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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Linux Kernel Makefiles
2
3This document describes the Linux kernel Makefiles.
4
5=== Table of Contents
6
7 === 1 Overview
8 === 2 Who does what
9 === 3 The kbuild files
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
20
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)
29
30 === 5 Kbuild clean infrastructure
31
32 === 6 Architecture Makefiles
33 --- 6.1 Set variables to tweak the build to the architecture
Sam Ravnborg5bb78262005-09-11 22:30:22 +020034 --- 6.2 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -070035 --- 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 $(CC) support functions
42
43 === 7 Kbuild Variables
44 === 8 Makefile language
45 === 9 Credits
46 === 10 TODO
47
48=== 1 Overview
49
50The Makefiles have five parts:
51
52 Makefile the top Makefile.
53 .config the kernel configuration file.
54 arch/$(ARCH)/Makefile the arch Makefile.
55 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
56 kbuild Makefiles there are about 500 of these.
57
58The top Makefile reads the .config file, which comes from the kernel
59configuration process.
60
61The top Makefile is responsible for building two major products: vmlinux
62(the resident kernel image) and modules (any module files).
63It builds these goals by recursively descending into the subdirectories of
64the kernel source tree.
65The list of subdirectories which are visited depends upon the kernel
66configuration. The top Makefile textually includes an arch Makefile
67with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
68architecture-specific information to the top Makefile.
69
70Each subdirectory has a kbuild Makefile which carries out the commands
71passed down from above. The kbuild Makefile uses information from the
72.config file to construct various file lists used by kbuild to build
73any built-in or modular targets.
74
75scripts/Makefile.* contains all the definitions/rules etc. that
76are used to build the kernel based on the kbuild makefiles.
77
78
79=== 2 Who does what
80
81People have four different relationships with the kernel Makefiles.
82
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
85any kernel Makefiles (or any other source files).
86
87*Normal developers* are people who work on features such as device
88drivers, file systems, and network protocols. These people need to
89maintain the kbuild Makefiles for the subsystem that they are
90working on. In order to do this effectively, they need some overall
91knowledge about the kernel Makefiles, plus detailed knowledge about the
92public interface for kbuild.
93
94*Arch developers* are people who work on an entire architecture, such
95as sparc or ia64. Arch developers need to know about the arch Makefile
96as well as kbuild Makefiles.
97
98*Kbuild developers* are people who work on the kernel build system itself.
99These people need to know about all aspects of the kernel Makefiles.
100
101This document is aimed towards normal developers and arch developers.
102
103
104=== 3 The kbuild files
105
106Most Makefiles within the kernel are kbuild Makefiles that use the
107kbuild infrastructure. This chapter introduce the syntax used in the
108kbuild makefiles.
109The preferred name for the kbuild files is 'Kbuild' but 'Makefile' will
110continue to be supported. All new developmen is expected to use the
111Kbuild filename.
112
113Section 3.1 "Goal definitions" is a quick intro, further chapters provide
114more details, with real examples.
115
116--- 3.1 Goal definitions
117
118 Goal definitions are the main part (heart) of the kbuild Makefile.
119 These lines define the files to be built, any special compilation
120 options, and any subdirectories to be entered recursively.
121
122 The most simple kbuild makefile contains one line:
123
124 Example:
125 obj-y += foo.o
126
127 This tell kbuild that there is one object in that directory named
128 foo.o. foo.o will be built from foo.c or foo.S.
129
130 If foo.o shall be built as a module, the variable obj-m is used.
131 Therefore the following pattern is often used:
132
133 Example:
134 obj-$(CONFIG_FOO) += foo.o
135
136 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
137 If CONFIG_FOO is neither y nor m, then the file will not be compiled
138 nor linked.
139
140--- 3.2 Built-in object goals - obj-y
141
142 The kbuild Makefile specifies object files for vmlinux
143 in the lists $(obj-y). These lists depend on the kernel
144 configuration.
145
146 Kbuild compiles all the $(obj-y) files. It then calls
147 "$(LD) -r" to merge these files into one built-in.o file.
148 built-in.o is later linked into vmlinux by the parent Makefile.
149
150 The order of files in $(obj-y) is significant. Duplicates in
151 the lists are allowed: the first instance will be linked into
152 built-in.o and succeeding instances will be ignored.
153
154 Link order is significant, because certain functions
155 (module_init() / __initcall) will be called during boot in the
156 order they appear. So keep in mind that changing the link
157 order may e.g. change the order in which your SCSI
158 controllers are detected, and thus you disks are renumbered.
159
160 Example:
161 #drivers/isdn/i4l/Makefile
162 # Makefile for the kernel ISDN subsystem and device drivers.
163 # Each configuration option enables a list of files.
