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 293 294$(extra-y) should only contain targets needed for vmlinux. 295 296Kbuild skips extra-y when vmlinux is apparently not a final goal. 297(e.g. ``make modules``, or building external modules) 298 299If you intend to build targets unconditionally, always-y (explained 300in the next section) is the correct syntax to use. 301 302Always built goals - always-y 303----------------------------- 304 305always-y specifies targets which are literally always built when 306Kbuild visits the Makefile. 307 308Example:: 309 310 # ./Kbuild 311 offsets-file := include/generated/asm-offsets.h 312 always-y += $(offsets-file) 313 314Compilation flags 315----------------- 316 317ccflags-y, asflags-y and ldflags-y 318 These three flags apply only to the kbuild makefile in which they 319 are assigned. They are used for all the normal cc, as and ld 320 invocations happening during a recursive build. 321 322 ccflags-y specifies options for compiling with $(CC). 323 324 Example:: 325 326 # drivers/acpi/acpica/Makefile 327 ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA 328 ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT 329 330 This variable is necessary because the top Makefile owns the 331 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the 332 entire tree. 333 334 asflags-y specifies assembler options. 335 336 Example:: 337 338 #arch/sparc/kernel/Makefile 339 asflags-y := -ansi 340 341 ldflags-y specifies options for linking with $(LD). 342 343 Example:: 344 345 #arch/cris/boot/compressed/Makefile 346 ldflags-y += -T $(src)/decompress_$(arch-y).lds 347 348subdir-ccflags-y, subdir-asflags-y 349 The two flags listed above are similar to ccflags-y and asflags-y. 350 The difference is that the subdir- variants have effect for the kbuild 351 file where they are present and all subdirectories. 352 Options specified using subdir-* are added to the commandline before 353 the options specified using the non-subdir variants. 354 355 Example:: 356 357 subdir-ccflags-y := -Werror 358 359ccflags-remove-y, asflags-remove-y 360 These flags are used to remove particular flags for the compiler, 361 assembler invocations. 362 363 Example:: 364 365 ccflags-remove-$(CONFIG_MCOUNT) += -pg 366 367CFLAGS_$@, AFLAGS_$@ 368 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current 369 kbuild makefile. 370 371 $(CFLAGS_$@) specifies per-file options for $(CC). The $@ 372 part has a literal value which specifies the file that it is for. 373 374 CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@ 375 can re-add compiler flags that were removed by ccflags-remove-y. 376 377 Example:: 378 379 # drivers/scsi/Makefile 380 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF 381 382 This line specify compilation flags for aha152x.o. 383 384 $(AFLAGS_$@) is a similar feature for source files in assembly 385 languages. 386 387 AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@ 388 can re-add assembler flags that were removed by asflags-remove-y. 389 390 Example:: 391 392 # arch/arm/kernel/Makefile 393 AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET) 394 AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312 395 AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt 396 397Dependency tracking 398------------------- 399 400Kbuild tracks dependencies on the following: 401 4021) All prerequisite files (both ``*.c`` and ``*.h``) 4032) ``CONFIG_`` options used in all prerequisite files 4043) Command-line used to compile target 405 406Thus, if you change an option to $(CC) all affected files will 407be re-compiled. 408 409Custom Rules 410------------ 411 412Custom rules are used when the kbuild infrastructure does 413not provide the required support. A typical example is 414header files generated during the build process. 415Another example are the architecture-specific Makefiles which 416need custom rules to prepare boot images etc. 417 418Custom rules are written as normal Make rules. 419Kbuild is not executing in the directory where the Makefile is 420located, so all custom rules shall use a relative 421path to prerequisite files and target files. 422 423Two variables are used when defining custom rules: 424 425$(src) 426 $(src) is the directory where the Makefile is located. Always use $(src) when 427 referring to files located in the src tree. 428 429$(obj) 430 $(obj) is the directory where the target is saved. Always use $(obj) when 431 referring to generated files. Use $(obj) for pattern rules that need to work 432 for both generated files and real sources (VPATH will help to find the 433 prerequisites not only in the object tree but also in the source tree). 434 435 Example:: 436 437 #drivers/scsi/Makefile 438 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl 439 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl 440 441 This is a custom rule, following the normal syntax 442 required by make. 443 444 The target file depends on two prerequisite files. References 445 to the target file are prefixed with $(obj), references 446 to prerequisites are referenced with $(src) (because they are not 447 generated files). 448 449$(srcroot) 450 $(srcroot) refers to the root of the source you are building, which can be 451 either the kernel source or the external modules source, depending on whether 452 KBUILD_EXTMOD is set. This can be either a relative or an absolute path, but 453 if KBUILD_ABS_SRCTREE=1 is set, it is always an absolute path. 454 455$(srctree) 456 $(srctree) refers to the root of the kernel source tree. When building the 457 kernel, this is the same as $(srcroot). 458 459$(objtree) 460 $(objtree) refers to the root of the kernel object tree. It is ``.`` when 461 building the kernel, but it is different when building external modules. 