3 This document describes the Linux kernel Makefiles.
9 === 3 The kbuild Makefiles
10 --- 3.1 Goal definitions
11 --- 3.2 Built-in object goals - obj-y
12 --- 3.3 Loadable module goals - obj-m
13 --- 3.4 Objects which export symbols
14 --- 3.5 Library file goals - lib-y
15 --- 3.6 Descending down in directories
16 --- 3.7 Compilation flags
17 --- 3.8 Command line dependency
18 --- 3.9 Dependency tracking
19 --- 3.10 Special Rules
21 === 4 Host Program support
22 --- 4.1 Simple Host Program
23 --- 4.2 Composite Host Programs
24 --- 4.3 Defining shared libraries
25 --- 4.4 Using C++ for host programs
26 --- 4.5 Controlling compiler options for host programs
27 --- 4.6 When host programs are actually built
28 --- 4.7 Using hostprogs-$(CONFIG_FOO)
30 === 5 Kbuild clean infrastructure
32 === 6 Architecture Makefiles
33 --- 6.1 Set variables to tweak the build to the architecture
34 --- 6.2 Add prerequisites to prepare:
35 --- 6.3 List directories to visit when descending
36 --- 6.4 Architecture specific boot images
37 --- 6.5 Building non-kbuild targets
38 --- 6.6 Commands useful for building a boot image
39 --- 6.7 Custom kbuild commands
40 --- 6.8 Preprocessing linker scripts
41 --- 6.9 $(CC) support functions
43 === 7 Kbuild Variables
44 === 8 Makefile language
50 The Makefiles have five parts:
52 Makefile the top Makefile.
53 .config the kernel configuration file.
54 arch/$(ARCH)/Makefile the arch Makefile.
55 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
56 kbuild Makefiles there are about 500 of these.
58 The top Makefile reads the .config file, which comes from the kernel
59 configuration process.
61 The top Makefile is responsible for building two major products: vmlinux
62 (the resident kernel image) and modules (any module files).
63 It builds these goals by recursively descending into the subdirectories of
64 the kernel source tree.
65 The list of subdirectories which are visited depends upon the kernel
66 configuration. The top Makefile textually includes an arch Makefile
67 with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
68 architecture-specific information to the top Makefile.
70 Each subdirectory has a kbuild Makefile which carries out the commands
71 passed down from above. The kbuild Makefile uses information from the
72 .config file to construct various file lists used by kbuild to build
73 any built-in or modular targets.
75 scripts/Makefile.* contains all the definitions/rules etc. that
76 are used to build the kernel based on the kbuild makefiles.
81 People have four different relationships with the kernel Makefiles.
83 *Users* are people who build kernels. These people type commands such as
84 "make menuconfig" or "make". They usually do not read or edit
85 any kernel Makefiles (or any other source files).
87 *Normal developers* are people who work on features such as device
88 drivers, file systems, and network protocols. These people need to
89 maintain the kbuild Makefiles for the subsystem that they are
90 working on. In order to do this effectively, they need some overall
91 knowledge about the kernel Makefiles, plus detailed knowledge about the
92 public interface for kbuild.
94 *Arch developers* are people who work on an entire architecture, such
95 as sparc or ia64. Arch developers need to know about the arch Makefile
96 as well as kbuild Makefiles.
98 *Kbuild developers* are people who work on the kernel build system itself.
99 These people need to know about all aspects of the kernel Makefiles.
101 This document is aimed towards normal developers and arch developers.
104 === 3 The kbuild Makefiles
106 Most Makefiles within the kernel are kbuild Makefiles that use the
107 kbuild infrastructure. This chapter introduce the syntax used in the
110 Section 3.1 "Goal definitions" is a quick intro, further chapters provide
111 more details, with real examples.
113 --- 3.1 Goal definitions
115 Goal definitions are the main part (heart) of the kbuild Makefile.
116 These lines define the files to be built, any special compilation
117 options, and any subdirectories to be entered recursively.
119 The most simple kbuild makefile contains one line:
124 This tell kbuild that there is one object in that directory named
125 foo.o. foo.o will be built from foo.c or foo.S.
