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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
Sam Ravnborg20a468b2006-01-22 13:34:15 +010020 --- 3.11 $(CC) support functions
Sam Ravnborg691ef3e2009-09-19 10:31:45 +020021 --- 3.12 $(LD) support functions
Linus Torvalds1da177e2005-04-16 15:20:36 -070022
23 === 4 Host Program support
24 --- 4.1 Simple Host Program
25 --- 4.2 Composite Host Programs
Bryce Harrington39e6e9c2006-09-20 12:43:37 -070026 --- 4.3 Defining shared libraries
Linus Torvalds1da177e2005-04-16 15:20:36 -070027 --- 4.4 Using C++ for host programs
28 --- 4.5 Controlling compiler options for host programs
29 --- 4.6 When host programs are actually built
30 --- 4.7 Using hostprogs-$(CONFIG_FOO)
31
32 === 5 Kbuild clean infrastructure
33
34 === 6 Architecture Makefiles
35 --- 6.1 Set variables to tweak the build to the architecture
Sam Ravnborg5bb78262005-09-11 22:30:22 +020036 --- 6.2 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -070037 --- 6.3 List directories to visit when descending
Randy Dunlap5c811e52007-02-17 20:03:14 +010038 --- 6.4 Architecture-specific boot images
Linus Torvalds1da177e2005-04-16 15:20:36 -070039 --- 6.5 Building non-kbuild targets
40 --- 6.6 Commands useful for building a boot image
41 --- 6.7 Custom kbuild commands
42 --- 6.8 Preprocessing linker scripts
Linus Torvalds1da177e2005-04-16 15:20:36 -070043
Sam Ravnborgc7bb3492009-04-10 08:52:43 +020044 === 7 Kbuild syntax for exported headers
45 --- 7.1 header-y
46 --- 7.2 objhdr-y
47 --- 7.3 destination-y
48 --- 7.4 unifdef-y (deprecated)
49
50 === 8 Kbuild Variables
51 === 9 Makefile language
52 === 10 Credits
53 === 11 TODO
Linus Torvalds1da177e2005-04-16 15:20:36 -070054
55=== 1 Overview
56
57The Makefiles have five parts:
58
59 Makefile the top Makefile.
60 .config the kernel configuration file.
61 arch/$(ARCH)/Makefile the arch Makefile.
62 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
63 kbuild Makefiles there are about 500 of these.
64
65The top Makefile reads the .config file, which comes from the kernel
66configuration process.
67
68The top Makefile is responsible for building two major products: vmlinux
69(the resident kernel image) and modules (any module files).
70It builds these goals by recursively descending into the subdirectories of
71the kernel source tree.
72The list of subdirectories which are visited depends upon the kernel
73configuration. The top Makefile textually includes an arch Makefile
74with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
75architecture-specific information to the top Makefile.
76
77Each subdirectory has a kbuild Makefile which carries out the commands
78passed down from above. The kbuild Makefile uses information from the
Bryce Harrington39e6e9c2006-09-20 12:43:37 -070079.config file to construct various file lists used by kbuild to build
Linus Torvalds1da177e2005-04-16 15:20:36 -070080any built-in or modular targets.
81
82scripts/Makefile.* contains all the definitions/rules etc. that
83are used to build the kernel based on the kbuild makefiles.
84
85
86=== 2 Who does what
87
88People have four different relationships with the kernel Makefiles.
89
90*Users* are people who build kernels. These people type commands such as
91"make menuconfig" or "make". They usually do not read or edit
92any kernel Makefiles (or any other source files).
93
94*Normal developers* are people who work on features such as device
95drivers, file systems, and network protocols. These people need to
Jan Engelhardta07f6032006-07-27 22:14:29 +020096maintain the kbuild Makefiles for the subsystem they are
Linus Torvalds1da177e2005-04-16 15:20:36 -070097working on. In order to do this effectively, they need some overall
98knowledge about the kernel Makefiles, plus detailed knowledge about the
99public interface for kbuild.
100
101*Arch developers* are people who work on an entire architecture, such
102as sparc or ia64. Arch developers need to know about the arch Makefile
103as well as kbuild Makefiles.
104
105*Kbuild developers* are people who work on the kernel build system itself.
106These people need to know about all aspects of the kernel Makefiles.
107
108This document is aimed towards normal developers and arch developers.
109
110
111=== 3 The kbuild files
112
113Most Makefiles within the kernel are kbuild Makefiles that use the
Jan Engelhardta07f6032006-07-27 22:14:29 +0200114kbuild infrastructure. This chapter introduces the syntax used in the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700115kbuild makefiles.
Sam Ravnborg172c3ae2006-03-10 00:23:32 +0100116The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
Jan Engelhardta07f6032006-07-27 22:14:29 +0200117be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
Sam Ravnborg172c3ae2006-03-10 00:23:32 +0100118file will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700119
120Section 3.1 "Goal definitions" is a quick intro, further chapters provide
121more details, with real examples.
122
123--- 3.1 Goal definitions
124
125 Goal definitions are the main part (heart) of the kbuild Makefile.
126 These lines define the files to be built, any special compilation
127 options, and any subdirectories to be entered recursively.
128
129 The most simple kbuild makefile contains one line:
130
131 Example:
132 obj-y += foo.o
133
Randy Dunlap5c811e52007-02-17 20:03:14 +0100134 This tells kbuild that there is one object in that directory, named
Linus Torvalds1da177e2005-04-16 15:20:36 -0700135 foo.o. foo.o will be built from foo.c or foo.S.
136
137 If foo.o shall be built as a module, the variable obj-m is used.
138 Therefore the following pattern is often used:
139
140 Example:
141 obj-$(CONFIG_FOO) += foo.o
142
143 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
144 If CONFIG_FOO is neither y nor m, then the file will not be compiled
145 nor linked.
146
147--- 3.2 Built-in object goals - obj-y
148
149 The kbuild Makefile specifies object files for vmlinux
Jan Engelhardta07f6032006-07-27 22:14:29 +0200150 in the $(obj-y) lists. These lists depend on the kernel
Linus Torvalds1da177e2005-04-16 15:20:36 -0700151 configuration.
152
153 Kbuild compiles all the $(obj-y) files. It then calls
154 "$(LD) -r" to merge these files into one built-in.o file.
155 built-in.o is later linked into vmlinux by the parent Makefile.
156
157 The order of files in $(obj-y) is significant. Duplicates in
158 the lists are allowed: the first instance will be linked into
159 built-in.o and succeeding instances will be ignored.
160
161 Link order is significant, because certain functions
162 (module_init() / __initcall) will be called during boot in the
163 order they appear. So keep in mind that changing the link
Jan Engelhardta07f6032006-07-27 22:14:29 +0200164 order may e.g. change the order in which your SCSI
165 controllers are detected, and thus your disks are renumbered.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700166
167 Example:
168 #drivers/isdn/i4l/Makefile
169 # Makefile for the kernel ISDN subsystem and device drivers.