462 463$(kecho) 464 echoing information to user in a rule is often a good practice 465 but when execution ``make -s`` one does not expect to see any output 466 except for warnings/errors. 467 To support this kbuild defines $(kecho) which will echo out the 468 text following $(kecho) to stdout except if ``make -s`` is used. 469 470 Example:: 471 472 # arch/arm/Makefile 473 $(BOOT_TARGETS): vmlinux 474 $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@ 475 @$(kecho) ' Kernel: $(boot)/$@ is ready' 476 477 When kbuild is executing with KBUILD_VERBOSE unset, then only a shorthand 478 of a command is normally displayed. 479 To enable this behaviour for custom commands kbuild requires 480 two variables to be set:: 481 482 quiet_cmd_<command> - what shall be echoed 483 cmd_<command> - the command to execute 484 485 Example:: 486 487 # lib/Makefile 488 quiet_cmd_crc32 = GEN $@ 489 cmd_crc32 = $< > $@ 490 491 $(obj)/crc32table.h: $(obj)/gen_crc32table 492 $(call cmd,crc32) 493 494 When updating the $(obj)/crc32table.h target, the line:: 495 496 GEN lib/crc32table.h 497 498 will be displayed with ``make KBUILD_VERBOSE=``. 499 500Command change detection 501------------------------ 502 503When the rule is evaluated, timestamps are compared between the target 504and its prerequisite files. GNU Make updates the target when any of the 505prerequisites is newer than that. 506 507The target should be rebuilt also when the command line has changed 508since the last invocation. This is not supported by Make itself, so 509Kbuild achieves this by a kind of meta-programming. 510 511if_changed is the macro used for this purpose, in the following form:: 512 513 quiet_cmd_<command> = ... 514 cmd_<command> = ... 515 516 <target>: <source(s)> FORCE 517 $(call if_changed,<command>) 518 519Any target that utilizes if_changed must be listed in $(targets), 520otherwise the command line check will fail, and the target will 521always be built. 522 523If the target is already listed in the recognized syntax such as 524obj-y/m, lib-y/m, extra-y/m, always-y/m, hostprogs, userprogs, Kbuild 525automatically adds it to $(targets). Otherwise, the target must be 526explicitly added to $(targets). 527 528Assignments to $(targets) are without $(obj)/ prefix. if_changed may be 529used in conjunction with custom rules as defined in `Custom Rules`_. 530 531Note: It is a typical mistake to forget the FORCE prerequisite. 532Another common pitfall is that whitespace is sometimes significant; for 533instance, the below will fail (note the extra space after the comma):: 534 535 target: source(s) FORCE 536 537**WRONG!** $(call if_changed, objcopy) 538 539Note: 540 if_changed should not be used more than once per target. 541 It stores the executed command in a corresponding .cmd 542 file and multiple calls would result in overwrites and 543 unwanted results when the target is up to date and only the 544 tests on changed commands trigger execution of commands. 545 546$(CC) support functions 547----------------------- 548 549The kernel may be built with several different versions of 550$(CC), each supporting a unique set of features and options. 551kbuild provides basic support to check for valid options for $(CC). 552$(CC) is usually the gcc compiler, but other alternatives are 553available. 554 555as-option 556 as-option is used to check if $(CC) -- when used to compile 557 assembler (``*.S``) files -- supports the given option. An optional 558 second option may be specified if the first option is not supported. 559 560 Example:: 561 562 #arch/sh/Makefile 563 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) 564 565 In the above example, cflags-y will be assigned the option 566 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). 567 The second argument is optional, and if supplied will be used 568 if first argument is not supported. 569 570as-instr 571 as-instr checks if the assembler reports a specific instruction 572 and then outputs either option1 or option2 573 C escapes are supported in the test instruction 574 Note: as-instr-option uses KBUILD_AFLAGS for assembler options 575 576cc-option 577 cc-option is used to check if $(CC) supports a given option, and if 578 not supported to use an optional second option. 579 580 Example:: 581 582 #arch/x86/Makefile 583 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) 584 585 In the above example, cflags-y will be assigned the option 586 -march=pentium-mmx if supported by $(CC), otherwise -march=i586. 587 The second argument to cc-option is optional, and if omitted, 588 cflags-y will be assigned no value if first option is not supported. 589 Note: cc-option uses KBUILD_CFLAGS for $(CC) options 590 591cc-option-yn 592 cc-option-yn is used to check if $(CC) supports a given option 593 and return "y" if supported, otherwise "n". 594 595 Example:: 596 597 #arch/ppc/Makefile 598 biarch := $(call cc-option-yn, -m32) 599 aflags-$(biarch) += -a32 600 cflags-$(biarch) += -m32 601 602 In the above example, $(biarch) is set to y if $(CC) supports the -m32 603 option. When $(biarch) equals "y", the expanded variables $(aflags-y) 604 and $(cflags-y) will be assigned the values -a32 and -m32, 605 respectively. 606 607 Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options 608 609cc-disable-warning 610 cc-disable-warning checks if $(CC) supports a given warning and returns 611 the commandline switch to disable it. This special function is needed, 612 because gcc 4.4 and later accept any unknown -Wno-* option and only 613 warn about it if there is another warning in the source file. 614 615 Example:: 616 617 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable) 618 619 In the above example, -Wno-unused-but-set-variable will be added to 620 KBUILD_CFLAGS only if $(CC) really accepts it. 621 622gcc-min-version 623 gcc-min-version tests if the value of $(CONFIG_GCC_VERSION) is greater than 624 or equal to the provided value and evaluates to y if so. 625 626 Example:: 627 628 cflags-$(call gcc-min-version, 70100) := -foo 629 630 In this example, cflags-y will be assigned the value -foo if $(CC) is gcc and 631 $(CONFIG_GCC_VERSION) is >= 7.1. 