5 This document describes the Linux kernel Makefiles.
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 $(LD) support functions
25 === 4 Host Program support
26 --- 4.1 Simple Host Program
27 --- 4.2 Composite Host Programs
28 --- 4.3 Using C++ for host programs
29 --- 4.4 Controlling compiler options for host programs
30 --- 4.5 When host programs are actually built
31 --- 4.6 Using hostprogs-$(CONFIG_FOO)
33 === 5 Kbuild clean infrastructure
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
48 === 7 Kbuild syntax for exported headers
49 --- 7.1 no-export-headers
54 === 8 Kbuild Variables
55 === 9 Makefile language
62 The Makefiles have five parts::
64 Makefile the top Makefile.
65 .config the kernel configuration file.
66 arch/$(ARCH)/Makefile the arch Makefile.
67 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
68 kbuild Makefiles there are about 500 of these.
70 The top Makefile reads the .config file, which comes from the kernel
71 configuration process.
73 The top Makefile is responsible for building two major products: vmlinux
74 (the resident kernel image) and modules (any module files).
75 It builds these goals by recursively descending into the subdirectories of
76 the kernel source tree.
77 The list of subdirectories which are visited depends upon the kernel
78 configuration. The top Makefile textually includes an arch Makefile
79 with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
80 architecture-specific information to the top Makefile.
82 Each subdirectory has a kbuild Makefile which carries out the commands
83 passed down from above. The kbuild Makefile uses information from the
84 .config file to construct various file lists used by kbuild to build
85 any built-in or modular targets.
87 scripts/Makefile.* contains all the definitions/rules etc. that
88 are used to build the kernel based on the kbuild makefiles.
94 People have four different relationships with the kernel Makefiles.
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
98 any kernel Makefiles (or any other source files).
100 *Normal developers* are people who work on features such as device
101 drivers, file systems, and network protocols. These people need to
102 maintain the kbuild Makefiles for the subsystem they are
103 working on. In order to do this effectively, they need some overall
104 knowledge about the kernel Makefiles, plus detailed knowledge about the
105 public interface for kbuild.
107 *Arch developers* are people who work on an entire architecture, such
108 as sparc or ia64. Arch developers need to know about the arch Makefile
109 as well as kbuild Makefiles.
111 *Kbuild developers* are people who work on the kernel build system itself.
112 These people need to know about all aspects of the kernel Makefiles.
114 This document is aimed towards normal developers and arch developers.
120 Most Makefiles within the kernel are kbuild Makefiles that use the
121 kbuild infrastructure. This chapter introduces the syntax used in the
123 The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
124 be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
127 Section 3.1 "Goal definitions" is a quick intro, further chapters provide
128 more details, with real examples.
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.
137 The most simple kbuild makefile contains one line:
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.
146 If foo.o shall be built as a module, the variable obj-m is used.
147 Therefore the following pattern is often used:
151 obj-$(CONFIG_FOO) += foo.o
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
157 3.2 Built-in object goals - obj-y
158 ---------------------------------
160 The kbuild Makefile specifies object files for vmlinux
161 in the $(obj-y) lists. These lists depend on the kernel
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
169 The order of files in $(obj-y) is significant. Duplicates in
170 the lists are allowed: the first instance will be linked into
171 built-in.a and succeeding instances will be ignored.
173 Link order is significant, because certain functions
174 (module_init() / __initcall) will be called during boot in the
175 order they appear. So keep in mind that changing the link
176 order may e.g. change the order in which your SCSI
177 controllers are detected, and thus your disks are renumbered.
181 #drivers/isdn/i4l/Makefile
182 # Makefile for the kernel ISDN subsystem and device drivers.
183 # Each configuration option enables a list of files.
184 obj-$(CONFIG_ISDN_I4L) += isdn.o
185 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
187 3.3 Loadable module goals - obj-m
188 ---------------------------------
190 $(obj-m) specifies object files which are built as loadable
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 $(obj-m).
199 #drivers/isdn/i4l/Makefile
200 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
202 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
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 $(<module_name>-y)
212 #drivers/isdn/i4l/Makefile
213 obj-$(CONFIG_ISDN_I4L) += isdn.o
214 isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
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.
220 Due to kbuild recognizing $(<module_name>-y) for composite objects,
221 you can use the value of a `CONFIG_` symbol to optionally include an
222 object file as part of a composite object.
227 obj-$(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 \
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)
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.
242 3.4 Objects which export symbols
243 --------------------------------
245 No special notation is required in the makefiles for
246 modules exporting symbols.