164 obj-$(CONFIG_ISDN) += isdn.o
165 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
166
167--- 3.3 Loadable module goals - obj-m
168
169 $(obj-m) specify object files which are built as loadable
170 kernel modules.
171
172 A module may be built from one source file or several source
173 files. In the case of one source file, the kbuild makefile
174 simply adds the file to $(obj-m).
175
176 Example:
177 #drivers/isdn/i4l/Makefile
178 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
179
180 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
181
182 If a kernel module is built from several source files, you specify
183 that you want to build a module in the same way as above.
184
185 Kbuild needs to know which the parts that you want to build your
186 module from, so you have to tell it by setting an
187 $(<module_name>-objs) variable.
188
189 Example:
190 #drivers/isdn/i4l/Makefile
191 obj-$(CONFIG_ISDN) += isdn.o
192 isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
193
194 In this example, the module name will be isdn.o. Kbuild will
195 compile the objects listed in $(isdn-objs) and then run
196 "$(LD) -r" on the list of these files to generate isdn.o.
197
198 Kbuild recognises objects used for composite objects by the suffix
199 -objs, and the suffix -y. This allows the Makefiles to use
200 the value of a CONFIG_ symbol to determine if an object is part
201 of a composite object.
202
203 Example:
204 #fs/ext2/Makefile
205 obj-$(CONFIG_EXT2_FS) += ext2.o
206 ext2-y := balloc.o bitmap.o
207 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
208
209 In this example xattr.o is only part of the composite object
210 ext2.o, if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
211
212 Note: Of course, when you are building objects into the kernel,
213 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
214 kbuild will build an ext2.o file for you out of the individual
215 parts and then link this into built-in.o, as you would expect.
216
217--- 3.4 Objects which export symbols
218
219 No special notation is required in the makefiles for
220 modules exporting symbols.
221
222--- 3.5 Library file goals - lib-y
223
224 Objects listed with obj-* are used for modules or
225 combined in a built-in.o for that specific directory.
226 There is also the possibility to list objects that will
227 be included in a library, lib.a.
228 All objects listed with lib-y are combined in a single
229 library for that directory.
230 Objects that are listed in obj-y and additional listed in
231 lib-y will not be included in the library, since they will anyway
232 be accessible.
233 For consistency objects listed in lib-m will be included in lib.a.
234
235 Note that the same kbuild makefile may list files to be built-in
236 and to be part of a library. Therefore the same directory
237 may contain both a built-in.o and a lib.a file.
238
239 Example:
240 #arch/i386/lib/Makefile
241 lib-y := checksum.o delay.o
242
243 This will create a library lib.a based on checksum.o and delay.o.
244 For kbuild to actually recognize that there is a lib.a being build
245 the directory shall be listed in libs-y.
246 See also "6.3 List directories to visit when descending".
247
248 Usage of lib-y is normally restricted to lib/ and arch/*/lib.
249
250--- 3.6 Descending down in directories
251
252 A Makefile is only responsible for building objects in its own
253 directory. Files in subdirectories should be taken care of by
254 Makefiles in these subdirs. The build system will automatically
255 invoke make recursively in subdirectories, provided you let it know of
256 them.
257
258 To do so obj-y and obj-m are used.
259 ext2 lives in a separate directory, and the Makefile present in fs/
260 tells kbuild to descend down using the following assignment.
261
262 Example:
263 #fs/Makefile
264 obj-$(CONFIG_EXT2_FS) += ext2/
265
266 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
267 the corresponding obj- variable will be set, and kbuild will descend
268 down in the ext2 directory.
269 Kbuild only uses this information to decide that it needs to visit
270 the directory, it is the Makefile in the subdirectory that
271 specifies what is modules and what is built-in.
272
273 It is good practice to use a CONFIG_ variable when assigning directory
274 names. This allows kbuild to totally skip the directory if the
275 corresponding CONFIG_ option is neither 'y' nor 'm'.
276
277--- 3.7 Compilation flags
278
279 EXTRA_CFLAGS, EXTRA_AFLAGS, EXTRA_LDFLAGS, EXTRA_ARFLAGS
280
281 All the EXTRA_ variables apply only to the kbuild makefile
282 where they are assigned. The EXTRA_ variables apply to all
283 commands executed in the kbuild makefile.
284
285 $(EXTRA_CFLAGS) specifies options for compiling C files with
286 $(CC).
287
288 Example:
289 # drivers/sound/emu10k1/Makefile
290 EXTRA_CFLAGS += -I$(obj)
291 ifdef DEBUG
292 EXTRA_CFLAGS += -DEMU10K1_DEBUG
293 endif
294
295
296 This variable is necessary because the top Makefile owns the
297 variable $(CFLAGS) and uses it for compilation flags for the
298 entire tree.
