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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
H. Peter Anvin052ad272011-11-17 13:17:35 -080036 --- 6.2 Add prerequisites to archheaders:
37 --- 6.3 Add prerequisites to archprepare:
38 --- 6.4 List directories to visit when descending
39 --- 6.5 Architecture-specific boot images
40 --- 6.6 Building non-kbuild targets
41 --- 6.7 Commands useful for building a boot image
42 --- 6.8 Custom kbuild commands
43 --- 6.9 Preprocessing linker scripts
44 --- 6.10 Generic header files
Linus Torvalds1da177e2005-04-16 15:20:36 -070045
Sam Ravnborgc7bb3492009-04-10 08:52:43 +020046 === 7 Kbuild syntax for exported headers
47 --- 7.1 header-y
48 --- 7.2 objhdr-y
49 --- 7.3 destination-y
Sam Ravnborgd8ecc5c2011-04-27 22:29:49 +020050 --- 7.4 generic-y
Sam Ravnborgc7bb3492009-04-10 08:52:43 +020051
52 === 8 Kbuild Variables
53 === 9 Makefile language
54 === 10 Credits
55 === 11 TODO
Linus Torvalds1da177e2005-04-16 15:20:36 -070056
57=== 1 Overview
58
59The Makefiles have five parts:
60
61 Makefile the top Makefile.
62 .config the kernel configuration file.
63 arch/$(ARCH)/Makefile the arch Makefile.
64 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
65 kbuild Makefiles there are about 500 of these.
66
67The top Makefile reads the .config file, which comes from the kernel
68configuration process.
69
70The top Makefile is responsible for building two major products: vmlinux
71(the resident kernel image) and modules (any module files).
72It builds these goals by recursively descending into the subdirectories of
73the kernel source tree.
74The list of subdirectories which are visited depends upon the kernel
75configuration. The top Makefile textually includes an arch Makefile
76with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
77architecture-specific information to the top Makefile.
78
79Each subdirectory has a kbuild Makefile which carries out the commands
80passed down from above. The kbuild Makefile uses information from the
Bryce Harrington39e6e9c2006-09-20 12:43:37 -070081.config file to construct various file lists used by kbuild to build
Linus Torvalds1da177e2005-04-16 15:20:36 -070082any built-in or modular targets.
83
84scripts/Makefile.* contains all the definitions/rules etc. that
85are used to build the kernel based on the kbuild makefiles.
86
87
88=== 2 Who does what
89
90People have four different relationships with the kernel Makefiles.
91
92*Users* are people who build kernels. These people type commands such as
93"make menuconfig" or "make". They usually do not read or edit
94any kernel Makefiles (or any other source files).
95
96*Normal developers* are people who work on features such as device
97drivers, file systems, and network protocols. These people need to
Jan Engelhardta07f6032006-07-27 22:14:29 +020098maintain the kbuild Makefiles for the subsystem they are
Linus Torvalds1da177e2005-04-16 15:20:36 -070099working on. In order to do this effectively, they need some overall
100knowledge about the kernel Makefiles, plus detailed knowledge about the
101public interface for kbuild.
102
103*Arch developers* are people who work on an entire architecture, such
104as sparc or ia64. Arch developers need to know about the arch Makefile
105as well as kbuild Makefiles.
106
107*Kbuild developers* are people who work on the kernel build system itself.
108These people need to know about all aspects of the kernel Makefiles.
109
110This document is aimed towards normal developers and arch developers.
111
112
113=== 3 The kbuild files
114
115Most Makefiles within the kernel are kbuild Makefiles that use the
Jan Engelhardta07f6032006-07-27 22:14:29 +0200116kbuild infrastructure. This chapter introduces the syntax used in the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700117kbuild makefiles.
Sam Ravnborg172c3ae2006-03-10 00:23:32 +0100118The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
Jan Engelhardta07f6032006-07-27 22:14:29 +0200119be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
Sam Ravnborg172c3ae2006-03-10 00:23:32 +0100120file will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700121
122Section 3.1 "Goal definitions" is a quick intro, further chapters provide
123more details, with real examples.
124
125--- 3.1 Goal definitions
126
127 Goal definitions are the main part (heart) of the kbuild Makefile.
128 These lines define the files to be built, any special compilation
129 options, and any subdirectories to be entered recursively.
130
131 The most simple kbuild makefile contains one line:
132
133 Example:
134 obj-y += foo.o
135
Randy Dunlap5c811e52007-02-17 20:03:14 +0100136 This tells kbuild that there is one object in that directory, named
Linus Torvalds1da177e2005-04-16 15:20:36 -0700137 foo.o. foo.o will be built from foo.c or foo.S.
138
139 If foo.o shall be built as a module, the variable obj-m is used.
140 Therefore the following pattern is often used:
141
142 Example:
143 obj-$(CONFIG_FOO) += foo.o
144
145 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
146 If CONFIG_FOO is neither y nor m, then the file will not be compiled
147 nor linked.