248 3.5 Library file goals - lib-y
249 ------------------------------
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.
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.
268 #arch/x86/lib/Makefile
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.
275 See also "6.4 List directories to visit when descending".
277 Use of lib-y is normally restricted to `lib/` and `arch/*/lib`.
279 3.6 Descending down in directories
280 ----------------------------------
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
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.
295 obj-$(CONFIG_EXT2_FS) += ext2/
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.
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'.
308 3.7 Compilation flags
309 ---------------------
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.
319 ccflags-y specifies options for compiling with $(CC).
323 # drivers/acpi/acpica/Makefile
324 ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA
325 ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT
327 This variable is necessary because the top Makefile owns the
328 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
331 asflags-y specifies options for assembling with $(AS).
335 #arch/sparc/kernel/Makefile
338 ldflags-y specifies options for linking with $(LD).
342 #arch/cris/boot/compressed/Makefile
343 ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
345 subdir-ccflags-y, subdir-asflags-y
346 The two flags listed above are similar to ccflags-y and asflags-y.
347 The difference is that the subdir- variants have effect for the kbuild
348 file where they are present and all subdirectories.
349 Options specified using subdir-* are added to the commandline before
350 the options specified using the non-subdir variants.
354 subdir-ccflags-y := -Werror
357 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
360 $(CFLAGS_$@) specifies per-file options for $(CC). The $@
361 part has a literal value which specifies the file that it is for.
365 # drivers/scsi/Makefile
366 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
367 CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
370 These two lines specify compilation flags for aha152x.o and gdth.o.
372 $(AFLAGS_$@) is a similar feature for source files in assembly
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
383 3.9 Dependency tracking
384 -----------------------
386 Kbuild tracks dependencies on the following:
387 1) All prerequisite files (both `*.c` and `*.h`)
388 2) `CONFIG_` options used in all prerequisite files
389 3) Command-line used to compile target
391 Thus, if you change an option to $(CC) all affected files will
397 Special rules are used when the kbuild infrastructure does
398 not provide the required support. A typical example is
399 header files generated during the build process.
400 Another example are the architecture-specific Makefiles which
401 need special rules to prepare boot images etc.
403 Special rules are written as normal Make rules.
404 Kbuild is not executing in the directory where the Makefile is
405 located, so all special rules shall provide a relative
406 path to prerequisite files and target files.
408 Two variables are used when defining special rules:
411 $(src) is a relative path which points to the directory
412 where the Makefile is located. Always use $(src) when
413 referring to files located in the src tree.
416 $(obj) is a relative path which points to the directory
417 where the target is saved. Always use $(obj) when
418 referring to generated files.
422 #drivers/scsi/Makefile
423 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
424 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
426 This is a special rule, following the normal syntax
429 The target file depends on two prerequisite files. References
430 to the target file are prefixed with $(obj), references
431 to prerequisites are referenced with $(src) (because they are not
435 echoing information to user in a rule is often a good practice
436 but when execution "make -s" one does not expect to see any output
437 except for warnings/errors.
438 To support this kbuild defines $(kecho) which will echo out the
439 text following $(kecho) to stdout except if "make -s" is used.
443 #arch/blackfin/boot/Makefile
444 $(obj)/vmImage: $(obj)/vmlinux.gz
445 $(call if_changed,uimage)
446 @$(kecho) 'Kernel: $@ is ready'
449 3.11 $(CC) support functions
450 ----------------------------
452 The kernel may be built with several different versions of
453 $(CC), each supporting a unique set of features and options.
454 kbuild provides basic support to check for valid options for $(CC).
455 $(CC) is usually the gcc compiler, but other alternatives are
459 as-option is used to check if $(CC) -- when used to compile
460 assembler (`*.S`) files -- supports the given option. An optional
461 second option may be specified if the first option is not supported.
466 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
468 In the above example, cflags-y will be assigned the option
469 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
470 The second argument is optional, and if supplied will be used
471 if first argument is not supported.
474 cc-ldoption is used to check if $(CC) when used to link object files
475 supports the given option. An optional second option may be
476 specified if first option are not supported.
480 #arch/x86/kernel/Makefile
481 vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
483 In the above example, vsyscall-flags will be assigned the option
484 -Wl$(comma)--hash-style=sysv if it is supported by $(CC).
485 The second argument is optional, and if supplied will be used
486 if first argument is not supported.
489 as-instr checks if the assembler reports a specific instruction
490 and then outputs either option1 or option2
491 C escapes are supported in the test instruction
492 Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options
495 cc-option is used to check if $(CC) supports a given option, and if
496 not supported to use an optional second option.
