blob: d76cfd8712e124905807332fba8b3feee89306f2 [file] [log] [blame]
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 $(CC) support functions
Linus Torvalds1da177e2005-04-16 15:20:36 -070021
22 === 4 Host Program support
23 --- 4.1 Simple Host Program
24 --- 4.2 Composite Host Programs
Bryce Harrington39e6e9c2006-09-20 12:43:37 -070025 --- 4.3 Defining shared libraries
Linus Torvalds1da177e2005-04-16 15:20:36 -070026 --- 4.4 Using C++ for host programs
27 --- 4.5 Controlling compiler options for host programs
28 --- 4.6 When host programs are actually built
29 --- 4.7 Using hostprogs-$(CONFIG_FOO)
30
31 === 5 Kbuild clean infrastructure
32
33 === 6 Architecture Makefiles
34 --- 6.1 Set variables to tweak the build to the architecture
Sam Ravnborg5bb78262005-09-11 22:30:22 +020035 --- 6.2 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -070036 --- 6.3 List directories to visit when descending
Randy Dunlap5c811e52007-02-17 20:03:14 +010037 --- 6.4 Architecture-specific boot images
Linus Torvalds1da177e2005-04-16 15:20:36 -070038 --- 6.5 Building non-kbuild targets
39 --- 6.6 Commands useful for building a boot image
40 --- 6.7 Custom kbuild commands
41 --- 6.8 Preprocessing linker scripts
Linus Torvalds1da177e2005-04-16 15:20:36 -070042
Sam Ravnborgc7bb3492009-04-10 08:52:43 +020043 === 7 Kbuild syntax for exported headers
44 --- 7.1 header-y
45 --- 7.2 objhdr-y
46 --- 7.3 destination-y
47 --- 7.4 unifdef-y (deprecated)
48
49 === 8 Kbuild Variables
50 === 9 Makefile language
51 === 10 Credits
52 === 11 TODO
Linus Torvalds1da177e2005-04-16 15:20:36 -070053
54=== 1 Overview
55
56The Makefiles have five parts:
57
58 Makefile the top Makefile.
59 .config the kernel configuration file.
60 arch/$(ARCH)/Makefile the arch Makefile.
61 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
62 kbuild Makefiles there are about 500 of these.
63
64The top Makefile reads the .config file, which comes from the kernel
65configuration process.
66
67The top Makefile is responsible for building two major products: vmlinux
68(the resident kernel image) and modules (any module files).
69It builds these goals by recursively descending into the subdirectories of
70the kernel source tree.
71The list of subdirectories which are visited depends upon the kernel
72configuration. The top Makefile textually includes an arch Makefile
73with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
74architecture-specific information to the top Makefile.
75
76Each subdirectory has a kbuild Makefile which carries out the commands
77passed down from above. The kbuild Makefile uses information from the
Bryce Harrington39e6e9c2006-09-20 12:43:37 -070078.config file to construct various file lists used by kbuild to build
Linus Torvalds1da177e2005-04-16 15:20:36 -070079any built-in or modular targets.
80
81scripts/Makefile.* contains all the definitions/rules etc. that
82are used to build the kernel based on the kbuild makefiles.
83
84
85=== 2 Who does what
86
87People have four different relationships with the kernel Makefiles.
88
89*Users* are people who build kernels. These people type commands such as
90"make menuconfig" or "make". They usually do not read or edit
91any kernel Makefiles (or any other source files).
92
93*Normal developers* are people who work on features such as device
94drivers, file systems, and network protocols. These people need to
Jan Engelhardta07f6032006-07-27 22:14:29 +020095maintain the kbuild Makefiles for the subsystem they are
Linus Torvalds1da177e2005-04-16 15:20:36 -070096working on. In order to do this effectively, they need some overall
97knowledge about the kernel Makefiles, plus detailed knowledge about the
98public interface for kbuild.
99
100*Arch developers* are people who work on an entire architecture, such
101as sparc or ia64. Arch developers need to know about the arch Makefile
102as well as kbuild Makefiles.
103
104*Kbuild developers* are people who work on the kernel build system itself.
105These people need to know about all aspects of the kernel Makefiles.
106
107This document is aimed towards normal developers and arch developers.
108
109
110=== 3 The kbuild files
111
112Most Makefiles within the kernel are kbuild Makefiles that use the
Jan Engelhardta07f6032006-07-27 22:14:29 +0200113kbuild infrastructure. This chapter introduces the syntax used in the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700114kbuild makefiles.
Sam Ravnborg172c3ae2006-03-10 00:23:32 +0100115The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
Jan Engelhardta07f6032006-07-27 22:14:29 +0200116be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
Sam Ravnborg172c3ae2006-03-10 00:23:32 +0100117file will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700118
119Section 3.1 "Goal definitions" is a quick intro, further chapters provide
120more details, with real examples.
121
122--- 3.1 Goal definitions
123
124 Goal definitions are the main part (heart) of the kbuild Makefile.
125 These lines define the files to be built, any special compilation
126 options, and any subdirectories to be entered recursively.
127
128 The most simple kbuild makefile contains one line:
129
130 Example:
131 obj-y += foo.o
132
Randy Dunlap5c811e52007-02-17 20:03:14 +0100133 This tells kbuild that there is one object in that directory, named
Linus Torvalds1da177e2005-04-16 15:20:36 -0700134 foo.o. foo.o will be built from foo.c or foo.S.
135
136 If foo.o shall be built as a module, the variable obj-m is used.
137 Therefore the following pattern is often used:
138
139 Example:
140 obj-$(CONFIG_FOO) += foo.o
141
142 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
143 If CONFIG_FOO is neither y nor m, then the file will not be compiled
144 nor linked.
145
146--- 3.2 Built-in object goals - obj-y
147
148 The kbuild Makefile specifies object files for vmlinux
Jan Engelhardta07f6032006-07-27 22:14:29 +0200149 in the $(obj-y) lists. These lists depend on the kernel
Linus Torvalds1da177e2005-04-16 15:20:36 -0700150 configuration.
151
152 Kbuild compiles all the $(obj-y) files. It then calls
153 "$(LD) -r" to merge these files into one built-in.o file.
154 built-in.o is later linked into vmlinux by the parent Makefile.
155
156 The order of files in $(obj-y) is significant. Duplicates in
157 the lists are allowed: the first instance will be linked into
158 built-in.o and succeeding instances will be ignored.
159
160 Link order is significant, because certain functions
161 (module_init() / __initcall) will be called during boot in the
162 order they appear. So keep in mind that changing the link
Jan Engelhardta07f6032006-07-27 22:14:29 +0200163 order may e.g. change the order in which your SCSI
164 controllers are detected, and thus your disks are renumbered.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700165
166 Example:
167 #drivers/isdn/i4l/Makefile
168 # Makefile for the kernel ISDN subsystem and device drivers.
