripts/Makefile.*          common rules etc. for all kbuild Makefiles.
  16	kbuild Makefiles            exist in every subdirectory
  17
  18The top Makefile reads the .config file, which comes from the kernel
  19configuration process.
  20
  21The top Makefile is responsible for building two major products: vmlinux
  22(the resident kernel image) and modules (any module files).
  23It builds these goals by recursively descending into the subdirectories of
  24the kernel source tree.
  25
  26The list of subdirectories which are visited depends upon the kernel
  27configuration. The top Makefile textually includes an arch Makefile
  28with the name arch/$(SRCARCH)/Makefile. The arch Makefile supplies
  29architecture-specific information to the top Makefile.
  30
  31Each subdirectory has a kbuild Makefile which carries out the commands
  32passed down from above. The kbuild Makefile uses information from the
  33.config file to construct various file lists used by kbuild to build
  34any built-in or modular targets.
  35
  36scripts/Makefile.* contains all the definitions/rules etc. that
  37are used to build the kernel based on the kbuild makefiles.
  38
  39Who does what
  40=============
  41
  42People have four different relationships with the kernel Makefiles.
  43
  44*Users* are people who build kernels.  These people type commands such as
  45``make menuconfig`` or ``make``.  They usually do not read or edit
  46any kernel Makefiles (or any other source files).
  47
  48*Normal developers* are people who work on features such as device
  49drivers, file systems, and network protocols.  These people need to
  50maintain the kbuild Makefiles for the subsystem they are
  51working on.  In order to do this effectively, they need some overall
  52knowledge about the kernel Makefiles, plus detailed knowledge about the
  53public interface for kbuild.
  54
  55*Arch developers* are people who work on an entire architecture, such
  56as sparc or x86.  Arch developers need to know about the arch Makefile
  57as well as kbuild Makefiles.
  58
  59*Kbuild developers* are people who work on the kernel build system itself.
  60These people need to know about all aspects of the kernel Makefiles.
  61
  62This document is aimed towards normal developers and arch developers.
  63
  64
  65The kbuild files
  66================
  67
  68Most Makefiles within the kernel are kbuild Makefiles that use the
  69kbuild infrastructure. This chapter introduces the syntax used in the
  70kbuild makefiles.
  71
  72The preferred name for the kbuild files are ``Makefile`` but ``Kbuild`` can
  73be used and if both a ``Makefile`` and a ``Kbuild`` file exists, then the ``Kbuild``
  74file will be used.
  75
  76Section `Goal definitions`_ is a quick intro; further chapters provide
  77more details, with real examples.
  78
  79Goal definitions
  80----------------
  81
  82Goal definitions are the main part (heart) of the kbuild Makefile.
  83These lines define the files to be built, any special compilation
  84options, and any subdirectories to be entered recursively.
  85
  86The most simple kbuild makefile contains one line:
  87
  88Example::
  89
  90  obj-y += foo.o
  91
  92This tells kbuild that there is one object in that directory, named
  93foo.o. foo.o will be built from foo.c or foo.S.
  94
  95If foo.o shall be built as a module, the variable obj-m is used.
  96Therefore the following pattern is often used:
  97
  98Example::
  99
 100  obj-$(CONFIG_FOO) += foo.o
 101
 102$(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
 103If CONFIG_FOO is neither y nor m, then the file will not be compiled
 104nor linked.
 105
 106Built-in object goals - obj-y
 107-----------------------------
 108
 109The kbuild Makefile specifies object files for vmlinux
 110in the $(obj-y) lists.  These lists depend on the kernel
 111configuration.
 112
 113Kbuild compiles all the $(obj-y) files.  It then calls
 114``$(AR) rcSTP`` to merge these files into one built-in.a file.
 115This is a thin archive without a symbol table. It will be later
 116linked into vmlinux by scripts/link-vmlinux.sh
 117
 118The order of files in $(obj-y) is significant.  Duplicates in
 119the lists are allowed: the first instance will be linked into
 120built-in.a and succeeding instances will be ignored.
 121
 122Link order is significant, because certain functions
 123(module_init() / __initcall) will be called during boot in the
 124order they appear. So keep in mind that changing the link
 125order may e.g. change the order in which your SCSI
 126controllers are detected, and thus your disks are renumbered.
 127
 128Example::
 129
 130  #drivers/isdn/i4l/Makefile
 131  # Makefile for the kernel ISDN subsystem and device drivers.
 132  # Each configuration option enables a list of files.
 133  obj-$(CONFIG_ISDN_I4L)         += isdn.o
 134  obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
 135
 136Loadable module goals - obj-m
 137-----------------------------
 138
 139$(obj-m) specifies object files which are built as loadable
 140kernel modules.
 141
 142A module may be built from one source file or several source
 143files. In the case of one source file, the kbuild makefile
 144simply adds the file to $(obj-m).
 145
 146Example::
 147
 148  #drivers/isdn/i4l/Makefile
 149  obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
 150
 151Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to "m"
 152
 153If a kernel module is built from several source files, you specify
 154that you want to build a module in the same way as above; however,
 155kbuild needs to know which object files you want to build your
 156module from, so you have to tell it by setting a $(<module_name>-y)
 157variable.
 158
 159Example::
 160
 161  #drivers/isdn/i4l/Makefile
 162  obj-$(CONFIG_ISDN_I4L) += isdn.o
 163  isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
 164
 165In this example, the module name will be isdn.o. Kbuild will
 166compile the objects listed in $(isdn-y) and then run
 167``$(LD) -r`` on the list of these files to generate isdn.o.
 168
 169Due to kbuild recognizing $(<module_name>-y) for composite objects,
 170you can use the value of a ``CONFIG_`` symbol to optionally include an
 171object file as part of a composite object.
 172
 173Example::
 174
 175  #fs/ext2/Makefile
 176  obj-$(CONFIG_EXT2_FS) += ext2.o
 177  ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
 178    namei.o super.o symlink.o
 179  ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
 180    xattr_trusted.o
 181
 182In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
 183part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
 184evaluates to "y".
 185
 186Note: Of course, when you are building objects into the kernel,
 187the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
 188kbuild will build an ext2.o file for you out of the individual
 189parts and then link this into built-in.a, as you would expect.
 190
 191Library file goals - lib-y
 192--------------------------
 193
 194Objects listed with obj-* are used for modules, or
 195combined in a built-in.a for that specific directory.
 196There is also the possibility to list objects that will
 197be included in a library, lib.a.
 198All objects listed with lib-y are combined in a single
 199library for that directory.
 200Objects that are listed in obj-y and additionally listed in
 201lib-y will not be included in the library, since they will
 202be accessible anyway.
 203For consistency, objects listed in lib-m will be included in lib.a.
 204
 205Note that the same kbuild makefile may list files to be built-in
 206and to be part of a library. Therefore the same directory
 207may contain both a built-in.a and a lib.a file.
 208
 209Example::
 210
 211  #arch/x86/lib/Makefile
 212  lib-y    := delay.o
 213
 214This will create a library lib.a based on delay.o. For kbuild to
 215actually recognize that there is a lib.a being built, the directory
 216shall be listed in libs-y.