501 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
503 In the above example, cflags-y will be assigned the option
504 -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
505 The second argument to cc-option is optional, and if omitted,
506 cflags-y will be assigned no value if first option is not supported.
507 Note: cc-option uses KBUILD_CFLAGS for $(CC) options
510 cc-option-yn is used to check if gcc supports a given option
511 and return 'y' if supported, otherwise 'n'.
516 biarch := $(call cc-option-yn, -m32)
517 aflags-$(biarch) += -a32
518 cflags-$(biarch) += -m32
520 In the above example, $(biarch) is set to y if $(CC) supports the -m32
521 option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
522 and $(cflags-y) will be assigned the values -a32 and -m32,
524 Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
527 cc-disable-warning checks if gcc supports a given warning and returns
528 the commandline switch to disable it. This special function is needed,
529 because gcc 4.4 and later accept any unknown -Wno-* option and only
530 warn about it if there is another warning in the source file.
534 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
536 In the above example, -Wno-unused-but-set-variable will be added to
537 KBUILD_CFLAGS only if gcc really accepts it.
540 cc-ifversion tests the version of $(CC) and equals the fourth parameter
541 if version expression is true, or the fifth (if given) if the version
546 #fs/reiserfs/Makefile
547 ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
549 In this example, ccflags-y will be assigned the value -O1 if the
550 $(CC) version is less than 4.2.
551 cc-ifversion takes all the shell operators:
552 -eq, -ne, -lt, -le, -gt, and -ge
553 The third parameter may be a text as in this example, but it may also
554 be an expanded variable or a macro.
557 cc-cross-prefix is used to check if there exists a $(CC) in path with
558 one of the listed prefixes. The first prefix where there exist a
559 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
560 then nothing is returned.
561 Additional prefixes are separated by a single space in the
562 call of cc-cross-prefix.
563 This functionality is useful for architecture Makefiles that try
564 to set CROSS_COMPILE to well-known values but may have several
565 values to select between.
566 It is recommended only to try to set CROSS_COMPILE if it is a cross
567 build (host arch is different from target arch). And if CROSS_COMPILE
568 is already set then leave it with the old value.
573 ifneq ($(SUBARCH),$(ARCH))
574 ifeq ($(CROSS_COMPILE),)
575 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
579 3.12 $(LD) support functions
580 ----------------------------
583 ld-option is used to check if $(LD) supports the supplied option.
584 ld-option takes two options as arguments.
585 The second argument is an optional option that can be used if the
586 first option is not supported by $(LD).
591 LDFLAGS_vmlinux += $(call ld-option, -X)
594 4 Host Program support
595 ======================
597 Kbuild supports building executables on the host for use during the
599 Two steps are required in order to use a host executable.
601 The first step is to tell kbuild that a host program exists. This is
602 done utilising the variable hostprogs-y.
604 The second step is to add an explicit dependency to the executable.
605 This can be done in two ways. Either add the dependency in a rule,
606 or utilise the variable $(always).
607 Both possibilities are described in the following.
609 4.1 Simple Host Program
610 -----------------------
612 In some cases there is a need to compile and run a program on the
613 computer where the build is running.
614 The following line tells kbuild that the program bin2hex shall be
615 built on the build host.
619 hostprogs-y := bin2hex
621 Kbuild assumes in the above example that bin2hex is made from a single
622 c-source file named bin2hex.c located in the same directory as
625 4.2 Composite Host Programs
626 ---------------------------
628 Host programs can be made up based on composite objects.
629 The syntax used to define composite objects for host programs is
630 similar to the syntax used for kernel objects.
631 $(<executable>-objs) lists all objects used to link the final
636 #scripts/lxdialog/Makefile
637 hostprogs-y := lxdialog
638 lxdialog-objs := checklist.o lxdialog.o
640 Objects with extension .o are compiled from the corresponding .c
641 files. In the above example, checklist.c is compiled to checklist.o
642 and lxdialog.c is compiled to lxdialog.o.
644 Finally, the two .o files are linked to the executable, lxdialog.
645 Note: The syntax <executable>-y is not permitted for host-programs.
647 4.3 Using C++ for host programs
648 -------------------------------
650 kbuild offers support for host programs written in C++. This was
651 introduced solely to support kconfig, and is not recommended
656 #scripts/kconfig/Makefile
658 qconf-cxxobjs := qconf.o
660 In the example above the executable is composed of the C++ file
661 qconf.cc - identified by $(qconf-cxxobjs).
663 If qconf is composed of a mixture of .c and .cc files, then an
664 additional line can be used to identify this.