170 # Each configuration option enables a list of files.
171 obj-$(CONFIG_ISDN) += isdn.o
172 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
173
174--- 3.3 Loadable module goals - obj-m
175
176 $(obj-m) specify object files which are built as loadable
177 kernel modules.
178
179 A module may be built from one source file or several source
180 files. In the case of one source file, the kbuild makefile
181 simply adds the file to $(obj-m).
182
183 Example:
184 #drivers/isdn/i4l/Makefile
185 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
186
187 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
188
189 If a kernel module is built from several source files, you specify
190 that you want to build a module in the same way as above.
191
192 Kbuild needs to know which the parts that you want to build your
193 module from, so you have to tell it by setting an
194 $(<module_name>-objs) variable.
195
196 Example:
197 #drivers/isdn/i4l/Makefile
198 obj-$(CONFIG_ISDN) += isdn.o
199 isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
200
201 In this example, the module name will be isdn.o. Kbuild will
202 compile the objects listed in $(isdn-objs) and then run
203 "$(LD) -r" on the list of these files to generate isdn.o.
204
205 Kbuild recognises objects used for composite objects by the suffix
206 -objs, and the suffix -y. This allows the Makefiles to use
207 the value of a CONFIG_ symbol to determine if an object is part
208 of a composite object.
209
210 Example:
211 #fs/ext2/Makefile
212 obj-$(CONFIG_EXT2_FS) += ext2.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700213 ext2-y := balloc.o bitmap.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700214 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700215
Jan Engelhardta07f6032006-07-27 22:14:29 +0200216 In this example, xattr.o is only part of the composite object
217 ext2.o if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700218
219 Note: Of course, when you are building objects into the kernel,
220 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
221 kbuild will build an ext2.o file for you out of the individual
222 parts and then link this into built-in.o, as you would expect.
223
224--- 3.4 Objects which export symbols
225
226 No special notation is required in the makefiles for
227 modules exporting symbols.
228
229--- 3.5 Library file goals - lib-y
230
Jan Engelhardta07f6032006-07-27 22:14:29 +0200231 Objects listed with obj-* are used for modules, or
Linus Torvalds1da177e2005-04-16 15:20:36 -0700232 combined in a built-in.o for that specific directory.
233 There is also the possibility to list objects that will
234 be included in a library, lib.a.
235 All objects listed with lib-y are combined in a single
236 library for that directory.
Matt LaPlante5d3f0832006-11-30 05:21:10 +0100237 Objects that are listed in obj-y and additionally listed in
238 lib-y will not be included in the library, since they will
239 be accessible anyway.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200240 For consistency, objects listed in lib-m will be included in lib.a.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700241
242 Note that the same kbuild makefile may list files to be built-in
243 and to be part of a library. Therefore the same directory
244 may contain both a built-in.o and a lib.a file.
245
246 Example:
247 #arch/i386/lib/Makefile
248 lib-y := checksum.o delay.o
249
250 This will create a library lib.a based on checksum.o and delay.o.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200251 For kbuild to actually recognize that there is a lib.a being built,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700252 the directory shall be listed in libs-y.
253 See also "6.3 List directories to visit when descending".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700254
Jan Engelhardta07f6032006-07-27 22:14:29 +0200255 Use of lib-y is normally restricted to lib/ and arch/*/lib.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700256
257--- 3.6 Descending down in directories
258
259 A Makefile is only responsible for building objects in its own
260 directory. Files in subdirectories should be taken care of by
261 Makefiles in these subdirs. The build system will automatically
262 invoke make recursively in subdirectories, provided you let it know of
263 them.
264
Jan Engelhardta07f6032006-07-27 22:14:29 +0200265 To do so, obj-y and obj-m are used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700266 ext2 lives in a separate directory, and the Makefile present in fs/
267 tells kbuild to descend down using the following assignment.
268
269 Example:
270 #fs/Makefile
271 obj-$(CONFIG_EXT2_FS) += ext2/
272
273 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
274 the corresponding obj- variable will be set, and kbuild will descend
275 down in the ext2 directory.
276 Kbuild only uses this information to decide that it needs to visit
277 the directory, it is the Makefile in the subdirectory that
278 specifies what is modules and what is built-in.
279
280 It is good practice to use a CONFIG_ variable when assigning directory
281 names. This allows kbuild to totally skip the directory if the
282 corresponding CONFIG_ option is neither 'y' nor 'm'.
283
284--- 3.7 Compilation flags
285
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200286 ccflags-y, asflags-y and ldflags-y
287 The three flags listed above applies only to the kbuild makefile
288 where they are assigned. They are used for all the normal
289 cc, as and ld invocation happenign during a recursive build.
290 Note: Flags with the same behaviour were previously named:
291 EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
292 They are yet supported but their use are deprecated.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700293
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200294 ccflags-y specifies options for compiling C files with $(CC).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700295
296 Example:
297 # drivers/sound/emu10k1/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200298 ccflags-y += -I$(obj)
299 ccflags-$(DEBUG) += -DEMU10K1_DEBUG
Linus Torvalds1da177e2005-04-16 15:20:36 -0700300
301
302 This variable is necessary because the top Makefile owns the
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200303 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700304 entire tree.
305
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200306 asflags-y is a similar string for per-directory options
Linus Torvalds1da177e2005-04-16 15:20:36 -0700307 when compiling assembly language source.
308
309 Example:
310 #arch/x86_64/kernel/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200311 asflags-y := -traditional
Linus Torvalds1da177e2005-04-16 15:20:36 -0700312
313
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200314 ldflags-y is a string for per-directory options to $(LD).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700315
316 Example:
317 #arch/m68k/fpsp040/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200318 ldflags-y := -x
Linus Torvalds1da177e2005-04-16 15:20:36 -0700319
Sam Ravnborg720097d2009-04-19 11:04:26 +0200320 subdir-ccflags-y, subdir-asflags-y
321 The two flags listed above are similar to ccflags-y and as-falgs-y.
322 The difference is that the subdir- variants has effect for the kbuild
323 file where tey are present and all subdirectories.
324 Options specified using subdir-* are added to the commandline before
325 the options specified using the non-subdir variants.
326
327 Example:
328 subdir-ccflags-y := -Werror
329
Linus Torvalds1da177e2005-04-16 15:20:36 -0700330 CFLAGS_$@, AFLAGS_$@
331
332 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
333 kbuild makefile.
334
335 $(CFLAGS_$@) specifies per-file options for $(CC). The $@
336 part has a literal value which specifies the file that it is for.
337
338 Example:
339 # drivers/scsi/Makefile
340 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
341 CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
342 -DGDTH_STATISTICS
343 CFLAGS_seagate.o = -DARBITRATE -DPARITY -DSEAGATE_USE_ASM
344
345 These three lines specify compilation flags for aha152x.o,
346 gdth.o, and seagate.o
347
348 $(AFLAGS_$@) is a similar feature for source files in assembly
349 languages.