169 # Each configuration option enables a list of files.
170 obj-$(CONFIG_ISDN) += isdn.o
171 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
172
173--- 3.3 Loadable module goals - obj-m
174
175 $(obj-m) specify object files which are built as loadable
176 kernel modules.
177
178 A module may be built from one source file or several source
179 files. In the case of one source file, the kbuild makefile
180 simply adds the file to $(obj-m).
181
182 Example:
183 #drivers/isdn/i4l/Makefile
184 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
185
186 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
187
188 If a kernel module is built from several source files, you specify
189 that you want to build a module in the same way as above.
190
191 Kbuild needs to know which the parts that you want to build your
192 module from, so you have to tell it by setting an
193 $(<module_name>-objs) variable.
194
195 Example:
196 #drivers/isdn/i4l/Makefile
197 obj-$(CONFIG_ISDN) += isdn.o
198 isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
199
200 In this example, the module name will be isdn.o. Kbuild will
201 compile the objects listed in $(isdn-objs) and then run
202 "$(LD) -r" on the list of these files to generate isdn.o.
203
204 Kbuild recognises objects used for composite objects by the suffix
205 -objs, and the suffix -y. This allows the Makefiles to use
206 the value of a CONFIG_ symbol to determine if an object is part
207 of a composite object.
208
209 Example:
210 #fs/ext2/Makefile
211 obj-$(CONFIG_EXT2_FS) += ext2.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700212 ext2-y := balloc.o bitmap.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700213 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700214
Jan Engelhardta07f6032006-07-27 22:14:29 +0200215 In this example, xattr.o is only part of the composite object
216 ext2.o if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700217
218 Note: Of course, when you are building objects into the kernel,
219 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
220 kbuild will build an ext2.o file for you out of the individual
221 parts and then link this into built-in.o, as you would expect.
222
223--- 3.4 Objects which export symbols
224
225 No special notation is required in the makefiles for
226 modules exporting symbols.
227
228--- 3.5 Library file goals - lib-y
229
Jan Engelhardta07f6032006-07-27 22:14:29 +0200230 Objects listed with obj-* are used for modules, or
Linus Torvalds1da177e2005-04-16 15:20:36 -0700231 combined in a built-in.o for that specific directory.
232 There is also the possibility to list objects that will
233 be included in a library, lib.a.
234 All objects listed with lib-y are combined in a single
235 library for that directory.
Matt LaPlante5d3f0832006-11-30 05:21:10 +0100236 Objects that are listed in obj-y and additionally listed in
237 lib-y will not be included in the library, since they will
238 be accessible anyway.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200239 For consistency, objects listed in lib-m will be included in lib.a.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700240
241 Note that the same kbuild makefile may list files to be built-in
242 and to be part of a library. Therefore the same directory
243 may contain both a built-in.o and a lib.a file.
244
245 Example:
246 #arch/i386/lib/Makefile
247 lib-y := checksum.o delay.o
248
249 This will create a library lib.a based on checksum.o and delay.o.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200250 For kbuild to actually recognize that there is a lib.a being built,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700251 the directory shall be listed in libs-y.
252 See also "6.3 List directories to visit when descending".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700253
Jan Engelhardta07f6032006-07-27 22:14:29 +0200254 Use of lib-y is normally restricted to lib/ and arch/*/lib.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700255
256--- 3.6 Descending down in directories
257
258 A Makefile is only responsible for building objects in its own
259 directory. Files in subdirectories should be taken care of by
260 Makefiles in these subdirs. The build system will automatically
261 invoke make recursively in subdirectories, provided you let it know of
262 them.
263
Jan Engelhardta07f6032006-07-27 22:14:29 +0200264 To do so, obj-y and obj-m are used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700265 ext2 lives in a separate directory, and the Makefile present in fs/
266 tells kbuild to descend down using the following assignment.
267
268 Example:
269 #fs/Makefile
270 obj-$(CONFIG_EXT2_FS) += ext2/
271
272 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
273 the corresponding obj- variable will be set, and kbuild will descend
274 down in the ext2 directory.
275 Kbuild only uses this information to decide that it needs to visit
276 the directory, it is the Makefile in the subdirectory that
277 specifies what is modules and what is built-in.
278
279 It is good practice to use a CONFIG_ variable when assigning directory
280 names. This allows kbuild to totally skip the directory if the
281 corresponding CONFIG_ option is neither 'y' nor 'm'.
282
283--- 3.7 Compilation flags
284
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200285 ccflags-y, asflags-y and ldflags-y
286 The three flags listed above applies only to the kbuild makefile
287 where they are assigned. They are used for all the normal
288 cc, as and ld invocation happenign during a recursive build.
289 Note: Flags with the same behaviour were previously named:
290 EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
291 They are yet supported but their use are deprecated.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700292
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200293 ccflags-y specifies options for compiling C files with $(CC).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700294
295 Example:
296 # drivers/sound/emu10k1/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200297 ccflags-y += -I$(obj)
298 ccflags-$(DEBUG) += -DEMU10K1_DEBUG
Linus Torvalds1da177e2005-04-16 15:20:36 -0700299
300
301 This variable is necessary because the top Makefile owns the
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200302 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700303 entire tree.
304
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200305 asflags-y is a similar string for per-directory options
Linus Torvalds1da177e2005-04-16 15:20:36 -0700306 when compiling assembly language source.
307
308 Example:
309 #arch/x86_64/kernel/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200310 asflags-y := -traditional
Linus Torvalds1da177e2005-04-16 15:20:36 -0700311
312
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200313 ldflags-y is a string for per-directory options to $(LD).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700314
315 Example:
316 #arch/m68k/fpsp040/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200317 ldflags-y := -x
Linus Torvalds1da177e2005-04-16 15:20:36 -0700318
Sam Ravnborg720097d2009-04-19 11:04:26 +0200319 subdir-ccflags-y, subdir-asflags-y
320 The two flags listed above are similar to ccflags-y and as-falgs-y.
321 The difference is that the subdir- variants has effect for the kbuild
322 file where tey are present and all subdirectories.
323 Options specified using subdir-* are added to the commandline before
324 the options specified using the non-subdir variants.
325
326 Example:
327 subdir-ccflags-y := -Werror
328
Linus Torvalds1da177e2005-04-16 15:20:36 -0700329 CFLAGS_$@, AFLAGS_$@
330
331 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
332 kbuild makefile.
333
334 $(CFLAGS_$@) specifies per-file options for $(CC). The $@
335 part has a literal value which specifies the file that it is for.
336
337 Example:
338 # drivers/scsi/Makefile
339 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
340 CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
341 -DGDTH_STATISTICS
342 CFLAGS_seagate.o = -DARBITRATE -DPARITY -DSEAGATE_USE_ASM
343
344 These three lines specify compilation flags for aha152x.o,
345 gdth.o, and seagate.o
346
347 $(AFLAGS_$@) is a similar feature for source files in assembly
348 languages.