 217
 218See also `List directories to visit when descending`_.
 219
 220Use of lib-y is normally restricted to ``lib/`` and ``arch/*/lib``.
 221
 222Descending down in directories
 223------------------------------
 224
 225A Makefile is only responsible for building objects in its own
 226directory. Files in subdirectories should be taken care of by
 227Makefiles in these subdirs. The build system will automatically
 228invoke make recursively in subdirectories, provided you let it know of
 229them.
 230
 231To do so, obj-y and obj-m are used.
 232ext2 lives in a separate directory, and the Makefile present in fs/
 233tells kbuild to descend down using the following assignment.
 234
 235Example::
 236
 237  #fs/Makefile
 238  obj-$(CONFIG_EXT2_FS) += ext2/
 239
 240If CONFIG_EXT2_FS is set to either "y" (built-in) or "m" (modular)
 241the corresponding obj- variable will be set, and kbuild will descend
 242down in the ext2 directory.
 243
 244Kbuild uses this information not only to decide that it needs to visit
 245the directory, but also to decide whether or not to link objects from
 246the directory into vmlinux.
 247
 248When Kbuild descends into the directory with "y", all built-in objects
 249from that directory are combined into the built-in.a, which will be
 250eventually linked into vmlinux.
 251
 252When Kbuild descends into the directory with "m", in contrast, nothing
 253from that directory will be linked into vmlinux. If the Makefile in
 254that directory specifies obj-y, those objects will be left orphan.
 255It is very likely a bug of the Makefile or of dependencies in Kconfig.
 256
 257Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
 258descending into subdirectories. It is a good fit when you know they
 259do not contain kernel-space objects at all. A typical usage is to let
 260Kbuild descend into subdirectories to build tools.
 261
 262Examples::
 263
 264  # scripts/Makefile
 265  subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
 266  subdir-$(CONFIG_MODVERSIONS) += genksyms
 267  subdir-$(CONFIG_SECURITY_SELINUX) += selinux
 268
 269Unlike obj-y/m, subdir-y/m does not need the trailing slash since this
 270syntax is always used for directories.
 271
 272It is good practice to use a ``CONFIG_`` variable when assigning directory
 273names. This allows kbuild to totally skip the directory if the
 274corresponding ``CONFIG_`` option is neither "y" nor "m".
 275
 276Non-builtin vmlinux targets - extra-y
 277-------------------------------------
 278
 279extra-y specifies targets which are needed for building vmlinux,
 280but not combined into built-in.a.
 281
 282Examples are:
 283
 2841) vmlinux linker script
 285
 286   The linker script for vmlinux is located at
 287   arch/$(SRCARCH)/kernel/vmlinux.lds
 288
 289Example::
 290
 291  # arch/x86/kernel/Makefile
 292  extra-y	+= vmlinux.lds
 293
 294extra-y is now deprecated because this is equivalent to:
 295
 296  always-$(KBUILD_BUILTIN) += vmlinux.lds
 297
 298$(extra-y) should only contain targets needed for vmlinux.
 299
 300Kbuild skips extra-y when vmlinux is apparently not a final goal.
 301(e.g. ``make modules``, or building external modules)
 302
 303If you intend to build targets unconditionally, always-y (explained
 304in the next section) is the correct syntax to use.
 305
 306Always built goals - always-y
 307-----------------------------
 308
 309always-y specifies targets which are literally always built when
 310Kbuild visits the Makefile.
 311
 312Example::
 313
 314  # ./Kbuild
 315  offsets-file := include/generated/asm-offsets.h
 316  always-y += $(offsets-file)
 317
 318Compilation flags
 319-----------------
 320
 321ccflags-y, asflags-y and ldflags-y
 322  These three flags apply only to the kbuild makefile in which they
 323  are assigned. They are used for all the normal cc, as and ld
 324  invocations happening during a recursive build.
 325
 326  ccflags-y specifies options for compiling with $(CC).
 327
 328  Example::
 329
 330    # drivers/acpi/acpica/Makefile
 331    ccflags-y				:= -Os -D_LINUX -DBUILDING_ACPICA
 332    ccflags-$(CONFIG_ACPI_DEBUG)	+= -DACPI_DEBUG_OUTPUT
 333
 334  This variable is necessary because the top Makefile owns the
 335  variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
 336  entire tree.
 337
 338  asflags-y specifies assembler options.
 339
 340  Example::
 341
 342    #arch/sparc/kernel/Makefile
 343    asflags-y := -ansi
 344
 345  ldflags-y specifies options for linking with $(LD).
 346
 347  Example::
 348
 349    #arch/cris/boot/compressed/Makefile
 350    ldflags-y += -T $(src)/decompress_$(arch-y).lds
 351
 352subdir-ccflags-y, subdir-asflags-y
 353  The two flags listed above are similar to ccflags-y and asflags-y.
 354  The difference is that the subdir- variants have effect for the kbuild
 355  file where they are present and all subdirectories.
 356  Options specified using subdir-* are added to the commandline before
 357  the options specified using the non-subdir variants.
 358
 359  Example::
 360
 361    subdir-ccflags-y := -Werror
 362
 363ccflags-remove-y, asflags-remove-y
 364  These flags are used to remove particular flags for the compiler,
 365  assembler invocations.
 366
 367  Example::
 368
 369    ccflags-remove-$(CONFIG_MCOUNT) += -pg
 370
 371CFLAGS_$@, AFLAGS_$@
 372  CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
 373  kbuild makefile.
 374
 375  $(CFLAGS_$@) specifies per-file options for $(CC).  The $@
 376  part has a literal value which specifies the file that it is for.
 377
 378  CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
 379  can re-add compiler flags that were removed by ccflags-remove-y.
 380
 381  Example::
 382
 383    # drivers/scsi/Makefile
 384    CFLAGS_aha152x.o =   -DAHA152X_STAT -DAUTOCONF
 385
 386  This line specify compilation flags for aha152x.o.
 387
 388  $(AFLAGS_$@) is a similar feature for source files in assembly
 389  languages.
 390
 391  AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
 392  can re-add assembler flags that were removed by asflags-remove-y.
 393
 394  Example::
 395
 396    # arch/arm/kernel/Makefile
 397    AFLAGS_head.o        := -DTEXT_OFFSET=$(TEXT_OFFSET)
 398    AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
 399    AFLAGS_iwmmxt.o      := -Wa,-mcpu=iwmmxt
 400
 401Dependency tracking
 402-------------------
 403
 404Kbuild tracks dependencies on the following:
 405
 4061) All prerequisite files (both ``*.c`` and ``*.h``)
 4072) ``CONFIG_`` options used in all prerequisite files
 4083) Command-line used to compile target
 409
 410Thus, if you change an option to $(CC) all affected files will
 411be re-compiled.
 412
 413Custom Rules
 414------------
 415
 416Custom rules are used when the kbuild infrastructure does
 417not provide the required support. A typical example is
 418header files generated during the build process.