350
351 Example:
352 # arch/arm/kernel/Makefile
353 AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional
354 AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional
355
356--- 3.9 Dependency tracking
357
358 Kbuild tracks dependencies on the following:
359 1) All prerequisite files (both *.c and *.h)
360 2) CONFIG_ options used in all prerequisite files
361 3) Command-line used to compile target
362
363 Thus, if you change an option to $(CC) all affected files will
364 be re-compiled.
365
366--- 3.10 Special Rules
367
368 Special rules are used when the kbuild infrastructure does
369 not provide the required support. A typical example is
370 header files generated during the build process.
Randy Dunlap5c811e52007-02-17 20:03:14 +0100371 Another example are the architecture-specific Makefiles which
Jan Engelhardta07f6032006-07-27 22:14:29 +0200372 need special rules to prepare boot images etc.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700373
374 Special rules are written as normal Make rules.
375 Kbuild is not executing in the directory where the Makefile is
376 located, so all special rules shall provide a relative
377 path to prerequisite files and target files.
378
379 Two variables are used when defining special rules:
380
381 $(src)
382 $(src) is a relative path which points to the directory
383 where the Makefile is located. Always use $(src) when
384 referring to files located in the src tree.
385
386 $(obj)
387 $(obj) is a relative path which points to the directory
388 where the target is saved. Always use $(obj) when
389 referring to generated files.
390
391 Example:
392 #drivers/scsi/Makefile
393 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
394 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
395
396 This is a special rule, following the normal syntax
397 required by make.
398 The target file depends on two prerequisite files. References
399 to the target file are prefixed with $(obj), references
400 to prerequisites are referenced with $(src) (because they are not
401 generated files).
402
Mike Frysinger5410ecc2008-11-06 03:31:34 -0500403 $(kecho)
404 echoing information to user in a rule is often a good practice
405 but when execution "make -s" one does not expect to see any output
406 except for warnings/errors.
407 To support this kbuild define $(kecho) which will echo out the
408 text following $(kecho) to stdout except if "make -s" is used.
409
410 Example:
411 #arch/blackfin/boot/Makefile
412 $(obj)/vmImage: $(obj)/vmlinux.gz
413 $(call if_changed,uimage)
414 @$(kecho) 'Kernel: $@ is ready'
415
416
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100417--- 3.11 $(CC) support functions
418
Jan Engelhardta07f6032006-07-27 22:14:29 +0200419 The kernel may be built with several different versions of
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100420 $(CC), each supporting a unique set of features and options.
421 kbuild provide basic support to check for valid options for $(CC).
Daniel Walkere95be9a2006-10-04 02:15:21 -0700422 $(CC) is usually the gcc compiler, but other alternatives are
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100423 available.
424
425 as-option
Jan Engelhardta07f6032006-07-27 22:14:29 +0200426 as-option is used to check if $(CC) -- when used to compile
427 assembler (*.S) files -- supports the given option. An optional
428 second option may be specified if the first option is not supported.
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100429
430 Example:
431 #arch/sh/Makefile
432 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
433
Jan Engelhardta07f6032006-07-27 22:14:29 +0200434 In the above example, cflags-y will be assigned the option
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100435 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
436 The second argument is optional, and if supplied will be used
437 if first argument is not supported.
438
Sam Ravnborgf86fd302009-09-19 10:14:33 +0200439 cc-ldoption
440 cc-ldoption is used to check if $(CC) when used to link object files
Roland McGrath0b0bf7a2006-07-30 03:04:06 -0700441 supports the given option. An optional second option may be
442 specified if first option are not supported.
443
444 Example:
445 #arch/i386/kernel/Makefile
Sam Ravnborgf86fd302009-09-19 10:14:33 +0200446 vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
Roland McGrath0b0bf7a2006-07-30 03:04:06 -0700447
Randy Dunlap5c811e52007-02-17 20:03:14 +0100448 In the above example, vsyscall-flags will be assigned the option
Roland McGrath0b0bf7a2006-07-30 03:04:06 -0700449 -Wl$(comma)--hash-style=sysv if it is supported by $(CC).
450 The second argument is optional, and if supplied will be used
451 if first argument is not supported.
452
Andi Kleene2414912006-09-26 10:52:30 +0200453 as-instr
454 as-instr checks if the assembler reports a specific instruction
455 and then outputs either option1 or option2
456 C escapes are supported in the test instruction
Sam Ravnborg222d3942007-10-15 21:59:31 +0200457 Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options
Andi Kleene2414912006-09-26 10:52:30 +0200458
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100459 cc-option
Jan Engelhardta07f6032006-07-27 22:14:29 +0200460 cc-option is used to check if $(CC) supports a given option, and not
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100461 supported to use an optional second option.
462
463 Example:
464 #arch/i386/Makefile
465 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
466
Randy Dunlap5c811e52007-02-17 20:03:14 +0100467 In the above example, cflags-y will be assigned the option
Jan Engelhardta07f6032006-07-27 22:14:29 +0200468 -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
469 The second argument to cc-option is optional, and if omitted,
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100470 cflags-y will be assigned no value if first option is not supported.
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200471 Note: cc-option uses KBUILD_CFLAGS for $(CC) options
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100472
473 cc-option-yn
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700474 cc-option-yn is used to check if gcc supports a given option
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100475 and return 'y' if supported, otherwise 'n'.
476
477 Example:
478 #arch/ppc/Makefile
479 biarch := $(call cc-option-yn, -m32)
480 aflags-$(biarch) += -a32
481 cflags-$(biarch) += -m32
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700482
Jan Engelhardta07f6032006-07-27 22:14:29 +0200483 In the above example, $(biarch) is set to y if $(CC) supports the -m32
484 option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
485 and $(cflags-y) will be assigned the values -a32 and -m32,
486 respectively.
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200487 Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100488
489 cc-option-align
Jan Engelhardta07f6032006-07-27 22:14:29 +0200490 gcc versions >= 3.0 changed the type of options used to specify
491 alignment of functions, loops etc. $(cc-option-align), when used
492 as prefix to the align options, will select the right prefix:
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100493 gcc < 3.00
494 cc-option-align = -malign
495 gcc >= 3.00
496 cc-option-align = -falign
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700497
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100498 Example:
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200499 KBUILD_CFLAGS += $(cc-option-align)-functions=4
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100500
Jan Engelhardta07f6032006-07-27 22:14:29 +0200501 In the above example, the option -falign-functions=4 is used for
502 gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used.
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200503 Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700504
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100505 cc-version
Jan Engelhardta07f6032006-07-27 22:14:29 +0200506 cc-version returns a numerical version of the $(CC) compiler version.
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100507 The format is <major><minor> where both are two digits. So for example
508 gcc 3.41 would return 0341.
509 cc-version is useful when a specific $(CC) version is faulty in one
Jan Engelhardta07f6032006-07-27 22:14:29 +0200510 area, for example -mregparm=3 was broken in some gcc versions
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100511 even though the option was accepted by gcc.
