1====================== 2Linux Kernel Makefiles 3====================== 4 5This document describes the Linux kernel Makefiles. 6 7.. Table of Contents 8 9 === 1 Overview 10 === 2 Who does what 11 === 3 The kbuild files 12 --- 3.1 Goal definitions 13 --- 3.2 Built-in object goals - obj-y 14 --- 3.3 Loadable module goals - obj-m 15 --- 3.4 Objects which export symbols 16 --- 3.5 Library file goals - lib-y 17 --- 3.6 Descending down in directories 18 --- 3.7 Compilation flags 19 --- 3.8 Command line dependency 20 --- 3.9 Dependency tracking 21 --- 3.10 Special Rules 22 --- 3.11 $(CC) support functions 23 --- 3.12 $(LD) support functions 24 25 === 4 Host Program support 26 --- 4.1 Simple Host Program 27 --- 4.2 Composite Host Programs 28 --- 4.3 Using C++ for host programs 29 --- 4.4 Controlling compiler options for host programs 30 --- 4.5 When host programs are actually built 31 --- 4.6 Using hostprogs-$(CONFIG_FOO) 32 33 === 5 Kbuild clean infrastructure 34 35 === 6 Architecture Makefiles 36 --- 6.1 Set variables to tweak the build to the architecture 37 --- 6.2 Add prerequisites to archheaders: 38 --- 6.3 Add prerequisites to archprepare: 39 --- 6.4 List directories to visit when descending 40 --- 6.5 Architecture-specific boot images 41 --- 6.6 Building non-kbuild targets 42 --- 6.7 Commands useful for building a boot image 43 --- 6.8 Custom kbuild commands 44 --- 6.9 Preprocessing linker scripts 45 --- 6.10 Generic header files 46 --- 6.11 Post-link pass 47 48 === 7 Kbuild syntax for exported headers 49 --- 7.1 no-export-headers 50 --- 7.2 generic-y 51 --- 7.3 generated-y 52 --- 7.4 mandatory-y 53 54 === 8 Kbuild Variables 55 === 9 Makefile language 56 === 10 Credits 57 === 11 TODO 58 591 Overview 60========== 61 62The Makefiles have five parts:: 63 64 Makefile the top Makefile. 65 .config the kernel configuration file. 66 arch/$(ARCH)/Makefile the arch Makefile. 67 scripts/Makefile.* common rules etc. for all kbuild Makefiles. 68 kbuild Makefiles there are about 500 of these. 69 70The top Makefile reads the .config file, which comes from the kernel 71configuration process. 72 73The top Makefile is responsible for building two major products: vmlinux 74(the resident kernel image) and modules (any module files). 75It builds these goals by recursively descending into the subdirectories of 76the kernel source tree. 77The list of subdirectories which are visited depends upon the kernel 78configuration. The top Makefile textually includes an arch Makefile 79with the name arch/$(ARCH)/Makefile. The arch Makefile supplies 80architecture-specific information to the top Makefile. 81 82Each subdirectory has a kbuild Makefile which carries out the commands 83passed down from above. The kbuild Makefile uses information from the 84.config file to construct various file lists used by kbuild to build 85any built-in or modular targets. 86 87scripts/Makefile.* contains all the definitions/rules etc. that 88are used to build the kernel based on the kbuild makefiles. 89 90 912 Who does what 92=============== 93 94People have four different relationships with the kernel Makefiles. 95 96*Users* are people who build kernels. These people type commands such as 97"make menuconfig" or "make". They usually do not read or edit 98any kernel Makefiles (or any other source files). 99 100*Normal developers* are people who work on features such as device 101drivers, file systems, and network protocols. These people need to 102maintain the kbuild Makefiles for the subsystem they are 103working on. In order to do this effectively, they need some overall 104knowledge about the kernel Makefiles, plus detailed knowledge about the 105public interface for kbuild. 106 107*Arch developers* are people who work on an entire architecture, such 108as sparc or ia64. Arch developers need to know about the arch Makefile 109as well as kbuild Makefiles. 110 111*Kbuild developers* are people who work on the kernel build system itself. 112These people need to know about all aspects of the kernel Makefiles. 113 114This document is aimed towards normal developers and arch developers. 115 116 1173 The kbuild files 118================== 119 120Most Makefiles within the kernel are kbuild Makefiles that use the 121kbuild infrastructure. This chapter introduces the syntax used in the 122kbuild makefiles. 123The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can 124be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild' 125file will be used. 126 127Section 3.1 "Goal definitions" is a quick intro, further chapters provide 128more details, with real examples. 129 1303.1 Goal definitions 131-------------------- 132 133 Goal definitions are the main part (heart) of the kbuild Makefile. 134 These lines define the files to be built, any special compilation 135 options, and any subdirectories to be entered recursively. 136 137 The most simple kbuild makefile contains one line: 138 139 Example:: 140 141 obj-y += foo.o 142 143 This tells kbuild that there is one object in that directory, named 144 foo.o. foo.o will be built from foo.c or foo.S. 145 146 If foo.o shall be built as a module, the variable obj-m is used. 147 Therefore the following pattern is often used: 148 149 Example:: 150 151 obj-$(CONFIG_FOO) += foo.o 152 153 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module). 154 If CONFIG_FOO is neither y nor m, then the file will not be compiled 155 nor linked. 156 1573.2 Built-in object goals - obj-y 158--------------------------------- 159 160 The kbuild Makefile specifies object files for vmlinux 161 in the $(obj-y) lists. These lists depend on the kernel 162 configuration. 163 164 Kbuild compiles all the $(obj-y) files. It then calls 165 "$(AR) rcSTP" to merge these files into one built-in.a file. 166 This is a thin archive without a symbol table. It will be later 167 linked into vmlinux by scripts/link-vmlinux.sh 168 169 The order of files in $(obj-y) is significant. Duplicates in 170 the lists are allowed: the first instance will be linked into 171 built-in.a and succeeding instances will be ignored. 172 173 Link order is significant, because certain functions 174 (module_init() / __initcall) will be called during boot in the 175 order they appear. So keep in mind that changing the link 176 order may e.g. change the order in which your SCSI 177 controllers are detected, and thus your disks are renumbered. 178 179 Example:: 180 181 #drivers/isdn/i4l/Makefile 182 # Makefile for the kernel ISDN subsystem and device drivers. 