127 If foo.o shall be built as a module, the variable obj-m is used.
128 Therefore the following pattern is often used:
131 obj-$(CONFIG_FOO) += foo.o
133 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
134 If CONFIG_FOO is neither y nor m, then the file will not be compiled
137 --- 3.2 Built-in object goals - obj-y
139 The kbuild Makefile specifies object files for vmlinux
140 in the lists $(obj-y). These lists depend on the kernel
143 Kbuild compiles all the $(obj-y) files. It then calls
144 "$(LD) -r" to merge these files into one built-in.o file.
145 built-in.o is later linked into vmlinux by the parent Makefile.
147 The order of files in $(obj-y) is significant. Duplicates in
148 the lists are allowed: the first instance will be linked into
149 built-in.o and succeeding instances will be ignored.
151 Link order is significant, because certain functions
152 (module_init() / __initcall) will be called during boot in the
153 order they appear. So keep in mind that changing the link
154 order may e.g. change the order in which your SCSI
155 controllers are detected, and thus you disks are renumbered.
158 #drivers/isdn/i4l/Makefile
159 # Makefile for the kernel ISDN subsystem and device drivers.
160 # Each configuration option enables a list of files.
161 obj-$(CONFIG_ISDN) += isdn.o
162 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
164 --- 3.3 Loadable module goals - obj-m
166 $(obj-m) specify object files which are built as loadable
169 A module may be built from one source file or several source
170 files. In the case of one source file, the kbuild makefile
171 simply adds the file to $(obj-m).
174 #drivers/isdn/i4l/Makefile
175 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
177 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
179 If a kernel module is built from several source files, you specify
180 that you want to build a module in the same way as above.
182 Kbuild needs to know which the parts that you want to build your
183 module from, so you have to tell it by setting an
184 $(<module_name>-objs) variable.
187 #drivers/isdn/i4l/Makefile
188 obj-$(CONFIG_ISDN) += isdn.o
189 isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
191 In this example, the module name will be isdn.o. Kbuild will
192 compile the objects listed in $(isdn-objs) and then run
193 "$(LD) -r" on the list of these files to generate isdn.o.
195 Kbuild recognises objects used for composite objects by the suffix
196 -objs, and the suffix -y. This allows the Makefiles to use
197 the value of a CONFIG_ symbol to determine if an object is part
198 of a composite object.
202 obj-$(CONFIG_EXT2_FS) += ext2.o
203 ext2-y := balloc.o bitmap.o
204 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
206 In this example xattr.o is only part of the composite object
207 ext2.o, if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
209 Note: Of course, when you are building objects into the kernel,
210 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
211 kbuild will build an ext2.o file for you out of the individual
212 parts and then link this into built-in.o, as you would expect.
214 --- 3.4 Objects which export symbols
216 No special notation is required in the makefiles for
217 modules exporting symbols.
219 --- 3.5 Library file goals - lib-y
221 Objects listed with obj-* are used for modules or
222 combined in a built-in.o for that specific directory.
223 There is also the possibility to list objects that will
224 be included in a library, lib.a.
225 All objects listed with lib-y are combined in a single
226 library for that directory.
227 Objects that are listed in obj-y and additional listed in
228 lib-y will not be included in the library, since they will anyway
230 For consistency objects listed in lib-m will be included in lib.a.
232 Note that the same kbuild makefile may list files to be built-in
233 and to be part of a library. Therefore the same directory
234 may contain both a built-in.o and a lib.a file.
237 #arch/i386/lib/Makefile
238 lib-y := checksum.o delay.o
240 This will create a library lib.a based on checksum.o and delay.o.
241 For kbuild to actually recognize that there is a lib.a being build
242 the directory shall be listed in libs-y.
243 See also "6.3 List directories to visit when descending".
245 Usage of lib-y is normally restricted to lib/ and arch/*/lib.
247 --- 3.6 Descending down in directories
249 A Makefile is only responsible for building objects in its own
250 directory. Files in subdirectories should be taken care of by
251 Makefiles in these subdirs. The build system will automatically
252 invoke make recursively in subdirectories, provided you let it know of
255 To do so obj-y and obj-m are used.
256 ext2 lives in a separate directory, and the Makefile present in fs/
257 tells kbuild to descend down using the following assignment.