668 #scripts/kconfig/Makefile
670 qconf-cxxobjs := qconf.o
671 qconf-objs := check.o
673 4.4 Controlling compiler options for host programs
674 --------------------------------------------------
676 When compiling host programs, it is possible to set specific flags.
677 The programs will always be compiled utilising $(HOSTCC) passed
678 the options specified in $(KBUILD_HOSTCFLAGS).
679 To set flags that will take effect for all host programs created
680 in that Makefile, use the variable HOST_EXTRACFLAGS.
684 #scripts/lxdialog/Makefile
685 HOST_EXTRACFLAGS += -I/usr/include/ncurses
687 To set specific flags for a single file the following construction
692 #arch/ppc64/boot/Makefile
693 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
695 It is also possible to specify additional options to the linker.
699 #scripts/kconfig/Makefile
700 HOSTLDLIBS_qconf := -L$(QTDIR)/lib
702 When linking qconf, it will be passed the extra option
705 4.5 When host programs are actually built
706 -----------------------------------------
708 Kbuild will only build host-programs when they are referenced
710 This is possible in two ways:
712 (1) List the prerequisite explicitly in a special rule.
716 #drivers/pci/Makefile
717 hostprogs-y := gen-devlist
718 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
719 ( cd $(obj); ./gen-devlist ) < $<
721 The target $(obj)/devlist.h will not be built before
722 $(obj)/gen-devlist is updated. Note that references to
723 the host programs in special rules must be prefixed with $(obj).
727 When there is no suitable special rule, and the host program
728 shall be built when a makefile is entered, the $(always)
729 variable shall be used.
733 #scripts/lxdialog/Makefile
734 hostprogs-y := lxdialog
735 always := $(hostprogs-y)
737 This will tell kbuild to build lxdialog even if not referenced in
740 4.6 Using hostprogs-$(CONFIG_FOO)
741 ---------------------------------
743 A typical pattern in a Kbuild file looks like this:
748 hostprogs-$(CONFIG_KALLSYMS) += kallsyms
750 Kbuild knows about both 'y' for built-in and 'm' for module.
751 So if a config symbol evaluates to 'm', kbuild will still build
752 the binary. In other words, Kbuild handles hostprogs-m exactly
753 like hostprogs-y. But only hostprogs-y is recommended to be used
754 when no CONFIG symbols are involved.
756 5 Kbuild clean infrastructure
757 =============================
759 "make clean" deletes most generated files in the obj tree where the kernel
760 is compiled. This includes generated files such as host programs.
761 Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
762 $(extra-y) and $(targets). They are all deleted during "make clean".
763 Files matching the patterns "*.[oas]", "*.ko", plus some additional files
764 generated by kbuild are deleted all over the kernel src tree when
765 "make clean" is executed.
767 Additional files can be specified in kbuild makefiles by use of $(clean-files).
772 clean-files := crc32table.h
774 When executing "make clean", the file "crc32table.h" will be deleted.
775 Kbuild will assume files to be in the same relative directory as the
776 Makefile, except if prefixed with $(objtree).
778 To delete a directory hierarchy use:
782 #scripts/package/Makefile
783 clean-dirs := $(objtree)/debian/
785 This will delete the directory debian in the toplevel directory, including all
788 To exclude certain files from make clean, use the $(no-clean-files) variable.
789 This is only a special case used in the top level Kbuild file:
794 no-clean-files := $(bounds-file) $(offsets-file)
796 Usually kbuild descends down in subdirectories due to "obj-* := dir/",
797 but in the architecture makefiles where the kbuild infrastructure
798 is not sufficient this sometimes needs to be explicit.
802 #arch/x86/boot/Makefile
803 subdir- := compressed/
805 The above assignment instructs kbuild to descend down in the
806 directory compressed/ when "make clean" is executed.
808 To support the clean infrastructure in the Makefiles that build the
809 final bootimage there is an optional target named archclean:
815 $(Q)$(MAKE) $(clean)=arch/x86/boot
817 When "make clean" is executed, make will descend down in arch/x86/boot,
818 and clean as usual. The Makefile located in arch/x86/boot/ may use
819 the subdir- trick to descend further down.
821 Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
822 included in the top level makefile, and the kbuild infrastructure
823 is not operational at that point.
825 Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
826 be visited during "make clean".
828 6 Architecture Makefiles
829 ========================
831 The top level Makefile sets up the environment and does the preparation,
832 before starting to descend down in the individual directories.
833 The top level makefile contains the generic part, whereas
834 arch/$(ARCH)/Makefile contains what is required to set up kbuild
835 for said architecture.