632 633clang-min-version 634 clang-min-version tests if the value of $(CONFIG_CLANG_VERSION) is greater 635 than or equal to the provided value and evaluates to y if so. 636 637 Example:: 638 639 cflags-$(call clang-min-version, 110000) := -foo 640 641 In this example, cflags-y will be assigned the value -foo if $(CC) is clang 642 and $(CONFIG_CLANG_VERSION) is >= 11.0.0. 643 644cc-cross-prefix 645 cc-cross-prefix is used to check if there exists a $(CC) in path with 646 one of the listed prefixes. The first prefix where there exist a 647 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found 648 then nothing is returned. 649 650 Additional prefixes are separated by a single space in the 651 call of cc-cross-prefix. 652 653 This functionality is useful for architecture Makefiles that try 654 to set CROSS_COMPILE to well-known values but may have several 655 values to select between. 656 657 It is recommended only to try to set CROSS_COMPILE if it is a cross 658 build (host arch is different from target arch). And if CROSS_COMPILE 659 is already set then leave it with the old value. 660 661 Example:: 662 663 #arch/m68k/Makefile 664 ifneq ($(SUBARCH),$(ARCH)) 665 ifeq ($(CROSS_COMPILE),) 666 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) 667 endif 668 endif 669 670$(LD) support functions 671----------------------- 672 673ld-option 674 ld-option is used to check if $(LD) supports the supplied option. 675 ld-option takes two options as arguments. 676 677 The second argument is an optional option that can be used if the 678 first option is not supported by $(LD). 679 680 Example:: 681 682 #Makefile 683 LDFLAGS_vmlinux += $(call ld-option, -X) 684 685Script invocation 686----------------- 687 688Make rules may invoke scripts to build the kernel. The rules shall 689always provide the appropriate interpreter to execute the script. They 690shall not rely on the execute bits being set, and shall not invoke the 691script directly. For the convenience of manual script invocation, such 692as invoking ./scripts/checkpatch.pl, it is recommended to set execute 693bits on the scripts nonetheless. 694 695Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL), 696and $(PYTHON3) to refer to interpreters for the respective 697scripts. 698 699Example:: 700 701 #Makefile 702 cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \ 703 $(KERNELRELEASE) 704 705Host Program support 706==================== 707 708Kbuild supports building executables on the host for use during the 709compilation stage. 710 711Two steps are required in order to use a host executable. 712 713The first step is to tell kbuild that a host program exists. This is 714done utilising the variable ``hostprogs``. 715 716The second step is to add an explicit dependency to the executable. 717This can be done in two ways. Either add the dependency in a rule, 718or utilise the variable ``always-y``. 719Both possibilities are described in the following. 720 721Simple Host Program 722------------------- 723 724In some cases there is a need to compile and run a program on the 725computer where the build is running. 726 727The following line tells kbuild that the program bin2hex shall be 728built on the build host. 729 730Example:: 731 732 hostprogs := bin2hex 733 734Kbuild assumes in the above example that bin2hex is made from a single 735c-source file named bin2hex.c located in the same directory as 736the Makefile. 737 738Composite Host Programs 739----------------------- 740 741Host programs can be made up based on composite objects. 742The syntax used to define composite objects for host programs is 743similar to the syntax used for kernel objects. 744$(<executable>-objs) lists all objects used to link the final 745executable. 746 747Example:: 748 749 #scripts/lxdialog/Makefile 750 hostprogs := lxdialog 751 lxdialog-objs := checklist.o lxdialog.o 752 753Objects with extension .o are compiled from the corresponding .c 754files. In the above example, checklist.c is compiled to checklist.o 755and lxdialog.c is compiled to lxdialog.o. 756 757Finally, the two .o files are linked to the executable, lxdialog. 758Note: The syntax <executable>-y is not permitted for host-programs. 759 760Using C++ for host programs 761--------------------------- 762 763kbuild offers support for host programs written in C++. This was 764introduced solely to support kconfig, and is not recommended 765for general use. 766 767Example:: 768 769 #scripts/kconfig/Makefile 770 hostprogs := qconf 771 qconf-cxxobjs := qconf.o 772 773In the example above the executable is composed of the C++ file 774qconf.cc - identified by $(qconf-cxxobjs). 775 776If qconf is composed of a mixture of .c and .cc files, then an 777additional line can be used to identify this. 778 779Example:: 780 781 #scripts/kconfig/Makefile 782 hostprogs := qconf 783 qconf-cxxobjs := qconf.o 784 qconf-objs := check.o 785 786Using Rust for host programs 787---------------------------- 788 789Kbuild offers support for host programs written in Rust. However, 790since a Rust toolchain is not mandatory for kernel compilation, 791it may only be used in scenarios where Rust is required to be 792available (e.g. when ``CONFIG_RUST`` is enabled). 793 794Example:: 795 796 hostprogs := target 797 target-rust := y 798 799Kbuild will compile ``target`` using ``target.rs`` as the crate root, 800located in the same directory as the ``Makefile``. The crate may 801consist of several source files (see ``samples/rust/hostprogs``). 