 419Another example are the architecture-specific Makefiles which
 420need custom rules to prepare boot images etc.
 421
 422Custom rules are written as normal Make rules.
 423Kbuild is not executing in the directory where the Makefile is
 424located, so all custom rules shall use a relative
 425path to prerequisite files and target files.
 426
 427Two variables are used when defining custom rules:
 428
 429$(src)
 430  $(src) is the directory where the Makefile is located. Always use $(src) when
 431  referring to files located in the src tree.
 432
 433$(obj)
 434  $(obj) is the directory where the target is saved. Always use $(obj) when
 435  referring to generated files. Use $(obj) for pattern rules that need to work
 436  for both generated files and real sources (VPATH will help to find the
 437  prerequisites not only in the object tree but also in the source tree).
 438
 439  Example::
 440
 441    #drivers/scsi/Makefile
 442    $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
 443    $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
 444
 445  This is a custom rule, following the normal syntax
 446  required by make.
 447
 448  The target file depends on two prerequisite files. References
 449  to the target file are prefixed with $(obj), references
 450  to prerequisites are referenced with $(src) (because they are not
 451  generated files).
 452
 453$(srcroot)
 454  $(srcroot) refers to the root of the source you are building, which can be
 455  either the kernel source or the external modules source, depending on whether
 456  KBUILD_EXTMOD is set. This can be either a relative or an absolute path, but
 457  if KBUILD_ABS_SRCTREE=1 is set, it is always an absolute path.
 458
 459$(srctree)
 460  $(srctree) refers to the root of the kernel source tree. When building the
 461  kernel, this is the same as $(srcroot).
 462
 463$(objtree)
 464  $(objtree) refers to the root of the kernel object tree. It is ``.`` when
 465  building the kernel, but it is different when building external modules.
 466
 467$(kecho)
 468  echoing information to user in a rule is often a good practice
 469  but when execution ``make -s`` one does not expect to see any output
 470  except for warnings/errors.
 471  To support this kbuild defines $(kecho) which will echo out the
 472  text following $(kecho) to stdout except if ``make -s`` is used.
 473
 474  Example::
 475
 476    # arch/arm/Makefile
 477    $(BOOT_TARGETS): vmlinux
 478            $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
 479            @$(kecho) '  Kernel: $(boot)/$@ is ready'
 480
 481  When kbuild is executing with KBUILD_VERBOSE unset, then only a shorthand
 482  of a command is normally displayed.
 483  To enable this behaviour for custom commands kbuild requires
 484  two variables to be set::
 485
 486    quiet_cmd_<command> - what shall be echoed
 487          cmd_<command> - the command to execute
 488
 489  Example::
 490
 491    # lib/Makefile
 492    quiet_cmd_crc32 = GEN     $@
 493          cmd_crc32 = $< > $@
 494
 495    $(obj)/crc32table.h: $(obj)/gen_crc32table
 496            $(call cmd,crc32)
 497
 498  When updating the $(obj)/crc32table.h target, the line::
 499
 500    GEN     lib/crc32table.h
 501
 502  will be displayed with ``make KBUILD_VERBOSE=``.
 503
 504Command change detection
 505------------------------
 506
 507When the rule is evaluated, timestamps are compared between the target
 508and its prerequisite files. GNU Make updates the target when any of the
 509prerequisites is newer than that.
 510
 511The target should be rebuilt also when the command line has changed
 512since the last invocation. This is not supported by Make itself, so
 513Kbuild achieves this by a kind of meta-programming.
 514
 515if_changed is the macro used for this purpose, in the following form::
 516
 517  quiet_cmd_<command> = ...
 518        cmd_<command> = ...
 519
 520  <target>: <source(s)> FORCE
 521          $(call if_changed,<command>)
 522
 523Any target that utilizes if_changed must be listed in $(targets),
 524otherwise the command line check will fail, and the target will
 525always be built.
 526
 527If the target is already listed in the recognized syntax such as
 528obj-y/m, lib-y/m, extra-y/m, always-y/m, hostprogs, userprogs, Kbuild
 529automatically adds it to $(targets). Otherwise, the target must be
 530explicitly added to $(targets).
 531
 532Assignments to $(targets) are without $(obj)/ prefix. if_changed may be
 533used in conjunction with custom rules as defined in `Custom Rules`_.
 534
 535Note: It is a typical mistake to forget the FORCE prerequisite.
 536Another common pitfall is that whitespace is sometimes significant; for
 537instance, the below will fail (note the extra space after the comma)::
 538
 539  target: source(s) FORCE
 540
 541**WRONG!**	$(call if_changed, objcopy)
 542
 543Note:
 544  if_changed should not be used more than once per target.
 545  It stores the executed command in a corresponding .cmd
 546  file and multiple calls would result in overwrites and
 547  unwanted results when the target is up to date and only the
 548  tests on changed commands trigger execution of commands.
 549
 550$(CC) support functions
 551-----------------------
 552
 553The kernel may be built with several different versions of
 554$(CC), each supporting a unique set of features and options.
 555kbuild provides basic support to check for valid options for $(CC).
 556$(CC) is usually the gcc compiler, but other alternatives are
 557available.
 558
 559as-option
 560  as-option is used to check if $(CC) -- when used to compile
 561  assembler (``*.S``) files -- supports the given option. An optional
 562  second option may be specified if the first option is not supported.
 563
 564  Example::
 565
 566    #arch/sh/Makefile
 567    cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
 568
 569  In the above example, cflags-y will be assigned the option
 570  -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
 571  The second argument is optional, and if supplied will be used
 572  if first argument is not supported.
 573
 574as-instr
 575  as-instr checks if the assembler reports a specific instruction
 576  and then outputs either option1 or option2
 577  C escapes are supported in the test instruction
 578  Note: as-instr-option uses KBUILD_AFLAGS for assembler options
 579
 580cc-option
 581  cc-option is used to check if $(CC) supports a given option, and if
 582  not supported to use an optional second option.
 583
 584  Example::
 585
 586    #arch/x86/Makefile
 587    cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
 588
 589  In the above example, cflags-y will be assigned the option
 590  -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
 591  The second argument to cc-option is optional, and if omitted,
 592  cflags-y will be assigned no value if first option is not supported.
 593  Note: cc-option uses KBUILD_CFLAGS for $(CC) options
 594
 595cc-option-yn
 596  cc-option-yn is used to check if $(CC) supports a given option
 597  and return "y" if supported, otherwise "n".
 598
 599  Example::
 600
 601    #arch/ppc/Makefile
 602    biarch := $(call cc-option-yn, -m32)
 603    aflags-$(biarch) += -a32
 604    cflags-$(biarch) += -m32
 605
 606  In the above example, $(biarch) is set to y if $(CC) supports the -m32
 607  option. When $(biarch) equals "y", the expanded variables $(aflags-y)
 608  and $(cflags-y) will be assigned the values -a32 and -m32,
 609  respectively.