148
149--- 3.2 Built-in object goals - obj-y
150
151 The kbuild Makefile specifies object files for vmlinux
Jan Engelhardta07f6032006-07-27 22:14:29 +0200152 in the $(obj-y) lists. These lists depend on the kernel
Linus Torvalds1da177e2005-04-16 15:20:36 -0700153 configuration.
154
155 Kbuild compiles all the $(obj-y) files. It then calls
156 "$(LD) -r" to merge these files into one built-in.o file.
157 built-in.o is later linked into vmlinux by the parent Makefile.
158
159 The order of files in $(obj-y) is significant. Duplicates in
160 the lists are allowed: the first instance will be linked into
161 built-in.o and succeeding instances will be ignored.
162
163 Link order is significant, because certain functions
164 (module_init() / __initcall) will be called during boot in the
165 order they appear. So keep in mind that changing the link
Jan Engelhardta07f6032006-07-27 22:14:29 +0200166 order may e.g. change the order in which your SCSI
167 controllers are detected, and thus your disks are renumbered.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700168
169 Example:
170 #drivers/isdn/i4l/Makefile
171 # Makefile for the kernel ISDN subsystem and device drivers.
172 # Each configuration option enables a list of files.
Matt Mooney2f5a2f82010-08-05 11:23:11 -0700173 obj-$(CONFIG_ISDN_I4L) += isdn.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700174 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
175
176--- 3.3 Loadable module goals - obj-m
177
178 $(obj-m) specify object files which are built as loadable
179 kernel modules.
180
181 A module may be built from one source file or several source
182 files. In the case of one source file, the kbuild makefile
183 simply adds the file to $(obj-m).
184
185 Example:
186 #drivers/isdn/i4l/Makefile
187 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
188
189 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
190
191 If a kernel module is built from several source files, you specify
Matt Mooney4f827282010-08-05 11:23:11 -0700192 that you want to build a module in the same way as above; however,
193 kbuild needs to know which object files you want to build your
194 module from, so you have to tell it by setting a $(<module_name>-y)
195 variable.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700196
197 Example:
198 #drivers/isdn/i4l/Makefile
Matt Mooney4f827282010-08-05 11:23:11 -0700199 obj-$(CONFIG_ISDN_I4L) += isdn.o
200 isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700201
202 In this example, the module name will be isdn.o. Kbuild will
Matt Mooney4f827282010-08-05 11:23:11 -0700203 compile the objects listed in $(isdn-y) and then run
Linus Torvalds1da177e2005-04-16 15:20:36 -0700204 "$(LD) -r" on the list of these files to generate isdn.o.
205
Matt Mooney4f827282010-08-05 11:23:11 -0700206 Due to kbuild recognizing $(<module_name>-y) for composite objects,
207 you can use the value of a CONFIG_ symbol to optionally include an
208 object file as part of a composite object.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700209
210 Example:
211 #fs/ext2/Makefile
Matt Mooney4f827282010-08-05 11:23:11 -0700212 obj-$(CONFIG_EXT2_FS) += ext2.o
213 ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
214 namei.o super.o symlink.o
215 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
216 xattr_trusted.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700217
Matt Mooney4f827282010-08-05 11:23:11 -0700218 In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
219 part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
220 evaluates to 'y'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700221
222 Note: Of course, when you are building objects into the kernel,
223 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
224 kbuild will build an ext2.o file for you out of the individual
225 parts and then link this into built-in.o, as you would expect.
226
227--- 3.4 Objects which export symbols
228
229 No special notation is required in the makefiles for
230 modules exporting symbols.
231
232--- 3.5 Library file goals - lib-y
233
Jan Engelhardta07f6032006-07-27 22:14:29 +0200234 Objects listed with obj-* are used for modules, or
Linus Torvalds1da177e2005-04-16 15:20:36 -0700235 combined in a built-in.o for that specific directory.
236 There is also the possibility to list objects that will
237 be included in a library, lib.a.
238 All objects listed with lib-y are combined in a single
239 library for that directory.
Matt LaPlante5d3f0832006-11-30 05:21:10 +0100240 Objects that are listed in obj-y and additionally listed in
241 lib-y will not be included in the library, since they will
242 be accessible anyway.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200243 For consistency, objects listed in lib-m will be included in lib.a.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700244
245 Note that the same kbuild makefile may list files to be built-in
246 and to be part of a library. Therefore the same directory
247 may contain both a built-in.o and a lib.a file.
248
249 Example:
Matt Mooney2f5a2f82010-08-05 11:23:11 -0700250 #arch/x86/lib/Makefile
251 lib-y := delay.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700252
Matt Mooney2f5a2f82010-08-05 11:23:11 -0700253 This will create a library lib.a based on delay.o. For kbuild to
254 actually recognize that there is a lib.a being built, the directory
255 shall be listed in libs-y.
H. Peter Anvin052ad272011-11-17 13:17:35 -0800256 See also "6.4 List directories to visit when descending".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700257
Jan Engelhardta07f6032006-07-27 22:14:29 +0200258 Use of lib-y is normally restricted to lib/ and arch/*/lib.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700259
260--- 3.6 Descending down in directories
261
262 A Makefile is only responsible for building objects in its own
263 directory. Files in subdirectories should be taken care of by
264 Makefiles in these subdirs. The build system will automatically
265 invoke make recursively in subdirectories, provided you let it know of
266 them.