512
513 Example:
514 #arch/i386/Makefile
515 cflags-y += $(shell \
516 if [ $(call cc-version) -ge 0300 ] ; then \
517 echo "-mregparm=3"; fi ;)
518
Jan Engelhardta07f6032006-07-27 22:14:29 +0200519 In the above example, -mregparm=3 is only used for gcc version greater
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100520 than or equal to gcc 3.0.
521
522 cc-ifversion
Jan Engelhardta07f6032006-07-27 22:14:29 +0200523 cc-ifversion tests the version of $(CC) and equals last argument if
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100524 version expression is true.
525
526 Example:
527 #fs/reiserfs/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200528 ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100529
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200530 In this example, ccflags-y will be assigned the value -O1 if the
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100531 $(CC) version is less than 4.2.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700532 cc-ifversion takes all the shell operators:
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100533 -eq, -ne, -lt, -le, -gt, and -ge
534 The third parameter may be a text as in this example, but it may also
535 be an expanded variable or a macro.
536
Sam Ravnborg70150302007-05-28 22:26:55 +0200537 cc-fullversion
538 cc-fullversion is useful when the exact version of gcc is needed.
539 One typical use-case is when a specific GCC version is broken.
540 cc-fullversion points out a more specific version than cc-version does.
541
542 Example:
543 #arch/powerpc/Makefile
544 $(Q)if test "$(call cc-fullversion)" = "040200" ; then \
545 echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \
546 false ; \
547 fi
548
549 In this example for a specific GCC version the build will error out explaining
550 to the user why it stops.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700551
Sam Ravnborg910b4042007-10-19 21:46:01 +0200552 cc-cross-prefix
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200553 cc-cross-prefix is used to check if there exists a $(CC) in path with
Sam Ravnborg910b4042007-10-19 21:46:01 +0200554 one of the listed prefixes. The first prefix where there exist a
555 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
556 then nothing is returned.
557 Additional prefixes are separated by a single space in the
558 call of cc-cross-prefix.
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200559 This functionality is useful for architecture Makefiles that try
560 to set CROSS_COMPILE to well-known values but may have several
Sam Ravnborg910b4042007-10-19 21:46:01 +0200561 values to select between.
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200562 It is recommended only to try to set CROSS_COMPILE if it is a cross
563 build (host arch is different from target arch). And if CROSS_COMPILE
Sam Ravnborg910b4042007-10-19 21:46:01 +0200564 is already set then leave it with the old value.
565
566 Example:
567 #arch/m68k/Makefile
568 ifneq ($(SUBARCH),$(ARCH))
569 ifeq ($(CROSS_COMPILE),)
570 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
571 endif
572 endif
573
Sam Ravnborg691ef3e2009-09-19 10:31:45 +0200574--- 3.12 $(LD) support functions
575
576 ld-option
577 ld-option is used to check if $(LD) supports the supplied option.
578 ld-option takes two options as arguments.
579 The second argument is an optional option that can be used if the
580 first option is not supported by $(LD).
581
582 Example:
583 #Makefile
584 LDFLAGS_vmlinux += $(call really-ld-option, -X)
585
586
Linus Torvalds1da177e2005-04-16 15:20:36 -0700587=== 4 Host Program support
588
589Kbuild supports building executables on the host for use during the
590compilation stage.
591Two steps are required in order to use a host executable.
592
593The first step is to tell kbuild that a host program exists. This is
594done utilising the variable hostprogs-y.
595
596The second step is to add an explicit dependency to the executable.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700597This can be done in two ways. Either add the dependency in a rule,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700598or utilise the variable $(always).
599Both possibilities are described in the following.
600
601--- 4.1 Simple Host Program
602
603 In some cases there is a need to compile and run a program on the
604 computer where the build is running.
605 The following line tells kbuild that the program bin2hex shall be
606 built on the build host.
607
608 Example:
609 hostprogs-y := bin2hex
610
611 Kbuild assumes in the above example that bin2hex is made from a single
612 c-source file named bin2hex.c located in the same directory as
613 the Makefile.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700614
Linus Torvalds1da177e2005-04-16 15:20:36 -0700615--- 4.2 Composite Host Programs
616
617 Host programs can be made up based on composite objects.
618 The syntax used to define composite objects for host programs is
619 similar to the syntax used for kernel objects.
Matt LaPlante5d3f0832006-11-30 05:21:10 +0100620 $(<executable>-objs) lists all objects used to link the final
Linus Torvalds1da177e2005-04-16 15:20:36 -0700621 executable.
622
623 Example:
624 #scripts/lxdialog/Makefile
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700625 hostprogs-y := lxdialog
Linus Torvalds1da177e2005-04-16 15:20:36 -0700626 lxdialog-objs := checklist.o lxdialog.o
627
628 Objects with extension .o are compiled from the corresponding .c
Jan Engelhardta07f6032006-07-27 22:14:29 +0200629 files. In the above example, checklist.c is compiled to checklist.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700630 and lxdialog.c is compiled to lxdialog.o.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200631 Finally, the two .o files are linked to the executable, lxdialog.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700632 Note: The syntax <executable>-y is not permitted for host-programs.
633
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700634--- 4.3 Defining shared libraries
635
Linus Torvalds1da177e2005-04-16 15:20:36 -0700636 Objects with extension .so are considered shared libraries, and
637 will be compiled as position independent objects.
638 Kbuild provides support for shared libraries, but the usage
639 shall be restricted.
640 In the following example the libkconfig.so shared library is used
641 to link the executable conf.
642
643 Example:
644 #scripts/kconfig/Makefile
645 hostprogs-y := conf
646 conf-objs := conf.o libkconfig.so
647 libkconfig-objs := expr.o type.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700648
Linus Torvalds1da177e2005-04-16 15:20:36 -0700649 Shared libraries always require a corresponding -objs line, and
650 in the example above the shared library libkconfig is composed by
651 the two objects expr.o and type.o.
652 expr.o and type.o will be built as position independent code and
653 linked as a shared library libkconfig.so. C++ is not supported for
654 shared libraries.
655
656--- 4.4 Using C++ for host programs
657
658 kbuild offers support for host programs written in C++. This was
659 introduced solely to support kconfig, and is not recommended
660 for general use.
661
662 Example:
663 #scripts/kconfig/Makefile
664 hostprogs-y := qconf
665 qconf-cxxobjs := qconf.o
666
667 In the example above the executable is composed of the C++ file
668 qconf.cc - identified by $(qconf-cxxobjs).
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700669
Linus Torvalds1da177e2005-04-16 15:20:36 -0700670 If qconf is composed by a mixture of .c and .cc files, then an
671 additional line can be used to identify this.