 610
 611  Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
 612
 613cc-disable-warning
 614  cc-disable-warning checks if $(CC) supports a given warning and returns
 615  the commandline switch to disable it. This special function is needed,
 616  because gcc 4.4 and later accept any unknown -Wno-* option and only
 617  warn about it if there is another warning in the source file.
 618
 619  Example::
 620
 621    KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 622
 623  In the above example, -Wno-unused-but-set-variable will be added to
 624  KBUILD_CFLAGS only if $(CC) really accepts it.
 625
 626gcc-min-version
 627  gcc-min-version tests if the value of $(CONFIG_GCC_VERSION) is greater than
 628  or equal to the provided value and evaluates to y if so.
 629
 630  Example::
 631
 632    cflags-$(call gcc-min-version, 110100) := -foo
 633
 634  In this example, cflags-y will be assigned the value -foo if $(CC) is gcc and
 635  $(CONFIG_GCC_VERSION) is >= 11.1.
 636
 637clang-min-version
 638  clang-min-version tests if the value of $(CONFIG_CLANG_VERSION) is greater
 639  than or equal to the provided value and evaluates to y if so.
 640
 641  Example::
 642
 643    cflags-$(call clang-min-version, 110000) := -foo
 644
 645  In this example, cflags-y will be assigned the value -foo if $(CC) is clang
 646  and $(CONFIG_CLANG_VERSION) is >= 11.0.0.
 647
 648cc-cross-prefix
 649  cc-cross-prefix is used to check if there exists a $(CC) in path with
 650  one of the listed prefixes. The first prefix where there exist a
 651  prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
 652  then nothing is returned.
 653
 654  Additional prefixes are separated by a single space in the
 655  call of cc-cross-prefix.
 656
 657  This functionality is useful for architecture Makefiles that try
 658  to set CROSS_COMPILE to well-known values but may have several
 659  values to select between.
 660
 661  It is recommended only to try to set CROSS_COMPILE if it is a cross
 662  build (host arch is different from target arch). And if CROSS_COMPILE
 663  is already set then leave it with the old value.
 664
 665  Example::
 666
 667    #arch/m68k/Makefile
 668    ifneq ($(SUBARCH),$(ARCH))
 669            ifeq ($(CROSS_COMPILE),)
 670                    CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
 671            endif
 672    endif
 673
 674$(RUSTC) support functions
 675--------------------------
 676
 677rustc-min-version
 678  rustc-min-version tests if the value of $(CONFIG_RUSTC_VERSION) is greater
 679  than or equal to the provided value and evaluates to y if so.
 680
 681  Example::
 682
 683    rustflags-$(call rustc-min-version, 108500) := -Cfoo
 684
 685  In this example, rustflags-y will be assigned the value -Cfoo if
 686  $(CONFIG_RUSTC_VERSION) is >= 1.85.0.
 687
 688$(LD) support functions
 689-----------------------
 690
 691ld-option
 692  ld-option is used to check if $(LD) supports the supplied option.
 693  ld-option takes two options as arguments.
 694
 695  The second argument is an optional option that can be used if the
 696  first option is not supported by $(LD).
 697
 698  Example::
 699
 700    #Makefile
 701    LDFLAGS_vmlinux += $(call ld-option, -X)
 702
 703Script invocation
 704-----------------
 705
 706Make rules may invoke scripts to build the kernel. The rules shall
 707always provide the appropriate interpreter to execute the script. They
 708shall not rely on the execute bits being set, and shall not invoke the
 709script directly. For the convenience of manual script invocation, such
 710as invoking ./scripts/checkpatch.pl, it is recommended to set execute
 711bits on the scripts nonetheless.
 712
 713Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
 714and $(PYTHON3) to refer to interpreters for the respective
 715scripts.
 716
 717Example::
 718
 719  #Makefile
 720  cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
 721          $(KERNELRELEASE)
 722
 723Host Program support
 724====================
 725
 726Kbuild supports building executables on the host for use during the
 727compilation stage.
 728
 729Two steps are required in order to use a host executable.
 730
 731The first step is to tell kbuild that a host program exists. This is
 732done utilising the variable ``hostprogs``.
 733
 734The second step is to add an explicit dependency to the executable.
 735This can be done in two ways. Either add the dependency in a rule,
 736or utilise the variable ``always-y``.
 737Both possibilities are described in the following.
 738
 739Simple Host Program
 740-------------------
 741
 742In some cases there is a need to compile and run a program on the
 743computer where the build is running.
 744
 745The following line tells kbuild that the program bin2hex shall be
 746built on the build host.
 747
 748Example::
 749
 750  hostprogs := bin2hex
 751
 752Kbuild assumes in the above example that bin2hex is made from a single
 753c-source file named bin2hex.c located in the same directory as
 754the Makefile.
 755
 756Composite Host Programs
 757-----------------------
 758
 759Host programs can be made up based on composite objects.
 760The syntax used to define composite objects for host programs is
 761similar to the syntax used for kernel objects.
 762$(<executable>-objs) lists all objects used to link the final
 763executable.
 764
 765Example::
 766
 767  #scripts/lxdialog/Makefile
 768  hostprogs     := lxdialog
 769  lxdialog-objs := checklist.o lxdialog.o
 770
 771Objects with extension .o are compiled from the corresponding .c
 772files. In the above example, checklist.c is compiled to checklist.o
 773and lxdialog.c is compiled to lxdialog.o.
 774
 775Finally, the two .o files are linked to the executable, lxdialog.
 776Note: The syntax <executable>-y is not permitted for host-programs.
 777
 778Using C++ for host programs
 779---------------------------
 780
 781kbuild offers support for host programs written in C++. This was
 782introduced solely to support kconfig, and is not recommended
 783for general use.
 784
 785Example::
 786
 787  #scripts/kconfig/Makefile
 788  hostprogs     := qconf
 789  qconf-cxxobjs := qconf.o
 790
 791In the example above the executable is composed of the C++ file
 792qconf.cc - identified by $(qconf-cxxobjs).
 793
 794If qconf is composed of a mixture of .c and .cc files, then an
 795additional line can be used to identify this.
 796
 797Example::
 798
 799  #scripts/kconfig/Makefile
 800  hostprogs     := qconf
 801  qconf-cxxobjs := qconf.o
 802  qconf-objs    := check.o
 803
 804Using Rust for host programs
 805----------------------------
 806
 807Kbuild offers support for host programs written in Rust. However,
 808since a Rust toolchain is not mandatory for kernel compilation,
 809it may only be used in scenarios where Rust is required to be
 810available (e.g. when  ``CONFIG_RUST`` is enabled).
 811
 812Example::
 813
 814  hostprogs     := target
 815  target-rust   := y
 816
 817Kbuild will compile ``target`` using ``target.rs`` as the crate root,
 818located in the same directory as the ``Makefile``. The crate may
 819consist of several source files (see ``samples/rust/hostprogs``).
 820
 821Controlling compiler options for host programs
 822----------------------------------------------
 823
 824When compiling host programs, it is possible to set specific flags.