183 # Each configuration option enables a list of files. 184 obj-$(CONFIG_ISDN_I4L) += isdn.o 185 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o 186 1873.3 Loadable module goals - obj-m 188--------------------------------- 189 190 $(obj-m) specifies object files which are built as loadable 191 kernel modules. 192 193 A module may be built from one source file or several source 194 files. In the case of one source file, the kbuild makefile 195 simply adds the file to $(obj-m). 196 197 Example:: 198 199 #drivers/isdn/i4l/Makefile 200 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o 201 202 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm' 203 204 If a kernel module is built from several source files, you specify 205 that you want to build a module in the same way as above; however, 206 kbuild needs to know which object files you want to build your 207 module from, so you have to tell it by setting a $(<module_name>-y) 208 variable. 209 210 Example:: 211 212 #drivers/isdn/i4l/Makefile 213 obj-$(CONFIG_ISDN_I4L) += isdn.o 214 isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o 215 216 In this example, the module name will be isdn.o. Kbuild will 217 compile the objects listed in $(isdn-y) and then run 218 "$(LD) -r" on the list of these files to generate isdn.o. 219 220 Due to kbuild recognizing $(<module_name>-y) for composite objects, 221 you can use the value of a `CONFIG_` symbol to optionally include an 222 object file as part of a composite object. 223 224 Example:: 225 226 #fs/ext2/Makefile 227 obj-$(CONFIG_EXT2_FS) += ext2.o 228 ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \ 229 namei.o super.o symlink.o 230 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \ 231 xattr_trusted.o 232 233 In this example, xattr.o, xattr_user.o and xattr_trusted.o are only 234 part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR) 235 evaluates to 'y'. 236 237 Note: Of course, when you are building objects into the kernel, 238 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y, 239 kbuild will build an ext2.o file for you out of the individual 240 parts and then link this into built-in.a, as you would expect. 241 2423.4 Objects which export symbols 243-------------------------------- 244 245 No special notation is required in the makefiles for 246 modules exporting symbols. 247 2483.5 Library file goals - lib-y 249------------------------------ 250 251 Objects listed with obj-* are used for modules, or 252 combined in a built-in.a for that specific directory. 253 There is also the possibility to list objects that will 254 be included in a library, lib.a. 255 All objects listed with lib-y are combined in a single 256 library for that directory. 257 Objects that are listed in obj-y and additionally listed in 258 lib-y will not be included in the library, since they will 259 be accessible anyway. 260 For consistency, objects listed in lib-m will be included in lib.a. 261 262 Note that the same kbuild makefile may list files to be built-in 263 and to be part of a library. Therefore the same directory 264 may contain both a built-in.a and a lib.a file. 265 266 Example:: 267 268 #arch/x86/lib/Makefile 269 lib-y := delay.o 270 271 This will create a library lib.a based on delay.o. For kbuild to 272 actually recognize that there is a lib.a being built, the directory 273 shall be listed in libs-y. 274 275 See also "6.4 List directories to visit when descending". 276 277 Use of lib-y is normally restricted to `lib/` and `arch/*/lib`. 278 2793.6 Descending down in directories 280---------------------------------- 281 282 A Makefile is only responsible for building objects in its own 283 directory. Files in subdirectories should be taken care of by 284 Makefiles in these subdirs. The build system will automatically 285 invoke make recursively in subdirectories, provided you let it know of 286 them. 287 288 To do so, obj-y and obj-m are used. 289 ext2 lives in a separate directory, and the Makefile present in fs/ 290 tells kbuild to descend down using the following assignment. 291 292 Example:: 293 294 #fs/Makefile 295 obj-$(CONFIG_EXT2_FS) += ext2/ 296 297 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular) 298 the corresponding obj- variable will be set, and kbuild will descend 299 down in the ext2 directory. 300 Kbuild only uses this information to decide that it needs to visit 301 the directory, it is the Makefile in the subdirectory that 302 specifies what is modular and what is built-in. 303 304 It is good practice to use a `CONFIG_` variable when assigning directory 305 names. This allows kbuild to totally skip the directory if the 306 corresponding `CONFIG_` option is neither 'y' nor 'm'. 307 3083.7 Compilation flags 309--------------------- 310 311 ccflags-y, asflags-y and ldflags-y 312 These three flags apply only to the kbuild makefile in which they 313 are assigned. They are used for all the normal cc, as and ld 314 invocations happening during a recursive build. 315 Note: Flags with the same behaviour were previously named: 316 EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS. 317 They are still supported but their usage is deprecated. 318 319 ccflags-y specifies options for compiling with $(CC). 320 321 Example:: 322 323 # drivers/acpi/acpica/Makefile 324 ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA 325 ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT 326 327 This variable is necessary because the top Makefile owns the 328 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the 329 entire tree. 330 331 asflags-y specifies assembler options. 332 333 Example:: 334 335 #arch/sparc/kernel/Makefile 336 asflags-y := -ansi 337 338 ldflags-y specifies options for linking with $(LD). 339 340 Example:: 341 342 #arch/cris/boot/compressed/Makefile 343 ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds 344 345 subdir-ccflags-y, subdir-asflags-y 346 The two flags listed above are similar to ccflags-y and asflags-y. 347 The difference is that the subdir- variants have effect for the kbuild 348 file where they are present and all subdirectories. 349 Options specified using subdir-* are added to the commandline before 350 the options specified using the non-subdir variants. 351 352 Example:: 353 354 subdir-ccflags-y := -Werror 355 356 CFLAGS_$@, AFLAGS_$@ 357 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current 358 kbuild makefile. 