802 803Controlling compiler options for host programs 804---------------------------------------------- 805 806When compiling host programs, it is possible to set specific flags. 807The programs will always be compiled utilising $(HOSTCC) passed 808the options specified in $(KBUILD_HOSTCFLAGS). 809 810To set flags that will take effect for all host programs created 811in that Makefile, use the variable HOST_EXTRACFLAGS. 812 813Example:: 814 815 #scripts/lxdialog/Makefile 816 HOST_EXTRACFLAGS += -I/usr/include/ncurses 817 818To set specific flags for a single file the following construction 819is used: 820 821Example:: 822 823 #arch/ppc64/boot/Makefile 824 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) 825 826It is also possible to specify additional options to the linker. 827 828Example:: 829 830 #scripts/kconfig/Makefile 831 HOSTLDLIBS_qconf := -L$(QTDIR)/lib 832 833When linking qconf, it will be passed the extra option 834``-L$(QTDIR)/lib``. 835 836When host programs are actually built 837------------------------------------- 838 839Kbuild will only build host-programs when they are referenced 840as a prerequisite. 841 842This is possible in two ways: 843 844(1) List the prerequisite explicitly in a custom rule. 845 846 Example:: 847 848 #drivers/pci/Makefile 849 hostprogs := gen-devlist 850 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist 851 ( cd $(obj); ./gen-devlist ) < $< 852 853 The target $(obj)/devlist.h will not be built before 854 $(obj)/gen-devlist is updated. Note that references to 855 the host programs in custom rules must be prefixed with $(obj). 856 857(2) Use always-y 858 859 When there is no suitable custom rule, and the host program 860 shall be built when a makefile is entered, the always-y 861 variable shall be used. 862 863 Example:: 864 865 #scripts/lxdialog/Makefile 866 hostprogs := lxdialog 867 always-y := $(hostprogs) 868 869 Kbuild provides the following shorthand for this:: 870 871 hostprogs-always-y := lxdialog 872 873 This will tell kbuild to build lxdialog even if not referenced in 874 any rule. 875 876Userspace Program support 877========================= 878 879Just like host programs, Kbuild also supports building userspace executables 880for the target architecture (i.e. the same architecture as you are building 881the kernel for). 882 883The syntax is quite similar. The difference is to use ``userprogs`` instead of 884``hostprogs``. 885 886Simple Userspace Program 887------------------------ 888 889The following line tells kbuild that the program bpf-direct shall be 890built for the target architecture. 891 892Example:: 893 894 userprogs := bpf-direct 895 896Kbuild assumes in the above example that bpf-direct is made from a 897single C source file named bpf-direct.c located in the same directory 898as the Makefile. 899 900Composite Userspace Programs 901---------------------------- 902 903Userspace programs can be made up based on composite objects. 904The syntax used to define composite objects for userspace programs is 905similar to the syntax used for kernel objects. 906$(<executable>-objs) lists all objects used to link the final 907executable. 908 909Example:: 910 911 #samples/seccomp/Makefile 912 userprogs := bpf-fancy 913 bpf-fancy-objs := bpf-fancy.o bpf-helper.o 914 915Objects with extension .o are compiled from the corresponding .c 916files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o 917and bpf-helper.c is compiled to bpf-helper.o. 918 919Finally, the two .o files are linked to the executable, bpf-fancy. 920Note: The syntax <executable>-y is not permitted for userspace programs. 921 922Controlling compiler options for userspace programs 923--------------------------------------------------- 924 925When compiling userspace programs, it is possible to set specific flags. 926The programs will always be compiled utilising $(CC) passed 927the options specified in $(KBUILD_USERCFLAGS). 928 929To set flags that will take effect for all userspace programs created 930in that Makefile, use the variable userccflags. 931 932Example:: 933 934 # samples/seccomp/Makefile 935 userccflags += -I usr/include 936 937To set specific flags for a single file the following construction 938is used: 939 940Example:: 941 942 bpf-helper-userccflags += -I user/include 943 944It is also possible to specify additional options to the linker. 945 946Example:: 947 948 # net/bpfilter/Makefile 949 bpfilter_umh-userldflags += -static 950 951To specify libraries linked to a userspace program, you can use 952``<executable>-userldlibs``. The ``userldlibs`` syntax specifies libraries 953linked to all userspace programs created in the current Makefile. 954 955When linking bpfilter_umh, it will be passed the extra option -static. 956 957From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used. 958 959When userspace programs are actually built 960------------------------------------------ 961 962Kbuild builds userspace programs only when told to do so. 963There are two ways to do this. 964 965(1) Add it as the prerequisite of another file 966 967 Example:: 968 969 #net/bpfilter/Makefile 970 userprogs := bpfilter_umh 971 $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh 972 973 $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o 974 975(2) Use always-y 976 977 Example:: 978 979 userprogs := binderfs_example 980 always-y := $(userprogs) 981 982 Kbuild provides the following shorthand for this:: 983 984 userprogs-always-y := binderfs_example 985 986 This will tell Kbuild to build binderfs_example when it visits this 987 Makefile. 988 989Kbuild clean infrastructure 990=========================== 991 992``make clean`` deletes most generated files in the obj tree where the kernel 993is compiled. This includes generated files such as host programs. 994Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m), 995$(always-), $(extra-y), $(extra-) and $(targets). They are all deleted 996during ``make clean``. Files matching the patterns ``*.[oas]``, ``*.ko``, plus 997some additional files generated by kbuild are deleted all over the kernel 998source tree when ``make clean`` is executed. 999 1000Additional files or directories can be specified in kbuild makefiles by use of 1001$(clean-files). 1002 1003Example:: 1004 1005 #lib/Makefile 1006 clean-files := crc32table.h 1007 1008When executing ``make clean``, the file ``crc32table.h`` will be deleted. 1009Kbuild will assume files to be in the same relative directory as the 1010Makefile. 1011 1012To exclude certain files or directories from make clean, use the 1013$(no-clean-files) variable. 1014 1015Usually kbuild descends down in subdirectories due to ``obj-* := dir/``, 1016but in the architecture makefiles where the kbuild infrastructure 1017is not sufficient this sometimes needs to be explicit. 1018 1019Example:: 1020 1021 #arch/x86/boot/Makefile 1022 subdir- := compressed 1023 1024The above assignment instructs kbuild to descend down in the 1025directory compressed/ when ``make clean`` is executed. 1026 1027Note 1: arch/$(SRCARCH)/Makefile cannot use ``subdir-``, because that file is 1028included in the top level makefile. Instead, arch/$(SRCARCH)/Kbuild can use 1029``subdir-``. 1030 1031Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will 1032be visited during ``make clean``. 1033 1034Architecture Makefiles 1035====================== 1036 1037The top level Makefile sets up the environment and does the preparation, 1038before starting to descend down in the individual directories. 1039 1040The top level makefile contains the generic part, whereas 1041arch/$(SRCARCH)/Makefile contains what is required to set up kbuild 1042for said architecture. 1043 1044To do so, arch/$(SRCARCH)/Makefile sets up a number of variables and defines 1045a few targets. 1046 1047When kbuild executes, the following steps are followed (roughly): 1048 10491) Configuration of the kernel => produce .config 1050 10512) Store kernel version in include/linux/version.h 1052 10533) Updating all other prerequisites to the target prepare: 1054 1055 - Additional prerequisites are specified in arch/$(SRCARCH)/Makefile 1056 10574) Recursively descend down in all directories listed in 1058 init-* core* drivers-* net-* libs-* and build all targets. 1059 1060 - The values of the above variables are expanded in arch/$(SRCARCH)/Makefile. 1061 10625) All object files are then linked and the resulting file vmlinux is 1063 located at the root of the obj tree. 1064 The very first objects linked are listed in scripts/head-object-list.txt. 1065 10666) Finally, the architecture-specific part does any required post processing 1067 and builds the final bootimage. 1068 1069 - This includes building boot records 1070 - Preparing initrd images and the like 1071 1072Set variables to tweak the build to the architecture 1073---------------------------------------------------- 1074 1075KBUILD_LDFLAGS 1076 Generic $(LD) options 1077 1078 Flags used for all invocations of the linker. 1079 Often specifying the emulation is sufficient. 1080 1081 Example:: 1082 1083 #arch/s390/Makefile 1084 KBUILD_LDFLAGS := -m elf_s390 1085 1086 Note: ldflags-y can be used to further customise 1087 the flags used. See `Non-builtin vmlinux targets - extra-y`_. 1088 1089LDFLAGS_vmlinux 1090 Options for $(LD) when linking vmlinux 1091 1092 LDFLAGS_vmlinux is used to specify additional flags to pass to 1093 the linker when linking the final vmlinux image. 1094 1095 LDFLAGS_vmlinux uses the LDFLAGS_$@ support. 1096 1097 Example:: 1098 1099 #arch/x86/Makefile 1100 LDFLAGS_vmlinux := -e stext 1101 1102OBJCOPYFLAGS 1103 objcopy flags 1104 1105 When $(call if_changed,objcopy) is used to translate a .o file, 1106 the flags specified in OBJCOPYFLAGS will be used. 1107 1108 $(call if_changed,objcopy) is often used to generate raw binaries on 1109 vmlinux. 1110 1111 Example:: 1112 1113 #arch/s390/Makefile 1114 OBJCOPYFLAGS := -O binary 1115 1116 #arch/s390/boot/Makefile 1117 $(obj)/image: vmlinux FORCE 1118 $(call if_changed,objcopy) 1119 1120 In this example, the binary $(obj)/image is a binary version of 1121 vmlinux. The usage of $(call if_changed,xxx) will be described later. 1122 1123KBUILD_AFLAGS 1124 Assembler flags 1125 1126 Default value - see top level Makefile. 1127 1128 Append or modify as required per architecture. 1129 1130 Example:: 1131 1132 #arch/sparc64/Makefile 1133 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc 1134 1135KBUILD_CFLAGS 1136 $(CC) compiler flags 1137 1138 Default value - see top level Makefile. 1139 1140 Append or modify as required per architecture. 1141 1142 Often, the KBUILD_CFLAGS variable depends on the configuration. 1143 1144 Example:: 1145 1146 #arch/x86/boot/compressed/Makefile 1147 cflags-$(CONFIG_X86_32) := -march=i386 1148 cflags-$(CONFIG_X86_64) := -mcmodel=small 1149 KBUILD_CFLAGS += $(cflags-y) 1150 1151 Many arch Makefiles dynamically run the target C compiler to 1152 probe supported options:: 1153 1154 #arch/x86/Makefile 1155 1156 ... 1157 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ 1158 -march=pentium2,-march=i686) 1159 ... 1160 # Disable unit-at-a-time mode ... 1161 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) 1162 ... 1163 1164 1165 The first example utilises the trick that a config option expands 1166 to "y" when selected. 1167 1168KBUILD_RUSTFLAGS 1169 $(RUSTC) compiler flags 1170 1171 Default value - see top level Makefile. 1172 1173 Append or modify as required per architecture. 1174 1175 Often, the KBUILD_RUSTFLAGS variable depends on the configuration. 