261 obj-$(CONFIG_EXT2_FS) += ext2/
263 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
264 the corresponding obj- variable will be set, and kbuild will descend
265 down in the ext2 directory.
266 Kbuild only uses this information to decide that it needs to visit
267 the directory, it is the Makefile in the subdirectory that
268 specifies what is modules and what is built-in.
270 It is good practice to use a CONFIG_ variable when assigning directory
271 names. This allows kbuild to totally skip the directory if the
272 corresponding CONFIG_ option is neither 'y' nor 'm'.
274 --- 3.7 Compilation flags
276 EXTRA_CFLAGS, EXTRA_AFLAGS, EXTRA_LDFLAGS, EXTRA_ARFLAGS
278 All the EXTRA_ variables apply only to the kbuild makefile
279 where they are assigned. The EXTRA_ variables apply to all
280 commands executed in the kbuild makefile.
282 $(EXTRA_CFLAGS) specifies options for compiling C files with
286 # drivers/sound/emu10k1/Makefile
287 EXTRA_CFLAGS += -I$(obj)
289 EXTRA_CFLAGS += -DEMU10K1_DEBUG
293 This variable is necessary because the top Makefile owns the
294 variable $(CFLAGS) and uses it for compilation flags for the
297 $(EXTRA_AFLAGS) is a similar string for per-directory options
298 when compiling assembly language source.
301 #arch/x86_64/kernel/Makefile
302 EXTRA_AFLAGS := -traditional
305 $(EXTRA_LDFLAGS) and $(EXTRA_ARFLAGS) are similar strings for
306 per-directory options to $(LD) and $(AR).
309 #arch/m68k/fpsp040/Makefile
314 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
317 $(CFLAGS_$@) specifies per-file options for $(CC). The $@
318 part has a literal value which specifies the file that it is for.
321 # drivers/scsi/Makefile
322 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
323 CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
325 CFLAGS_seagate.o = -DARBITRATE -DPARITY -DSEAGATE_USE_ASM
327 These three lines specify compilation flags for aha152x.o,
328 gdth.o, and seagate.o
330 $(AFLAGS_$@) is a similar feature for source files in assembly
334 # arch/arm/kernel/Makefile
335 AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional
336 AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional
338 --- 3.9 Dependency tracking
340 Kbuild tracks dependencies on the following:
341 1) All prerequisite files (both *.c and *.h)
342 2) CONFIG_ options used in all prerequisite files
343 3) Command-line used to compile target
345 Thus, if you change an option to $(CC) all affected files will
348 --- 3.10 Special Rules
350 Special rules are used when the kbuild infrastructure does
351 not provide the required support. A typical example is
352 header files generated during the build process.
353 Another example is the architecture specific Makefiles which
354 needs special rules to prepare boot images etc.
356 Special rules are written as normal Make rules.
357 Kbuild is not executing in the directory where the Makefile is
358 located, so all special rules shall provide a relative
359 path to prerequisite files and target files.
361 Two variables are used when defining special rules:
364 $(src) is a relative path which points to the directory
365 where the Makefile is located. Always use $(src) when
366 referring to files located in the src tree.
369 $(obj) is a relative path which points to the directory
370 where the target is saved. Always use $(obj) when
371 referring to generated files.
374 #drivers/scsi/Makefile
375 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
376 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
378 This is a special rule, following the normal syntax
380 The target file depends on two prerequisite files. References
381 to the target file are prefixed with $(obj), references
382 to prerequisites are referenced with $(src) (because they are not
386 === 4 Host Program support
388 Kbuild supports building executables on the host for use during the
390 Two steps are required in order to use a host executable.
392 The first step is to tell kbuild that a host program exists. This is
393 done utilising the variable hostprogs-y.
395 The second step is to add an explicit dependency to the executable.
396 This can be done in two ways. Either add the dependency in a rule,
397 or utilise the variable $(always).
398 Both possibilities are described in the following.
400 --- 4.1 Simple Host Program
402 In some cases there is a need to compile and run a program on the
403 computer where the build is running.
404 The following line tells kbuild that the program bin2hex shall be
405 built on the build host.