349
350 Example:
351 # arch/arm/kernel/Makefile
352 AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional
353 AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional
354
355--- 3.9 Dependency tracking
356
357 Kbuild tracks dependencies on the following:
358 1) All prerequisite files (both *.c and *.h)
359 2) CONFIG_ options used in all prerequisite files
360 3) Command-line used to compile target
361
362 Thus, if you change an option to $(CC) all affected files will
363 be re-compiled.
364
365--- 3.10 Special Rules
366
367 Special rules are used when the kbuild infrastructure does
368 not provide the required support. A typical example is
369 header files generated during the build process.
Randy Dunlap5c811e52007-02-17 20:03:14 +0100370 Another example are the architecture-specific Makefiles which
Jan Engelhardta07f6032006-07-27 22:14:29 +0200371 need special rules to prepare boot images etc.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700372
373 Special rules are written as normal Make rules.
374 Kbuild is not executing in the directory where the Makefile is
375 located, so all special rules shall provide a relative
376 path to prerequisite files and target files.
377
378 Two variables are used when defining special rules:
379
380 $(src)
381 $(src) is a relative path which points to the directory
382 where the Makefile is located. Always use $(src) when
383 referring to files located in the src tree.
384
385 $(obj)
386 $(obj) is a relative path which points to the directory
387 where the target is saved. Always use $(obj) when
388 referring to generated files.
389
390 Example:
391 #drivers/scsi/Makefile
392 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
393 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
394
395 This is a special rule, following the normal syntax
396 required by make.
397 The target file depends on two prerequisite files. References
398 to the target file are prefixed with $(obj), references
399 to prerequisites are referenced with $(src) (because they are not
400 generated files).
401
Mike Frysinger5410ecc2008-11-06 03:31:34 -0500402 $(kecho)
403 echoing information to user in a rule is often a good practice
404 but when execution "make -s" one does not expect to see any output
405 except for warnings/errors.
406 To support this kbuild define $(kecho) which will echo out the
407 text following $(kecho) to stdout except if "make -s" is used.
408
409 Example:
410 #arch/blackfin/boot/Makefile
411 $(obj)/vmImage: $(obj)/vmlinux.gz
412 $(call if_changed,uimage)
413 @$(kecho) 'Kernel: $@ is ready'
414
415
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100416--- 3.11 $(CC) support functions
417
Jan Engelhardta07f6032006-07-27 22:14:29 +0200418 The kernel may be built with several different versions of
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100419 $(CC), each supporting a unique set of features and options.
420 kbuild provide basic support to check for valid options for $(CC).
Daniel Walkere95be9a2006-10-04 02:15:21 -0700421 $(CC) is usually the gcc compiler, but other alternatives are
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100422 available.
423
424 as-option
Jan Engelhardta07f6032006-07-27 22:14:29 +0200425 as-option is used to check if $(CC) -- when used to compile
426 assembler (*.S) files -- supports the given option. An optional
427 second option may be specified if the first option is not supported.
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100428
429 Example:
430 #arch/sh/Makefile
431 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
432
Jan Engelhardta07f6032006-07-27 22:14:29 +0200433 In the above example, cflags-y will be assigned the option
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100434 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
435 The second argument is optional, and if supplied will be used
436 if first argument is not supported.
437
Roland McGrath0b0bf7a2006-07-30 03:04:06 -0700438 ld-option
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700439 ld-option is used to check if $(CC) when used to link object files
Roland McGrath0b0bf7a2006-07-30 03:04:06 -0700440 supports the given option. An optional second option may be
441 specified if first option are not supported.
442
443 Example:
444 #arch/i386/kernel/Makefile
445 vsyscall-flags += $(call ld-option, -Wl$(comma)--hash-style=sysv)
446
Randy Dunlap5c811e52007-02-17 20:03:14 +0100447 In the above example, vsyscall-flags will be assigned the option
Roland McGrath0b0bf7a2006-07-30 03:04:06 -0700448 -Wl$(comma)--hash-style=sysv if it is supported by $(CC).
449 The second argument is optional, and if supplied will be used
450 if first argument is not supported.
451
Andi Kleene2414912006-09-26 10:52:30 +0200452 as-instr
453 as-instr checks if the assembler reports a specific instruction
454 and then outputs either option1 or option2
455 C escapes are supported in the test instruction
Sam Ravnborg222d3942007-10-15 21:59:31 +0200456 Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options
Andi Kleene2414912006-09-26 10:52:30 +0200457
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100458 cc-option
Jan Engelhardta07f6032006-07-27 22:14:29 +0200459 cc-option is used to check if $(CC) supports a given option, and not
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100460 supported to use an optional second option.
461
462 Example:
463 #arch/i386/Makefile
464 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
465
Randy Dunlap5c811e52007-02-17 20:03:14 +0100466 In the above example, cflags-y will be assigned the option
Jan Engelhardta07f6032006-07-27 22:14:29 +0200467 -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
468 The second argument to cc-option is optional, and if omitted,
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100469 cflags-y will be assigned no value if first option is not supported.
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200470 Note: cc-option uses KBUILD_CFLAGS for $(CC) options
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100471
472 cc-option-yn
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700473 cc-option-yn is used to check if gcc supports a given option
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100474 and return 'y' if supported, otherwise 'n'.
475
476 Example:
477 #arch/ppc/Makefile
478 biarch := $(call cc-option-yn, -m32)
479 aflags-$(biarch) += -a32
480 cflags-$(biarch) += -m32
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700481
Jan Engelhardta07f6032006-07-27 22:14:29 +0200482 In the above example, $(biarch) is set to y if $(CC) supports the -m32
483 option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
484 and $(cflags-y) will be assigned the values -a32 and -m32,
485 respectively.
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200486 Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100487
488 cc-option-align
Jan Engelhardta07f6032006-07-27 22:14:29 +0200489 gcc versions >= 3.0 changed the type of options used to specify
490 alignment of functions, loops etc. $(cc-option-align), when used
491 as prefix to the align options, will select the right prefix:
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100492 gcc < 3.00
493 cc-option-align = -malign
494 gcc >= 3.00
495 cc-option-align = -falign
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700496
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100497 Example:
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200498 KBUILD_CFLAGS += $(cc-option-align)-functions=4
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100499
Jan Engelhardta07f6032006-07-27 22:14:29 +0200500 In the above example, the option -falign-functions=4 is used for
501 gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used.
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200502 Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700503
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100504 cc-version
Jan Engelhardta07f6032006-07-27 22:14:29 +0200505 cc-version returns a numerical version of the $(CC) compiler version.
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100506 The format is <major><minor> where both are two digits. So for example
507 gcc 3.41 would return 0341.