267
Jan Engelhardta07f6032006-07-27 22:14:29 +0200268 To do so, obj-y and obj-m are used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700269 ext2 lives in a separate directory, and the Makefile present in fs/
270 tells kbuild to descend down using the following assignment.
271
272 Example:
273 #fs/Makefile
274 obj-$(CONFIG_EXT2_FS) += ext2/
275
276 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
277 the corresponding obj- variable will be set, and kbuild will descend
278 down in the ext2 directory.
279 Kbuild only uses this information to decide that it needs to visit
280 the directory, it is the Makefile in the subdirectory that
281 specifies what is modules and what is built-in.
282
283 It is good practice to use a CONFIG_ variable when assigning directory
284 names. This allows kbuild to totally skip the directory if the
285 corresponding CONFIG_ option is neither 'y' nor 'm'.
286
287--- 3.7 Compilation flags
288
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200289 ccflags-y, asflags-y and ldflags-y
Nicolas Kaiserc95940f2010-08-05 11:23:11 -0700290 These three flags apply only to the kbuild makefile in which they
291 are assigned. They are used for all the normal cc, as and ld
292 invocations happening during a recursive build.
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200293 Note: Flags with the same behaviour were previously named:
Nicolas Kaiserc95940f2010-08-05 11:23:11 -0700294 EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
295 They are still supported but their usage is deprecated.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700296
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700297 ccflags-y specifies options for compiling with $(CC).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700298
299 Example:
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700300 # drivers/acpi/Makefile
301 ccflags-y := -Os
302 ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT
Linus Torvalds1da177e2005-04-16 15:20:36 -0700303
304 This variable is necessary because the top Makefile owns the
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200305 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700306 entire tree.
307
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700308 asflags-y specifies options for assembling with $(AS).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700309
310 Example:
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700311 #arch/sparc/kernel/Makefile
312 asflags-y := -ansi
Linus Torvalds1da177e2005-04-16 15:20:36 -0700313
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700314 ldflags-y specifies options for linking with $(LD).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700315
316 Example:
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700317 #arch/cris/boot/compressed/Makefile
318 ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
Linus Torvalds1da177e2005-04-16 15:20:36 -0700319
Sam Ravnborg720097d2009-04-19 11:04:26 +0200320 subdir-ccflags-y, subdir-asflags-y
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700321 The two flags listed above are similar to ccflags-y and asflags-y.
Nicolas Kaiserc95940f2010-08-05 11:23:11 -0700322 The difference is that the subdir- variants have effect for the kbuild
323 file where they are present and all subdirectories.
324 Options specified using subdir-* are added to the commandline before
325 the options specified using the non-subdir variants.
Sam Ravnborg720097d2009-04-19 11:04:26 +0200326
327 Example:
328 subdir-ccflags-y := -Werror
329
Linus Torvalds1da177e2005-04-16 15:20:36 -0700330 CFLAGS_$@, 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
Linus Torvalds1da177e2005-04-16 15:20:36 -0700343
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700344 These two lines specify compilation flags for aha152x.o and gdth.o.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700345
346 $(AFLAGS_$@) is a similar feature for source files in assembly
347 languages.
348
349 Example:
350 # arch/arm/kernel/Makefile
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700351 AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
352 AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
353 AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt
354
Linus Torvalds1da177e2005-04-16 15:20:36 -0700355
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:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800445 #arch/x86/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:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800464 #arch/x86/Makefile
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100465 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
Michal Marek8417da6f2011-05-02 12:51:15 +0200505 cc-disable-warning
506 cc-disable-warning checks if gcc supports a given warning and returns
507 the commandline switch to disable it. This special function is needed,
508 because gcc 4.4 and later accept any unknown -Wno-* option and only
509 warn about it if there is another warning in the source file.
510
511 Example:
512 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
513
514 In the above example, -Wno-unused-but-set-variable will be added to
515 KBUILD_CFLAGS only if gcc really accepts it.
516
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100517 cc-version
Jan Engelhardta07f6032006-07-27 22:14:29 +0200518 cc-version returns a numerical version of the $(CC) compiler version.
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100519 The format is <major><minor> where both are two digits. So for example
520 gcc 3.41 would return 0341.
521 cc-version is useful when a specific $(CC) version is faulty in one
Jan Engelhardta07f6032006-07-27 22:14:29 +0200522 area, for example -mregparm=3 was broken in some gcc versions
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100523 even though the option was accepted by gcc.
524
525 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800526 #arch/x86/Makefile
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100527 cflags-y += $(shell \
528 if [ $(call cc-version) -ge 0300 ] ; then \
529 echo "-mregparm=3"; fi ;)
530
Jan Engelhardta07f6032006-07-27 22:14:29 +0200531 In the above example, -mregparm=3 is only used for gcc version greater
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100532 than or equal to gcc 3.0.