1176 1177 Note that target specification file generation (for ``--target``) 1178 is handled in ``scripts/generate_rust_target.rs``. 1179 1180KBUILD_AFLAGS_KERNEL 1181 Assembler options specific for built-in 1182 1183 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile 1184 resident kernel code. 1185 1186KBUILD_AFLAGS_MODULE 1187 Assembler options specific for modules 1188 1189 $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that 1190 are used for assembler. 1191 1192 From commandline AFLAGS_MODULE shall be used (see kbuild.rst). 1193 1194KBUILD_CFLAGS_KERNEL 1195 $(CC) options specific for built-in 1196 1197 $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile 1198 resident kernel code. 1199 1200KBUILD_CFLAGS_MODULE 1201 Options for $(CC) when building modules 1202 1203 $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that 1204 are used for $(CC). 1205 1206 From commandline CFLAGS_MODULE shall be used (see kbuild.rst). 1207 1208KBUILD_RUSTFLAGS_KERNEL 1209 $(RUSTC) options specific for built-in 1210 1211 $(KBUILD_RUSTFLAGS_KERNEL) contains extra Rust compiler flags used to 1212 compile resident kernel code. 1213 1214KBUILD_RUSTFLAGS_MODULE 1215 Options for $(RUSTC) when building modules 1216 1217 $(KBUILD_RUSTFLAGS_MODULE) is used to add arch-specific options that 1218 are used for $(RUSTC). 1219 1220 From commandline RUSTFLAGS_MODULE shall be used (see kbuild.rst). 1221 1222KBUILD_LDFLAGS_MODULE 1223 Options for $(LD) when linking modules 1224 1225 $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options 1226 used when linking modules. This is often a linker script. 1227 1228 From commandline LDFLAGS_MODULE shall be used (see kbuild.rst). 1229 1230KBUILD_LDS 1231 The linker script with full path. Assigned by the top-level Makefile. 1232 1233KBUILD_VMLINUX_OBJS 1234 All object files for vmlinux. They are linked to vmlinux in the same 1235 order as listed in KBUILD_VMLINUX_OBJS. 1236 1237 The objects listed in scripts/head-object-list.txt are exceptions; 1238 they are placed before the other objects. 1239 1240KBUILD_VMLINUX_LIBS 1241 All .a ``lib`` files for vmlinux. KBUILD_VMLINUX_OBJS and 1242 KBUILD_VMLINUX_LIBS together specify all the object files used to 1243 link vmlinux. 1244 1245Add prerequisites to archheaders 1246-------------------------------- 1247 1248The archheaders: rule is used to generate header files that 1249may be installed into user space by ``make header_install``. 1250 1251It is run before ``make archprepare`` when run on the 1252architecture itself. 1253 1254Add prerequisites to archprepare 1255-------------------------------- 1256 1257The archprepare: rule is used to list prerequisites that need to be 1258built before starting to descend down in the subdirectories. 1259 1260This is usually used for header files containing assembler constants. 1261 1262Example:: 1263 1264 #arch/arm/Makefile 1265 archprepare: maketools 1266 1267In this example, the file target maketools will be processed 1268before descending down in the subdirectories. 1269 1270See also chapter XXX-TODO that describes how kbuild supports 1271generating offset header files. 1272 1273List directories to visit when descending 1274----------------------------------------- 1275 1276An arch Makefile cooperates with the top Makefile to define variables 1277which specify how to build the vmlinux file. Note that there is no 1278corresponding arch-specific section for modules; the module-building 1279machinery is all architecture-independent. 1280 1281core-y, libs-y, drivers-y 1282 $(libs-y) lists directories where a lib.a archive can be located. 1283 1284 The rest list directories where a built-in.a object file can be 1285 located. 1286 1287 Then the rest follows in this order: 1288 1289 $(core-y), $(libs-y), $(drivers-y) 1290 1291 The top level Makefile defines values for all generic directories, 1292 and arch/$(SRCARCH)/Makefile only adds architecture-specific 1293 directories. 1294 1295 Example:: 1296 1297 # arch/sparc/Makefile 1298 core-y += arch/sparc/ 1299 1300 libs-y += arch/sparc/prom/ 1301 libs-y += arch/sparc/lib/ 1302 1303 drivers-$(CONFIG_PM) += arch/sparc/power/ 1304 1305Architecture-specific boot images 1306--------------------------------- 1307 1308An arch Makefile specifies goals that take the vmlinux file, compress 1309it, wrap it in bootstrapping code, and copy the resulting files 1310somewhere. This includes various kinds of installation commands. 1311The actual goals are not standardized across architectures. 1312 1313It is common to locate any additional processing in a boot/ 1314directory below arch/$(SRCARCH)/. 1315 1316Kbuild does not provide any smart way to support building a 1317target specified in boot/. Therefore arch/$(SRCARCH)/Makefile shall 1318call make manually to build a target in boot/. 1319 1320The recommended approach is to include shortcuts in 1321arch/$(SRCARCH)/Makefile, and use the full path when calling down 1322into the arch/$(SRCARCH)/boot/Makefile. 1323 1324Example:: 1325 1326 #arch/x86/Makefile 1327 boot := arch/x86/boot 1328 bzImage: vmlinux 1329 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ 1330 1331``$(Q)$(MAKE) $(build)=<dir>`` is the recommended way to invoke 1332make in a subdirectory. 1333 1334There are no rules for naming architecture-specific targets, 1335but executing ``make help`` will list all relevant targets. 1336To support this, $(archhelp) must be defined. 1337 1338Example:: 1339 1340 #arch/x86/Makefile 1341 define archhelp 1342 echo '* bzImage - Compressed kernel image (arch/x86/boot/bzImage)' 1343 endif 1344 1345When make is executed without arguments, the first goal encountered 1346will be built. In the top level Makefile the first goal present 1347is all:. 