508 cc-version is useful when a specific $(CC) version is faulty in one
Jan Engelhardta07f6032006-07-27 22:14:29 +0200509 area, for example -mregparm=3 was broken in some gcc versions
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100510 even though the option was accepted by gcc.
511
512 Example:
513 #arch/i386/Makefile
514 cflags-y += $(shell \
515 if [ $(call cc-version) -ge 0300 ] ; then \
516 echo "-mregparm=3"; fi ;)
517
Jan Engelhardta07f6032006-07-27 22:14:29 +0200518 In the above example, -mregparm=3 is only used for gcc version greater
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100519 than or equal to gcc 3.0.
520
521 cc-ifversion
Jan Engelhardta07f6032006-07-27 22:14:29 +0200522 cc-ifversion tests the version of $(CC) and equals last argument if
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100523 version expression is true.
524
525 Example:
526 #fs/reiserfs/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200527 ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100528
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200529 In this example, ccflags-y will be assigned the value -O1 if the
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100530 $(CC) version is less than 4.2.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700531 cc-ifversion takes all the shell operators:
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100532 -eq, -ne, -lt, -le, -gt, and -ge
533 The third parameter may be a text as in this example, but it may also
534 be an expanded variable or a macro.
535
Sam Ravnborg70150302007-05-28 22:26:55 +0200536 cc-fullversion
537 cc-fullversion is useful when the exact version of gcc is needed.
538 One typical use-case is when a specific GCC version is broken.
539 cc-fullversion points out a more specific version than cc-version does.
540
541 Example:
542 #arch/powerpc/Makefile
543 $(Q)if test "$(call cc-fullversion)" = "040200" ; then \
544 echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \
545 false ; \
546 fi
547
548 In this example for a specific GCC version the build will error out explaining
549 to the user why it stops.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700550
Sam Ravnborg910b4042007-10-19 21:46:01 +0200551 cc-cross-prefix
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200552 cc-cross-prefix is used to check if there exists a $(CC) in path with
Sam Ravnborg910b4042007-10-19 21:46:01 +0200553 one of the listed prefixes. The first prefix where there exist a
554 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
555 then nothing is returned.
556 Additional prefixes are separated by a single space in the
557 call of cc-cross-prefix.
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200558 This functionality is useful for architecture Makefiles that try
559 to set CROSS_COMPILE to well-known values but may have several
Sam Ravnborg910b4042007-10-19 21:46:01 +0200560 values to select between.
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200561 It is recommended only to try to set CROSS_COMPILE if it is a cross
562 build (host arch is different from target arch). And if CROSS_COMPILE
Sam Ravnborg910b4042007-10-19 21:46:01 +0200563 is already set then leave it with the old value.
564
565 Example:
566 #arch/m68k/Makefile
567 ifneq ($(SUBARCH),$(ARCH))
568 ifeq ($(CROSS_COMPILE),)
569 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
570 endif
571 endif
572
Linus Torvalds1da177e2005-04-16 15:20:36 -0700573=== 4 Host Program support
574
575Kbuild supports building executables on the host for use during the
576compilation stage.
577Two steps are required in order to use a host executable.
578
579The first step is to tell kbuild that a host program exists. This is
580done utilising the variable hostprogs-y.
581
582The second step is to add an explicit dependency to the executable.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700583This can be done in two ways. Either add the dependency in a rule,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700584or utilise the variable $(always).
585Both possibilities are described in the following.
586
587--- 4.1 Simple Host Program
588
589 In some cases there is a need to compile and run a program on the
590 computer where the build is running.
591 The following line tells kbuild that the program bin2hex shall be
592 built on the build host.
593
594 Example:
595 hostprogs-y := bin2hex
596
597 Kbuild assumes in the above example that bin2hex is made from a single
598 c-source file named bin2hex.c located in the same directory as
599 the Makefile.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700600
Linus Torvalds1da177e2005-04-16 15:20:36 -0700601--- 4.2 Composite Host Programs
602
603 Host programs can be made up based on composite objects.
604 The syntax used to define composite objects for host programs is
605 similar to the syntax used for kernel objects.
Matt LaPlante5d3f0832006-11-30 05:21:10 +0100606 $(<executable>-objs) lists all objects used to link the final
Linus Torvalds1da177e2005-04-16 15:20:36 -0700607 executable.
608
609 Example:
610 #scripts/lxdialog/Makefile
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700611 hostprogs-y := lxdialog
Linus Torvalds1da177e2005-04-16 15:20:36 -0700612 lxdialog-objs := checklist.o lxdialog.o
613
614 Objects with extension .o are compiled from the corresponding .c
Jan Engelhardta07f6032006-07-27 22:14:29 +0200615 files. In the above example, checklist.c is compiled to checklist.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700616 and lxdialog.c is compiled to lxdialog.o.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200617 Finally, the two .o files are linked to the executable, lxdialog.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700618 Note: The syntax <executable>-y is not permitted for host-programs.
619
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700620--- 4.3 Defining shared libraries
621
Linus Torvalds1da177e2005-04-16 15:20:36 -0700622 Objects with extension .so are considered shared libraries, and
623 will be compiled as position independent objects.
624 Kbuild provides support for shared libraries, but the usage
625 shall be restricted.
626 In the following example the libkconfig.so shared library is used
627 to link the executable conf.
628
629 Example:
630 #scripts/kconfig/Makefile
631 hostprogs-y := conf
632 conf-objs := conf.o libkconfig.so
633 libkconfig-objs := expr.o type.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700634
Linus Torvalds1da177e2005-04-16 15:20:36 -0700635 Shared libraries always require a corresponding -objs line, and
636 in the example above the shared library libkconfig is composed by
637 the two objects expr.o and type.o.
638 expr.o and type.o will be built as position independent code and
639 linked as a shared library libkconfig.so. C++ is not supported for
640 shared libraries.
641
642--- 4.4 Using C++ for host programs
643
644 kbuild offers support for host programs written in C++. This was
645 introduced solely to support kconfig, and is not recommended
646 for general use.
647
648 Example:
649 #scripts/kconfig/Makefile
650 hostprogs-y := qconf
651 qconf-cxxobjs := qconf.o
652
653 In the example above the executable is composed of the C++ file
654 qconf.cc - identified by $(qconf-cxxobjs).
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700655
Linus Torvalds1da177e2005-04-16 15:20:36 -0700656 If qconf is composed by a mixture of .c and .cc files, then an
657 additional line can be used to identify this.
658
659 Example:
660 #scripts/kconfig/Makefile
661 hostprogs-y := qconf
662 qconf-cxxobjs := qconf.o
663 qconf-objs := check.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700664
Linus Torvalds1da177e2005-04-16 15:20:36 -0700665--- 4.5 Controlling compiler options for host programs
666
667 When compiling host programs, it is possible to set specific flags.
668 The programs will always be compiled utilising $(HOSTCC) passed
669 the options specified in $(HOSTCFLAGS).