299
300 $(EXTRA_AFLAGS) is a similar string for per-directory options
301 when compiling assembly language source.
302
303 Example:
304 #arch/x86_64/kernel/Makefile
305 EXTRA_AFLAGS := -traditional
306
307
308 $(EXTRA_LDFLAGS) and $(EXTRA_ARFLAGS) are similar strings for
309 per-directory options to $(LD) and $(AR).
310
311 Example:
312 #arch/m68k/fpsp040/Makefile
313 EXTRA_LDFLAGS := -x
314
315 CFLAGS_$@, AFLAGS_$@
316
317 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
318 kbuild makefile.
319
320 $(CFLAGS_$@) specifies per-file options for $(CC). The $@
321 part has a literal value which specifies the file that it is for.
322
323 Example:
324 # drivers/scsi/Makefile
325 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
326 CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
327 -DGDTH_STATISTICS
328 CFLAGS_seagate.o = -DARBITRATE -DPARITY -DSEAGATE_USE_ASM
329
330 These three lines specify compilation flags for aha152x.o,
331 gdth.o, and seagate.o
332
333 $(AFLAGS_$@) is a similar feature for source files in assembly
334 languages.
335
336 Example:
337 # arch/arm/kernel/Makefile
338 AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional
339 AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional
340
341--- 3.9 Dependency tracking
342
343 Kbuild tracks dependencies on the following:
344 1) All prerequisite files (both *.c and *.h)
345 2) CONFIG_ options used in all prerequisite files
346 3) Command-line used to compile target
347
348 Thus, if you change an option to $(CC) all affected files will
349 be re-compiled.
350
351--- 3.10 Special Rules
352
353 Special rules are used when the kbuild infrastructure does
354 not provide the required support. A typical example is
355 header files generated during the build process.
356 Another example is the architecture specific Makefiles which
357 needs special rules to prepare boot images etc.
358
359 Special rules are written as normal Make rules.
360 Kbuild is not executing in the directory where the Makefile is
361 located, so all special rules shall provide a relative
362 path to prerequisite files and target files.
363
364 Two variables are used when defining special rules:
365
366 $(src)
367 $(src) is a relative path which points to the directory
368 where the Makefile is located. Always use $(src) when
369 referring to files located in the src tree.
370
371 $(obj)
372 $(obj) is a relative path which points to the directory
373 where the target is saved. Always use $(obj) when
374 referring to generated files.
375
376 Example:
377 #drivers/scsi/Makefile
378 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
379 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
380
381 This is a special rule, following the normal syntax
382 required by make.
383 The target file depends on two prerequisite files. References
384 to the target file are prefixed with $(obj), references
385 to prerequisites are referenced with $(src) (because they are not
386 generated files).
387
388
389=== 4 Host Program support
390
391Kbuild supports building executables on the host for use during the
392compilation stage.
393Two steps are required in order to use a host executable.
394
395The first step is to tell kbuild that a host program exists. This is
396done utilising the variable hostprogs-y.
397
398The second step is to add an explicit dependency to the executable.
399This can be done in two ways. Either add the dependency in a rule,
400or utilise the variable $(always).
401Both possibilities are described in the following.
402
403--- 4.1 Simple Host Program
404
405 In some cases there is a need to compile and run a program on the
406 computer where the build is running.
407 The following line tells kbuild that the program bin2hex shall be
408 built on the build host.
409
410 Example:
411 hostprogs-y := bin2hex
412
413 Kbuild assumes in the above example that bin2hex is made from a single
414 c-source file named bin2hex.c located in the same directory as
415 the Makefile.
416
417--- 4.2 Composite Host Programs
418
419 Host programs can be made up based on composite objects.
420 The syntax used to define composite objects for host programs is
421 similar to the syntax used for kernel objects.
422 $(<executeable>-objs) list all objects used to link the final
423 executable.
424
425 Example:
426 #scripts/lxdialog/Makefile
427 hostprogs-y := lxdialog
428 lxdialog-objs := checklist.o lxdialog.o
429
430 Objects with extension .o are compiled from the corresponding .c
431 files. In the above example checklist.c is compiled to checklist.o
432 and lxdialog.c is compiled to lxdialog.o.
433 Finally the two .o files are linked to the executable, lxdialog.
434 Note: The syntax <executable>-y is not permitted for host-programs.
435
436--- 4.3 Defining shared libraries
437
438 Objects with extension .so are considered shared libraries, and
439 will be compiled as position independent objects.
440 Kbuild provides support for shared libraries, but the usage
441 shall be restricted.
442 In the following example the libkconfig.so shared library is used
443 to link the executable conf.