836 To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
839 When kbuild executes, the following steps are followed (roughly):
841 1) Configuration of the kernel => produce .config
842 2) Store kernel version in include/linux/version.h
843 3) Updating all other prerequisites to the target prepare:
844 - Additional prerequisites are specified in arch/$(ARCH)/Makefile
845 4) Recursively descend down in all directories listed in
846 init-* core* drivers-* net-* libs-* and build all targets.
847 - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
848 5) All object files are then linked and the resulting file vmlinux is
849 located at the root of the obj tree.
850 The very first objects linked are listed in head-y, assigned by
851 arch/$(ARCH)/Makefile.
852 6) Finally, the architecture-specific part does any required post processing
853 and builds the final bootimage.
854 - This includes building boot records
855 - Preparing initrd images and the like
858 6.1 Set variables to tweak the build to the architecture
859 --------------------------------------------------------
862 Generic $(LD) options
864 Flags used for all invocations of the linker.
865 Often specifying the emulation is sufficient.
870 LDFLAGS := -m elf_s390
872 Note: ldflags-y can be used to further customise
873 the flags used. See chapter 3.7.
876 Options for $(LD) when linking vmlinux
878 LDFLAGS_vmlinux is used to specify additional flags to pass to
879 the linker when linking the final vmlinux image.
880 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
885 LDFLAGS_vmlinux := -e stext
890 When $(call if_changed,objcopy) is used to translate a .o file,
891 the flags specified in OBJCOPYFLAGS will be used.
892 $(call if_changed,objcopy) is often used to generate raw binaries on
898 OBJCOPYFLAGS := -O binary
900 #arch/s390/boot/Makefile
901 $(obj)/image: vmlinux FORCE
902 $(call if_changed,objcopy)
904 In this example, the binary $(obj)/image is a binary version of
905 vmlinux. The usage of $(call if_changed,xxx) will be described later.
908 $(AS) assembler flags
910 Default value - see top level Makefile
911 Append or modify as required per architecture.
915 #arch/sparc64/Makefile
916 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
921 Default value - see top level Makefile
922 Append or modify as required per architecture.
924 Often, the KBUILD_CFLAGS variable depends on the configuration.
928 #arch/x86/boot/compressed/Makefile
929 cflags-$(CONFIG_X86_32) := -march=i386
930 cflags-$(CONFIG_X86_64) := -mcmodel=small
931 KBUILD_CFLAGS += $(cflags-y)
933 Many arch Makefiles dynamically run the target C compiler to
934 probe supported options::
939 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
940 -march=pentium2,-march=i686)
942 # Disable unit-at-a-time mode ...
943 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
947 The first example utilises the trick that a config option expands
948 to 'y' when selected.
951 $(AS) options specific for built-in
953 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
954 resident kernel code.
957 Options for $(AS) when building modules
959 $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
962 From commandline AFLAGS_MODULE shall be used (see kbuild.txt).
965 $(CC) options specific for built-in
967 $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
968 resident kernel code.
971 Options for $(CC) when building modules
973 $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
975 From commandline CFLAGS_MODULE shall be used (see kbuild.txt).
977 KBUILD_LDFLAGS_MODULE
978 Options for $(LD) when linking modules
980 $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
981 used when linking modules. This is often a linker script.
983 From commandline LDFLAGS_MODULE shall be used (see kbuild.txt).
985 KBUILD_ARFLAGS Options for $(AR) when creating archives
987 $(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic
988 mode) if this option is supported by $(AR).
990 ARCH_CPPFLAGS, ARCH_AFLAGS, ARCH_CFLAGS Overrides the kbuild defaults
992 These variables are appended to the KBUILD_CPPFLAGS,
993 KBUILD_AFLAGS, and KBUILD_CFLAGS, respectively, after the
994 top-level Makefile has set any other flags. This provides a
995 means for an architecture to override the defaults.
998 6.2 Add prerequisites to archheaders
999 ------------------------------------
1001 The archheaders: rule is used to generate header files that
1002 may be installed into user space by "make header_install".
1004 It is run before "make archprepare" when run on the
1005 architecture itself.
1008 6.3 Add prerequisites to archprepare
1009 ------------------------------------
1011 The archprepare: rule is used to list prerequisites that need to be
1012 built before starting to descend down in the subdirectories.
1013 This is usually used for header files containing assembler constants.
1018 archprepare: maketools
1020 In this example, the file target maketools will be processed
1021 before descending down in the subdirectories.