359 360 $(CFLAGS_$@) specifies per-file options for $(CC). The $@ 361 part has a literal value which specifies the file that it is for. 362 363 Example:: 364 365 # drivers/scsi/Makefile 366 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF 367 CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \ 368 -DGDTH_STATISTICS 369 370 These two lines specify compilation flags for aha152x.o and gdth.o. 371 372 $(AFLAGS_$@) is a similar feature for source files in assembly 373 languages. 374 375 Example:: 376 377 # arch/arm/kernel/Makefile 378 AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET) 379 AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312 380 AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt 381 382 3833.9 Dependency tracking 384----------------------- 385 386 Kbuild tracks dependencies on the following: 387 388 1) All prerequisite files (both `*.c` and `*.h`) 389 2) `CONFIG_` options used in all prerequisite files 390 3) Command-line used to compile target 391 392 Thus, if you change an option to $(CC) all affected files will 393 be re-compiled. 394 3953.10 Special Rules 396------------------ 397 398 Special rules are used when the kbuild infrastructure does 399 not provide the required support. A typical example is 400 header files generated during the build process. 401 Another example are the architecture-specific Makefiles which 402 need special rules to prepare boot images etc. 403 404 Special rules are written as normal Make rules. 405 Kbuild is not executing in the directory where the Makefile is 406 located, so all special rules shall provide a relative 407 path to prerequisite files and target files. 408 409 Two variables are used when defining special rules: 410 411 $(src) 412 $(src) is a relative path which points to the directory 413 where the Makefile is located. Always use $(src) when 414 referring to files located in the src tree. 415 416 $(obj) 417 $(obj) is a relative path which points to the directory 418 where the target is saved. Always use $(obj) when 419 referring to generated files. 420 421 Example:: 422 423 #drivers/scsi/Makefile 424 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl 425 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl 426 427 This is a special rule, following the normal syntax 428 required by make. 429 430 The target file depends on two prerequisite files. References 431 to the target file are prefixed with $(obj), references 432 to prerequisites are referenced with $(src) (because they are not 433 generated files). 434 435 $(kecho) 436 echoing information to user in a rule is often a good practice 437 but when execution "make -s" one does not expect to see any output 438 except for warnings/errors. 439 To support this kbuild defines $(kecho) which will echo out the 440 text following $(kecho) to stdout except if "make -s" is used. 441 442 Example:: 443 444 #arch/blackfin/boot/Makefile 445 $(obj)/vmImage: $(obj)/vmlinux.gz 446 $(call if_changed,uimage) 447 @$(kecho) 'Kernel: $@ is ready' 448 449 4503.11 $(CC) support functions 451---------------------------- 452 453 The kernel may be built with several different versions of 454 $(CC), each supporting a unique set of features and options. 455 kbuild provides basic support to check for valid options for $(CC). 456 $(CC) is usually the gcc compiler, but other alternatives are 457 available. 458 459 as-option 460 as-option is used to check if $(CC) -- when used to compile 461 assembler (`*.S`) files -- supports the given option. An optional 462 second option may be specified if the first option is not supported. 463 464 Example:: 465 466 #arch/sh/Makefile 467 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) 468 469 In the above example, cflags-y will be assigned the option 470 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). 471 The second argument is optional, and if supplied will be used 472 if first argument is not supported. 473 474 as-instr 475 as-instr checks if the assembler reports a specific instruction 476 and then outputs either option1 or option2 477 C escapes are supported in the test instruction 478 Note: as-instr-option uses KBUILD_AFLAGS for assembler options 479 480 cc-option 481 cc-option is used to check if $(CC) supports a given option, and if 482 not supported to use an optional second option. 483 484 Example:: 485 486 #arch/x86/Makefile 487 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) 488 489 In the above example, cflags-y will be assigned the option 490 -march=pentium-mmx if supported by $(CC), otherwise -march=i586. 491 The second argument to cc-option is optional, and if omitted, 492 cflags-y will be assigned no value if first option is not supported. 493 Note: cc-option uses KBUILD_CFLAGS for $(CC) options 494 495 cc-option-yn 496 cc-option-yn is used to check if gcc supports a given option 497 and return 'y' if supported, otherwise 'n'. 498 499 Example:: 500 501 #arch/ppc/Makefile 502 biarch := $(call cc-option-yn, -m32) 503 aflags-$(biarch) += -a32 504 cflags-$(biarch) += -m32 505 506 In the above example, $(biarch) is set to y if $(CC) supports the -m32 507 option. When $(biarch) equals 'y', the expanded variables $(aflags-y) 508 and $(cflags-y) will be assigned the values -a32 and -m32, 509 respectively. 510 Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options 511 512 cc-disable-warning 513 cc-disable-warning checks if gcc supports a given warning and returns 514 the commandline switch to disable it. This special function is needed, 515 because gcc 4.4 and later accept any unknown -Wno-* option and only 516 warn about it if there is another warning in the source file. 517 518 Example:: 519 520 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable) 521 522 In the above example, -Wno-unused-but-set-variable will be added to 523 KBUILD_CFLAGS only if gcc really accepts it. 524 525 cc-ifversion 526 cc-ifversion tests the version of $(CC) and equals the fourth parameter 527 if version expression is true, or the fifth (if given) if the version 528 expression is false. 529 530 Example:: 531 532 #fs/reiserfs/Makefile 533 ccflags-y := $(call cc-ifversion, -lt, 0402, -O1) 534 535 In this example, ccflags-y will be assigned the value -O1 if the 536 $(CC) version is less than 4.2. 537 cc-ifversion takes all the shell operators: 538 -eq, -ne, -lt, -le, -gt, and -ge 539 The third parameter may be a text as in this example, but it may also 540 be an expanded variable or a macro. 