444
445 Example:
446 #scripts/kconfig/Makefile
447 hostprogs-y := conf
448 conf-objs := conf.o libkconfig.so
449 libkconfig-objs := expr.o type.o
450
451 Shared libraries always require a corresponding -objs line, and
452 in the example above the shared library libkconfig is composed by
453 the two objects expr.o and type.o.
454 expr.o and type.o will be built as position independent code and
455 linked as a shared library libkconfig.so. C++ is not supported for
456 shared libraries.
457
458--- 4.4 Using C++ for host programs
459
460 kbuild offers support for host programs written in C++. This was
461 introduced solely to support kconfig, and is not recommended
462 for general use.
463
464 Example:
465 #scripts/kconfig/Makefile
466 hostprogs-y := qconf
467 qconf-cxxobjs := qconf.o
468
469 In the example above the executable is composed of the C++ file
470 qconf.cc - identified by $(qconf-cxxobjs).
471
472 If qconf is composed by a mixture of .c and .cc files, then an
473 additional line can be used to identify this.
474
475 Example:
476 #scripts/kconfig/Makefile
477 hostprogs-y := qconf
478 qconf-cxxobjs := qconf.o
479 qconf-objs := check.o
480
481--- 4.5 Controlling compiler options for host programs
482
483 When compiling host programs, it is possible to set specific flags.
484 The programs will always be compiled utilising $(HOSTCC) passed
485 the options specified in $(HOSTCFLAGS).
486 To set flags that will take effect for all host programs created
487 in that Makefile use the variable HOST_EXTRACFLAGS.
488
489 Example:
490 #scripts/lxdialog/Makefile
491 HOST_EXTRACFLAGS += -I/usr/include/ncurses
492
493 To set specific flags for a single file the following construction
494 is used:
495
496 Example:
497 #arch/ppc64/boot/Makefile
498 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
499
500 It is also possible to specify additional options to the linker.
501
502 Example:
503 #scripts/kconfig/Makefile
504 HOSTLOADLIBES_qconf := -L$(QTDIR)/lib
505
506 When linking qconf it will be passed the extra option "-L$(QTDIR)/lib".
507
508--- 4.6 When host programs are actually built
509
510 Kbuild will only build host-programs when they are referenced
511 as a prerequisite.
512 This is possible in two ways:
513
514 (1) List the prerequisite explicitly in a special rule.
515
516 Example:
517 #drivers/pci/Makefile
518 hostprogs-y := gen-devlist
519 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
520 ( cd $(obj); ./gen-devlist ) < $<
521
522 The target $(obj)/devlist.h will not be built before
523 $(obj)/gen-devlist is updated. Note that references to
524 the host programs in special rules must be prefixed with $(obj).
525
526 (2) Use $(always)
527 When there is no suitable special rule, and the host program
528 shall be built when a makefile is entered, the $(always)
529 variable shall be used.
530
531 Example:
532 #scripts/lxdialog/Makefile
533 hostprogs-y := lxdialog
534 always := $(hostprogs-y)
535
536 This will tell kbuild to build lxdialog even if not referenced in
537 any rule.
538
539--- 4.7 Using hostprogs-$(CONFIG_FOO)
540
541 A typcal pattern in a Kbuild file lok like this:
542
543 Example:
544 #scripts/Makefile
545 hostprogs-$(CONFIG_KALLSYMS) += kallsyms
546
547 Kbuild knows about both 'y' for built-in and 'm' for module.
548 So if a config symbol evaluate to 'm', kbuild will still build
549 the binary. In other words Kbuild handle hostprogs-m exactly
550 like hostprogs-y. But only hostprogs-y is recommend used
551 when no CONFIG symbol are involved.
552
553=== 5 Kbuild clean infrastructure
554
555"make clean" deletes most generated files in the src tree where the kernel
556is compiled. This includes generated files such as host programs.
557Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
558$(extra-y) and $(targets). They are all deleted during "make clean".
559Files matching the patterns "*.[oas]", "*.ko", plus some additional files
560generated by kbuild are deleted all over the kernel src tree when
561"make clean" is executed.
562
563Additional files can be specified in kbuild makefiles by use of $(clean-files).
564
565 Example:
566 #drivers/pci/Makefile
567 clean-files := devlist.h classlist.h
568
569When executing "make clean", the two files "devlist.h classlist.h" will
570be deleted. Kbuild will assume files to be in same relative directory as the
571Makefile except if an absolute path is specified (path starting with '/').
572
573To delete a directory hirachy use:
574 Example:
575 #scripts/package/Makefile
576 clean-dirs := $(objtree)/debian/
577
578This will delete the directory debian, including all subdirectories.