533
534 cc-ifversion
Jan Engelhardta07f6032006-07-27 22:14:29 +0200535 cc-ifversion tests the version of $(CC) and equals last argument if
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100536 version expression is true.
537
538 Example:
539 #fs/reiserfs/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200540 ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100541
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200542 In this example, ccflags-y will be assigned the value -O1 if the
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100543 $(CC) version is less than 4.2.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700544 cc-ifversion takes all the shell operators:
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100545 -eq, -ne, -lt, -le, -gt, and -ge
546 The third parameter may be a text as in this example, but it may also
547 be an expanded variable or a macro.
548
Sam Ravnborg70150302007-05-28 22:26:55 +0200549 cc-fullversion
550 cc-fullversion is useful when the exact version of gcc is needed.
551 One typical use-case is when a specific GCC version is broken.
552 cc-fullversion points out a more specific version than cc-version does.
553
554 Example:
555 #arch/powerpc/Makefile
556 $(Q)if test "$(call cc-fullversion)" = "040200" ; then \
557 echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \
558 false ; \
559 fi
560
561 In this example for a specific GCC version the build will error out explaining
562 to the user why it stops.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700563
Sam Ravnborg910b4042007-10-19 21:46:01 +0200564 cc-cross-prefix
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200565 cc-cross-prefix is used to check if there exists a $(CC) in path with
Sam Ravnborg910b4042007-10-19 21:46:01 +0200566 one of the listed prefixes. The first prefix where there exist a
567 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
568 then nothing is returned.
569 Additional prefixes are separated by a single space in the
570 call of cc-cross-prefix.
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200571 This functionality is useful for architecture Makefiles that try
572 to set CROSS_COMPILE to well-known values but may have several
Sam Ravnborg910b4042007-10-19 21:46:01 +0200573 values to select between.
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200574 It is recommended only to try to set CROSS_COMPILE if it is a cross
575 build (host arch is different from target arch). And if CROSS_COMPILE
Sam Ravnborg910b4042007-10-19 21:46:01 +0200576 is already set then leave it with the old value.
577
578 Example:
579 #arch/m68k/Makefile
580 ifneq ($(SUBARCH),$(ARCH))
581 ifeq ($(CROSS_COMPILE),)
582 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
583 endif
584 endif
585
Sam Ravnborg691ef3e2009-09-19 10:31:45 +0200586--- 3.12 $(LD) support functions
587
588 ld-option
589 ld-option is used to check if $(LD) supports the supplied option.
590 ld-option takes two options as arguments.
591 The second argument is an optional option that can be used if the
592 first option is not supported by $(LD).
593
594 Example:
595 #Makefile
596 LDFLAGS_vmlinux += $(call really-ld-option, -X)
597
598
Linus Torvalds1da177e2005-04-16 15:20:36 -0700599=== 4 Host Program support
600
601Kbuild supports building executables on the host for use during the
602compilation stage.
603Two steps are required in order to use a host executable.
604
605The first step is to tell kbuild that a host program exists. This is
606done utilising the variable hostprogs-y.
607
608The second step is to add an explicit dependency to the executable.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700609This can be done in two ways. Either add the dependency in a rule,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700610or utilise the variable $(always).
611Both possibilities are described in the following.
612
613--- 4.1 Simple Host Program
614
615 In some cases there is a need to compile and run a program on the
616 computer where the build is running.
617 The following line tells kbuild that the program bin2hex shall be
618 built on the build host.
619
620 Example:
621 hostprogs-y := bin2hex
622
623 Kbuild assumes in the above example that bin2hex is made from a single
624 c-source file named bin2hex.c located in the same directory as
625 the Makefile.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700626
Linus Torvalds1da177e2005-04-16 15:20:36 -0700627--- 4.2 Composite Host Programs
628
629 Host programs can be made up based on composite objects.
630 The syntax used to define composite objects for host programs is
631 similar to the syntax used for kernel objects.
Matt LaPlante5d3f0832006-11-30 05:21:10 +0100632 $(<executable>-objs) lists all objects used to link the final
Linus Torvalds1da177e2005-04-16 15:20:36 -0700633 executable.
634
635 Example:
636 #scripts/lxdialog/Makefile
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700637 hostprogs-y := lxdialog
Linus Torvalds1da177e2005-04-16 15:20:36 -0700638 lxdialog-objs := checklist.o lxdialog.o
639
640 Objects with extension .o are compiled from the corresponding .c
Jan Engelhardta07f6032006-07-27 22:14:29 +0200641 files. In the above example, checklist.c is compiled to checklist.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700642 and lxdialog.c is compiled to lxdialog.o.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200643 Finally, the two .o files are linked to the executable, lxdialog.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700644 Note: The syntax <executable>-y is not permitted for host-programs.
645
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700646--- 4.3 Defining shared libraries
647
Linus Torvalds1da177e2005-04-16 15:20:36 -0700648 Objects with extension .so are considered shared libraries, and
649 will be compiled as position independent objects.
650 Kbuild provides support for shared libraries, but the usage
651 shall be restricted.