1022 See also chapter XXX-TODO that describe how kbuild supports
1023 generating offset header files.
1026 6.4 List directories to visit when descending
1027 ---------------------------------------------
1029 An arch Makefile cooperates with the top Makefile to define variables
1030 which specify how to build the vmlinux file. Note that there is no
1031 corresponding arch-specific section for modules; the module-building
1032 machinery is all architecture-independent.
1035 head-y, init-y, core-y, libs-y, drivers-y, net-y
1036 $(head-y) lists objects to be linked first in vmlinux.
1038 $(libs-y) lists directories where a lib.a archive can be located.
1040 The rest list directories where a built-in.a object file can be
1043 $(init-y) objects will be located after $(head-y).
1045 Then the rest follows in this order:
1047 $(core-y), $(libs-y), $(drivers-y) and $(net-y).
1049 The top level Makefile defines values for all generic directories,
1050 and arch/$(ARCH)/Makefile only adds architecture-specific
1055 #arch/sparc64/Makefile
1056 core-y += arch/sparc64/kernel/
1057 libs-y += arch/sparc64/prom/ arch/sparc64/lib/
1058 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
1061 6.5 Architecture-specific boot images
1062 -------------------------------------
1064 An arch Makefile specifies goals that take the vmlinux file, compress
1065 it, wrap it in bootstrapping code, and copy the resulting files
1066 somewhere. This includes various kinds of installation commands.
1067 The actual goals are not standardized across architectures.
1069 It is common to locate any additional processing in a boot/
1070 directory below arch/$(ARCH)/.
1072 Kbuild does not provide any smart way to support building a
1073 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
1074 call make manually to build a target in boot/.
1076 The recommended approach is to include shortcuts in
1077 arch/$(ARCH)/Makefile, and use the full path when calling down
1078 into the arch/$(ARCH)/boot/Makefile.
1083 boot := arch/x86/boot
1085 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
1087 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
1088 make in a subdirectory.
1090 There are no rules for naming architecture-specific targets,
1091 but executing "make help" will list all relevant targets.
1092 To support this, $(archhelp) must be defined.
1098 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
1101 When make is executed without arguments, the first goal encountered
1102 will be built. In the top level Makefile the first goal present
1104 An architecture shall always, per default, build a bootable image.
1105 In "make help", the default goal is highlighted with a '*'.
1106 Add a new prerequisite to all: to select a default goal different
1114 When "make" is executed without arguments, bzImage will be built.
1116 6.6 Building non-kbuild targets
1117 -------------------------------
1120 extra-y specifies additional targets created in the current
1121 directory, in addition to any targets specified by `obj-*`.
1123 Listing all targets in extra-y is required for two purposes:
1125 1) Enable kbuild to check changes in command lines
1127 - When $(call if_changed,xxx) is used
1129 2) kbuild knows what files to delete during "make clean"
1133 #arch/x86/kernel/Makefile
1134 extra-y := head.o init_task.o
1136 In this example, extra-y is used to list object files that
1137 shall be built, but shall not be linked as part of built-in.a.
1141 header-test-y specifies headers (*.h) in the current directory that
1142 should be compile tested to ensure they are self-contained,
1143 i.e. compilable as standalone units. If CONFIG_HEADER_TEST is enabled,
1144 this builds them as part of extra-y.
1146 header-test-pattern-y
1148 This works as a weaker version of header-test-y, and accepts wildcard
1149 patterns. The typical usage is:
1151 header-test-pattern-y += *.h
1153 This specifies all the files that matches to '*.h' in the current
1154 directory, but the files in 'header-test-' are excluded.
1156 6.7 Commands useful for building a boot image
1157 ---------------------------------------------
1159 Kbuild provides a few macros that are useful when building a
1163 if_changed is the infrastructure used for the following commands.
1167 target: source(s) FORCE
1168 $(call if_changed,ld/objcopy/gzip/...)
1170 When the rule is evaluated, it is checked to see if any files
1171 need an update, or the command line has changed since the last
1172 invocation. The latter will force a rebuild if any options
1173 to the executable have changed.
1174 Any target that utilises if_changed must be listed in $(targets),
1175 otherwise the command line check will fail, and the target will
1177 Assignments to $(targets) are without $(obj)/ prefix.
1178 if_changed may be used in conjunction with custom commands as
1179 defined in 6.8 "Custom kbuild commands".
1181 Note: It is a typical mistake to forget the FORCE prerequisite.
1182 Another common pitfall is that whitespace is sometimes
1183 significant; for instance, the below will fail (note the extra space
1186 target: source(s) FORCE
1188 **WRONG!** $(call if_changed, ld/objcopy/gzip/...)