579Kbuild will assume the directories to be in the same relative path as the
580Makefile if no absolute path is specified (path does not start with '/').
581
582Usually kbuild descends down in subdirectories due to "obj-* := dir/",
583but in the architecture makefiles where the kbuild infrastructure
584is not sufficient this sometimes needs to be explicit.
585
586 Example:
587 #arch/i386/boot/Makefile
588 subdir- := compressed/
589
590The above assignment instructs kbuild to descend down in the
591directory compressed/ when "make clean" is executed.
592
593To support the clean infrastructure in the Makefiles that builds the
594final bootimage there is an optional target named archclean:
595
596 Example:
597 #arch/i386/Makefile
598 archclean:
599 $(Q)$(MAKE) $(clean)=arch/i386/boot
600
601When "make clean" is executed, make will descend down in arch/i386/boot,
602and clean as usual. The Makefile located in arch/i386/boot/ may use
603the subdir- trick to descend further down.
604
605Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
606included in the top level makefile, and the kbuild infrastructure
607is not operational at that point.
608
609Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
610be visited during "make clean".
611
612=== 6 Architecture Makefiles
613
614The top level Makefile sets up the environment and does the preparation,
615before starting to descend down in the individual directories.
616The top level makefile contains the generic part, whereas the
617arch/$(ARCH)/Makefile contains what is required to set-up kbuild
618to the said architecture.
619To do so arch/$(ARCH)/Makefile sets a number of variables, and defines
620a few targets.
621
622When kbuild executes the following steps are followed (roughly):
6231) Configuration of the kernel => produced .config
6242) Store kernel version in include/linux/version.h
6253) Symlink include/asm to include/asm-$(ARCH)
6264) Updating all other prerequisites to the target prepare:
627 - Additional prerequisites are specified in arch/$(ARCH)/Makefile
6285) Recursively descend down in all directories listed in
629 init-* core* drivers-* net-* libs-* and build all targets.
630 - The value of the above variables are extended in arch/$(ARCH)/Makefile.
6316) All object files are then linked and the resulting file vmlinux is
632 located at the root of the src tree.
633 The very first objects linked are listed in head-y, assigned by
634 arch/$(ARCH)/Makefile.
6357) Finally the architecture specific part does any required post processing
636 and builds the final bootimage.
637 - This includes building boot records
638 - Preparing initrd images and the like
639
640
641--- 6.1 Set variables to tweak the build to the architecture
642
643 LDFLAGS Generic $(LD) options
644
645 Flags used for all invocations of the linker.
646 Often specifying the emulation is sufficient.
647
648 Example:
649 #arch/s390/Makefile
650 LDFLAGS := -m elf_s390
651 Note: EXTRA_LDFLAGS and LDFLAGS_$@ can be used to further customise
652 the flags used. See chapter 7.
653
654 LDFLAGS_MODULE Options for $(LD) when linking modules
655
656 LDFLAGS_MODULE is used to set specific flags for $(LD) when
657 linking the .ko files used for modules.
658 Default is "-r", for relocatable output.
659
660 LDFLAGS_vmlinux Options for $(LD) when linking vmlinux
661
662 LDFLAGS_vmlinux is used to specify additional flags to pass to
663 the linker when linking the final vmlinux.
664 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
665
666 Example:
667 #arch/i386/Makefile
668 LDFLAGS_vmlinux := -e stext
669
670 OBJCOPYFLAGS objcopy flags
671
672 When $(call if_changed,objcopy) is used to translate a .o file,
673 then the flags specified in OBJCOPYFLAGS will be used.
674 $(call if_changed,objcopy) is often used to generate raw binaries on
675 vmlinux.
676
677 Example:
678 #arch/s390/Makefile
679 OBJCOPYFLAGS := -O binary
680
681 #arch/s390/boot/Makefile
682 $(obj)/image: vmlinux FORCE
683 $(call if_changed,objcopy)
684
685 In this example the binary $(obj)/image is a binary version of
686 vmlinux. The usage of $(call if_changed,xxx) will be described later.
687
688 AFLAGS $(AS) assembler flags
689
690 Default value - see top level Makefile
691 Append or modify as required per architecture.
692
693 Example:
694 #arch/sparc64/Makefile
695 AFLAGS += -m64 -mcpu=ultrasparc
696
697 CFLAGS $(CC) compiler flags
698
699 Default value - see top level Makefile
700 Append or modify as required per architecture.
701
702 Often the CFLAGS variable depends on the configuration.
703
704 Example:
705 #arch/i386/Makefile
706 cflags-$(CONFIG_M386) += -march=i386
707 CFLAGS += $(cflags-y)
708
709 Many arch Makefiles dynamically run the target C compiler to
710 probe supported options:
711
712 #arch/i386/Makefile
713
714 ...