670 To set flags that will take effect for all host programs created
Jan Engelhardta07f6032006-07-27 22:14:29 +0200671 in that Makefile, use the variable HOST_EXTRACFLAGS.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700672
673 Example:
674 #scripts/lxdialog/Makefile
675 HOST_EXTRACFLAGS += -I/usr/include/ncurses
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700676
Linus Torvalds1da177e2005-04-16 15:20:36 -0700677 To set specific flags for a single file the following construction
678 is used:
679
680 Example:
681 #arch/ppc64/boot/Makefile
682 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700683
Linus Torvalds1da177e2005-04-16 15:20:36 -0700684 It is also possible to specify additional options to the linker.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700685
Linus Torvalds1da177e2005-04-16 15:20:36 -0700686 Example:
687 #scripts/kconfig/Makefile
688 HOSTLOADLIBES_qconf := -L$(QTDIR)/lib
689
Jan Engelhardta07f6032006-07-27 22:14:29 +0200690 When linking qconf, it will be passed the extra option
691 "-L$(QTDIR)/lib".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700692
Linus Torvalds1da177e2005-04-16 15:20:36 -0700693--- 4.6 When host programs are actually built
694
695 Kbuild will only build host-programs when they are referenced
696 as a prerequisite.
697 This is possible in two ways:
698
699 (1) List the prerequisite explicitly in a special rule.
700
701 Example:
702 #drivers/pci/Makefile
703 hostprogs-y := gen-devlist
704 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
705 ( cd $(obj); ./gen-devlist ) < $<
706
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700707 The target $(obj)/devlist.h will not be built before
Linus Torvalds1da177e2005-04-16 15:20:36 -0700708 $(obj)/gen-devlist is updated. Note that references to
709 the host programs in special rules must be prefixed with $(obj).
710
711 (2) Use $(always)
712 When there is no suitable special rule, and the host program
713 shall be built when a makefile is entered, the $(always)
714 variable shall be used.
715
716 Example:
717 #scripts/lxdialog/Makefile
718 hostprogs-y := lxdialog
719 always := $(hostprogs-y)
720
721 This will tell kbuild to build lxdialog even if not referenced in
722 any rule.
723
724--- 4.7 Using hostprogs-$(CONFIG_FOO)
725
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700726 A typical pattern in a Kbuild file looks like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700727
728 Example:
729 #scripts/Makefile
730 hostprogs-$(CONFIG_KALLSYMS) += kallsyms
731
732 Kbuild knows about both 'y' for built-in and 'm' for module.
733 So if a config symbol evaluate to 'm', kbuild will still build
Jan Engelhardta07f6032006-07-27 22:14:29 +0200734 the binary. In other words, Kbuild handles hostprogs-m exactly
735 like hostprogs-y. But only hostprogs-y is recommended to be used
736 when no CONFIG symbols are involved.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700737
738=== 5 Kbuild clean infrastructure
739
Jan Engelhardta07f6032006-07-27 22:14:29 +0200740"make clean" deletes most generated files in the obj tree where the kernel
Linus Torvalds1da177e2005-04-16 15:20:36 -0700741is compiled. This includes generated files such as host programs.
742Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
743$(extra-y) and $(targets). They are all deleted during "make clean".
744Files matching the patterns "*.[oas]", "*.ko", plus some additional files
745generated by kbuild are deleted all over the kernel src tree when
746"make clean" is executed.
747
748Additional files can be specified in kbuild makefiles by use of $(clean-files).
749
750 Example:
751 #drivers/pci/Makefile
752 clean-files := devlist.h classlist.h
753
754When executing "make clean", the two files "devlist.h classlist.h" will
755be deleted. Kbuild will assume files to be in same relative directory as the
756Makefile except if an absolute path is specified (path starting with '/').
757
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700758To delete a directory hierarchy use:
759
Linus Torvalds1da177e2005-04-16 15:20:36 -0700760 Example:
761 #scripts/package/Makefile
762 clean-dirs := $(objtree)/debian/
763
764This will delete the directory debian, including all subdirectories.
765Kbuild will assume the directories to be in the same relative path as the
766Makefile if no absolute path is specified (path does not start with '/').
767
768Usually kbuild descends down in subdirectories due to "obj-* := dir/",
769but in the architecture makefiles where the kbuild infrastructure
770is not sufficient this sometimes needs to be explicit.
771
772 Example:
773 #arch/i386/boot/Makefile
774 subdir- := compressed/
775
776The above assignment instructs kbuild to descend down in the
777directory compressed/ when "make clean" is executed.
778
779To support the clean infrastructure in the Makefiles that builds the
780final bootimage there is an optional target named archclean:
781
782 Example:
783 #arch/i386/Makefile
784 archclean:
785 $(Q)$(MAKE) $(clean)=arch/i386/boot
786
787When "make clean" is executed, make will descend down in arch/i386/boot,
788and clean as usual. The Makefile located in arch/i386/boot/ may use
789the subdir- trick to descend further down.
790
791Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
792included in the top level makefile, and the kbuild infrastructure
793is not operational at that point.
794
795Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
796be visited during "make clean".
797
798=== 6 Architecture Makefiles
799
800The top level Makefile sets up the environment and does the preparation,
801before starting to descend down in the individual directories.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200802The top level makefile contains the generic part, whereas
803arch/$(ARCH)/Makefile contains what is required to set up kbuild
804for said architecture.
805To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
Linus Torvalds1da177e2005-04-16 15:20:36 -0700806a few targets.
807
Jan Engelhardta07f6032006-07-27 22:14:29 +0200808When kbuild executes, the following steps are followed (roughly):
8091) Configuration of the kernel => produce .config
Linus Torvalds1da177e2005-04-16 15:20:36 -07008102) Store kernel version in include/linux/version.h
8113) Symlink include/asm to include/asm-$(ARCH)
8124) Updating all other prerequisites to the target prepare:
813 - Additional prerequisites are specified in arch/$(ARCH)/Makefile
8145) Recursively descend down in all directories listed in
815 init-* core* drivers-* net-* libs-* and build all targets.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200816 - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07008176) All object files are then linked and the resulting file vmlinux is
Jan Engelhardta07f6032006-07-27 22:14:29 +0200818 located at the root of the obj tree.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700819 The very first objects linked are listed in head-y, assigned by
820 arch/$(ARCH)/Makefile.
Randy Dunlap5c811e52007-02-17 20:03:14 +01008217) Finally, the architecture-specific part does any required post processing
Linus Torvalds1da177e2005-04-16 15:20:36 -0700822 and builds the final bootimage.