408 hostprogs-y := bin2hex
410 Kbuild assumes in the above example that bin2hex is made from a single
411 c-source file named bin2hex.c located in the same directory as
414 --- 4.2 Composite Host Programs
416 Host programs can be made up based on composite objects.
417 The syntax used to define composite objects for host programs is
418 similar to the syntax used for kernel objects.
419 $(<executeable>-objs) list all objects used to link the final
423 #scripts/lxdialog/Makefile
424 hostprogs-y := lxdialog
425 lxdialog-objs := checklist.o lxdialog.o
427 Objects with extension .o are compiled from the corresponding .c
428 files. In the above example checklist.c is compiled to checklist.o
429 and lxdialog.c is compiled to lxdialog.o.
430 Finally the two .o files are linked to the executable, lxdialog.
431 Note: The syntax <executable>-y is not permitted for host-programs.
433 --- 4.3 Defining shared libraries
435 Objects with extension .so are considered shared libraries, and
436 will be compiled as position independent objects.
437 Kbuild provides support for shared libraries, but the usage
439 In the following example the libkconfig.so shared library is used
440 to link the executable conf.
443 #scripts/kconfig/Makefile
445 conf-objs := conf.o libkconfig.so
446 libkconfig-objs := expr.o type.o
448 Shared libraries always require a corresponding -objs line, and
449 in the example above the shared library libkconfig is composed by
450 the two objects expr.o and type.o.
451 expr.o and type.o will be built as position independent code and
452 linked as a shared library libkconfig.so. C++ is not supported for
455 --- 4.4 Using C++ for host programs
457 kbuild offers support for host programs written in C++. This was
458 introduced solely to support kconfig, and is not recommended
462 #scripts/kconfig/Makefile
464 qconf-cxxobjs := qconf.o
466 In the example above the executable is composed of the C++ file
467 qconf.cc - identified by $(qconf-cxxobjs).
469 If qconf is composed by a mixture of .c and .cc files, then an
470 additional line can be used to identify this.
473 #scripts/kconfig/Makefile
475 qconf-cxxobjs := qconf.o
476 qconf-objs := check.o
478 --- 4.5 Controlling compiler options for host programs
480 When compiling host programs, it is possible to set specific flags.
481 The programs will always be compiled utilising $(HOSTCC) passed
482 the options specified in $(HOSTCFLAGS).
483 To set flags that will take effect for all host programs created
484 in that Makefile use the variable HOST_EXTRACFLAGS.
487 #scripts/lxdialog/Makefile
488 HOST_EXTRACFLAGS += -I/usr/include/ncurses
490 To set specific flags for a single file the following construction
494 #arch/ppc64/boot/Makefile
495 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
497 It is also possible to specify additional options to the linker.
500 #scripts/kconfig/Makefile
501 HOSTLOADLIBES_qconf := -L$(QTDIR)/lib
503 When linking qconf it will be passed the extra option "-L$(QTDIR)/lib".
505 --- 4.6 When host programs are actually built
507 Kbuild will only build host-programs when they are referenced
509 This is possible in two ways:
511 (1) List the prerequisite explicitly in a special rule.
514 #drivers/pci/Makefile
515 hostprogs-y := gen-devlist
516 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
517 ( cd $(obj); ./gen-devlist ) < $<
519 The target $(obj)/devlist.h will not be built before
520 $(obj)/gen-devlist is updated. Note that references to
521 the host programs in special rules must be prefixed with $(obj).
524 When there is no suitable special rule, and the host program
525 shall be built when a makefile is entered, the $(always)
526 variable shall be used.
529 #scripts/lxdialog/Makefile
530 hostprogs-y := lxdialog
531 always := $(hostprogs-y)
533 This will tell kbuild to build lxdialog even if not referenced in
536 --- 4.7 Using hostprogs-$(CONFIG_FOO)
538 A typcal pattern in a Kbuild file lok like this:
542 hostprogs-$(CONFIG_KALLSYMS) += kallsyms
544 Kbuild knows about both 'y' for built-in and 'm' for module.
545 So if a config symbol evaluate to 'm', kbuild will still build
546 the binary. In other words Kbuild handle hostprogs-m exactly
547 like hostprogs-y. But only hostprogs-y is recommend used
548 when no CONFIG symbol are involved.