541 542 cc-cross-prefix 543 cc-cross-prefix is used to check if there exists a $(CC) in path with 544 one of the listed prefixes. The first prefix where there exist a 545 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found 546 then nothing is returned. 547 Additional prefixes are separated by a single space in the 548 call of cc-cross-prefix. 549 This functionality is useful for architecture Makefiles that try 550 to set CROSS_COMPILE to well-known values but may have several 551 values to select between. 552 It is recommended only to try to set CROSS_COMPILE if it is a cross 553 build (host arch is different from target arch). And if CROSS_COMPILE 554 is already set then leave it with the old value. 555 556 Example:: 557 558 #arch/m68k/Makefile 559 ifneq ($(SUBARCH),$(ARCH)) 560 ifeq ($(CROSS_COMPILE),) 561 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) 562 endif 563 endif 564 5653.12 $(LD) support functions 566---------------------------- 567 568 ld-option 569 ld-option is used to check if $(LD) supports the supplied option. 570 ld-option takes two options as arguments. 571 The second argument is an optional option that can be used if the 572 first option is not supported by $(LD). 573 574 Example:: 575 576 #Makefile 577 LDFLAGS_vmlinux += $(call ld-option, -X) 578 579 5804 Host Program support 581====================== 582 583Kbuild supports building executables on the host for use during the 584compilation stage. 585Two steps are required in order to use a host executable. 586 587The first step is to tell kbuild that a host program exists. This is 588done utilising the variable hostprogs-y. 589 590The second step is to add an explicit dependency to the executable. 591This can be done in two ways. Either add the dependency in a rule, 592or utilise the variable $(always). 593Both possibilities are described in the following. 594 5954.1 Simple Host Program 596----------------------- 597 598 In some cases there is a need to compile and run a program on the 599 computer where the build is running. 600 The following line tells kbuild that the program bin2hex shall be 601 built on the build host. 602 603 Example:: 604 605 hostprogs-y := bin2hex 606 607 Kbuild assumes in the above example that bin2hex is made from a single 608 c-source file named bin2hex.c located in the same directory as 609 the Makefile. 610 6114.2 Composite Host Programs 612--------------------------- 613 614 Host programs can be made up based on composite objects. 615 The syntax used to define composite objects for host programs is 616 similar to the syntax used for kernel objects. 617 $(<executable>-objs) lists all objects used to link the final 618 executable. 619 620 Example:: 621 622 #scripts/lxdialog/Makefile 623 hostprogs-y := lxdialog 624 lxdialog-objs := checklist.o lxdialog.o 625 626 Objects with extension .o are compiled from the corresponding .c 627 files. In the above example, checklist.c is compiled to checklist.o 628 and lxdialog.c is compiled to lxdialog.o. 629 630 Finally, the two .o files are linked to the executable, lxdialog. 631 Note: The syntax <executable>-y is not permitted for host-programs. 632 6334.3 Using C++ for host programs 634------------------------------- 635 636 kbuild offers support for host programs written in C++. This was 637 introduced solely to support kconfig, and is not recommended 638 for general use. 639 640 Example:: 641 642 #scripts/kconfig/Makefile 643 hostprogs-y := qconf 644 qconf-cxxobjs := qconf.o 645 646 In the example above the executable is composed of the C++ file 647 qconf.cc - identified by $(qconf-cxxobjs). 648 649 If qconf is composed of a mixture of .c and .cc files, then an 650 additional line can be used to identify this. 651 652 Example:: 653 654 #scripts/kconfig/Makefile 655 hostprogs-y := qconf 656 qconf-cxxobjs := qconf.o 657 qconf-objs := check.o 658 6594.4 Controlling compiler options for host programs 660-------------------------------------------------- 661 662 When compiling host programs, it is possible to set specific flags. 663 The programs will always be compiled utilising $(HOSTCC) passed 664 the options specified in $(KBUILD_HOSTCFLAGS). 665 To set flags that will take effect for all host programs created 666 in that Makefile, use the variable HOST_EXTRACFLAGS. 667 668 Example:: 669 670 #scripts/lxdialog/Makefile 671 HOST_EXTRACFLAGS += -I/usr/include/ncurses 672 673 To set specific flags for a single file the following construction 674 is used: 675 676 Example:: 677 678 #arch/ppc64/boot/Makefile 679 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) 680 681 It is also possible to specify additional options to the linker. 682 683 Example:: 684 685 #scripts/kconfig/Makefile 686 HOSTLDLIBS_qconf := -L$(QTDIR)/lib 687 688 When linking qconf, it will be passed the extra option 689 "-L$(QTDIR)/lib". 690 6914.5 When host programs are actually built 692----------------------------------------- 693 694 Kbuild will only build host-programs when they are referenced 695 as a prerequisite. 696 This is possible in two ways: 697 698 (1) List the prerequisite explicitly in a special rule. 699 700 Example:: 701 702 #drivers/pci/Makefile 703 hostprogs-y := gen-devlist 704 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist 705 ( cd $(obj); ./gen-devlist ) < $< 706 707 The target $(obj)/devlist.h will not be built before 708 $(obj)/gen-devlist is updated. Note that references to 709 the host programs in special rules must be prefixed with $(obj). 710 711 (2) Use $(always) 712 713 When there is no suitable special rule, and the host program 714 shall be built when a makefile is entered, the $(always) 715 variable shall be used. 716 717 Example:: 718 719 #scripts/lxdialog/Makefile 720 hostprogs-y := lxdialog 721 always := $(hostprogs-y) 722 723 This will tell kbuild to build lxdialog even if not referenced in 724 any rule. 725 7264.6 Using hostprogs-$(CONFIG_FOO) 727--------------------------------- 728 729 A typical pattern in a Kbuild file looks like this: 730 731 Example:: 732 733 #scripts/Makefile 734 hostprogs-$(CONFIG_KALLSYMS) += kallsyms 735 736 Kbuild knows about both 'y' for built-in and 'm' for module. 737 So if a config symbol evaluates to 'm', kbuild will still build 738 the binary. In other words, Kbuild handles hostprogs-m exactly 739 like hostprogs-y. But only hostprogs-y is recommended to be used 740 when no CONFIG symbols are involved. 