823 - This includes building boot records
Randy Dunlap5c811e52007-02-17 20:03:14 +0100824 - Preparing initrd images and the like
Linus Torvalds1da177e2005-04-16 15:20:36 -0700825
826
827--- 6.1 Set variables to tweak the build to the architecture
828
829 LDFLAGS Generic $(LD) options
830
831 Flags used for all invocations of the linker.
832 Often specifying the emulation is sufficient.
833
834 Example:
835 #arch/s390/Makefile
836 LDFLAGS := -m elf_s390
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200837 Note: ldflags-y can be used to further customise
Sam Ravnborga9af3302007-09-30 20:08:25 +0200838 the flags used. See chapter 3.7.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700839
Linus Torvalds1da177e2005-04-16 15:20:36 -0700840 LDFLAGS_MODULE Options for $(LD) when linking modules
841
842 LDFLAGS_MODULE is used to set specific flags for $(LD) when
843 linking the .ko files used for modules.
844 Default is "-r", for relocatable output.
845
846 LDFLAGS_vmlinux Options for $(LD) when linking vmlinux
847
848 LDFLAGS_vmlinux is used to specify additional flags to pass to
Jan Engelhardta07f6032006-07-27 22:14:29 +0200849 the linker when linking the final vmlinux image.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700850 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
851
852 Example:
853 #arch/i386/Makefile
854 LDFLAGS_vmlinux := -e stext
855
856 OBJCOPYFLAGS objcopy flags
857
858 When $(call if_changed,objcopy) is used to translate a .o file,
Jan Engelhardta07f6032006-07-27 22:14:29 +0200859 the flags specified in OBJCOPYFLAGS will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700860 $(call if_changed,objcopy) is often used to generate raw binaries on
861 vmlinux.
862
863 Example:
864 #arch/s390/Makefile
865 OBJCOPYFLAGS := -O binary
866
867 #arch/s390/boot/Makefile
868 $(obj)/image: vmlinux FORCE
869 $(call if_changed,objcopy)
870
Jan Engelhardta07f6032006-07-27 22:14:29 +0200871 In this example, the binary $(obj)/image is a binary version of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700872 vmlinux. The usage of $(call if_changed,xxx) will be described later.
873
Sam Ravnborg222d3942007-10-15 21:59:31 +0200874 KBUILD_AFLAGS $(AS) assembler flags
Linus Torvalds1da177e2005-04-16 15:20:36 -0700875
876 Default value - see top level Makefile
877 Append or modify as required per architecture.
878
879 Example:
880 #arch/sparc64/Makefile
Sam Ravnborg222d3942007-10-15 21:59:31 +0200881 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
Linus Torvalds1da177e2005-04-16 15:20:36 -0700882
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200883 KBUILD_CFLAGS $(CC) compiler flags
Linus Torvalds1da177e2005-04-16 15:20:36 -0700884
885 Default value - see top level Makefile
886 Append or modify as required per architecture.
887
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200888 Often, the KBUILD_CFLAGS variable depends on the configuration.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700889
890 Example:
891 #arch/i386/Makefile
892 cflags-$(CONFIG_M386) += -march=i386
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200893 KBUILD_CFLAGS += $(cflags-y)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700894
895 Many arch Makefiles dynamically run the target C compiler to
896 probe supported options:
897
898 #arch/i386/Makefile
899
900 ...
901 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
902 -march=pentium2,-march=i686)
903 ...
904 # Disable unit-at-a-time mode ...
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200905 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700906 ...
907
908
Jan Engelhardta07f6032006-07-27 22:14:29 +0200909 The first example utilises the trick that a config option expands
Linus Torvalds1da177e2005-04-16 15:20:36 -0700910 to 'y' when selected.
911
912 CFLAGS_KERNEL $(CC) options specific for built-in
913
914 $(CFLAGS_KERNEL) contains extra C compiler flags used to compile
915 resident kernel code.
916
917 CFLAGS_MODULE $(CC) options specific for modules
918
919 $(CFLAGS_MODULE) contains extra C compiler flags used to compile code
920 for loadable kernel modules.
921
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700922
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200923--- 6.2 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700924
Jan Engelhardta07f6032006-07-27 22:14:29 +0200925 The archprepare: rule is used to list prerequisites that need to be
Linus Torvalds1da177e2005-04-16 15:20:36 -0700926 built before starting to descend down in the subdirectories.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200927 This is usually used for header files containing assembler constants.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700928
929 Example:
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200930 #arch/arm/Makefile
931 archprepare: maketools
Linus Torvalds1da177e2005-04-16 15:20:36 -0700932
Jan Engelhardta07f6032006-07-27 22:14:29 +0200933 In this example, the file target maketools will be processed
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200934 before descending down in the subdirectories.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700935 See also chapter XXX-TODO that describe how kbuild supports
936 generating offset header files.
937
938
939--- 6.3 List directories to visit when descending
940
941 An arch Makefile cooperates with the top Makefile to define variables
942 which specify how to build the vmlinux file. Note that there is no
943 corresponding arch-specific section for modules; the module-building
944 machinery is all architecture-independent.
945
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700946
Linus Torvalds1da177e2005-04-16 15:20:36 -0700947 head-y, init-y, core-y, libs-y, drivers-y, net-y
948
Jan Engelhardta07f6032006-07-27 22:14:29 +0200949 $(head-y) lists objects to be linked first in vmlinux.
950 $(libs-y) lists directories where a lib.a archive can be located.
Randy Dunlap5c811e52007-02-17 20:03:14 +0100951 The rest list directories where a built-in.o object file can be
Jan Engelhardta07f6032006-07-27 22:14:29 +0200952 located.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700953
954 $(init-y) objects will be located after $(head-y).
955 Then the rest follows in this order:
956 $(core-y), $(libs-y), $(drivers-y) and $(net-y).
957
Jan Engelhardta07f6032006-07-27 22:14:29 +0200958 The top level Makefile defines values for all generic directories,
Randy Dunlap5c811e52007-02-17 20:03:14 +0100959 and arch/$(ARCH)/Makefile only adds architecture-specific directories.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700960
961 Example:
962 #arch/sparc64/Makefile
963 core-y += arch/sparc64/kernel/
964 libs-y += arch/sparc64/prom/ arch/sparc64/lib/
965 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
966
967
Randy Dunlap5c811e52007-02-17 20:03:14 +0100968--- 6.4 Architecture-specific boot images
Linus Torvalds1da177e2005-04-16 15:20:36 -0700969
970 An arch Makefile specifies goals that take the vmlinux file, compress
971 it, wrap it in bootstrapping code, and copy the resulting files
972 somewhere. This includes various kinds of installation commands.
973 The actual goals are not standardized across architectures.
974
975 It is common to locate any additional processing in a boot/
976 directory below arch/$(ARCH)/.
977
978 Kbuild does not provide any smart way to support building a
979 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
980 call make manually to build a target in boot/.
981
982 The recommended approach is to include shortcuts in
983 arch/$(ARCH)/Makefile, and use the full path when calling down
984 into the arch/$(ARCH)/boot/Makefile.