 825The programs will always be compiled utilising $(HOSTCC) passed
 826the options specified in $(KBUILD_HOSTCFLAGS).
 827
 828To set flags that will take effect for all host programs created
 829in that Makefile, use the variable HOST_EXTRACFLAGS.
 830
 831Example::
 832
 833  #scripts/lxdialog/Makefile
 834  HOST_EXTRACFLAGS += -I/usr/include/ncurses
 835
 836To set specific flags for a single file the following construction
 837is used:
 838
 839Example::
 840
 841  #arch/ppc64/boot/Makefile
 842  HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
 843
 844It is also possible to specify additional options to the linker.
 845
 846Example::
 847
 848  #scripts/kconfig/Makefile
 849  HOSTLDLIBS_qconf := -L$(QTDIR)/lib
 850
 851When linking qconf, it will be passed the extra option
 852``-L$(QTDIR)/lib``.
 853
 854When host programs are actually built
 855-------------------------------------
 856
 857Kbuild will only build host-programs when they are referenced
 858as a prerequisite.
 859
 860This is possible in two ways:
 861
 862(1) List the prerequisite explicitly in a custom rule.
 863
 864    Example::
 865
 866      #drivers/pci/Makefile
 867      hostprogs := gen-devlist
 868      $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
 869      ( cd $(obj); ./gen-devlist ) < $<
 870
 871    The target $(obj)/devlist.h will not be built before
 872    $(obj)/gen-devlist is updated. Note that references to
 873    the host programs in custom rules must be prefixed with $(obj).
 874
 875(2) Use always-y
 876
 877    When there is no suitable custom rule, and the host program
 878    shall be built when a makefile is entered, the always-y
 879    variable shall be used.
 880
 881    Example::
 882
 883      #scripts/lxdialog/Makefile
 884      hostprogs     := lxdialog
 885      always-y      := $(hostprogs)
 886
 887    Kbuild provides the following shorthand for this::
 888
 889      hostprogs-always-y := lxdialog
 890
 891    This will tell kbuild to build lxdialog even if not referenced in
 892    any rule.
 893
 894Userspace Program support
 895=========================
 896
 897Just like host programs, Kbuild also supports building userspace executables
 898for the target architecture (i.e. the same architecture as you are building
 899the kernel for).
 900
 901The syntax is quite similar. The difference is to use ``userprogs`` instead of
 902``hostprogs``.
 903
 904Simple Userspace Program
 905------------------------
 906
 907The following line tells kbuild that the program bpf-direct shall be
 908built for the target architecture.
 909
 910Example::
 911
 912  userprogs := bpf-direct
 913
 914Kbuild assumes in the above example that bpf-direct is made from a
 915single C source file named bpf-direct.c located in the same directory
 916as the Makefile.
 917
 918Composite Userspace Programs
 919----------------------------
 920
 921Userspace programs can be made up based on composite objects.
 922The syntax used to define composite objects for userspace programs is
 923similar to the syntax used for kernel objects.
 924$(<executable>-objs) lists all objects used to link the final
 925executable.
 926
 927Example::
 928
 929  #samples/seccomp/Makefile
 930  userprogs      := bpf-fancy
 931  bpf-fancy-objs := bpf-fancy.o bpf-helper.o
 932
 933Objects with extension .o are compiled from the corresponding .c
 934files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
 935and bpf-helper.c is compiled to bpf-helper.o.
 936
 937Finally, the two .o files are linked to the executable, bpf-fancy.
 938Note: The syntax <executable>-y is not permitted for userspace programs.
 939
 940Controlling compiler options for userspace programs
 941---------------------------------------------------
 942
 943When compiling userspace programs, it is possible to set specific flags.
 944The programs will always be compiled utilising $(CC) passed
 945the options specified in $(KBUILD_USERCFLAGS).
 946
 947To set flags that will take effect for all userspace programs created
 948in that Makefile, use the variable userccflags.
 949
 950Example::
 951
 952  # samples/seccomp/Makefile
 953  userccflags += -I usr/include
 954
 955To set specific flags for a single file the following construction
 956is used:
 957
 958Example::
 959
 960  bpf-helper-userccflags += -I user/include
 961
 962It is also possible to specify additional options to the linker.
 963
 964Example::
 965
 966  # net/bpfilter/Makefile
 967  bpfilter_umh-userldflags += -static
 968
 969To specify libraries linked to a userspace program, you can use
 970``<executable>-userldlibs``. The ``userldlibs`` syntax specifies libraries
 971linked to all userspace programs created in the current Makefile.
 972
 973When linking bpfilter_umh, it will be passed the extra option -static.
 974
 975From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used.
 976
 977When userspace programs are actually built
 978------------------------------------------
 979
 980Kbuild builds userspace programs only when told to do so.
 981There are two ways to do this.
 982
 983(1) Add it as the prerequisite of another file
 984
 985    Example::
 986
 987      #net/bpfilter/Makefile
 988      userprogs := bpfilter_umh
 989      $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
 990
 991    $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o
 992
 993(2) Use always-y
 994
 995    Example::
 996
 997      userprogs := binderfs_example
 998      always-y := $(userprogs)
 999
1000    Kbuild provides the following shorthand for this::
1001
1002      userprogs-always-y := binderfs_example
1003
1004    This will tell Kbuild to build binderfs_example when it visits this
1005    Makefile.
1006
1007Kbuild clean infrastructure
1008===========================
1009
1010``make clean`` deletes most generated files in the obj tree where the kernel
1011is compiled. This includes generated files such as host programs.
1012Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m),
1013$(always-), $(extra-y), $(extra-) and $(targets). They are all deleted
1014during ``make clean``. Files matching the patterns ``*.[oas]``, ``*.ko``, plus
1015some additional files generated by kbuild are deleted all over the kernel
1016source tree when ``make clean`` is executed.
1017
1018Additional files or directories can be specified in kbuild makefiles by use of
1019$(clean-files).
1020
1021Example::
1022
1023  #lib/Makefile
1024  clean-files := crc32table.h
1025
1026When executing ``make clean``, the file ``crc32table.h`` will be deleted.
1027Kbuild will assume files to be in the same relative directory as the
1028Makefile.
1029
1030To exclude certain files or directories from make clean, use the
1031$(no-clean-files) variable.
1032
1033Usually kbuild descends down in subdirectories due to ``obj-* := dir/``,
1034but in the architecture makefiles where the kbuild infrastructure
1035is not sufficient this sometimes needs to be explicit.
1036
1037Example::
1038
1039  #arch/x86/boot/Makefile
1040  subdir- := compressed
1041
1042The above assignment instructs kbuild to descend down in the
1043directory compressed/ when ``make clean`` is executed.
1044
1045Note 1: arch/$(SRCARCH)/Makefile cannot use ``subdir-``, because that file is
1046included in the top level makefile. Instead, arch/$(SRCARCH)/Kbuild can use
1047``subdir-``.
1048
1049Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
1050be visited during ``make clean``.
1051
1052Architecture Makefiles
1053======================
1054
1055The top level Makefile sets up the environment and does the preparation,
1056before starting to descend down in the individual directories.