672
673 Example:
674 #scripts/kconfig/Makefile
675 hostprogs-y := qconf
676 qconf-cxxobjs := qconf.o
677 qconf-objs := check.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700678
Linus Torvalds1da177e2005-04-16 15:20:36 -0700679--- 4.5 Controlling compiler options for host programs
680
681 When compiling host programs, it is possible to set specific flags.
682 The programs will always be compiled utilising $(HOSTCC) passed
683 the options specified in $(HOSTCFLAGS).
684 To set flags that will take effect for all host programs created
Jan Engelhardta07f6032006-07-27 22:14:29 +0200685 in that Makefile, use the variable HOST_EXTRACFLAGS.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700686
687 Example:
688 #scripts/lxdialog/Makefile
689 HOST_EXTRACFLAGS += -I/usr/include/ncurses
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700690
Linus Torvalds1da177e2005-04-16 15:20:36 -0700691 To set specific flags for a single file the following construction
692 is used:
693
694 Example:
695 #arch/ppc64/boot/Makefile
696 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700697
Linus Torvalds1da177e2005-04-16 15:20:36 -0700698 It is also possible to specify additional options to the linker.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700699
Linus Torvalds1da177e2005-04-16 15:20:36 -0700700 Example:
701 #scripts/kconfig/Makefile
702 HOSTLOADLIBES_qconf := -L$(QTDIR)/lib
703
Jan Engelhardta07f6032006-07-27 22:14:29 +0200704 When linking qconf, it will be passed the extra option
705 "-L$(QTDIR)/lib".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700706
Linus Torvalds1da177e2005-04-16 15:20:36 -0700707--- 4.6 When host programs are actually built
708
709 Kbuild will only build host-programs when they are referenced
710 as a prerequisite.
711 This is possible in two ways:
712
713 (1) List the prerequisite explicitly in a special rule.
714
715 Example:
716 #drivers/pci/Makefile
717 hostprogs-y := gen-devlist
718 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
719 ( cd $(obj); ./gen-devlist ) < $<
720
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700721 The target $(obj)/devlist.h will not be built before
Linus Torvalds1da177e2005-04-16 15:20:36 -0700722 $(obj)/gen-devlist is updated. Note that references to
723 the host programs in special rules must be prefixed with $(obj).
724
725 (2) Use $(always)
726 When there is no suitable special rule, and the host program
727 shall be built when a makefile is entered, the $(always)
728 variable shall be used.
729
730 Example:
731 #scripts/lxdialog/Makefile
732 hostprogs-y := lxdialog
733 always := $(hostprogs-y)
734
735 This will tell kbuild to build lxdialog even if not referenced in
736 any rule.
737
738--- 4.7 Using hostprogs-$(CONFIG_FOO)
739
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700740 A typical pattern in a Kbuild file looks like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700741
742 Example:
743 #scripts/Makefile
744 hostprogs-$(CONFIG_KALLSYMS) += kallsyms
745
746 Kbuild knows about both 'y' for built-in and 'm' for module.
747 So if a config symbol evaluate to 'm', kbuild will still build
Jan Engelhardta07f6032006-07-27 22:14:29 +0200748 the binary. In other words, Kbuild handles hostprogs-m exactly
749 like hostprogs-y. But only hostprogs-y is recommended to be used
750 when no CONFIG symbols are involved.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700751
752=== 5 Kbuild clean infrastructure
753
Jan Engelhardta07f6032006-07-27 22:14:29 +0200754"make clean" deletes most generated files in the obj tree where the kernel
Linus Torvalds1da177e2005-04-16 15:20:36 -0700755is compiled. This includes generated files such as host programs.
756Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
757$(extra-y) and $(targets). They are all deleted during "make clean".
758Files matching the patterns "*.[oas]", "*.ko", plus some additional files
759generated by kbuild are deleted all over the kernel src tree when
760"make clean" is executed.
761
762Additional files can be specified in kbuild makefiles by use of $(clean-files).
763
764 Example:
765 #drivers/pci/Makefile
766 clean-files := devlist.h classlist.h
767
768When executing "make clean", the two files "devlist.h classlist.h" will
769be deleted. Kbuild will assume files to be in same relative directory as the
770Makefile except if an absolute path is specified (path starting with '/').
771
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700772To delete a directory hierarchy use:
773
Linus Torvalds1da177e2005-04-16 15:20:36 -0700774 Example:
775 #scripts/package/Makefile
776 clean-dirs := $(objtree)/debian/
777
778This will delete the directory debian, including all subdirectories.
779Kbuild will assume the directories to be in the same relative path as the
780Makefile if no absolute path is specified (path does not start with '/').
781
782Usually kbuild descends down in subdirectories due to "obj-* := dir/",
783but in the architecture makefiles where the kbuild infrastructure
784is not sufficient this sometimes needs to be explicit.
785
786 Example:
787 #arch/i386/boot/Makefile
788 subdir- := compressed/
789
790The above assignment instructs kbuild to descend down in the
791directory compressed/ when "make clean" is executed.
792
793To support the clean infrastructure in the Makefiles that builds the
794final bootimage there is an optional target named archclean:
795
796 Example:
797 #arch/i386/Makefile
798 archclean:
799 $(Q)$(MAKE) $(clean)=arch/i386/boot
800
801When "make clean" is executed, make will descend down in arch/i386/boot,
802and clean as usual. The Makefile located in arch/i386/boot/ may use
803the subdir- trick to descend further down.
804
805Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
806included in the top level makefile, and the kbuild infrastructure
807is not operational at that point.
808
809Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
810be visited during "make clean".
811
812=== 6 Architecture Makefiles
813
814The top level Makefile sets up the environment and does the preparation,
815before starting to descend down in the individual directories.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200816The top level makefile contains the generic part, whereas
817arch/$(ARCH)/Makefile contains what is required to set up kbuild
818for said architecture.
819To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
Linus Torvalds1da177e2005-04-16 15:20:36 -0700820a few targets.
821
Jan Engelhardta07f6032006-07-27 22:14:29 +0200822When kbuild executes, the following steps are followed (roughly):
8231) Configuration of the kernel => produce .config
Linus Torvalds1da177e2005-04-16 15:20:36 -07008242) Store kernel version in include/linux/version.h
8253) Symlink include/asm to include/asm-$(ARCH)
8264) Updating all other prerequisites to the target prepare:
827 - Additional prerequisites are specified in arch/$(ARCH)/Makefile
8285) Recursively descend down in all directories listed in
829 init-* core* drivers-* net-* libs-* and build all targets.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200830 - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07008316) All object files are then linked and the resulting file vmlinux is
Jan Engelhardta07f6032006-07-27 22:14:29 +0200832 located at the root of the obj tree.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700833 The very first objects linked are listed in head-y, assigned by
834 arch/$(ARCH)/Makefile.
Randy Dunlap5c811e52007-02-17 20:03:14 +01008357) Finally, the architecture-specific part does any required post processing
Linus Torvalds1da177e2005-04-16 15:20:36 -0700836 and builds the final bootimage.