550 === 5 Kbuild clean infrastructure
552 "make clean" deletes most generated files in the src tree where the kernel
553 is compiled. This includes generated files such as host programs.
554 Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
555 $(extra-y) and $(targets). They are all deleted during "make clean".
556 Files matching the patterns "*.[oas]", "*.ko", plus some additional files
557 generated by kbuild are deleted all over the kernel src tree when
558 "make clean" is executed.
560 Additional files can be specified in kbuild makefiles by use of $(clean-files).
563 #drivers/pci/Makefile
564 clean-files := devlist.h classlist.h
566 When executing "make clean", the two files "devlist.h classlist.h" will
567 be deleted. Kbuild will assume files to be in same relative directory as the
568 Makefile except if an absolute path is specified (path starting with '/').
570 To delete a directory hirachy use:
572 #scripts/package/Makefile
573 clean-dirs := $(objtree)/debian/
575 This will delete the directory debian, including all subdirectories.
576 Kbuild will assume the directories to be in the same relative path as the
577 Makefile if no absolute path is specified (path does not start with '/').
579 Usually kbuild descends down in subdirectories due to "obj-* := dir/",
580 but in the architecture makefiles where the kbuild infrastructure
581 is not sufficient this sometimes needs to be explicit.
584 #arch/i386/boot/Makefile
585 subdir- := compressed/
587 The above assignment instructs kbuild to descend down in the
588 directory compressed/ when "make clean" is executed.
590 To support the clean infrastructure in the Makefiles that builds the
591 final bootimage there is an optional target named archclean:
596 $(Q)$(MAKE) $(clean)=arch/i386/boot
598 When "make clean" is executed, make will descend down in arch/i386/boot,
599 and clean as usual. The Makefile located in arch/i386/boot/ may use
600 the subdir- trick to descend further down.
602 Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
603 included in the top level makefile, and the kbuild infrastructure
604 is not operational at that point.
606 Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
607 be visited during "make clean".
609 === 6 Architecture Makefiles
611 The top level Makefile sets up the environment and does the preparation,
612 before starting to descend down in the individual directories.
613 The top level makefile contains the generic part, whereas the
614 arch/$(ARCH)/Makefile contains what is required to set-up kbuild
615 to the said architecture.
616 To do so arch/$(ARCH)/Makefile sets a number of variables, and defines
619 When kbuild executes the following steps are followed (roughly):
620 1) Configuration of the kernel => produced .config
621 2) Store kernel version in include/linux/version.h
622 3) Symlink include/asm to include/asm-$(ARCH)
623 4) Updating all other prerequisites to the target prepare:
624 - Additional prerequisites are specified in arch/$(ARCH)/Makefile
625 5) Recursively descend down in all directories listed in
626 init-* core* drivers-* net-* libs-* and build all targets.
627 - The value of the above variables are extended in arch/$(ARCH)/Makefile.
628 6) All object files are then linked and the resulting file vmlinux is
629 located at the root of the src tree.
630 The very first objects linked are listed in head-y, assigned by
631 arch/$(ARCH)/Makefile.
632 7) Finally the architecture specific part does any required post processing
633 and builds the final bootimage.
634 - This includes building boot records
635 - Preparing initrd images and the like
638 --- 6.1 Set variables to tweak the build to the architecture
640 LDFLAGS Generic $(LD) options
642 Flags used for all invocations of the linker.
643 Often specifying the emulation is sufficient.
647 LDFLAGS := -m elf_s390
648 Note: EXTRA_LDFLAGS and LDFLAGS_$@ can be used to further customise
649 the flags used. See chapter 7.
651 LDFLAGS_MODULE Options for $(LD) when linking modules
653 LDFLAGS_MODULE is used to set specific flags for $(LD) when
654 linking the .ko files used for modules.
655 Default is "-r", for relocatable output.
657 LDFLAGS_vmlinux Options for $(LD) when linking vmlinux
659 LDFLAGS_vmlinux is used to specify additional flags to pass to
660 the linker when linking the final vmlinux.
661 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
665 LDFLAGS_vmlinux := -e stext
667 OBJCOPYFLAGS objcopy flags
669 When $(call if_changed,objcopy) is used to translate a .o file,
670 then the flags specified in OBJCOPYFLAGS will be used.