985
986 Example:
987 #arch/i386/Makefile
988 boot := arch/i386/boot
989 bzImage: vmlinux
990 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
991
992 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
993 make in a subdirectory.
994
Randy Dunlap5c811e52007-02-17 20:03:14 +0100995 There are no rules for naming architecture-specific targets,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700996 but executing "make help" will list all relevant targets.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200997 To support this, $(archhelp) must be defined.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700998
999 Example:
1000 #arch/i386/Makefile
1001 define archhelp
1002 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001003 endif
Linus Torvalds1da177e2005-04-16 15:20:36 -07001004
1005 When make is executed without arguments, the first goal encountered
1006 will be built. In the top level Makefile the first goal present
1007 is all:.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001008 An architecture shall always, per default, build a bootable image.
1009 In "make help", the default goal is highlighted with a '*'.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001010 Add a new prerequisite to all: to select a default goal different
1011 from vmlinux.
1012
1013 Example:
1014 #arch/i386/Makefile
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001015 all: bzImage
Linus Torvalds1da177e2005-04-16 15:20:36 -07001016
1017 When "make" is executed without arguments, bzImage will be built.
1018
1019--- 6.5 Building non-kbuild targets
1020
1021 extra-y
1022
1023 extra-y specify additional targets created in the current
1024 directory, in addition to any targets specified by obj-*.
1025
1026 Listing all targets in extra-y is required for two purposes:
1027 1) Enable kbuild to check changes in command lines
1028 - When $(call if_changed,xxx) is used
1029 2) kbuild knows what files to delete during "make clean"
1030
1031 Example:
1032 #arch/i386/kernel/Makefile
1033 extra-y := head.o init_task.o
1034
Jan Engelhardta07f6032006-07-27 22:14:29 +02001035 In this example, extra-y is used to list object files that
Linus Torvalds1da177e2005-04-16 15:20:36 -07001036 shall be built, but shall not be linked as part of built-in.o.
1037
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001038
Linus Torvalds1da177e2005-04-16 15:20:36 -07001039--- 6.6 Commands useful for building a boot image
1040
1041 Kbuild provides a few macros that are useful when building a
1042 boot image.
1043
1044 if_changed
1045
1046 if_changed is the infrastructure used for the following commands.
1047
1048 Usage:
1049 target: source(s) FORCE
1050 $(call if_changed,ld/objcopy/gzip)
1051
Jan Engelhardta07f6032006-07-27 22:14:29 +02001052 When the rule is evaluated, it is checked to see if any files
Randy Dunlap5c811e52007-02-17 20:03:14 +01001053 need an update, or the command line has changed since the last
Linus Torvalds1da177e2005-04-16 15:20:36 -07001054 invocation. The latter will force a rebuild if any options
1055 to the executable have changed.
1056 Any target that utilises if_changed must be listed in $(targets),
1057 otherwise the command line check will fail, and the target will
1058 always be built.
1059 Assignments to $(targets) are without $(obj)/ prefix.
1060 if_changed may be used in conjunction with custom commands as
1061 defined in 6.7 "Custom kbuild commands".
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +02001062
Linus Torvalds1da177e2005-04-16 15:20:36 -07001063 Note: It is a typical mistake to forget the FORCE prerequisite.
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +02001064 Another common pitfall is that whitespace is sometimes
1065 significant; for instance, the below will fail (note the extra space
1066 after the comma):
1067 target: source(s) FORCE
1068 #WRONG!# $(call if_changed, ld/objcopy/gzip)
Linus Torvalds1da177e2005-04-16 15:20:36 -07001069
1070 ld
Jan Engelhardta07f6032006-07-27 22:14:29 +02001071 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001072
Linus Torvalds1da177e2005-04-16 15:20:36 -07001073 objcopy
1074 Copy binary. Uses OBJCOPYFLAGS usually specified in
1075 arch/$(ARCH)/Makefile.
1076 OBJCOPYFLAGS_$@ may be used to set additional options.
1077
1078 gzip
1079 Compress target. Use maximum compression to compress target.
1080
1081 Example:
1082 #arch/i386/boot/Makefile
1083 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1084 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
1085
1086 targets += setup setup.o bootsect bootsect.o
1087 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1088 $(call if_changed,ld)
1089
Jan Engelhardta07f6032006-07-27 22:14:29 +02001090 In this example, there are two possible targets, requiring different
1091 options to the linker. The linker options are specified using the
Linus Torvalds1da177e2005-04-16 15:20:36 -07001092 LDFLAGS_$@ syntax - one for each potential target.
Matt LaPlante5d3f0832006-11-30 05:21:10 +01001093 $(targets) are assigned all potential targets, by which kbuild knows
Linus Torvalds1da177e2005-04-16 15:20:36 -07001094 the targets and will:
1095 1) check for commandline changes
1096 2) delete target during make clean
1097
1098 The ": %: %.o" part of the prerequisite is a shorthand that
1099 free us from listing the setup.o and bootsect.o files.
1100 Note: It is a common mistake to forget the "target :=" assignment,
1101 resulting in the target file being recompiled for no
1102 obvious reason.
1103
1104
1105--- 6.7 Custom kbuild commands
1106
Jan Engelhardta07f6032006-07-27 22:14:29 +02001107 When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
Linus Torvalds1da177e2005-04-16 15:20:36 -07001108 of a command is normally displayed.
1109 To enable this behaviour for custom commands kbuild requires
1110 two variables to be set:
1111 quiet_cmd_<command> - what shall be echoed
1112 cmd_<command> - the command to execute
1113
1114 Example:
1115 #
1116 quiet_cmd_image = BUILD $@
1117 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
1118 $(obj)/vmlinux.bin > $@
1119
1120 targets += bzImage
1121 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
1122 $(call if_changed,image)
1123 @echo 'Kernel: $@ is ready'
1124
Jan Engelhardta07f6032006-07-27 22:14:29 +02001125 When updating the $(obj)/bzImage target, the line
Linus Torvalds1da177e2005-04-16 15:20:36 -07001126
1127 BUILD arch/i386/boot/bzImage
1128
1129 will be displayed with "make KBUILD_VERBOSE=0".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001130
Linus Torvalds1da177e2005-04-16 15:20:36 -07001131
1132--- 6.8 Preprocessing linker scripts
1133
Jan Engelhardta07f6032006-07-27 22:14:29 +02001134 When the vmlinux image is built, the linker script
Linus Torvalds1da177e2005-04-16 15:20:36 -07001135 arch/$(ARCH)/kernel/vmlinux.lds is used.
1136 The script is a preprocessed variant of the file vmlinux.lds.S
1137 located in the same directory.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001138 kbuild knows .lds files and includes a rule *lds.S -> *lds.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001139
Linus Torvalds1da177e2005-04-16 15:20:36 -07001140 Example:
1141 #arch/i386/kernel/Makefile
1142 always := vmlinux.lds
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001143
Linus Torvalds1da177e2005-04-16 15:20:36 -07001144 #Makefile
1145 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001146
1147 The assignment to $(always) is used to tell kbuild to build the
Jan Engelhardta07f6032006-07-27 22:14:29 +02001148 target vmlinux.lds.