741 7425 Kbuild clean infrastructure 743============================= 744 745"make clean" deletes most generated files in the obj tree where the kernel 746is compiled. This includes generated files such as host programs. 747Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always), 748$(extra-y) and $(targets). They are all deleted during "make clean". 749Files matching the patterns "*.[oas]", "*.ko", plus some additional files 750generated by kbuild are deleted all over the kernel src tree when 751"make clean" is executed. 752 753Additional files or directories can be specified in kbuild makefiles by use of 754$(clean-files). 755 756 Example:: 757 758 #lib/Makefile 759 clean-files := crc32table.h 760 761When executing "make clean", the file "crc32table.h" will be deleted. 762Kbuild will assume files to be in the same relative directory as the 763Makefile, except if prefixed with $(objtree). 764 765To exclude certain files or directories from make clean, use the 766$(no-clean-files) variable. 767 768Usually kbuild descends down in subdirectories due to "obj-* := dir/", 769but in the architecture makefiles where the kbuild infrastructure 770is not sufficient this sometimes needs to be explicit. 771 772 Example:: 773 774 #arch/x86/boot/Makefile 775 subdir- := compressed/ 776 777The above assignment instructs kbuild to descend down in the 778directory compressed/ when "make clean" is executed. 779 780To support the clean infrastructure in the Makefiles that build the 781final bootimage there is an optional target named archclean: 782 783 Example:: 784 785 #arch/x86/Makefile 786 archclean: 787 $(Q)$(MAKE) $(clean)=arch/x86/boot 788 789When "make clean" is executed, make will descend down in arch/x86/boot, 790and clean as usual. The Makefile located in arch/x86/boot/ may use 791the subdir- trick to descend further down. 792 793Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is 794included in the top level makefile, and the kbuild infrastructure 795is not operational at that point. 796 797Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will 798be visited during "make clean". 799 8006 Architecture Makefiles 801======================== 802 803The top level Makefile sets up the environment and does the preparation, 804before starting to descend down in the individual directories. 805The top level makefile contains the generic part, whereas 806arch/$(ARCH)/Makefile contains what is required to set up kbuild 807for said architecture. 808To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines 809a few targets. 810 811When kbuild executes, the following steps are followed (roughly): 812 8131) Configuration of the kernel => produce .config 8142) Store kernel version in include/linux/version.h 8153) Updating all other prerequisites to the target prepare: 816 - Additional prerequisites are specified in arch/$(ARCH)/Makefile 8174) Recursively descend down in all directories listed in 818 init-* core* drivers-* net-* libs-* and build all targets. 819 - The values of the above variables are expanded in arch/$(ARCH)/Makefile. 8205) All object files are then linked and the resulting file vmlinux is 821 located at the root of the obj tree. 822 The very first objects linked are listed in head-y, assigned by 823 arch/$(ARCH)/Makefile. 8246) Finally, the architecture-specific part does any required post processing 825 and builds the final bootimage. 826 - This includes building boot records 827 - Preparing initrd images and the like 828 829 8306.1 Set variables to tweak the build to the architecture 831-------------------------------------------------------- 832 833 LDFLAGS 834 Generic $(LD) options 835 836 Flags used for all invocations of the linker. 837 Often specifying the emulation is sufficient. 838 839 Example:: 840 841 #arch/s390/Makefile 842 LDFLAGS := -m elf_s390 843 844 Note: ldflags-y can be used to further customise 845 the flags used. See chapter 3.7. 846 847 LDFLAGS_vmlinux 848 Options for $(LD) when linking vmlinux 849 850 LDFLAGS_vmlinux is used to specify additional flags to pass to 851 the linker when linking the final vmlinux image. 852 LDFLAGS_vmlinux uses the LDFLAGS_$@ support. 853 854 Example:: 855 856 #arch/x86/Makefile 857 LDFLAGS_vmlinux := -e stext 858 859 OBJCOPYFLAGS 860 objcopy flags 861 862 When $(call if_changed,objcopy) is used to translate a .o file, 863 the flags specified in OBJCOPYFLAGS will be used. 864 $(call if_changed,objcopy) is often used to generate raw binaries on 865 vmlinux. 866 867 Example:: 868 869 #arch/s390/Makefile 870 OBJCOPYFLAGS := -O binary 871 872 #arch/s390/boot/Makefile 873 $(obj)/image: vmlinux FORCE 874 $(call if_changed,objcopy) 875 876 In this example, the binary $(obj)/image is a binary version of 877 vmlinux. The usage of $(call if_changed,xxx) will be described later. 878 879 KBUILD_AFLAGS 880 Assembler flags 881 882 Default value - see top level Makefile 883 Append or modify as required per architecture. 884 885 Example:: 886 887 #arch/sparc64/Makefile 888 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc 889 890 KBUILD_CFLAGS 891 $(CC) compiler flags 892 893 Default value - see top level Makefile 894 Append or modify as required per architecture. 895 896 Often, the KBUILD_CFLAGS variable depends on the configuration. 897 898 Example:: 899 900 #arch/x86/boot/compressed/Makefile 901 cflags-$(CONFIG_X86_32) := -march=i386 902 cflags-$(CONFIG_X86_64) := -mcmodel=small 903 KBUILD_CFLAGS += $(cflags-y) 904 905 Many arch Makefiles dynamically run the target C compiler to 906 probe supported options:: 907 908 #arch/x86/Makefile 909 910 ... 911 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ 912 -march=pentium2,-march=i686) 913 ... 914 # Disable unit-at-a-time mode ... 915 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) 916 ... 917 918 919 The first example utilises the trick that a config option expands 920 to 'y' when selected. 921 922 KBUILD_AFLAGS_KERNEL 923 Assembler options specific for built-in 924 925 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile 926 resident kernel code. 927 928 KBUILD_AFLAGS_MODULE 929 Assembler options specific for modules 930 931 $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that 932 are used for assembler. 933 934 From commandline AFLAGS_MODULE shall be used (see kbuild.txt). 935 936 KBUILD_CFLAGS_KERNEL 937 $(CC) options specific for built-in 938 939 $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile 940 resident kernel code. 941 942 KBUILD_CFLAGS_MODULE 943 Options for $(CC) when building modules 944 945 $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that 946 are used for $(CC). 