1057
1058The top level makefile contains the generic part, whereas
1059arch/$(SRCARCH)/Makefile contains what is required to set up kbuild
1060for said architecture.
1061
1062To do so, arch/$(SRCARCH)/Makefile sets up a number of variables and defines
1063a few targets.
1064
1065When kbuild executes, the following steps are followed (roughly):
1066
10671) Configuration of the kernel => produce .config
1068
10692) Store kernel version in include/linux/version.h
1070
10713) Updating all other prerequisites to the target prepare:
1072
1073   - Additional prerequisites are specified in arch/$(SRCARCH)/Makefile
1074
10754) Recursively descend down in all directories listed in
1076   init-* core* drivers-* net-* libs-* and build all targets.
1077
1078   - The values of the above variables are expanded in arch/$(SRCARCH)/Makefile.
1079
10805) All object files are then linked and the resulting file vmlinux is
1081   located at the root of the obj tree.
1082   The very first objects linked are listed in scripts/head-object-list.txt.
1083
10846) Finally, the architecture-specific part does any required post processing
1085   and builds the final bootimage.
1086
1087   - This includes building boot records
1088   - Preparing initrd images and the like
1089
1090Set variables to tweak the build to the architecture
1091----------------------------------------------------
1092
1093KBUILD_LDFLAGS
1094  Generic $(LD) options
1095
1096  Flags used for all invocations of the linker.
1097  Often specifying the emulation is sufficient.
1098
1099  Example::
1100
1101    #arch/s390/Makefile
1102    KBUILD_LDFLAGS         := -m elf_s390
1103
1104  Note: ldflags-y can be used to further customise
1105  the flags used. See `Non-builtin vmlinux targets - extra-y`_.
1106
1107LDFLAGS_vmlinux
1108  Options for $(LD) when linking vmlinux
1109
1110  LDFLAGS_vmlinux is used to specify additional flags to pass to
1111  the linker when linking the final vmlinux image.
1112
1113  LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
1114
1115  Example::
1116
1117    #arch/x86/Makefile
1118    LDFLAGS_vmlinux := -e stext
1119
1120OBJCOPYFLAGS
1121  objcopy flags
1122
1123  When $(call if_changed,objcopy) is used to translate a .o file,
1124  the flags specified in OBJCOPYFLAGS will be used.
1125
1126  $(call if_changed,objcopy) is often used to generate raw binaries on
1127  vmlinux.
1128
1129  Example::
1130
1131    #arch/s390/Makefile
1132    OBJCOPYFLAGS := -O binary
1133
1134    #arch/s390/boot/Makefile
1135    $(obj)/image: vmlinux FORCE
1136            $(call if_changed,objcopy)
1137
1138  In this example, the binary $(obj)/image is a binary version of
1139  vmlinux. The usage of $(call if_changed,xxx) will be described later.
1140
1141KBUILD_AFLAGS
1142  Assembler flags
1143
1144  Default value - see top level Makefile.
1145
1146  Append or modify as required per architecture.
1147
1148  Example::
1149
1150    #arch/sparc64/Makefile
1151    KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
1152
1153KBUILD_CFLAGS
1154  $(CC) compiler flags
1155
1156  Default value - see top level Makefile.
1157
1158  Append or modify as required per architecture.
1159
1160  Often, the KBUILD_CFLAGS variable depends on the configuration.
1161
1162  Example::
1163
1164    #arch/x86/boot/compressed/Makefile
1165    cflags-$(CONFIG_X86_32) := -march=i386
1166    cflags-$(CONFIG_X86_64) := -mcmodel=small
1167    KBUILD_CFLAGS += $(cflags-y)
1168
1169  Many arch Makefiles dynamically run the target C compiler to
1170  probe supported options::
1171
1172    #arch/x86/Makefile
1173
1174    ...
1175    cflags-$(CONFIG_MPENTIUMII)     += $(call cc-option,\
1176						-march=pentium2,-march=i686)
1177    ...
1178    # Disable unit-at-a-time mode ...
1179    KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
1180    ...
1181
1182
1183  The first example utilises the trick that a config option expands
1184  to "y" when selected.
1185
1186KBUILD_RUSTFLAGS
1187  $(RUSTC) compiler flags
1188
1189  Default value - see top level Makefile.
1190
1191  Append or modify as required per architecture.
1192
1193  Often, the KBUILD_RUSTFLAGS variable depends on the configuration.
1194
1195  Note that target specification file generation (for ``--target``)
1196  is handled in ``scripts/generate_rust_target.rs``.
1197
1198KBUILD_AFLAGS_KERNEL
1199  Assembler options specific for built-in
1200
1201  $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
1202  resident kernel code.
1203
1204KBUILD_AFLAGS_MODULE
1205  Assembler options specific for modules
1206
1207  $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
1208  are used for assembler.
1209
1210  From commandline AFLAGS_MODULE shall be used (see kbuild.rst).
1211
1212KBUILD_CFLAGS_KERNEL
1213  $(CC) options specific for built-in
1214
1215  $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
1216  resident kernel code.
1217
1218KBUILD_CFLAGS_MODULE
1219  Options for $(CC) when building modules
1220
1221  $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
1222  are used for $(CC).
1223
1224  From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
1225
1226KBUILD_RUSTFLAGS_KERNEL
1227  $(RUSTC) options specific for built-in
1228
1229  $(KBUILD_RUSTFLAGS_KERNEL) contains extra Rust compiler flags used to
1230  compile resident kernel code.
1231
1232KBUILD_RUSTFLAGS_MODULE
1233  Options for $(RUSTC) when building modules
1234
1235  $(KBUILD_RUSTFLAGS_MODULE) is used to add arch-specific options that
1236  are used for $(RUSTC).
1237
1238  From commandline RUSTFLAGS_MODULE shall be used (see kbuild.rst).
1239
1240KBUILD_LDFLAGS_MODULE
1241  Options for $(LD) when linking modules
1242
1243  $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
1244  used when linking modules. This is often a linker script.
1245
1246  From commandline LDFLAGS_MODULE shall be used (see kbuild.rst).
1247
1248KBUILD_LDS
1249  The linker script with full path. Assigned by the top-level Makefile.
1250
1251KBUILD_VMLINUX_OBJS
1252  All object files for vmlinux. They are linked to vmlinux in the same
1253  order as listed in KBUILD_VMLINUX_OBJS.
1254
1255  The objects listed in scripts/head-object-list.txt are exceptions;
1256  they are placed before the other objects.
1257
1258KBUILD_VMLINUX_LIBS
1259  All .a ``lib`` files for vmlinux. KBUILD_VMLINUX_OBJS and
1260  KBUILD_VMLINUX_LIBS together specify all the object files used to
1261  link vmlinux.
1262
1263Add prerequisites to archheaders
1264--------------------------------
1265
1266The archheaders: rule is used to generate header files that
1267may be installed into user space by ``make header_install``.
1268
1269It is run before ``make archprepare`` when run on the
1270architecture itself.