652 In the following example the libkconfig.so shared library is used
653 to link the executable conf.
654
655 Example:
656 #scripts/kconfig/Makefile
657 hostprogs-y := conf
658 conf-objs := conf.o libkconfig.so
659 libkconfig-objs := expr.o type.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700660
Linus Torvalds1da177e2005-04-16 15:20:36 -0700661 Shared libraries always require a corresponding -objs line, and
662 in the example above the shared library libkconfig is composed by
663 the two objects expr.o and type.o.
664 expr.o and type.o will be built as position independent code and
665 linked as a shared library libkconfig.so. C++ is not supported for
666 shared libraries.
667
668--- 4.4 Using C++ for host programs
669
670 kbuild offers support for host programs written in C++. This was
671 introduced solely to support kconfig, and is not recommended
672 for general use.
673
674 Example:
675 #scripts/kconfig/Makefile
676 hostprogs-y := qconf
677 qconf-cxxobjs := qconf.o
678
679 In the example above the executable is composed of the C++ file
680 qconf.cc - identified by $(qconf-cxxobjs).
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700681
Linus Torvalds1da177e2005-04-16 15:20:36 -0700682 If qconf is composed by a mixture of .c and .cc files, then an
683 additional line can be used to identify this.
684
685 Example:
686 #scripts/kconfig/Makefile
687 hostprogs-y := qconf
688 qconf-cxxobjs := qconf.o
689 qconf-objs := check.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700690
Linus Torvalds1da177e2005-04-16 15:20:36 -0700691--- 4.5 Controlling compiler options for host programs
692
693 When compiling host programs, it is possible to set specific flags.
694 The programs will always be compiled utilising $(HOSTCC) passed
695 the options specified in $(HOSTCFLAGS).
696 To set flags that will take effect for all host programs created
Jan Engelhardta07f6032006-07-27 22:14:29 +0200697 in that Makefile, use the variable HOST_EXTRACFLAGS.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700698
699 Example:
700 #scripts/lxdialog/Makefile
701 HOST_EXTRACFLAGS += -I/usr/include/ncurses
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700702
Linus Torvalds1da177e2005-04-16 15:20:36 -0700703 To set specific flags for a single file the following construction
704 is used:
705
706 Example:
707 #arch/ppc64/boot/Makefile
708 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700709
Linus Torvalds1da177e2005-04-16 15:20:36 -0700710 It is also possible to specify additional options to the linker.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700711
Linus Torvalds1da177e2005-04-16 15:20:36 -0700712 Example:
713 #scripts/kconfig/Makefile
714 HOSTLOADLIBES_qconf := -L$(QTDIR)/lib
715
Jan Engelhardta07f6032006-07-27 22:14:29 +0200716 When linking qconf, it will be passed the extra option
717 "-L$(QTDIR)/lib".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700718
Linus Torvalds1da177e2005-04-16 15:20:36 -0700719--- 4.6 When host programs are actually built
720
721 Kbuild will only build host-programs when they are referenced
722 as a prerequisite.
723 This is possible in two ways:
724
725 (1) List the prerequisite explicitly in a special rule.
726
727 Example:
728 #drivers/pci/Makefile
729 hostprogs-y := gen-devlist
730 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
731 ( cd $(obj); ./gen-devlist ) < $<
732
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700733 The target $(obj)/devlist.h will not be built before
Linus Torvalds1da177e2005-04-16 15:20:36 -0700734 $(obj)/gen-devlist is updated. Note that references to
735 the host programs in special rules must be prefixed with $(obj).
736
737 (2) Use $(always)
738 When there is no suitable special rule, and the host program
739 shall be built when a makefile is entered, the $(always)
740 variable shall be used.
741
742 Example:
743 #scripts/lxdialog/Makefile
744 hostprogs-y := lxdialog
745 always := $(hostprogs-y)
746
747 This will tell kbuild to build lxdialog even if not referenced in
748 any rule.
749
750--- 4.7 Using hostprogs-$(CONFIG_FOO)
751
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700752 A typical pattern in a Kbuild file looks like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700753
754 Example:
755 #scripts/Makefile
756 hostprogs-$(CONFIG_KALLSYMS) += kallsyms
757
758 Kbuild knows about both 'y' for built-in and 'm' for module.
759 So if a config symbol evaluate to 'm', kbuild will still build
Jan Engelhardta07f6032006-07-27 22:14:29 +0200760 the binary. In other words, Kbuild handles hostprogs-m exactly
761 like hostprogs-y. But only hostprogs-y is recommended to be used
762 when no CONFIG symbols are involved.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700763
764=== 5 Kbuild clean infrastructure
765
Jan Engelhardta07f6032006-07-27 22:14:29 +0200766"make clean" deletes most generated files in the obj tree where the kernel
Linus Torvalds1da177e2005-04-16 15:20:36 -0700767is compiled. This includes generated files such as host programs.
768Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
769$(extra-y) and $(targets). They are all deleted during "make clean".
770Files matching the patterns "*.[oas]", "*.ko", plus some additional files
771generated by kbuild are deleted all over the kernel src tree when
772"make clean" is executed.