715 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
716 -march=pentium2,-march=i686)
717 ...
718 # Disable unit-at-a-time mode ...
719 CFLAGS += $(call cc-option,-fno-unit-at-a-time)
720 ...
721
722
723 The first examples utilises the trick that a config option expands
724 to 'y' when selected.
725
726 CFLAGS_KERNEL $(CC) options specific for built-in
727
728 $(CFLAGS_KERNEL) contains extra C compiler flags used to compile
729 resident kernel code.
730
731 CFLAGS_MODULE $(CC) options specific for modules
732
733 $(CFLAGS_MODULE) contains extra C compiler flags used to compile code
734 for loadable kernel modules.
735
736
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200737--- 6.2 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700738
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200739 The archprepare: rule is used to list prerequisites that needs to be
Linus Torvalds1da177e2005-04-16 15:20:36 -0700740 built before starting to descend down in the subdirectories.
741 This is usual header files containing assembler constants.
742
743 Example:
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200744 #arch/arm/Makefile
745 archprepare: maketools
Linus Torvalds1da177e2005-04-16 15:20:36 -0700746
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200747 In this example the file target maketools will be processed
748 before descending down in the subdirectories.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700749 See also chapter XXX-TODO that describe how kbuild supports
750 generating offset header files.
751
752
753--- 6.3 List directories to visit when descending
754
755 An arch Makefile cooperates with the top Makefile to define variables
756 which specify how to build the vmlinux file. Note that there is no
757 corresponding arch-specific section for modules; the module-building
758 machinery is all architecture-independent.
759
760
761 head-y, init-y, core-y, libs-y, drivers-y, net-y
762
763 $(head-y) list objects to be linked first in vmlinux.
764 $(libs-y) list directories where a lib.a archive can be located.
765 The rest list directories where a built-in.o object file can be located.
766
767 $(init-y) objects will be located after $(head-y).
768 Then the rest follows in this order:
769 $(core-y), $(libs-y), $(drivers-y) and $(net-y).
770
771 The top level Makefile define values for all generic directories,
772 and arch/$(ARCH)/Makefile only adds architecture specific directories.
773
774 Example:
775 #arch/sparc64/Makefile
776 core-y += arch/sparc64/kernel/
777 libs-y += arch/sparc64/prom/ arch/sparc64/lib/
778 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
779
780
781--- 6.4 Architecture specific boot images
782
783 An arch Makefile specifies goals that take the vmlinux file, compress
784 it, wrap it in bootstrapping code, and copy the resulting files
785 somewhere. This includes various kinds of installation commands.
786 The actual goals are not standardized across architectures.
787
788 It is common to locate any additional processing in a boot/
789 directory below arch/$(ARCH)/.
790
791 Kbuild does not provide any smart way to support building a
792 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
793 call make manually to build a target in boot/.
794
795 The recommended approach is to include shortcuts in
796 arch/$(ARCH)/Makefile, and use the full path when calling down
797 into the arch/$(ARCH)/boot/Makefile.
798
799 Example:
800 #arch/i386/Makefile
801 boot := arch/i386/boot
802 bzImage: vmlinux
803 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
804
805 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
806 make in a subdirectory.
807
808 There are no rules for naming of the architecture specific targets,
809 but executing "make help" will list all relevant targets.
810 To support this $(archhelp) must be defined.
811
812 Example:
813 #arch/i386/Makefile
814 define archhelp
815 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
816 endef
817
818 When make is executed without arguments, the first goal encountered
819 will be built. In the top level Makefile the first goal present
820 is all:.
821 An architecture shall always per default build a bootable image.
822 In "make help" the default goal is highlighted with a '*'.
823 Add a new prerequisite to all: to select a default goal different
824 from vmlinux.
825
826 Example:
827 #arch/i386/Makefile
828 all: bzImage
829
830 When "make" is executed without arguments, bzImage will be built.
831
832--- 6.5 Building non-kbuild targets
833
834 extra-y
835
836 extra-y specify additional targets created in the current
837 directory, in addition to any targets specified by obj-*.
838
839 Listing all targets in extra-y is required for two purposes:
840 1) Enable kbuild to check changes in command lines
841 - When $(call if_changed,xxx) is used
842 2) kbuild knows what files to delete during "make clean"
843
844 Example:
845 #arch/i386/kernel/Makefile
846 extra-y := head.o init_task.o
847
848 In this example extra-y is used to list object files that
849 shall be built, but shall not be linked as part of built-in.o.
850
851
852--- 6.6 Commands useful for building a boot image
853
854 Kbuild provides a few macros that are useful when building a
855 boot image.