1348 1349An architecture shall always, per default, build a bootable image. 1350In ``make help``, the default goal is highlighted with a ``*``. 1351 1352Add a new prerequisite to all: to select a default goal different 1353from vmlinux. 1354 1355Example:: 1356 1357 #arch/x86/Makefile 1358 all: bzImage 1359 1360When ``make`` is executed without arguments, bzImage will be built. 1361 1362Commands useful for building a boot image 1363----------------------------------------- 1364 1365Kbuild provides a few macros that are useful when building a 1366boot image. 1367 1368ld 1369 Link target. Often, LDFLAGS_$@ is used to set specific options to ld. 1370 1371 Example:: 1372 1373 #arch/x86/boot/Makefile 1374 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary 1375 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext 1376 1377 targets += setup setup.o bootsect bootsect.o 1378 $(obj)/setup $(obj)/bootsect: %: %.o FORCE 1379 $(call if_changed,ld) 1380 1381 In this example, there are two possible targets, requiring different 1382 options to the linker. The linker options are specified using the 1383 LDFLAGS_$@ syntax - one for each potential target. 1384 1385 $(targets) are assigned all potential targets, by which kbuild knows 1386 the targets and will: 1387 1388 1) check for commandline changes 1389 2) delete target during make clean 1390 1391 The ``: %: %.o`` part of the prerequisite is a shorthand that 1392 frees us from listing the setup.o and bootsect.o files. 1393 1394 Note: 1395 It is a common mistake to forget the ``targets :=`` assignment, 1396 resulting in the target file being recompiled for no 1397 obvious reason. 1398 1399objcopy 1400 Copy binary. Uses OBJCOPYFLAGS usually specified in 1401 arch/$(SRCARCH)/Makefile. 1402 1403 OBJCOPYFLAGS_$@ may be used to set additional options. 1404 1405gzip 1406 Compress target. Use maximum compression to compress target. 1407 1408 Example:: 1409 1410 #arch/x86/boot/compressed/Makefile 1411 $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE 1412 $(call if_changed,gzip) 1413 1414dtc 1415 Create flattened device tree blob object suitable for linking 1416 into vmlinux. Device tree blobs linked into vmlinux are placed 1417 in an init section in the image. Platform code *must* copy the 1418 blob to non-init memory prior to calling unflatten_device_tree(). 1419 1420 To use this command, simply add ``*.dtb`` into obj-y or targets, or make 1421 some other target depend on ``%.dtb`` 1422 1423 A central rule exists to create ``$(obj)/%.dtb`` from ``$(src)/%.dts``; 1424 architecture Makefiles do no need to explicitly write out that rule. 1425 1426 Example:: 1427 1428 targets += $(dtb-y) 1429 DTC_FLAGS ?= -p 1024 1430 1431Preprocessing linker scripts 1432---------------------------- 1433 1434When the vmlinux image is built, the linker script 1435arch/$(SRCARCH)/kernel/vmlinux.lds is used. 1436 1437The script is a preprocessed variant of the file vmlinux.lds.S 1438located in the same directory. 1439 1440kbuild knows .lds files and includes a rule ``*lds.S`` -> ``*lds``. 1441 1442Example:: 1443 1444 #arch/x86/kernel/Makefile 1445 extra-y := vmlinux.lds 1446 1447The assignment to extra-y is used to tell kbuild to build the 1448target vmlinux.lds. 1449 1450The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the 1451specified options when building the target vmlinux.lds. 1452 1453When building the ``*.lds`` target, kbuild uses the variables:: 1454 1455 KBUILD_CPPFLAGS : Set in top-level Makefile 1456 cppflags-y : May be set in the kbuild makefile 1457 CPPFLAGS_$(@F) : Target-specific flags. 1458 Note that the full filename is used in this 1459 assignment. 1460 1461The kbuild infrastructure for ``*lds`` files is used in several 1462architecture-specific files. 1463 1464Generic header files 1465-------------------- 1466 1467The directory include/asm-generic contains the header files 1468that may be shared between individual architectures. 1469 1470The recommended approach how to use a generic header file is 1471to list the file in the Kbuild file. 1472 1473See `generic-y`_ for further info on syntax etc. 1474 1475Post-link pass 1476-------------- 1477 1478If the file arch/xxx/Makefile.postlink exists, this makefile 1479will be invoked for post-link objects (vmlinux and modules.ko) 1480for architectures to run post-link passes on. Must also handle 1481the clean target. 1482 1483This pass runs after kallsyms generation. If the architecture 1484needs to modify symbol locations, rather than manipulate the 1485kallsyms, it may be easier to add another postlink target for 1486.tmp_vmlinux? targets to be called from link-vmlinux.sh. 1487 1488For example, powerpc uses this to check relocation sanity of 1489the linked vmlinux file. 1490 1491Kbuild syntax for exported headers 1492================================== 1493 1494The kernel includes a set of headers that is exported to userspace. 1495Many headers can be exported as-is but other headers require a 1496minimal pre-processing before they are ready for user-space. 1497 1498The pre-processing does: 1499 1500- drop kernel-specific annotations 1501- drop include of compiler.h 1502- drop all sections that are kernel internal (guarded by ``ifdef __KERNEL__``) 1503 1504All headers under include/uapi/, include/generated/uapi/, 1505arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/ 1506are exported. 1507 1508A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and 1509arch/<arch>/include/asm/ to list asm files coming from asm-generic. 1510 1511See subsequent chapter for the syntax of the Kbuild file. 1512 1513no-export-headers 1514----------------- 1515 1516no-export-headers is essentially used by include/uapi/linux/Kbuild to 1517avoid exporting specific headers (e.g. kvm.h) on architectures that do 1518not support it. It should be avoided as much as possible. 