773
774Additional files can be specified in kbuild makefiles by use of $(clean-files).
775
776 Example:
777 #drivers/pci/Makefile
778 clean-files := devlist.h classlist.h
779
780When executing "make clean", the two files "devlist.h classlist.h" will
781be deleted. Kbuild will assume files to be in same relative directory as the
782Makefile except if an absolute path is specified (path starting with '/').
783
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700784To delete a directory hierarchy use:
785
Linus Torvalds1da177e2005-04-16 15:20:36 -0700786 Example:
787 #scripts/package/Makefile
788 clean-dirs := $(objtree)/debian/
789
790This will delete the directory debian, including all subdirectories.
791Kbuild will assume the directories to be in the same relative path as the
792Makefile if no absolute path is specified (path does not start with '/').
793
Michal Marekef8ff892010-03-09 16:00:20 +0100794To exclude certain files from make clean, use the $(no-clean-files) variable.
795This is only a special case used in the top level Kbuild file:
796
797 Example:
798 #Kbuild
799 no-clean-files := $(bounds-file) $(offsets-file)
800
Linus Torvalds1da177e2005-04-16 15:20:36 -0700801Usually kbuild descends down in subdirectories due to "obj-* := dir/",
802but in the architecture makefiles where the kbuild infrastructure
803is not sufficient this sometimes needs to be explicit.
804
805 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800806 #arch/x86/boot/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700807 subdir- := compressed/
808
809The above assignment instructs kbuild to descend down in the
810directory compressed/ when "make clean" is executed.
811
812To support the clean infrastructure in the Makefiles that builds the
813final bootimage there is an optional target named archclean:
814
815 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800816 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700817 archclean:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800818 $(Q)$(MAKE) $(clean)=arch/x86/boot
Linus Torvalds1da177e2005-04-16 15:20:36 -0700819
Wanlong Gao25eb6502011-06-13 17:53:53 +0800820When "make clean" is executed, make will descend down in arch/x86/boot,
821and clean as usual. The Makefile located in arch/x86/boot/ may use
Linus Torvalds1da177e2005-04-16 15:20:36 -0700822the subdir- trick to descend further down.
823
824Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
825included in the top level makefile, and the kbuild infrastructure
826is not operational at that point.
827
828Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
829be visited during "make clean".
830
831=== 6 Architecture Makefiles
832
833The top level Makefile sets up the environment and does the preparation,
834before starting to descend down in the individual directories.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200835The top level makefile contains the generic part, whereas
836arch/$(ARCH)/Makefile contains what is required to set up kbuild
837for said architecture.
838To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
Linus Torvalds1da177e2005-04-16 15:20:36 -0700839a few targets.
840
Jan Engelhardta07f6032006-07-27 22:14:29 +0200841When kbuild executes, the following steps are followed (roughly):
8421) Configuration of the kernel => produce .config
Linus Torvalds1da177e2005-04-16 15:20:36 -07008432) Store kernel version in include/linux/version.h
8443) Symlink include/asm to include/asm-$(ARCH)
8454) Updating all other prerequisites to the target prepare:
846 - Additional prerequisites are specified in arch/$(ARCH)/Makefile
8475) Recursively descend down in all directories listed in
848 init-* core* drivers-* net-* libs-* and build all targets.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200849 - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07008506) All object files are then linked and the resulting file vmlinux is
Jan Engelhardta07f6032006-07-27 22:14:29 +0200851 located at the root of the obj tree.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700852 The very first objects linked are listed in head-y, assigned by
853 arch/$(ARCH)/Makefile.
Randy Dunlap5c811e52007-02-17 20:03:14 +01008547) Finally, the architecture-specific part does any required post processing
Linus Torvalds1da177e2005-04-16 15:20:36 -0700855 and builds the final bootimage.
856 - This includes building boot records
Randy Dunlap5c811e52007-02-17 20:03:14 +0100857 - Preparing initrd images and the like
Linus Torvalds1da177e2005-04-16 15:20:36 -0700858
859
860--- 6.1 Set variables to tweak the build to the architecture
861
862 LDFLAGS Generic $(LD) options
863
864 Flags used for all invocations of the linker.
865 Often specifying the emulation is sufficient.
866
867 Example:
868 #arch/s390/Makefile
869 LDFLAGS := -m elf_s390
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200870 Note: ldflags-y can be used to further customise
Sam Ravnborga9af3302007-09-30 20:08:25 +0200871 the flags used. See chapter 3.7.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700872
Linus Torvalds1da177e2005-04-16 15:20:36 -0700873 LDFLAGS_MODULE Options for $(LD) when linking modules
874
875 LDFLAGS_MODULE is used to set specific flags for $(LD) when
876 linking the .ko files used for modules.