947 From commandline CFLAGS_MODULE shall be used (see kbuild.txt). 948 949 KBUILD_LDFLAGS_MODULE 950 Options for $(LD) when linking modules 951 952 $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options 953 used when linking modules. This is often a linker script. 954 955 From commandline LDFLAGS_MODULE shall be used (see kbuild.txt). 956 957 KBUILD_LDS 958 959 The linker script with full path. Assigned by the top-level Makefile. 960 961 KBUILD_LDS_MODULE 962 963 The module linker script with full path. Assigned by the top-level 964 Makefile and additionally by the arch Makefile. 965 966 KBUILD_VMLINUX_OBJS 967 968 All object files for vmlinux. They are linked to vmlinux in the same 969 order as listed in KBUILD_VMLINUX_OBJS. 970 971 KBUILD_VMLINUX_LIBS 972 973 All .a "lib" files for vmlinux. KBUILD_VMLINUX_OBJS and 974 KBUILD_VMLINUX_LIBS together specify all the object files used to 975 link vmlinux. 976 9776.2 Add prerequisites to archheaders 978------------------------------------ 979 980 The archheaders: rule is used to generate header files that 981 may be installed into user space by "make header_install". 982 983 It is run before "make archprepare" when run on the 984 architecture itself. 985 986 9876.3 Add prerequisites to archprepare 988------------------------------------ 989 990 The archprepare: rule is used to list prerequisites that need to be 991 built before starting to descend down in the subdirectories. 992 This is usually used for header files containing assembler constants. 993 994 Example:: 995 996 #arch/arm/Makefile 997 archprepare: maketools 998 999 In this example, the file target maketools will be processed 1000 before descending down in the subdirectories. 1001 See also chapter XXX-TODO that describe how kbuild supports 1002 generating offset header files. 1003 1004 10056.4 List directories to visit when descending 1006--------------------------------------------- 1007 1008 An arch Makefile cooperates with the top Makefile to define variables 1009 which specify how to build the vmlinux file. Note that there is no 1010 corresponding arch-specific section for modules; the module-building 1011 machinery is all architecture-independent. 1012 1013 1014 head-y, init-y, core-y, libs-y, drivers-y, net-y 1015 $(head-y) lists objects to be linked first in vmlinux. 1016 1017 $(libs-y) lists directories where a lib.a archive can be located. 1018 1019 The rest list directories where a built-in.a object file can be 1020 located. 1021 1022 $(init-y) objects will be located after $(head-y). 1023 1024 Then the rest follows in this order: 1025 1026 $(core-y), $(libs-y), $(drivers-y) and $(net-y). 1027 1028 The top level Makefile defines values for all generic directories, 1029 and arch/$(ARCH)/Makefile only adds architecture-specific 1030 directories. 1031 1032 Example:: 1033 1034 #arch/sparc64/Makefile 1035 core-y += arch/sparc64/kernel/ 1036 libs-y += arch/sparc64/prom/ arch/sparc64/lib/ 1037 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ 1038 1039 10406.5 Architecture-specific boot images 1041------------------------------------- 1042 1043 An arch Makefile specifies goals that take the vmlinux file, compress 1044 it, wrap it in bootstrapping code, and copy the resulting files 1045 somewhere. This includes various kinds of installation commands. 1046 The actual goals are not standardized across architectures. 1047 1048 It is common to locate any additional processing in a boot/ 1049 directory below arch/$(ARCH)/. 1050 1051 Kbuild does not provide any smart way to support building a 1052 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall 1053 call make manually to build a target in boot/. 1054 1055 The recommended approach is to include shortcuts in 1056 arch/$(ARCH)/Makefile, and use the full path when calling down 1057 into the arch/$(ARCH)/boot/Makefile. 1058 1059 Example:: 1060 1061 #arch/x86/Makefile 1062 boot := arch/x86/boot 1063 bzImage: vmlinux 1064 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ 1065 1066 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke 1067 make in a subdirectory. 1068 1069 There are no rules for naming architecture-specific targets, 1070 but executing "make help" will list all relevant targets. 1071 To support this, $(archhelp) must be defined. 1072 1073 Example:: 1074 1075 #arch/x86/Makefile 1076 define archhelp 1077 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' 1078 endif 1079 1080 When make is executed without arguments, the first goal encountered 1081 will be built. In the top level Makefile the first goal present 1082 is all:. 1083 An architecture shall always, per default, build a bootable image. 1084 In "make help", the default goal is highlighted with a '*'. 1085 Add a new prerequisite to all: to select a default goal different 1086 from vmlinux. 1087 1088 Example:: 1089 1090 #arch/x86/Makefile 1091 all: bzImage 1092 1093 When "make" is executed without arguments, bzImage will be built. 1094 10956.6 Building non-kbuild targets 1096------------------------------- 1097 1098 extra-y 1099 extra-y specifies additional targets created in the current 1100 directory, in addition to any targets specified by `obj-*`. 1101 1102 Listing all targets in extra-y is required for two purposes: 1103 1104 1) Enable kbuild to check changes in command lines 1105 1106 - When $(call if_changed,xxx) is used 1107 1108 2) kbuild knows what files to delete during "make clean" 1109 1110 Example:: 1111 1112 #arch/x86/kernel/Makefile 1113 extra-y := head.o init_task.o 1114 1115 In this example, extra-y is used to list object files that 1116 shall be built, but shall not be linked as part of built-in.a. 1117 1118 header-test-y 1119 1120 header-test-y specifies headers (`*.h`) in the current directory that 1121 should be compile tested to ensure they are self-contained, 1122 i.e. compilable as standalone units. If CONFIG_HEADER_TEST is enabled, 1123 this builds them as part of extra-y. 1124 1125 header-test-pattern-y 1126 1127 This works as a weaker version of header-test-y, and accepts wildcard 1128 patterns. The typical usage is:: 1129 1130 header-test-pattern-y += *.h 1131 1132 This specifies all the files that matches to `*.h` in the current 1133 directory, but the files in 'header-test-' are excluded. 