1519 1520generic-y 1521--------- 1522 1523If an architecture uses a verbatim copy of a header from 1524include/asm-generic then this is listed in the file 1525arch/$(SRCARCH)/include/asm/Kbuild like this: 1526 1527Example:: 1528 1529 #arch/x86/include/asm/Kbuild 1530 generic-y += termios.h 1531 generic-y += rtc.h 1532 1533During the prepare phase of the build a wrapper include 1534file is generated in the directory:: 1535 1536 arch/$(SRCARCH)/include/generated/asm 1537 1538When a header is exported where the architecture uses 1539the generic header a similar wrapper is generated as part 1540of the set of exported headers in the directory:: 1541 1542 usr/include/asm 1543 1544The generated wrapper will in both cases look like the following: 1545 1546Example: termios.h:: 1547 1548 #include <asm-generic/termios.h> 1549 1550generated-y 1551----------- 1552 1553If an architecture generates other header files alongside generic-y 1554wrappers, generated-y specifies them. 1555 1556This prevents them being treated as stale asm-generic wrappers and 1557removed. 1558 1559Example:: 1560 1561 #arch/x86/include/asm/Kbuild 1562 generated-y += syscalls_32.h 1563 1564mandatory-y 1565----------- 1566 1567mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild 1568to define the minimum set of ASM headers that all architectures must have. 1569 1570This works like optional generic-y. If a mandatory header is missing 1571in arch/$(SRCARCH)/include/(uapi/)/asm, Kbuild will automatically 1572generate a wrapper of the asm-generic one. 1573 1574Kbuild Variables 1575================ 1576 1577The top Makefile exports the following variables: 1578 1579VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION 1580 These variables define the current kernel version. A few arch 1581 Makefiles actually use these values directly; they should use 1582 $(KERNELRELEASE) instead. 1583 1584 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic 1585 three-part version number, such as "2", "4", and "0". These three 1586 values are always numeric. 1587 1588 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches 1589 or additional patches. It is usually some non-numeric string 1590 such as "-pre4", and is often blank. 1591 1592KERNELRELEASE 1593 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable 1594 for constructing installation directory names or showing in 1595 version strings. Some arch Makefiles use it for this purpose. 1596 1597ARCH 1598 This variable defines the target architecture, such as "i386", 1599 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to 1600 determine which files to compile. 1601 1602 By default, the top Makefile sets $(ARCH) to be the same as the 1603 host system architecture. For a cross build, a user may 1604 override the value of $(ARCH) on the command line:: 1605 1606 make ARCH=m68k ... 1607 1608SRCARCH 1609 This variable specifies the directory in arch/ to build. 1610 1611 ARCH and SRCARCH may not necessarily match. A couple of arch 1612 directories are biarch, that is, a single ``arch/*/`` directory supports 1613 both 32-bit and 64-bit. 1614 1615 For example, you can pass in ARCH=i386, ARCH=x86_64, or ARCH=x86. 1616 For all of them, SRCARCH=x86 because arch/x86/ supports both i386 and 1617 x86_64. 1618 1619INSTALL_PATH 1620 This variable defines a place for the arch Makefiles to install 1621 the resident kernel image and System.map file. 1622 Use this for architecture-specific install targets. 1623 1624INSTALL_MOD_PATH, MODLIB 1625 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module 1626 installation. This variable is not defined in the Makefile but 1627 may be passed in by the user if desired. 1628 1629 $(MODLIB) specifies the directory for module installation. 1630 The top Makefile defines $(MODLIB) to 1631 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may 1632 override this value on the command line if desired. 1633 1634INSTALL_MOD_STRIP 1635 If this variable is specified, it will cause modules to be stripped 1636 after they are installed. If INSTALL_MOD_STRIP is "1", then the 1637 default option --strip-debug will be used. Otherwise, the 1638 INSTALL_MOD_STRIP value will be used as the option(s) to the strip 1639 command. 1640 1641INSTALL_DTBS_PATH 1642 This variable specifies a prefix for relocations required by build 1643 roots. It defines a place for installing the device tree blobs. Like 1644 INSTALL_MOD_PATH, it isn't defined in the Makefile, but can be passed 1645 by the user if desired. Otherwise it defaults to the kernel install 1646 path. 1647 1648Makefile language 1649================= 1650 1651The kernel Makefiles are designed to be run with GNU Make. The Makefiles 1652use only the documented features of GNU Make, but they do use many 1653GNU extensions. 1654 1655GNU Make supports elementary list-processing functions. The kernel 1656Makefiles use a novel style of list building and manipulation with few 1657``if`` statements. 1658 1659GNU Make has two assignment operators, ``:=`` and ``=``. ``:=`` performs 1660immediate evaluation of the right-hand side and stores an actual string 1661into the left-hand side. ``=`` is like a formula definition; it stores the 1662right-hand side in an unevaluated form and then evaluates this form each 1663time the left-hand side is used. 1664 1665There are some cases where ``=`` is appropriate. Usually, though, ``:=`` 1666is the right choice. 1667 1668Credits 1669======= 1670 1671- Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> 1672- Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> 1673- Updates by Sam Ravnborg <sam@ravnborg.org> 1674- Language QA by Jan Engelhardt <jengelh@gmx.de> 1675 1676TODO 1677==== 1678 1679- Generating offset header files. 1680- Add more variables to chapters 7 or 9? 1681