877 Default is "-r", for relocatable output.
878
879 LDFLAGS_vmlinux Options for $(LD) when linking vmlinux
880
881 LDFLAGS_vmlinux is used to specify additional flags to pass to
Jan Engelhardta07f6032006-07-27 22:14:29 +0200882 the linker when linking the final vmlinux image.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700883 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
884
885 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800886 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700887 LDFLAGS_vmlinux := -e stext
888
889 OBJCOPYFLAGS objcopy flags
890
891 When $(call if_changed,objcopy) is used to translate a .o file,
Jan Engelhardta07f6032006-07-27 22:14:29 +0200892 the flags specified in OBJCOPYFLAGS will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700893 $(call if_changed,objcopy) is often used to generate raw binaries on
894 vmlinux.
895
896 Example:
897 #arch/s390/Makefile
898 OBJCOPYFLAGS := -O binary
899
900 #arch/s390/boot/Makefile
901 $(obj)/image: vmlinux FORCE
902 $(call if_changed,objcopy)
903
Jan Engelhardta07f6032006-07-27 22:14:29 +0200904 In this example, the binary $(obj)/image is a binary version of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700905 vmlinux. The usage of $(call if_changed,xxx) will be described later.
906
Sam Ravnborg222d3942007-10-15 21:59:31 +0200907 KBUILD_AFLAGS $(AS) assembler flags
Linus Torvalds1da177e2005-04-16 15:20:36 -0700908
909 Default value - see top level Makefile
910 Append or modify as required per architecture.
911
912 Example:
913 #arch/sparc64/Makefile
Sam Ravnborg222d3942007-10-15 21:59:31 +0200914 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
Linus Torvalds1da177e2005-04-16 15:20:36 -0700915
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200916 KBUILD_CFLAGS $(CC) compiler flags
Linus Torvalds1da177e2005-04-16 15:20:36 -0700917
918 Default value - see top level Makefile
919 Append or modify as required per architecture.
920
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200921 Often, the KBUILD_CFLAGS variable depends on the configuration.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700922
923 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800924 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700925 cflags-$(CONFIG_M386) += -march=i386
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200926 KBUILD_CFLAGS += $(cflags-y)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700927
928 Many arch Makefiles dynamically run the target C compiler to
929 probe supported options:
930
Wanlong Gao25eb6502011-06-13 17:53:53 +0800931 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700932
933 ...
934 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
935 -march=pentium2,-march=i686)
936 ...
937 # Disable unit-at-a-time mode ...
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200938 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700939 ...
940
941
Jan Engelhardta07f6032006-07-27 22:14:29 +0200942 The first example utilises the trick that a config option expands
Linus Torvalds1da177e2005-04-16 15:20:36 -0700943 to 'y' when selected.
944
Sam Ravnborg80c00ba2010-07-28 19:11:27 +0200945 KBUILD_AFLAGS_KERNEL $(AS) options specific for built-in
Linus Torvalds1da177e2005-04-16 15:20:36 -0700946
Sam Ravnborg80c00ba2010-07-28 19:11:27 +0200947 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700948 resident kernel code.
949
Sam Ravnborg65881692010-07-28 17:33:09 +0200950 KBUILD_AFLAGS_MODULE Options for $(AS) when building modules
Linus Torvalds1da177e2005-04-16 15:20:36 -0700951
Sam Ravnborg65881692010-07-28 17:33:09 +0200952 $(KBUILD_AFLAGS_MODULE) is used to add arch specific options that
953 are used for $(AS).
954 From commandline AFLAGS_MODULE shall be used (see kbuild.txt).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700955
Sam Ravnborg80c00ba2010-07-28 19:11:27 +0200956 KBUILD_CFLAGS_KERNEL $(CC) options specific for built-in
957
958 $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
959 resident kernel code.
960
Sam Ravnborg65881692010-07-28 17:33:09 +0200961 KBUILD_CFLAGS_MODULE Options for $(CC) when building modules
962
963 $(KBUILD_CFLAGS_MODULE) is used to add arch specific options that
964 are used for $(CC).
965 From commandline CFLAGS_MODULE shall be used (see kbuild.txt).
966
967 KBUILD_LDFLAGS_MODULE Options for $(LD) when linking modules
968
969 $(KBUILD_LDFLAGS_MODULE) is used to add arch specific options
970 used when linking modules. This is often a linker script.
971 From commandline LDFLAGS_MODULE shall be used (see kbuild.txt).
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700972
Michal Marek40df7592011-04-20 13:45:30 +0200973 KBUILD_ARFLAGS Options for $(AR) when creating archives
974
975 $(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic
976 mode) if this option is supported by $(AR).
977
H. Peter Anvin052ad272011-11-17 13:17:35 -0800978--- 6.2 Add prerequisites to archheaders:
979
980 The archheaders: rule is used to generate header files that
981 may be installed into user space by "make header_install" or
982 "make headers_install_all". In order to support
983 "make headers_install_all", this target has to be able to run
984 on an unconfigured tree, or a tree configured for another
985 architecture.
986
987 It is run before "make archprepare" when run on the
988 architecture itself.