1149 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
Linus Torvalds1da177e2005-04-16 15:20:36 -07001150 specified options when building the target vmlinux.lds.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001151
Jan Engelhardta07f6032006-07-27 22:14:29 +02001152 When building the *.lds target, kbuild uses the variables:
Sam Ravnborg06c50402007-10-15 22:17:25 +02001153 KBUILD_CPPFLAGS : Set in top-level Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +02001154 cppflags-y : May be set in the kbuild makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001155 CPPFLAGS_$(@F) : Target specific flags.
1156 Note that the full filename is used in this
1157 assignment.
1158
1159 The kbuild infrastructure for *lds file are used in several
Randy Dunlap5c811e52007-02-17 20:03:14 +01001160 architecture-specific files.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001161
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001162=== 7 Kbuild syntax for exported headers
Linus Torvalds1da177e2005-04-16 15:20:36 -07001163
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001164The kernel include a set of headers that is exported to userspace.
1165Many headers can be exported as-is but other headers requires a
1166minimal pre-processing before they are ready for user-space.
1167The pre-processing does:
1168- drop kernel specific annotations
1169- drop include of compiler.h
1170- drop all sections that is kernel internat (guarded by ifdef __KERNEL__)
1171
1172Each relevant directory contain a file name "Kbuild" which specify the
1173headers to be exported.
1174See subsequent chapter for the syntax of the Kbuild file.
1175
1176 --- 7.1 header-y
1177
1178 header-y specify header files to be exported.
1179
1180 Example:
1181 #include/linux/Kbuild
1182 header-y += usb/
1183 header-y += aio_abi.h
1184
1185 The convention is to list one file per line and
1186 preferably in alphabetic order.
1187
1188 header-y also specify which subdirectories to visit.
1189 A subdirectory is identified by a trailing '/' which
1190 can be seen in the example above for the usb subdirectory.
1191
1192 Subdirectories are visited before their parent directories.
1193
1194 --- 7.2 objhdr-y
1195
1196 objhdr-y specifies generated files to be exported.
1197 Generated files are special as they need to be looked
1198 up in another directory when doing 'make O=...' builds.
1199
1200 Example:
1201 #include/linux/Kbuild
1202 objhdr-y += version.h
1203
1204 --- 7.3 destination-y
1205
1206 When an architecture have a set of exported headers that needs to be
1207 exported to a different directory destination-y is used.
1208 destination-y specify the destination directory for all exported
1209 headers in the file where it is present.
1210
1211 Example:
1212 #arch/xtensa/platforms/s6105/include/platform/Kbuild
1213 destination-y := include/linux
1214
1215 In the example above all exported headers in the Kbuild file
1216 will be located in the directory "include/linux" when exported.
1217
1218
1219 --- 7.4 unifdef-y (deprecated)
1220
1221 unifdef-y is deprecated. A direct replacement is header-y.
1222
1223
1224=== 8 Kbuild Variables
Linus Torvalds1da177e2005-04-16 15:20:36 -07001225
1226The top Makefile exports the following variables:
1227
1228 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1229
1230 These variables define the current kernel version. A few arch
1231 Makefiles actually use these values directly; they should use
1232 $(KERNELRELEASE) instead.
1233
1234 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1235 three-part version number, such as "2", "4", and "0". These three
1236 values are always numeric.
1237
1238 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1239 or additional patches. It is usually some non-numeric string
1240 such as "-pre4", and is often blank.
1241
1242 KERNELRELEASE
1243
1244 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1245 for constructing installation directory names or showing in
1246 version strings. Some arch Makefiles use it for this purpose.
1247
1248 ARCH
1249
1250 This variable defines the target architecture, such as "i386",
1251 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1252 determine which files to compile.
1253
1254 By default, the top Makefile sets $(ARCH) to be the same as the
1255 host system architecture. For a cross build, a user may
1256 override the value of $(ARCH) on the command line:
1257
1258 make ARCH=m68k ...
1259
1260
1261 INSTALL_PATH
1262
1263 This variable defines a place for the arch Makefiles to install
1264 the resident kernel image and System.map file.
Randy Dunlap5c811e52007-02-17 20:03:14 +01001265 Use this for architecture-specific install targets.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001266
1267 INSTALL_MOD_PATH, MODLIB
1268
1269 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1270 installation. This variable is not defined in the Makefile but
1271 may be passed in by the user if desired.
1272
1273 $(MODLIB) specifies the directory for module installation.
1274 The top Makefile defines $(MODLIB) to
1275 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1276 override this value on the command line if desired.
1277
Theodore Ts'oac031f22006-06-21 20:53:09 -04001278 INSTALL_MOD_STRIP
1279
1280 If this variable is specified, will cause modules to be stripped
1281 after they are installed. If INSTALL_MOD_STRIP is '1', then the
1282 default option --strip-debug will be used. Otherwise,
1283 INSTALL_MOD_STRIP will used as the option(s) to the strip command.
1284
1285
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001286=== 9 Makefile language
Linus Torvalds1da177e2005-04-16 15:20:36 -07001287
Jan Engelhardta07f6032006-07-27 22:14:29 +02001288The kernel Makefiles are designed to be run with GNU Make. The Makefiles
Linus Torvalds1da177e2005-04-16 15:20:36 -07001289use only the documented features of GNU Make, but they do use many
1290GNU extensions.
1291
1292GNU Make supports elementary list-processing functions. The kernel
1293Makefiles use a novel style of list building and manipulation with few
1294"if" statements.
1295
1296GNU Make has two assignment operators, ":=" and "=". ":=" performs
1297immediate evaluation of the right-hand side and stores an actual string
1298into the left-hand side. "=" is like a formula definition; it stores the
1299right-hand side in an unevaluated form and then evaluates this form each
1300time the left-hand side is used.
1301
1302There are some cases where "=" is appropriate. Usually, though, ":="
1303is the right choice.
1304
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001305=== 10 Credits
Linus Torvalds1da177e2005-04-16 15:20:36 -07001306
1307Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1308Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1309Updates by Sam Ravnborg <sam@ravnborg.org>
Jan Engelhardta07f6032006-07-27 22:14:29 +02001310Language QA by Jan Engelhardt <jengelh@gmx.de>
Linus Torvalds1da177e2005-04-16 15:20:36 -07001311
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001312=== 11 TODO
Linus Torvalds1da177e2005-04-16 15:20:36 -07001313
Jan Engelhardta07f6032006-07-27 22:14:29 +02001314- Describe how kbuild supports shipped files with _shipped.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001315- Generating offset header files.
1316- Add more variables to section 7?
1317
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001318
1319