856
857 if_changed
858
859 if_changed is the infrastructure used for the following commands.
860
861 Usage:
862 target: source(s) FORCE
863 $(call if_changed,ld/objcopy/gzip)
864
865 When the rule is evaluated it is checked to see if any files
866 needs an update, or the commandline has changed since last
867 invocation. The latter will force a rebuild if any options
868 to the executable have changed.
869 Any target that utilises if_changed must be listed in $(targets),
870 otherwise the command line check will fail, and the target will
871 always be built.
872 Assignments to $(targets) are without $(obj)/ prefix.
873 if_changed may be used in conjunction with custom commands as
874 defined in 6.7 "Custom kbuild commands".
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +0200875
Linus Torvalds1da177e2005-04-16 15:20:36 -0700876 Note: It is a typical mistake to forget the FORCE prerequisite.
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +0200877 Another common pitfall is that whitespace is sometimes
878 significant; for instance, the below will fail (note the extra space
879 after the comma):
880 target: source(s) FORCE
881 #WRONG!# $(call if_changed, ld/objcopy/gzip)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700882
883 ld
884 Link target. Often LDFLAGS_$@ is used to set specific options to ld.
885
886 objcopy
887 Copy binary. Uses OBJCOPYFLAGS usually specified in
888 arch/$(ARCH)/Makefile.
889 OBJCOPYFLAGS_$@ may be used to set additional options.
890
891 gzip
892 Compress target. Use maximum compression to compress target.
893
894 Example:
895 #arch/i386/boot/Makefile
896 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
897 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
898
899 targets += setup setup.o bootsect bootsect.o
900 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
901 $(call if_changed,ld)
902
903 In this example there are two possible targets, requiring different
904 options to the linker. the linker options are specified using the
905 LDFLAGS_$@ syntax - one for each potential target.
906 $(targets) are assinged all potential targets, herby kbuild knows
907 the targets and will:
908 1) check for commandline changes
909 2) delete target during make clean
910
911 The ": %: %.o" part of the prerequisite is a shorthand that
912 free us from listing the setup.o and bootsect.o files.
913 Note: It is a common mistake to forget the "target :=" assignment,
914 resulting in the target file being recompiled for no
915 obvious reason.
916
917
918--- 6.7 Custom kbuild commands
919
920 When kbuild is executing with KBUILD_VERBOSE=0 then only a shorthand
921 of a command is normally displayed.
922 To enable this behaviour for custom commands kbuild requires
923 two variables to be set:
924 quiet_cmd_<command> - what shall be echoed
925 cmd_<command> - the command to execute
926
927 Example:
928 #
929 quiet_cmd_image = BUILD $@
930 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
931 $(obj)/vmlinux.bin > $@
932
933 targets += bzImage
934 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
935 $(call if_changed,image)
936 @echo 'Kernel: $@ is ready'
937
938 When updating the $(obj)/bzImage target the line:
939
940 BUILD arch/i386/boot/bzImage
941
942 will be displayed with "make KBUILD_VERBOSE=0".
943
944
945--- 6.8 Preprocessing linker scripts
946
947 When the vmlinux image is build the linker script:
948 arch/$(ARCH)/kernel/vmlinux.lds is used.
949 The script is a preprocessed variant of the file vmlinux.lds.S
950 located in the same directory.
951 kbuild knows .lds file and includes a rule *lds.S -> *lds.
952
953 Example:
954 #arch/i386/kernel/Makefile
955 always := vmlinux.lds
956
957 #Makefile
958 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
959
960 The assigment to $(always) is used to tell kbuild to build the
961 target: vmlinux.lds.
962 The assignment to $(CPPFLAGS_vmlinux.lds) tell kbuild to use the
963 specified options when building the target vmlinux.lds.
964
965 When building the *.lds target kbuild used the variakles:
966 CPPFLAGS : Set in top-level Makefile
967 EXTRA_CPPFLAGS : May be set in the kbuild makefile
968 CPPFLAGS_$(@F) : Target specific flags.
969 Note that the full filename is used in this
970 assignment.
971
972 The kbuild infrastructure for *lds file are used in several
973 architecture specific files.
974
975
976--- 6.9 $(CC) support functions
977
978 The kernel may be build with several different versions of
979 $(CC), each supporting a unique set of features and options.
980 kbuild provide basic support to check for valid options for $(CC).
981 $(CC) is useally the gcc compiler, but other alternatives are
982 available.
983
984 cc-option
985 cc-option is used to check if $(CC) support a given option, and not
986 supported to use an optional second option.
987
988 Example:
989 #arch/i386/Makefile
990 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
991
992 In the above example cflags-y will be assigned the option
993 -march=pentium-mmx if supported by $(CC), otherwise -march-i586.