837 - This includes building boot records
Randy Dunlap5c811e52007-02-17 20:03:14 +0100838 - Preparing initrd images and the like
Linus Torvalds1da177e2005-04-16 15:20:36 -0700839
840
841--- 6.1 Set variables to tweak the build to the architecture
842
843 LDFLAGS Generic $(LD) options
844
845 Flags used for all invocations of the linker.
846 Often specifying the emulation is sufficient.
847
848 Example:
849 #arch/s390/Makefile
850 LDFLAGS := -m elf_s390
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200851 Note: ldflags-y can be used to further customise
Sam Ravnborga9af3302007-09-30 20:08:25 +0200852 the flags used. See chapter 3.7.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700853
Linus Torvalds1da177e2005-04-16 15:20:36 -0700854 LDFLAGS_MODULE Options for $(LD) when linking modules
855
856 LDFLAGS_MODULE is used to set specific flags for $(LD) when
857 linking the .ko files used for modules.
858 Default is "-r", for relocatable output.
859
860 LDFLAGS_vmlinux Options for $(LD) when linking vmlinux
861
862 LDFLAGS_vmlinux is used to specify additional flags to pass to
Jan Engelhardta07f6032006-07-27 22:14:29 +0200863 the linker when linking the final vmlinux image.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700864 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
865
866 Example:
867 #arch/i386/Makefile
868 LDFLAGS_vmlinux := -e stext
869
870 OBJCOPYFLAGS objcopy flags
871
872 When $(call if_changed,objcopy) is used to translate a .o file,
Jan Engelhardta07f6032006-07-27 22:14:29 +0200873 the flags specified in OBJCOPYFLAGS will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700874 $(call if_changed,objcopy) is often used to generate raw binaries on
875 vmlinux.
876
877 Example:
878 #arch/s390/Makefile
879 OBJCOPYFLAGS := -O binary
880
881 #arch/s390/boot/Makefile
882 $(obj)/image: vmlinux FORCE
883 $(call if_changed,objcopy)
884
Jan Engelhardta07f6032006-07-27 22:14:29 +0200885 In this example, the binary $(obj)/image is a binary version of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700886 vmlinux. The usage of $(call if_changed,xxx) will be described later.
887
Sam Ravnborg222d3942007-10-15 21:59:31 +0200888 KBUILD_AFLAGS $(AS) assembler flags
Linus Torvalds1da177e2005-04-16 15:20:36 -0700889
890 Default value - see top level Makefile
891 Append or modify as required per architecture.
892
893 Example:
894 #arch/sparc64/Makefile
Sam Ravnborg222d3942007-10-15 21:59:31 +0200895 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
Linus Torvalds1da177e2005-04-16 15:20:36 -0700896
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200897 KBUILD_CFLAGS $(CC) compiler flags
Linus Torvalds1da177e2005-04-16 15:20:36 -0700898
899 Default value - see top level Makefile
900 Append or modify as required per architecture.
901
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200902 Often, the KBUILD_CFLAGS variable depends on the configuration.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700903
904 Example:
905 #arch/i386/Makefile
906 cflags-$(CONFIG_M386) += -march=i386
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200907 KBUILD_CFLAGS += $(cflags-y)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700908
909 Many arch Makefiles dynamically run the target C compiler to
910 probe supported options:
911
912 #arch/i386/Makefile
913
914 ...
915 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
916 -march=pentium2,-march=i686)
917 ...
918 # Disable unit-at-a-time mode ...
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200919 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700920 ...
921
922
Jan Engelhardta07f6032006-07-27 22:14:29 +0200923 The first example utilises the trick that a config option expands
Linus Torvalds1da177e2005-04-16 15:20:36 -0700924 to 'y' when selected.
925
926 CFLAGS_KERNEL $(CC) options specific for built-in
927
928 $(CFLAGS_KERNEL) contains extra C compiler flags used to compile
929 resident kernel code.
930
931 CFLAGS_MODULE $(CC) options specific for modules
932
933 $(CFLAGS_MODULE) contains extra C compiler flags used to compile code
934 for loadable kernel modules.
935
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700936
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200937--- 6.2 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700938
Jan Engelhardta07f6032006-07-27 22:14:29 +0200939 The archprepare: rule is used to list prerequisites that need to be
Linus Torvalds1da177e2005-04-16 15:20:36 -0700940 built before starting to descend down in the subdirectories.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200941 This is usually used for header files containing assembler constants.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700942
943 Example:
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200944 #arch/arm/Makefile
945 archprepare: maketools
Linus Torvalds1da177e2005-04-16 15:20:36 -0700946
Jan Engelhardta07f6032006-07-27 22:14:29 +0200947 In this example, the file target maketools will be processed
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200948 before descending down in the subdirectories.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700949 See also chapter XXX-TODO that describe how kbuild supports
950 generating offset header files.
951
952
953--- 6.3 List directories to visit when descending
954
955 An arch Makefile cooperates with the top Makefile to define variables
956 which specify how to build the vmlinux file. Note that there is no
957 corresponding arch-specific section for modules; the module-building
958 machinery is all architecture-independent.
959
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700960
Linus Torvalds1da177e2005-04-16 15:20:36 -0700961 head-y, init-y, core-y, libs-y, drivers-y, net-y
962
Jan Engelhardta07f6032006-07-27 22:14:29 +0200963 $(head-y) lists objects to be linked first in vmlinux.
964 $(libs-y) lists directories where a lib.a archive can be located.
Randy Dunlap5c811e52007-02-17 20:03:14 +0100965 The rest list directories where a built-in.o object file can be
Jan Engelhardta07f6032006-07-27 22:14:29 +0200966 located.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700967
968 $(init-y) objects will be located after $(head-y).
969 Then the rest follows in this order:
970 $(core-y), $(libs-y), $(drivers-y) and $(net-y).
971
Jan Engelhardta07f6032006-07-27 22:14:29 +0200972 The top level Makefile defines values for all generic directories,
Randy Dunlap5c811e52007-02-17 20:03:14 +0100973 and arch/$(ARCH)/Makefile only adds architecture-specific directories.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700974
975 Example:
976 #arch/sparc64/Makefile
977 core-y += arch/sparc64/kernel/
978 libs-y += arch/sparc64/prom/ arch/sparc64/lib/
979 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
980
981
Randy Dunlap5c811e52007-02-17 20:03:14 +0100982--- 6.4 Architecture-specific boot images
Linus Torvalds1da177e2005-04-16 15:20:36 -0700983
984 An arch Makefile specifies goals that take the vmlinux file, compress
985 it, wrap it in bootstrapping code, and copy the resulting files
986 somewhere. This includes various kinds of installation commands.
987 The actual goals are not standardized across architectures.
988
989 It is common to locate any additional processing in a boot/
990 directory below arch/$(ARCH)/.
991
992 Kbuild does not provide any smart way to support building a
993 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
994 call make manually to build a target in boot/.
995
996 The recommended approach is to include shortcuts in
997 arch/$(ARCH)/Makefile, and use the full path when calling down
998 into the arch/$(ARCH)/boot/Makefile.