671 $(call if_changed,objcopy) is often used to generate raw binaries on
676 OBJCOPYFLAGS := -O binary
678 #arch/s390/boot/Makefile
679 $(obj)/image: vmlinux FORCE
680 $(call if_changed,objcopy)
682 In this example the binary $(obj)/image is a binary version of
683 vmlinux. The usage of $(call if_changed,xxx) will be described later.
685 AFLAGS $(AS) assembler flags
687 Default value - see top level Makefile
688 Append or modify as required per architecture.
691 #arch/sparc64/Makefile
692 AFLAGS += -m64 -mcpu=ultrasparc
694 CFLAGS $(CC) compiler flags
696 Default value - see top level Makefile
697 Append or modify as required per architecture.
699 Often the CFLAGS variable depends on the configuration.
703 cflags-$(CONFIG_M386) += -march=i386
704 CFLAGS += $(cflags-y)
706 Many arch Makefiles dynamically run the target C compiler to
707 probe supported options:
710 check_gcc = $(shell if $(CC) $(1) -S -o /dev/null -xc \
711 /dev/null\ > /dev/null 2>&1; then echo "$(1)"; \
712 else echo "$(2)"; fi)
713 cflags-$(CONFIG_MCYRIXIII) += $(call check_gcc,\
714 -march=c3,-march=i486)
716 CFLAGS += $(cflags-y)
718 The above examples both utilise the trick that a config option expands
719 to 'y' when selected.
721 CFLAGS_KERNEL $(CC) options specific for built-in
723 $(CFLAGS_KERNEL) contains extra C compiler flags used to compile
724 resident kernel code.
726 CFLAGS_MODULE $(CC) options specific for modules
728 $(CFLAGS_MODULE) contains extra C compiler flags used to compile code
729 for loadable kernel modules.
732 --- 6.2 Add prerequisites to prepare:
734 The prepare: rule is used to list prerequisites that needs to be
735 built before starting to descend down in the subdirectories.
736 This is usual header files containing assembler constants.
740 prepare: include/asm-$(ARCH)/offsets.h
742 In this example the file include/asm-$(ARCH)/offsets.h will
743 be built before descending down in the subdirectories.
744 See also chapter XXX-TODO that describe how kbuild supports
745 generating offset header files.
748 --- 6.3 List directories to visit when descending
750 An arch Makefile cooperates with the top Makefile to define variables
751 which specify how to build the vmlinux file. Note that there is no
752 corresponding arch-specific section for modules; the module-building
753 machinery is all architecture-independent.
756 head-y, init-y, core-y, libs-y, drivers-y, net-y
758 $(head-y) list objects to be linked first in vmlinux.
759 $(libs-y) list directories where a lib.a archive can be located.
760 The rest list directories where a built-in.o object file can be located.
762 $(init-y) objects will be located after $(head-y).
763 Then the rest follows in this order:
764 $(core-y), $(libs-y), $(drivers-y) and $(net-y).
766 The top level Makefile define values for all generic directories,
767 and arch/$(ARCH)/Makefile only adds architecture specific directories.
770 #arch/sparc64/Makefile
771 core-y += arch/sparc64/kernel/
772 libs-y += arch/sparc64/prom/ arch/sparc64/lib/
773 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
776 --- 6.4 Architecture specific boot images
778 An arch Makefile specifies goals that take the vmlinux file, compress
779 it, wrap it in bootstrapping code, and copy the resulting files
780 somewhere. This includes various kinds of installation commands.
781 The actual goals are not standardized across architectures.
783 It is common to locate any additional processing in a boot/
784 directory below arch/$(ARCH)/.
786 Kbuild does not provide any smart way to support building a
787 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
788 call make manually to build a target in boot/.
790 The recommended approach is to include shortcuts in
791 arch/$(ARCH)/Makefile, and use the full path when calling down
792 into the arch/$(ARCH)/boot/Makefile.
796 boot := arch/i386/boot
798 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
800 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
801 make in a subdirectory.
803 There are no rules for naming of the architecture specific targets,
804 but executing "make help" will list all relevant targets.