1271
1272Add prerequisites to archprepare
1273--------------------------------
1274
1275The archprepare: rule is used to list prerequisites that need to be
1276built before starting to descend down in the subdirectories.
1277
1278This is usually used for header files containing assembler constants.
1279
1280Example::
1281
1282  #arch/arm/Makefile
1283  archprepare: maketools
1284
1285In this example, the file target maketools will be processed
1286before descending down in the subdirectories.
1287
1288See also chapter XXX-TODO that describes how kbuild supports
1289generating offset header files.
1290
1291List directories to visit when descending
1292-----------------------------------------
1293
1294An arch Makefile cooperates with the top Makefile to define variables
1295which specify how to build the vmlinux file.  Note that there is no
1296corresponding arch-specific section for modules; the module-building
1297machinery is all architecture-independent.
1298
1299core-y, libs-y, drivers-y
1300  $(libs-y) lists directories where a lib.a archive can be located.
1301
1302  The rest list directories where a built-in.a object file can be
1303  located.
1304
1305  Then the rest follows in this order:
1306
1307    $(core-y), $(libs-y), $(drivers-y)
1308
1309  The top level Makefile defines values for all generic directories,
1310  and arch/$(SRCARCH)/Makefile only adds architecture-specific
1311  directories.
1312
1313  Example::
1314
1315    # arch/sparc/Makefile
1316    core-y                 += arch/sparc/
1317
1318    libs-y                 += arch/sparc/prom/
1319    libs-y                 += arch/sparc/lib/
1320
1321    drivers-$(CONFIG_PM) += arch/sparc/power/
1322
1323Architecture-specific boot images
1324---------------------------------
1325
1326An arch Makefile specifies goals that take the vmlinux file, compress
1327it, wrap it in bootstrapping code, and copy the resulting files
1328somewhere. This includes various kinds of installation commands.
1329The actual goals are not standardized across architectures.
1330
1331It is common to locate any additional processing in a boot/
1332directory below arch/$(SRCARCH)/.
1333
1334Kbuild does not provide any smart way to support building a
1335target specified in boot/. Therefore arch/$(SRCARCH)/Makefile shall
1336call make manually to build a target in boot/.
1337
1338The recommended approach is to include shortcuts in
1339arch/$(SRCARCH)/Makefile, and use the full path when calling down
1340into the arch/$(SRCARCH)/boot/Makefile.
1341
1342Example::
1343
1344  #arch/x86/Makefile
1345  boot := arch/x86/boot
1346  bzImage: vmlinux
1347          $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
1348
1349``$(Q)$(MAKE) $(build)=<dir>`` is the recommended way to invoke
1350make in a subdirectory.
1351
1352There are no rules for naming architecture-specific targets,
1353but executing ``make help`` will list all relevant targets.
1354To support this, $(archhelp) must be defined.
1355
1356Example::
1357
1358  #arch/x86/Makefile
1359  define archhelp
1360    echo  '* bzImage      - Compressed kernel image (arch/x86/boot/bzImage)'
1361  endif
1362
1363When make is executed without arguments, the first goal encountered
1364will be built. In the top level Makefile the first goal present
1365is all:.
1366
1367An architecture shall always, per default, build a bootable image.
1368In ``make help``, the default goal is highlighted with a ``*``.
1369
1370Add a new prerequisite to all: to select a default goal different
1371from vmlinux.
1372
1373Example::
1374
1375  #arch/x86/Makefile
1376  all: bzImage
1377
1378When ``make`` is executed without arguments, bzImage will be built.
1379
1380Commands useful for building a boot image
1381-----------------------------------------
1382
1383Kbuild provides a few macros that are useful when building a
1384boot image.
1385
1386ld
1387  Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
1388
1389  Example::
1390
1391    #arch/x86/boot/Makefile
1392    LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1393    LDFLAGS_setup    := -Ttext 0x0 -s --oformat binary -e begtext
1394
1395    targets += setup setup.o bootsect bootsect.o
1396    $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1397            $(call if_changed,ld)
1398
1399  In this example, there are two possible targets, requiring different
1400  options to the linker. The linker options are specified using the
1401  LDFLAGS_$@ syntax - one for each potential target.
1402
1403  $(targets) are assigned all potential targets, by which kbuild knows
1404  the targets and will:
1405
1406  1) check for commandline changes
1407  2) delete target during make clean
1408
1409  The ``: %: %.o`` part of the prerequisite is a shorthand that
1410  frees us from listing the setup.o and bootsect.o files.
1411
1412  Note:
1413  It is a common mistake to forget the ``targets :=`` assignment,
1414  resulting in the target file being recompiled for no
1415  obvious reason.
1416
1417objcopy
1418  Copy binary. Uses OBJCOPYFLAGS usually specified in
1419  arch/$(SRCARCH)/Makefile.
1420
1421  OBJCOPYFLAGS_$@ may be used to set additional options.
1422
1423gzip
1424  Compress target. Use maximum compression to compress target.
1425
1426  Example::
1427
1428    #arch/x86/boot/compressed/Makefile
1429    $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
1430            $(call if_changed,gzip)
1431
1432dtc
1433  Create flattened device tree blob object suitable for linking
1434  into vmlinux. Device tree blobs linked into vmlinux are placed
1435  in an init section in the image. Platform code *must* copy the
1436  blob to non-init memory prior to calling unflatten_device_tree().
1437
1438  To use this command, simply add ``*.dtb`` into obj-y or targets, or make
1439  some other target depend on ``%.dtb``
1440
1441  A central rule exists to create ``$(obj)/%.dtb`` from ``$(src)/%.dts``;
1442  architecture Makefiles do no need to explicitly write out that rule.
1443
1444  Example::
1445
1446    targets += $(dtb-y)
1447    DTC_FLAGS ?= -p 1024
1448
1449Preprocessing linker scripts
1450----------------------------
1451
1452When the vmlinux image is built, the linker script
1453arch/$(SRCARCH)/kernel/vmlinux.lds is used.
1454
1455The script is a preprocessed variant of the file vmlinux.lds.S
1456located in the same directory.
1457
1458kbuild knows .lds files and includes a rule ``*lds.S`` -> ``*lds``.
1459
1460Example::
1461
1462  #arch/x86/kernel/Makefile
1463  extra-y := vmlinux.lds
1464
1465The assignment to extra-y is used to tell kbuild to build the
1466target vmlinux.lds.
1467
1468The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
1469specified options when building the target vmlinux.lds.
1470
1471When building the ``*.lds`` target, kbuild uses the variables::
1472
1473  KBUILD_CPPFLAGS      : Set in top-level Makefile
1474  cppflags-y           : May be set in the kbuild makefile
1475  CPPFLAGS_$(@F)       : Target-specific flags.
1476                         Note that the full filename is used in this
1477                         assignment.
1478
1479The kbuild infrastructure for ``*lds`` files is used in several
1480architecture-specific files.
1481
1482Generic header files
1483--------------------
1484
1485The directory include/asm-generic contains the header files
1486that may be shared between individual architectures.