1191 if_changed should not be used more than once per target.
1192 It stores the executed command in a corresponding .cmd
1194 file and multiple calls would result in overwrites and
1195 unwanted results when the target is up to date and only the
1196 tests on changed commands trigger execution of commands.
1199 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
1203 #arch/x86/boot/Makefile
1204 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1205 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
1207 targets += setup setup.o bootsect bootsect.o
1208 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1209 $(call if_changed,ld)
1211 In this example, there are two possible targets, requiring different
1212 options to the linker. The linker options are specified using the
1213 LDFLAGS_$@ syntax - one for each potential target.
1214 $(targets) are assigned all potential targets, by which kbuild knows
1215 the targets and will:
1217 1) check for commandline changes
1218 2) delete target during make clean
1220 The ": %: %.o" part of the prerequisite is a shorthand that
1221 frees us from listing the setup.o and bootsect.o files.
1224 It is a common mistake to forget the "targets :=" assignment,
1225 resulting in the target file being recompiled for no
1229 Copy binary. Uses OBJCOPYFLAGS usually specified in
1230 arch/$(ARCH)/Makefile.
1231 OBJCOPYFLAGS_$@ may be used to set additional options.
1234 Compress target. Use maximum compression to compress target.
1238 #arch/x86/boot/compressed/Makefile
1239 $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
1240 $(call if_changed,gzip)
1243 Create flattened device tree blob object suitable for linking
1244 into vmlinux. Device tree blobs linked into vmlinux are placed
1245 in an init section in the image. Platform code *must* copy the
1246 blob to non-init memory prior to calling unflatten_device_tree().
1248 To use this command, simply add `*.dtb` into obj-y or targets, or make
1249 some other target depend on `%.dtb`
1251 A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
1252 architecture Makefiles do no need to explicitly write out that rule.
1257 DTC_FLAGS ?= -p 1024
1259 6.8 Custom kbuild commands
1260 --------------------------
1262 When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
1263 of a command is normally displayed.
1264 To enable this behaviour for custom commands kbuild requires
1265 two variables to be set::
1267 quiet_cmd_<command> - what shall be echoed
1268 cmd_<command> - the command to execute
1273 quiet_cmd_image = BUILD $@
1274 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
1275 $(obj)/vmlinux.bin > $@
1278 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
1279 $(call if_changed,image)
1280 @echo 'Kernel: $@ is ready'
1282 When updating the $(obj)/bzImage target, the line:
1284 BUILD arch/x86/boot/bzImage
1286 will be displayed with "make KBUILD_VERBOSE=0".
1289 --- 6.9 Preprocessing linker scripts
1291 When the vmlinux image is built, the linker script
1292 arch/$(ARCH)/kernel/vmlinux.lds is used.
1293 The script is a preprocessed variant of the file vmlinux.lds.S
1294 located in the same directory.
1295 kbuild knows .lds files and includes a rule `*lds.S` -> `*lds`.
1299 #arch/x86/kernel/Makefile
1300 always := vmlinux.lds
1303 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
1305 The assignment to $(always) is used to tell kbuild to build the
1307 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
1308 specified options when building the target vmlinux.lds.
1310 When building the `*.lds` target, kbuild uses the variables::
1312 KBUILD_CPPFLAGS : Set in top-level Makefile
1313 cppflags-y : May be set in the kbuild makefile
1314 CPPFLAGS_$(@F) : Target-specific flags.
1315 Note that the full filename is used in this
1318 The kbuild infrastructure for `*lds` files is used in several
1319 architecture-specific files.
1321 6.10 Generic header files
1322 -------------------------
1324 The directory include/asm-generic contains the header files
1325 that may be shared between individual architectures.
1326 The recommended approach how to use a generic header file is
1327 to list the file in the Kbuild file.
1328 See "7.2 generic-y" for further info on syntax etc.
1333 If the file arch/xxx/Makefile.postlink exists, this makefile
1334 will be invoked for post-link objects (vmlinux and modules.ko)
1335 for architectures to run post-link passes on. Must also handle
1338 This pass runs after kallsyms generation. If the architecture
1339 needs to modify symbol locations, rather than manipulate the
1340 kallsyms, it may be easier to add another postlink target for
1341 .tmp_vmlinux? targets to be called from link-vmlinux.sh.
1343 For example, powerpc uses this to check relocation sanity of
1344 the linked vmlinux file.