994 The second argument to cc-option is optional, and if omitted
995 cflags-y will be assigned no value if first option is not supported.
996
997 cc-option-yn
998 cc-option-yn is used to check if gcc supports a given option
999 and return 'y' if supported, otherwise 'n'.
1000
1001 Example:
1002 #arch/ppc/Makefile
1003 biarch := $(call cc-option-yn, -m32)
1004 aflags-$(biarch) += -a32
1005 cflags-$(biarch) += -m32
1006
1007 In the above example $(biarch) is set to y if $(CC) supports the -m32
1008 option. When $(biarch) equals to y the expanded variables $(aflags-y)
1009 and $(cflags-y) will be assigned the values -a32 and -m32.
1010
1011 cc-option-align
1012 gcc version >= 3.0 shifted type of options used to speify
1013 alignment of functions, loops etc. $(cc-option-align) whrn used
1014 as prefix to the align options will select the right prefix:
1015 gcc < 3.00
1016 cc-option-align = -malign
1017 gcc >= 3.00
1018 cc-option-align = -falign
1019
1020 Example:
1021 CFLAGS += $(cc-option-align)-functions=4
1022
1023 In the above example the option -falign-functions=4 is used for
1024 gcc >= 3.00. For gcc < 3.00 -malign-functions=4 is used.
1025
1026 cc-version
1027 cc-version return a numerical version of the $(CC) compiler version.
1028 The format is <major><minor> where both are two digits. So for example
1029 gcc 3.41 would return 0341.
1030 cc-version is useful when a specific $(CC) version is faulty in one
1031 area, for example the -mregparm=3 were broken in some gcc version
1032 even though the option was accepted by gcc.
1033
1034 Example:
1035 #arch/i386/Makefile
1036 GCC_VERSION := $(call cc-version)
1037 cflags-y += $(shell \
1038 if [ $(GCC_VERSION) -ge 0300 ] ; then echo "-mregparm=3"; fi ;)
1039
1040 In the above example -mregparm=3 is only used for gcc version greater
1041 than or equal to gcc 3.0.
1042
1043
1044=== 7 Kbuild Variables
1045
1046The top Makefile exports the following variables:
1047
1048 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1049
1050 These variables define the current kernel version. A few arch
1051 Makefiles actually use these values directly; they should use
1052 $(KERNELRELEASE) instead.
1053
1054 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1055 three-part version number, such as "2", "4", and "0". These three
1056 values are always numeric.
1057
1058 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1059 or additional patches. It is usually some non-numeric string
1060 such as "-pre4", and is often blank.
1061
1062 KERNELRELEASE
1063
1064 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1065 for constructing installation directory names or showing in
1066 version strings. Some arch Makefiles use it for this purpose.
1067
1068 ARCH
1069
1070 This variable defines the target architecture, such as "i386",
1071 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1072 determine which files to compile.
1073
1074 By default, the top Makefile sets $(ARCH) to be the same as the
1075 host system architecture. For a cross build, a user may
1076 override the value of $(ARCH) on the command line:
1077
1078 make ARCH=m68k ...
1079
1080
1081 INSTALL_PATH
1082
1083 This variable defines a place for the arch Makefiles to install
1084 the resident kernel image and System.map file.
1085 Use this for architecture specific install targets.
1086
1087 INSTALL_MOD_PATH, MODLIB
1088
1089 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1090 installation. This variable is not defined in the Makefile but
1091 may be passed in by the user if desired.
1092
1093 $(MODLIB) specifies the directory for module installation.
1094 The top Makefile defines $(MODLIB) to
1095 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1096 override this value on the command line if desired.
1097
1098=== 8 Makefile language
1099
1100The kernel Makefiles are designed to run with GNU Make. The Makefiles
1101use only the documented features of GNU Make, but they do use many
1102GNU extensions.
1103
1104GNU Make supports elementary list-processing functions. The kernel
1105Makefiles use a novel style of list building and manipulation with few
1106"if" statements.
1107
1108GNU Make has two assignment operators, ":=" and "=". ":=" performs
1109immediate evaluation of the right-hand side and stores an actual string
1110into the left-hand side. "=" is like a formula definition; it stores the
1111right-hand side in an unevaluated form and then evaluates this form each
1112time the left-hand side is used.
1113
1114There are some cases where "=" is appropriate. Usually, though, ":="
1115is the right choice.
1116
1117=== 9 Credits
1118
1119Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1120Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1121Updates by Sam Ravnborg <sam@ravnborg.org>
1122
1123=== 10 TODO
1124
1125- Describe how kbuild support shipped files with _shipped.
1126- Generating offset header files.
1127- Add more variables to section 7?
1128