805 To support this $(archhelp) must be defined.
810 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
813 When make is executed without arguments, the first goal encountered
814 will be built. In the top level Makefile the first goal present
816 An architecture shall always per default build a bootable image.
817 In "make help" the default goal is highlighted with a '*'.
818 Add a new prerequisite to all: to select a default goal different
825 When "make" is executed without arguments, bzImage will be built.
827 --- 6.5 Building non-kbuild targets
831 extra-y specify additional targets created in the current
832 directory, in addition to any targets specified by obj-*.
834 Listing all targets in extra-y is required for two purposes:
835 1) Enable kbuild to check changes in command lines
836 - When $(call if_changed,xxx) is used
837 2) kbuild knows what files to delete during "make clean"
840 #arch/i386/kernel/Makefile
841 extra-y := head.o init_task.o
843 In this example extra-y is used to list object files that
844 shall be built, but shall not be linked as part of built-in.o.
847 --- 6.6 Commands useful for building a boot image
849 Kbuild provides a few macros that are useful when building a
854 if_changed is the infrastructure used for the following commands.
857 target: source(s) FORCE
858 $(call if_changed,ld/objcopy/gzip)
860 When the rule is evaluated it is checked to see if any files
861 needs an update, or the commandline has changed since last
862 invocation. The latter will force a rebuild if any options
863 to the executable have changed.
864 Any target that utilises if_changed must be listed in $(targets),
865 otherwise the command line check will fail, and the target will
867 Assignments to $(targets) are without $(obj)/ prefix.
868 if_changed may be used in conjunction with custom commands as
869 defined in 6.7 "Custom kbuild commands".
870 Note: It is a typical mistake to forget the FORCE prerequisite.
873 Link target. Often LDFLAGS_$@ is used to set specific options to ld.
876 Copy binary. Uses OBJCOPYFLAGS usually specified in
877 arch/$(ARCH)/Makefile.
878 OBJCOPYFLAGS_$@ may be used to set additional options.
881 Compress target. Use maximum compression to compress target.
884 #arch/i386/boot/Makefile
885 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
886 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
888 targets += setup setup.o bootsect bootsect.o
889 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
890 $(call if_changed,ld)
892 In this example there are two possible targets, requiring different
893 options to the linker. the linker options are specified using the
894 LDFLAGS_$@ syntax - one for each potential target.
895 $(targets) are assinged all potential targets, herby kbuild knows
896 the targets and will:
897 1) check for commandline changes
898 2) delete target during make clean
900 The ": %: %.o" part of the prerequisite is a shorthand that
901 free us from listing the setup.o and bootsect.o files.
902 Note: It is a common mistake to forget the "target :=" assignment,
903 resulting in the target file being recompiled for no
907 --- 6.7 Custom kbuild commands
909 When kbuild is executing with KBUILD_VERBOSE=0 then only a shorthand
910 of a command is normally displayed.
911 To enable this behaviour for custom commands kbuild requires
912 two variables to be set:
913 quiet_cmd_<command> - what shall be echoed
914 cmd_<command> - the command to execute
918 quiet_cmd_image = BUILD $@
919 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
920 $(obj)/vmlinux.bin > $@
923 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
924 $(call if_changed,image)
925 @echo 'Kernel: $@ is ready'
927 When updating the $(obj)/bzImage target the line:
929 BUILD arch/i386/boot/bzImage
931 will be displayed with "make KBUILD_VERBOSE=0".
934 --- 6.8 Preprocessing linker scripts
936 When the vmlinux image is build the linker script:
937 arch/$(ARCH)/kernel/vmlinux.lds is used.
938 The script is a preprocessed variant of the file vmlinux.lds.S
939 located in the same directory.
940 kbuild knows .lds file and includes a rule *lds.S -> *lds.
943 #arch/i386/kernel/Makefile
944 always := vmlinux.lds
947 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
949 The assigment to $(always) is used to tell kbuild to build the
951 The assignment to $(CPPFLAGS_vmlinux.lds) tell kbuild to use the
952 specified options when building the target vmlinux.lds.