1346 7 Kbuild syntax for exported headers
1347 ------------------------------------
1349 The kernel includes a set of headers that is exported to userspace.
1350 Many headers can be exported as-is but other headers require a
1351 minimal pre-processing before they are ready for user-space.
1352 The pre-processing does:
1354 - drop kernel-specific annotations
1355 - drop include of compiler.h
1356 - drop all sections that are kernel internal (guarded by `ifdef __KERNEL__`)
1358 All headers under include/uapi/, include/generated/uapi/,
1359 arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
1362 A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
1363 arch/<arch>/include/asm/ to list asm files coming from asm-generic.
1364 See subsequent chapter for the syntax of the Kbuild file.
1366 7.1 no-export-headers
1367 ---------------------
1369 no-export-headers is essentially used by include/uapi/linux/Kbuild to
1370 avoid exporting specific headers (e.g. kvm.h) on architectures that do
1371 not support it. It should be avoided as much as possible.
1376 If an architecture uses a verbatim copy of a header from
1377 include/asm-generic then this is listed in the file
1378 arch/$(ARCH)/include/asm/Kbuild like this:
1382 #arch/x86/include/asm/Kbuild
1383 generic-y += termios.h
1386 During the prepare phase of the build a wrapper include
1387 file is generated in the directory::
1389 arch/$(ARCH)/include/generated/asm
1391 When a header is exported where the architecture uses
1392 the generic header a similar wrapper is generated as part
1393 of the set of exported headers in the directory::
1397 The generated wrapper will in both cases look like the following:
1399 Example: termios.h::
1401 #include <asm-generic/termios.h>
1406 If an architecture generates other header files alongside generic-y
1407 wrappers, generated-y specifies them.
1409 This prevents them being treated as stale asm-generic wrappers and
1414 #arch/x86/include/asm/Kbuild
1415 generated-y += syscalls_32.h
1420 mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
1421 to define the minimum set of ASM headers that all architectures must have.
1423 This works like optional generic-y. If a mandatory header is missing
1424 in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate
1425 a wrapper of the asm-generic one.
1427 The convention is to list one subdir per line and
1428 preferably in alphabetic order.
1433 The top Makefile exports the following variables:
1435 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1436 These variables define the current kernel version. A few arch
1437 Makefiles actually use these values directly; they should use
1438 $(KERNELRELEASE) instead.
1440 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1441 three-part version number, such as "2", "4", and "0". These three
1442 values are always numeric.
1444 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1445 or additional patches. It is usually some non-numeric string
1446 such as "-pre4", and is often blank.
1449 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1450 for constructing installation directory names or showing in
1451 version strings. Some arch Makefiles use it for this purpose.
1454 This variable defines the target architecture, such as "i386",
1455 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1456 determine which files to compile.
1458 By default, the top Makefile sets $(ARCH) to be the same as the
1459 host system architecture. For a cross build, a user may
1460 override the value of $(ARCH) on the command line::
1466 This variable defines a place for the arch Makefiles to install
1467 the resident kernel image and System.map file.
1468 Use this for architecture-specific install targets.
1470 INSTALL_MOD_PATH, MODLIB
1471 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1472 installation. This variable is not defined in the Makefile but
1473 may be passed in by the user if desired.
1475 $(MODLIB) specifies the directory for module installation.
1476 The top Makefile defines $(MODLIB) to
1477 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1478 override this value on the command line if desired.
1481 If this variable is specified, it will cause modules to be stripped
1482 after they are installed. If INSTALL_MOD_STRIP is '1', then the
1483 default option --strip-debug will be used. Otherwise, the
1484 INSTALL_MOD_STRIP value will be used as the option(s) to the strip
1491 The kernel Makefiles are designed to be run with GNU Make. The Makefiles
1492 use only the documented features of GNU Make, but they do use many
1495 GNU Make supports elementary list-processing functions. The kernel
1496 Makefiles use a novel style of list building and manipulation with few
1499 GNU Make has two assignment operators, ":=" and "=". ":=" performs
1500 immediate evaluation of the right-hand side and stores an actual string
1501 into the left-hand side. "=" is like a formula definition; it stores the
1502 right-hand side in an unevaluated form and then evaluates this form each
1503 time the left-hand side is used.
1505 There are some cases where "=" is appropriate. Usually, though, ":="
1506 is the right choice.
1511 - Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1512 - Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1513 - Updates by Sam Ravnborg <sam@ravnborg.org>
1514 - Language QA by Jan Engelhardt <jengelh@gmx.de>
1519 - Describe how kbuild supports shipped files with _shipped.
1520 - Generating offset header files.
1521 - Add more variables to section 7?