999
1000 Example:
1001 #arch/i386/Makefile
1002 boot := arch/i386/boot
1003 bzImage: vmlinux
1004 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
1005
1006 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
1007 make in a subdirectory.
1008
Randy Dunlap5c811e52007-02-17 20:03:14 +01001009 There are no rules for naming architecture-specific targets,
Linus Torvalds1da177e2005-04-16 15:20:36 -07001010 but executing "make help" will list all relevant targets.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001011 To support this, $(archhelp) must be defined.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001012
1013 Example:
1014 #arch/i386/Makefile
1015 define archhelp
1016 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001017 endif
Linus Torvalds1da177e2005-04-16 15:20:36 -07001018
1019 When make is executed without arguments, the first goal encountered
1020 will be built. In the top level Makefile the first goal present
1021 is all:.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001022 An architecture shall always, per default, build a bootable image.
1023 In "make help", the default goal is highlighted with a '*'.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001024 Add a new prerequisite to all: to select a default goal different
1025 from vmlinux.
1026
1027 Example:
1028 #arch/i386/Makefile
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001029 all: bzImage
Linus Torvalds1da177e2005-04-16 15:20:36 -07001030
1031 When "make" is executed without arguments, bzImage will be built.
1032
1033--- 6.5 Building non-kbuild targets
1034
1035 extra-y
1036
1037 extra-y specify additional targets created in the current
1038 directory, in addition to any targets specified by obj-*.
1039
1040 Listing all targets in extra-y is required for two purposes:
1041 1) Enable kbuild to check changes in command lines
1042 - When $(call if_changed,xxx) is used
1043 2) kbuild knows what files to delete during "make clean"
1044
1045 Example:
1046 #arch/i386/kernel/Makefile
1047 extra-y := head.o init_task.o
1048
Jan Engelhardta07f6032006-07-27 22:14:29 +02001049 In this example, extra-y is used to list object files that
Linus Torvalds1da177e2005-04-16 15:20:36 -07001050 shall be built, but shall not be linked as part of built-in.o.
1051
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001052
Linus Torvalds1da177e2005-04-16 15:20:36 -07001053--- 6.6 Commands useful for building a boot image
1054
1055 Kbuild provides a few macros that are useful when building a
1056 boot image.
1057
1058 if_changed
1059
1060 if_changed is the infrastructure used for the following commands.
1061
1062 Usage:
1063 target: source(s) FORCE
1064 $(call if_changed,ld/objcopy/gzip)
1065
Jan Engelhardta07f6032006-07-27 22:14:29 +02001066 When the rule is evaluated, it is checked to see if any files
Randy Dunlap5c811e52007-02-17 20:03:14 +01001067 need an update, or the command line has changed since the last
Linus Torvalds1da177e2005-04-16 15:20:36 -07001068 invocation. The latter will force a rebuild if any options
1069 to the executable have changed.
1070 Any target that utilises if_changed must be listed in $(targets),
1071 otherwise the command line check will fail, and the target will
1072 always be built.
1073 Assignments to $(targets) are without $(obj)/ prefix.
1074 if_changed may be used in conjunction with custom commands as
1075 defined in 6.7 "Custom kbuild commands".
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +02001076
Linus Torvalds1da177e2005-04-16 15:20:36 -07001077 Note: It is a typical mistake to forget the FORCE prerequisite.
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +02001078 Another common pitfall is that whitespace is sometimes
1079 significant; for instance, the below will fail (note the extra space
1080 after the comma):
1081 target: source(s) FORCE
1082 #WRONG!# $(call if_changed, ld/objcopy/gzip)
Linus Torvalds1da177e2005-04-16 15:20:36 -07001083
1084 ld
Jan Engelhardta07f6032006-07-27 22:14:29 +02001085 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001086
Linus Torvalds1da177e2005-04-16 15:20:36 -07001087 objcopy
1088 Copy binary. Uses OBJCOPYFLAGS usually specified in
1089 arch/$(ARCH)/Makefile.
1090 OBJCOPYFLAGS_$@ may be used to set additional options.
1091
1092 gzip
1093 Compress target. Use maximum compression to compress target.
1094
1095 Example:
1096 #arch/i386/boot/Makefile
1097 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1098 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
1099
1100 targets += setup setup.o bootsect bootsect.o
1101 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1102 $(call if_changed,ld)
1103
Jan Engelhardta07f6032006-07-27 22:14:29 +02001104 In this example, there are two possible targets, requiring different
1105 options to the linker. The linker options are specified using the
Linus Torvalds1da177e2005-04-16 15:20:36 -07001106 LDFLAGS_$@ syntax - one for each potential target.
Matt LaPlante5d3f0832006-11-30 05:21:10 +01001107 $(targets) are assigned all potential targets, by which kbuild knows
Linus Torvalds1da177e2005-04-16 15:20:36 -07001108 the targets and will:
1109 1) check for commandline changes
1110 2) delete target during make clean
1111
1112 The ": %: %.o" part of the prerequisite is a shorthand that
1113 free us from listing the setup.o and bootsect.o files.
1114 Note: It is a common mistake to forget the "target :=" assignment,
1115 resulting in the target file being recompiled for no
1116 obvious reason.
1117
1118
1119--- 6.7 Custom kbuild commands
1120
Jan Engelhardta07f6032006-07-27 22:14:29 +02001121 When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
Linus Torvalds1da177e2005-04-16 15:20:36 -07001122 of a command is normally displayed.
1123 To enable this behaviour for custom commands kbuild requires
1124 two variables to be set:
1125 quiet_cmd_<command> - what shall be echoed
1126 cmd_<command> - the command to execute
1127
1128 Example:
1129 #
1130 quiet_cmd_image = BUILD $@
1131 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
1132 $(obj)/vmlinux.bin > $@
1133
1134 targets += bzImage
1135 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
1136 $(call if_changed,image)
1137 @echo 'Kernel: $@ is ready'
1138
Jan Engelhardta07f6032006-07-27 22:14:29 +02001139 When updating the $(obj)/bzImage target, the line
Linus Torvalds1da177e2005-04-16 15:20:36 -07001140
1141 BUILD arch/i386/boot/bzImage
1142
1143 will be displayed with "make KBUILD_VERBOSE=0".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001144
Linus Torvalds1da177e2005-04-16 15:20:36 -07001145
1146--- 6.8 Preprocessing linker scripts
1147
Jan Engelhardta07f6032006-07-27 22:14:29 +02001148 When the vmlinux image is built, the linker script
Linus Torvalds1da177e2005-04-16 15:20:36 -07001149 arch/$(ARCH)/kernel/vmlinux.lds is used.