954 When building the *.lds target kbuild used the variakles:
955 CPPFLAGS : Set in top-level Makefile
956 EXTRA_CPPFLAGS : May be set in the kbuild makefile
957 CPPFLAGS_$(@F) : Target specific flags.
958 Note that the full filename is used in this
961 The kbuild infrastructure for *lds file are used in several
962 architecture specific files.
965 --- 6.9 $(CC) support functions
967 The kernel may be build with several different versions of
968 $(CC), each supporting a unique set of features and options.
969 kbuild provide basic support to check for valid options for $(CC).
970 $(CC) is useally the gcc compiler, but other alternatives are
974 cc-option is used to check if $(CC) support a given option, and not
975 supported to use an optional second option.
979 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
981 In the above example cflags-y will be assigned the option
982 -march=pentium-mmx if supported by $(CC), otherwise -march-i586.
983 The second argument to cc-option is optional, and if omitted
984 cflags-y will be assigned no value if first option is not supported.
987 cc-option-yn is used to check if gcc supports a given option
988 and return 'y' if supported, otherwise 'n'.
992 biarch := $(call cc-option-yn, -m32)
993 aflags-$(biarch) += -a32
994 cflags-$(biarch) += -m32
996 In the above example $(biarch) is set to y if $(CC) supports the -m32
997 option. When $(biarch) equals to y the expanded variables $(aflags-y)
998 and $(cflags-y) will be assigned the values -a32 and -m32.
1001 cc-version return a numerical version of the $(CC) compiler version.
1002 The format is <major><minor> where both are two digits. So for example
1003 gcc 3.41 would return 0341.
1004 cc-version is useful when a specific $(CC) version is faulty in one
1005 area, for example the -mregparm=3 were broken in some gcc version
1006 even though the option was accepted by gcc.
1010 GCC_VERSION := $(call cc-version)
1011 cflags-y += $(shell \
1012 if [ $(GCC_VERSION) -ge 0300 ] ; then echo "-mregparm=3"; fi ;)
1014 In the above example -mregparm=3 is only used for gcc version greater
1015 than or equal to gcc 3.0.
1018 === 7 Kbuild Variables
1020 The top Makefile exports the following variables:
1022 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1024 These variables define the current kernel version. A few arch
1025 Makefiles actually use these values directly; they should use
1026 $(KERNELRELEASE) instead.
1028 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1029 three-part version number, such as "2", "4", and "0". These three
1030 values are always numeric.
1032 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1033 or additional patches. It is usually some non-numeric string
1034 such as "-pre4", and is often blank.
1038 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1039 for constructing installation directory names or showing in
1040 version strings. Some arch Makefiles use it for this purpose.
1044 This variable defines the target architecture, such as "i386",
1045 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1046 determine which files to compile.
1048 By default, the top Makefile sets $(ARCH) to be the same as the
1049 host system architecture. For a cross build, a user may
1050 override the value of $(ARCH) on the command line:
1057 This variable defines a place for the arch Makefiles to install
1058 the resident kernel image and System.map file.
1059 Use this for architecture specific install targets.
1061 INSTALL_MOD_PATH, MODLIB
1063 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1064 installation. This variable is not defined in the Makefile but
1065 may be passed in by the user if desired.
1067 $(MODLIB) specifies the directory for module installation.
1068 The top Makefile defines $(MODLIB) to
1069 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1070 override this value on the command line if desired.
1072 === 8 Makefile language
1074 The kernel Makefiles are designed to run with GNU Make. The Makefiles
1075 use only the documented features of GNU Make, but they do use many
1078 GNU Make supports elementary list-processing functions. The kernel
1079 Makefiles use a novel style of list building and manipulation with few
1082 GNU Make has two assignment operators, ":=" and "=". ":=" performs
1083 immediate evaluation of the right-hand side and stores an actual string
1084 into the left-hand side. "=" is like a formula definition; it stores the
1085 right-hand side in an unevaluated form and then evaluates this form each
1086 time the left-hand side is used.
1088 There are some cases where "=" is appropriate. Usually, though, ":="
1089 is the right choice.
1093 Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1094 Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1095 Updates by Sam Ravnborg <sam@ravnborg.org>
1099 - Describe how kbuild support shipped files with _shipped.
1100 - Generating offset header files.
1101 - Add more variables to section 7?