1134 11356.7 Commands useful for building a boot image 1136--------------------------------------------- 1137 1138 Kbuild provides a few macros that are useful when building a 1139 boot image. 1140 1141 if_changed 1142 if_changed is the infrastructure used for the following commands. 1143 1144 Usage:: 1145 1146 target: source(s) FORCE 1147 $(call if_changed,ld/objcopy/gzip/...) 1148 1149 When the rule is evaluated, it is checked to see if any files 1150 need an update, or the command line has changed since the last 1151 invocation. The latter will force a rebuild if any options 1152 to the executable have changed. 1153 Any target that utilises if_changed must be listed in $(targets), 1154 otherwise the command line check will fail, and the target will 1155 always be built. 1156 Assignments to $(targets) are without $(obj)/ prefix. 1157 if_changed may be used in conjunction with custom commands as 1158 defined in 6.8 "Custom kbuild commands". 1159 1160 Note: It is a typical mistake to forget the FORCE prerequisite. 1161 Another common pitfall is that whitespace is sometimes 1162 significant; for instance, the below will fail (note the extra space 1163 after the comma):: 1164 1165 target: source(s) FORCE 1166 1167 **WRONG!** $(call if_changed, ld/objcopy/gzip/...) 1168 1169 Note: 1170 if_changed should not be used more than once per target. 1171 It stores the executed command in a corresponding .cmd 1172 1173 file and multiple calls would result in overwrites and 1174 unwanted results when the target is up to date and only the 1175 tests on changed commands trigger execution of commands. 1176 1177 ld 1178 Link target. Often, LDFLAGS_$@ is used to set specific options to ld. 1179 1180 Example:: 1181 1182 #arch/x86/boot/Makefile 1183 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary 1184 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext 1185 1186 targets += setup setup.o bootsect bootsect.o 1187 $(obj)/setup $(obj)/bootsect: %: %.o FORCE 1188 $(call if_changed,ld) 1189 1190 In this example, there are two possible targets, requiring different 1191 options to the linker. The linker options are specified using the 1192 LDFLAGS_$@ syntax - one for each potential target. 1193 $(targets) are assigned all potential targets, by which kbuild knows 1194 the targets and will: 1195 1196 1) check for commandline changes 1197 2) delete target during make clean 1198 1199 The ": %: %.o" part of the prerequisite is a shorthand that 1200 frees us from listing the setup.o and bootsect.o files. 1201 1202 Note: 1203 It is a common mistake to forget the "targets :=" assignment, 1204 resulting in the target file being recompiled for no 1205 obvious reason. 1206 1207 objcopy 1208 Copy binary. Uses OBJCOPYFLAGS usually specified in 1209 arch/$(ARCH)/Makefile. 1210 OBJCOPYFLAGS_$@ may be used to set additional options. 1211 1212 gzip 1213 Compress target. Use maximum compression to compress target. 1214 1215 Example:: 1216 1217 #arch/x86/boot/compressed/Makefile 1218 $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE 1219 $(call if_changed,gzip) 1220 1221 dtc 1222 Create flattened device tree blob object suitable for linking 1223 into vmlinux. Device tree blobs linked into vmlinux are placed 1224 in an init section in the image. Platform code *must* copy the 1225 blob to non-init memory prior to calling unflatten_device_tree(). 1226 1227 To use this command, simply add `*.dtb` into obj-y or targets, or make 1228 some other target depend on `%.dtb` 1229 1230 A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`; 1231 architecture Makefiles do no need to explicitly write out that rule. 1232 1233 Example:: 1234 1235 targets += $(dtb-y) 1236 DTC_FLAGS ?= -p 1024 1237 12386.8 Custom kbuild commands 1239-------------------------- 1240 1241 When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand 1242 of a command is normally displayed. 1243 To enable this behaviour for custom commands kbuild requires 1244 two variables to be set:: 1245 1246 quiet_cmd_<command> - what shall be echoed 1247 cmd_<command> - the command to execute 1248 1249 Example:: 1250 1251 # 1252 quiet_cmd_image = BUILD $@ 1253 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \ 1254 $(obj)/vmlinux.bin > $@ 1255 1256 targets += bzImage 1257 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE 1258 $(call if_changed,image) 1259 @echo 'Kernel: $@ is ready' 1260 1261 When updating the $(obj)/bzImage target, the line: 1262 1263 BUILD arch/x86/boot/bzImage 1264 1265 will be displayed with "make KBUILD_VERBOSE=0". 1266 1267 1268--- 6.9 Preprocessing linker scripts 1269 1270 When the vmlinux image is built, the linker script 1271 arch/$(ARCH)/kernel/vmlinux.lds is used. 1272 The script is a preprocessed variant of the file vmlinux.lds.S 1273 located in the same directory. 1274 kbuild knows .lds files and includes a rule `*lds.S` -> `*lds`. 1275 1276 Example:: 1277 1278 #arch/x86/kernel/Makefile 1279 always := vmlinux.lds 1280 1281 #Makefile 1282 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) 1283 1284 The assignment to $(always) is used to tell kbuild to build the 1285 target vmlinux.lds. 1286 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the 1287 specified options when building the target vmlinux.lds. 1288 1289 When building the `*.lds` target, kbuild uses the variables:: 1290 1291 KBUILD_CPPFLAGS : Set in top-level Makefile 1292 cppflags-y : May be set in the kbuild makefile 1293 CPPFLAGS_$(@F) : Target-specific flags. 1294 Note that the full filename is used in this 1295 assignment. 1296 1297 The kbuild infrastructure for `*lds` files is used in several 1298 architecture-specific files. 1299 13006.10 Generic header files 1301------------------------- 1302 1303 The directory include/asm-generic contains the header files 1304 that may be shared between individual architectures. 1305 The recommended approach how to use a generic header file is 1306 to list the file in the Kbuild file. 1307 See "7.2 generic-y" for further info on syntax etc. 1308 13096.11 Post-link pass 1310------------------- 1311 1312 If the file arch/xxx/Makefile.postlink exists, this makefile 1313 will be invoked for post-link objects (vmlinux and modules.ko) 1314 for architectures to run post-link passes on. Must also handle 1315 the clean target. 