1150 The script is a preprocessed variant of the file vmlinux.lds.S
1151 located in the same directory.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001152 kbuild knows .lds files and includes a rule *lds.S -> *lds.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001153
Linus Torvalds1da177e2005-04-16 15:20:36 -07001154 Example:
1155 #arch/i386/kernel/Makefile
1156 always := vmlinux.lds
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001157
Linus Torvalds1da177e2005-04-16 15:20:36 -07001158 #Makefile
1159 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001160
1161 The assignment to $(always) is used to tell kbuild to build the
Jan Engelhardta07f6032006-07-27 22:14:29 +02001162 target vmlinux.lds.
1163 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
Linus Torvalds1da177e2005-04-16 15:20:36 -07001164 specified options when building the target vmlinux.lds.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001165
Jan Engelhardta07f6032006-07-27 22:14:29 +02001166 When building the *.lds target, kbuild uses the variables:
Sam Ravnborg06c50402007-10-15 22:17:25 +02001167 KBUILD_CPPFLAGS : Set in top-level Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +02001168 cppflags-y : May be set in the kbuild makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001169 CPPFLAGS_$(@F) : Target specific flags.
1170 Note that the full filename is used in this
1171 assignment.
1172
1173 The kbuild infrastructure for *lds file are used in several
Randy Dunlap5c811e52007-02-17 20:03:14 +01001174 architecture-specific files.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001175
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001176=== 7 Kbuild syntax for exported headers
Linus Torvalds1da177e2005-04-16 15:20:36 -07001177
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001178The kernel include a set of headers that is exported to userspace.
1179Many headers can be exported as-is but other headers requires a
1180minimal pre-processing before they are ready for user-space.
1181The pre-processing does:
1182- drop kernel specific annotations
1183- drop include of compiler.h
1184- drop all sections that is kernel internat (guarded by ifdef __KERNEL__)
1185
1186Each relevant directory contain a file name "Kbuild" which specify the
1187headers to be exported.
1188See subsequent chapter for the syntax of the Kbuild file.
1189
1190 --- 7.1 header-y
1191
1192 header-y specify header files to be exported.
1193
1194 Example:
1195 #include/linux/Kbuild
1196 header-y += usb/
1197 header-y += aio_abi.h
1198
1199 The convention is to list one file per line and
1200 preferably in alphabetic order.
1201
1202 header-y also specify which subdirectories to visit.
1203 A subdirectory is identified by a trailing '/' which
1204 can be seen in the example above for the usb subdirectory.
1205
1206 Subdirectories are visited before their parent directories.
1207
1208 --- 7.2 objhdr-y
1209
1210 objhdr-y specifies generated files to be exported.
1211 Generated files are special as they need to be looked
1212 up in another directory when doing 'make O=...' builds.
1213
1214 Example:
1215 #include/linux/Kbuild
1216 objhdr-y += version.h
1217
1218 --- 7.3 destination-y
1219
1220 When an architecture have a set of exported headers that needs to be
1221 exported to a different directory destination-y is used.
1222 destination-y specify the destination directory for all exported
1223 headers in the file where it is present.
1224
1225 Example:
1226 #arch/xtensa/platforms/s6105/include/platform/Kbuild
1227 destination-y := include/linux
1228
1229 In the example above all exported headers in the Kbuild file
1230 will be located in the directory "include/linux" when exported.
1231
1232
1233 --- 7.4 unifdef-y (deprecated)
1234
1235 unifdef-y is deprecated. A direct replacement is header-y.
1236
1237
1238=== 8 Kbuild Variables
Linus Torvalds1da177e2005-04-16 15:20:36 -07001239
1240The top Makefile exports the following variables:
1241
1242 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1243
1244 These variables define the current kernel version. A few arch
1245 Makefiles actually use these values directly; they should use
1246 $(KERNELRELEASE) instead.
1247
1248 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1249 three-part version number, such as "2", "4", and "0". These three
1250 values are always numeric.
1251
1252 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1253 or additional patches. It is usually some non-numeric string
1254 such as "-pre4", and is often blank.
1255
1256 KERNELRELEASE
1257
1258 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1259 for constructing installation directory names or showing in
1260 version strings. Some arch Makefiles use it for this purpose.
1261
1262 ARCH
1263
1264 This variable defines the target architecture, such as "i386",
1265 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1266 determine which files to compile.
1267
1268 By default, the top Makefile sets $(ARCH) to be the same as the
1269 host system architecture. For a cross build, a user may
1270 override the value of $(ARCH) on the command line:
1271
1272 make ARCH=m68k ...
1273
1274
1275 INSTALL_PATH
1276
1277 This variable defines a place for the arch Makefiles to install
1278 the resident kernel image and System.map file.
Randy Dunlap5c811e52007-02-17 20:03:14 +01001279 Use this for architecture-specific install targets.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001280
1281 INSTALL_MOD_PATH, MODLIB
1282
1283 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1284 installation. This variable is not defined in the Makefile but
1285 may be passed in by the user if desired.
1286
1287 $(MODLIB) specifies the directory for module installation.
1288 The top Makefile defines $(MODLIB) to
1289 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1290 override this value on the command line if desired.
1291
Theodore Ts'oac031f22006-06-21 20:53:09 -04001292 INSTALL_MOD_STRIP
1293
1294 If this variable is specified, will cause modules to be stripped
1295 after they are installed. If INSTALL_MOD_STRIP is '1', then the
1296 default option --strip-debug will be used. Otherwise,
1297 INSTALL_MOD_STRIP will used as the option(s) to the strip command.
1298
1299
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001300=== 9 Makefile language
Linus Torvalds1da177e2005-04-16 15:20:36 -07001301
Jan Engelhardta07f6032006-07-27 22:14:29 +02001302The kernel Makefiles are designed to be run with GNU Make. The Makefiles
Linus Torvalds1da177e2005-04-16 15:20:36 -07001303use only the documented features of GNU Make, but they do use many
1304GNU extensions.
1305
1306GNU Make supports elementary list-processing functions. The kernel
1307Makefiles use a novel style of list building and manipulation with few
1308"if" statements.
1309
1310GNU Make has two assignment operators, ":=" and "=". ":=" performs
1311immediate evaluation of the right-hand side and stores an actual string
1312into the left-hand side. "=" is like a formula definition; it stores the
1313right-hand side in an unevaluated form and then evaluates this form each
1314time the left-hand side is used.
1315
1316There are some cases where "=" is appropriate. Usually, though, ":="
1317is the right choice.
1318
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001319=== 10 Credits
Linus Torvalds1da177e2005-04-16 15:20:36 -07001320
1321Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1322Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1323Updates by Sam Ravnborg <sam@ravnborg.org>
Jan Engelhardta07f6032006-07-27 22:14:29 +02001324Language QA by Jan Engelhardt <jengelh@gmx.de>
Linus Torvalds1da177e2005-04-16 15:20:36 -07001325
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001326=== 11 TODO
Linus Torvalds1da177e2005-04-16 15:20:36 -07001327
Jan Engelhardta07f6032006-07-27 22:14:29 +02001328- Describe how kbuild supports shipped files with _shipped.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001329- Generating offset header files.
1330- Add more variables to section 7?
1331
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001332
1333