1487
1488The recommended approach how to use a generic header file is
1489to list the file in the Kbuild file.
1490
1491See `generic-y`_ for further info on syntax etc.
1492
1493Post-link pass
1494--------------
1495
1496If the file arch/xxx/Makefile.postlink exists, this makefile
1497will be invoked for post-link objects (vmlinux and modules.ko)
1498for architectures to run post-link passes on. Must also handle
1499the clean target.
1500
1501This pass runs after kallsyms generation. If the architecture
1502needs to modify symbol locations, rather than manipulate the
1503kallsyms, it may be easier to add another postlink target for
1504.tmp_vmlinux? targets to be called from link-vmlinux.sh.
1505
1506For example, powerpc uses this to check relocation sanity of
1507the linked vmlinux file.
1508
1509Kbuild syntax for exported headers
1510==================================
1511
1512The kernel includes a set of headers that is exported to userspace.
1513Many headers can be exported as-is but other headers require a
1514minimal pre-processing before they are ready for user-space.
1515
1516The pre-processing does:
1517
1518- drop kernel-specific annotations
1519- drop include of compiler.h
1520- drop all sections that are kernel internal (guarded by ``ifdef __KERNEL__``)
1521
1522All headers under include/uapi/, include/generated/uapi/,
1523arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
1524are exported.
1525
1526A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
1527arch/<arch>/include/asm/ to list asm files coming from asm-generic.
1528
1529See subsequent chapter for the syntax of the Kbuild file.
1530
1531no-export-headers
1532-----------------
1533
1534no-export-headers is essentially used by include/uapi/linux/Kbuild to
1535avoid exporting specific headers (e.g. kvm.h) on architectures that do
1536not support it. It should be avoided as much as possible.
1537
1538generic-y
1539---------
1540
1541If an architecture uses a verbatim copy of a header from
1542include/asm-generic then this is listed in the file
1543arch/$(SRCARCH)/include/asm/Kbuild like this:
1544
1545Example::
1546
1547  #arch/x86/include/asm/Kbuild
1548  generic-y += termios.h
1549  generic-y += rtc.h
1550
1551During the prepare phase of the build a wrapper include
1552file is generated in the directory::
1553
1554  arch/$(SRCARCH)/include/generated/asm
1555
1556When a header is exported where the architecture uses
1557the generic header a similar wrapper is generated as part
1558of the set of exported headers in the directory::
1559
1560  usr/include/asm
1561
1562The generated wrapper will in both cases look like the following:
1563
1564Example: termios.h::
1565
1566  #include <asm-generic/termios.h>
1567
1568generated-y
1569-----------
1570
1571If an architecture generates other header files alongside generic-y
1572wrappers, generated-y specifies them.
1573
1574This prevents them being treated as stale asm-generic wrappers and
1575removed.
1576
1577Example::
1578
1579  #arch/x86/include/asm/Kbuild
1580  generated-y += syscalls_32.h
1581
1582mandatory-y
1583-----------
1584
1585mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
1586to define the minimum set of ASM headers that all architectures must have.
1587
1588This works like optional generic-y. If a mandatory header is missing
1589in arch/$(SRCARCH)/include/(uapi/)/asm, Kbuild will automatically
1590generate a wrapper of the asm-generic one.
1591
1592Kbuild Variables
1593================
1594
1595The top Makefile exports the following variables:
1596
1597VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1598  These variables define the current kernel version.  A few arch
1599  Makefiles actually use these values directly; they should use
1600  $(KERNELRELEASE) instead.
1601
1602  $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1603  three-part version number, such as "2", "4", and "0".  These three
1604  values are always numeric.
1605
1606  $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1607  or additional patches.	It is usually some non-numeric string
1608  such as "-pre4", and is often blank.
1609
1610KERNELRELEASE
1611  $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1612  for constructing installation directory names or showing in
1613  version strings.  Some arch Makefiles use it for this purpose.
1614
1615ARCH
1616  This variable defines the target architecture, such as "i386",
1617  "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1618  determine which files to compile.
1619
1620  By default, the top Makefile sets $(ARCH) to be the same as the
1621  host system architecture.  For a cross build, a user may
1622  override the value of $(ARCH) on the command line::
1623
1624    make ARCH=m68k ...
1625
1626SRCARCH
1627  This variable specifies the directory in arch/ to build.
1628
1629  ARCH and SRCARCH may not necessarily match. A couple of arch
1630  directories are biarch, that is, a single ``arch/*/`` directory supports
1631  both 32-bit and 64-bit.
1632
1633  For example, you can pass in ARCH=i386, ARCH=x86_64, or ARCH=x86.
1634  For all of them, SRCARCH=x86 because arch/x86/ supports both i386 and
1635  x86_64.
1636
1637INSTALL_PATH
1638  This variable defines a place for the arch Makefiles to install
1639  the resident kernel image and System.map file.
1640  Use this for architecture-specific install targets.
1641
1642INSTALL_MOD_PATH, MODLIB
1643  $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1644  installation.  This variable is not defined in the Makefile but
1645  may be passed in by the user if desired.
1646
1647  $(MODLIB) specifies the directory for module installation.
1648  The top Makefile defines $(MODLIB) to
1649  $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE).  The user may
1650  override this value on the command line if desired.
1651
1652INSTALL_MOD_STRIP
1653  If this variable is specified, it will cause modules to be stripped
1654  after they are installed.  If INSTALL_MOD_STRIP is "1", then the
1655  default option --strip-debug will be used.  Otherwise, the
1656  INSTALL_MOD_STRIP value will be used as the option(s) to the strip
1657  command.
1658
1659INSTALL_DTBS_PATH
1660  This variable specifies a prefix for relocations required by build
1661  roots. It defines a place for installing the device tree blobs. Like
1662  INSTALL_MOD_PATH, it isn't defined in the Makefile, but can be passed
1663  by the user if desired. Otherwise it defaults to the kernel install
1664  path.
1665
1666Makefile language
1667=================
1668
1669The kernel Makefiles are designed to be run with GNU Make.  The Makefiles
1670use only the documented features of GNU Make, but they do use many
1671GNU extensions.
1672
1673GNU Make supports elementary list-processing functions.  The kernel
1674Makefiles use a novel style of list building and manipulation with few
1675``if`` statements.
1676
1677GNU Make has two assignment operators, ``:=`` and ``=``.  ``:=`` performs
1678immediate evaluation of the right-hand side and stores an actual string
1679into the left-hand side.  ``=`` is like a formula definition; it stores the
1680right-hand side in an unevaluated form and then evaluates this form each
1681time the left-hand side is used.
1682
1683There are some cases where ``=`` is appropriate.  Usually, though, ``:=``
1684is the right choice.
1685
1686Credits
1687=======
1688
1689- Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1690- Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1691- Updates by Sam Ravnborg <sam@ravnborg.org>
1692- Language QA by Jan Engelhardt <jengelh@gmx.de>
1693
1694TODO
1695====
1696
1697- Generating offset header files.
1698- Add more variables to chapters 7 or 9?