1316 1317 This pass runs after kallsyms generation. If the architecture 1318 needs to modify symbol locations, rather than manipulate the 1319 kallsyms, it may be easier to add another postlink target for 1320 .tmp_vmlinux? targets to be called from link-vmlinux.sh. 1321 1322 For example, powerpc uses this to check relocation sanity of 1323 the linked vmlinux file. 1324 13257 Kbuild syntax for exported headers 1326------------------------------------ 1327 1328The kernel includes a set of headers that is exported to userspace. 1329Many headers can be exported as-is but other headers require a 1330minimal pre-processing before they are ready for user-space. 1331The pre-processing does: 1332 1333- drop kernel-specific annotations 1334- drop include of compiler.h 1335- drop all sections that are kernel internal (guarded by `ifdef __KERNEL__`) 1336 1337All headers under include/uapi/, include/generated/uapi/, 1338arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/ 1339are exported. 1340 1341A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and 1342arch/<arch>/include/asm/ to list asm files coming from asm-generic. 1343See subsequent chapter for the syntax of the Kbuild file. 1344 13457.1 no-export-headers 1346--------------------- 1347 1348 no-export-headers is essentially used by include/uapi/linux/Kbuild to 1349 avoid exporting specific headers (e.g. kvm.h) on architectures that do 1350 not support it. It should be avoided as much as possible. 1351 13527.2 generic-y 1353------------- 1354 1355 If an architecture uses a verbatim copy of a header from 1356 include/asm-generic then this is listed in the file 1357 arch/$(ARCH)/include/asm/Kbuild like this: 1358 1359 Example:: 1360 1361 #arch/x86/include/asm/Kbuild 1362 generic-y += termios.h 1363 generic-y += rtc.h 1364 1365 During the prepare phase of the build a wrapper include 1366 file is generated in the directory:: 1367 1368 arch/$(ARCH)/include/generated/asm 1369 1370 When a header is exported where the architecture uses 1371 the generic header a similar wrapper is generated as part 1372 of the set of exported headers in the directory:: 1373 1374 usr/include/asm 1375 1376 The generated wrapper will in both cases look like the following: 1377 1378 Example: termios.h:: 1379 1380 #include <asm-generic/termios.h> 1381 13827.3 generated-y 1383--------------- 1384 1385 If an architecture generates other header files alongside generic-y 1386 wrappers, generated-y specifies them. 1387 1388 This prevents them being treated as stale asm-generic wrappers and 1389 removed. 1390 1391 Example:: 1392 1393 #arch/x86/include/asm/Kbuild 1394 generated-y += syscalls_32.h 1395 13967.4 mandatory-y 1397--------------- 1398 1399 mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild 1400 to define the minimum set of ASM headers that all architectures must have. 1401 1402 This works like optional generic-y. If a mandatory header is missing 1403 in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate 1404 a wrapper of the asm-generic one. 1405 1406 The convention is to list one subdir per line and 1407 preferably in alphabetic order. 1408 14098 Kbuild Variables 1410================== 1411 1412The top Makefile exports the following variables: 1413 1414 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION 1415 These variables define the current kernel version. A few arch 1416 Makefiles actually use these values directly; they should use 1417 $(KERNELRELEASE) instead. 1418 1419 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic 1420 three-part version number, such as "2", "4", and "0". These three 1421 values are always numeric. 1422 1423 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches 1424 or additional patches. It is usually some non-numeric string 1425 such as "-pre4", and is often blank. 1426 1427 KERNELRELEASE 1428 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable 1429 for constructing installation directory names or showing in 1430 version strings. Some arch Makefiles use it for this purpose. 1431 1432 ARCH 1433 This variable defines the target architecture, such as "i386", 1434 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to 1435 determine which files to compile. 1436 1437 By default, the top Makefile sets $(ARCH) to be the same as the 1438 host system architecture. For a cross build, a user may 1439 override the value of $(ARCH) on the command line:: 1440 1441 make ARCH=m68k ... 1442 1443 1444 INSTALL_PATH 1445 This variable defines a place for the arch Makefiles to install 1446 the resident kernel image and System.map file. 1447 Use this for architecture-specific install targets. 1448 1449 INSTALL_MOD_PATH, MODLIB 1450 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module 1451 installation. This variable is not defined in the Makefile but 1452 may be passed in by the user if desired. 1453 1454 $(MODLIB) specifies the directory for module installation. 1455 The top Makefile defines $(MODLIB) to 1456 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may 1457 override this value on the command line if desired. 1458 1459 INSTALL_MOD_STRIP 1460 If this variable is specified, it will cause modules to be stripped 1461 after they are installed. If INSTALL_MOD_STRIP is '1', then the 1462 default option --strip-debug will be used. Otherwise, the 1463 INSTALL_MOD_STRIP value will be used as the option(s) to the strip 1464 command. 1465 1466 14679 Makefile language 1468=================== 1469 1470The kernel Makefiles are designed to be run with GNU Make. The Makefiles 1471use only the documented features of GNU Make, but they do use many 1472GNU extensions. 1473 1474GNU Make supports elementary list-processing functions. The kernel 1475Makefiles use a novel style of list building and manipulation with few 1476"if" statements. 1477 1478GNU Make has two assignment operators, ":=" and "=". ":=" performs 1479immediate evaluation of the right-hand side and stores an actual string 1480into the left-hand side. "=" is like a formula definition; it stores the 1481right-hand side in an unevaluated form and then evaluates this form each 1482time the left-hand side is used. 1483 1484There are some cases where "=" is appropriate. Usually, though, ":=" 1485is the right choice. 1486 148710 Credits 1488========== 1489 1490- Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> 1491- Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> 1492- Updates by Sam Ravnborg <sam@ravnborg.org> 1493- Language QA by Jan Engelhardt <jengelh@gmx.de> 1494 149511 TODO 1496======= 1497 1498- Describe how kbuild supports shipped files with _shipped. 1499- Generating offset header files. 1500- Add more variables to section 7? 1501