989
990
991--- 6.3 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700992
Jan Engelhardta07f6032006-07-27 22:14:29 +0200993 The archprepare: rule is used to list prerequisites that need to be
Linus Torvalds1da177e2005-04-16 15:20:36 -0700994 built before starting to descend down in the subdirectories.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200995 This is usually used for header files containing assembler constants.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700996
997 Example:
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200998 #arch/arm/Makefile
999 archprepare: maketools
Linus Torvalds1da177e2005-04-16 15:20:36 -07001000
Jan Engelhardta07f6032006-07-27 22:14:29 +02001001 In this example, the file target maketools will be processed
Sam Ravnborg5bb78262005-09-11 22:30:22 +02001002 before descending down in the subdirectories.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001003 See also chapter XXX-TODO that describe how kbuild supports
1004 generating offset header files.
1005
1006
H. Peter Anvin052ad272011-11-17 13:17:35 -08001007--- 6.4 List directories to visit when descending
Linus Torvalds1da177e2005-04-16 15:20:36 -07001008
1009 An arch Makefile cooperates with the top Makefile to define variables
1010 which specify how to build the vmlinux file. Note that there is no
1011 corresponding arch-specific section for modules; the module-building
1012 machinery is all architecture-independent.
1013
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001014
Linus Torvalds1da177e2005-04-16 15:20:36 -07001015 head-y, init-y, core-y, libs-y, drivers-y, net-y
1016
Jan Engelhardta07f6032006-07-27 22:14:29 +02001017 $(head-y) lists objects to be linked first in vmlinux.
1018 $(libs-y) lists directories where a lib.a archive can be located.
Randy Dunlap5c811e52007-02-17 20:03:14 +01001019 The rest list directories where a built-in.o object file can be
Jan Engelhardta07f6032006-07-27 22:14:29 +02001020 located.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001021
1022 $(init-y) objects will be located after $(head-y).
1023 Then the rest follows in this order:
1024 $(core-y), $(libs-y), $(drivers-y) and $(net-y).
1025
Jan Engelhardta07f6032006-07-27 22:14:29 +02001026 The top level Makefile defines values for all generic directories,
Randy Dunlap5c811e52007-02-17 20:03:14 +01001027 and arch/$(ARCH)/Makefile only adds architecture-specific directories.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001028
1029 Example:
1030 #arch/sparc64/Makefile
1031 core-y += arch/sparc64/kernel/
1032 libs-y += arch/sparc64/prom/ arch/sparc64/lib/
1033 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
1034
1035
H. Peter Anvin052ad272011-11-17 13:17:35 -08001036--- 6.5 Architecture-specific boot images
Linus Torvalds1da177e2005-04-16 15:20:36 -07001037
1038 An arch Makefile specifies goals that take the vmlinux file, compress
1039 it, wrap it in bootstrapping code, and copy the resulting files
1040 somewhere. This includes various kinds of installation commands.
1041 The actual goals are not standardized across architectures.
1042
1043 It is common to locate any additional processing in a boot/
1044 directory below arch/$(ARCH)/.
1045
1046 Kbuild does not provide any smart way to support building a
1047 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
1048 call make manually to build a target in boot/.
1049
1050 The recommended approach is to include shortcuts in
1051 arch/$(ARCH)/Makefile, and use the full path when calling down
1052 into the arch/$(ARCH)/boot/Makefile.
1053
1054 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001055 #arch/x86/Makefile
1056 boot := arch/x86/boot
Linus Torvalds1da177e2005-04-16 15:20:36 -07001057 bzImage: vmlinux
1058 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
1059
1060 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
1061 make in a subdirectory.
1062
Randy Dunlap5c811e52007-02-17 20:03:14 +01001063 There are no rules for naming architecture-specific targets,
Linus Torvalds1da177e2005-04-16 15:20:36 -07001064 but executing "make help" will list all relevant targets.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001065 To support this, $(archhelp) must be defined.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001066
1067 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001068 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001069 define archhelp
1070 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001071 endif
Linus Torvalds1da177e2005-04-16 15:20:36 -07001072
1073 When make is executed without arguments, the first goal encountered
1074 will be built. In the top level Makefile the first goal present
1075 is all:.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001076 An architecture shall always, per default, build a bootable image.
1077 In "make help", the default goal is highlighted with a '*'.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001078 Add a new prerequisite to all: to select a default goal different
1079 from vmlinux.
1080
1081 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001082 #arch/x86/Makefile
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001083 all: bzImage
Linus Torvalds1da177e2005-04-16 15:20:36 -07001084
1085 When "make" is executed without arguments, bzImage will be built.
1086
H. Peter Anvin052ad272011-11-17 13:17:35 -08001087--- 6.6 Building non-kbuild targets
Linus Torvalds1da177e2005-04-16 15:20:36 -07001088
1089 extra-y
1090
1091 extra-y specify additional targets created in the current
1092 directory, in addition to any targets specified by obj-*.
1093
1094 Listing all targets in extra-y is required for two purposes:
1095 1) Enable kbuild to check changes in command lines
1096 - When $(call if_changed,xxx) is used
1097 2) kbuild knows what files to delete during "make clean"
1098
1099 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001100 #arch/x86/kernel/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001101 extra-y := head.o init_task.o
1102
Jan Engelhardta07f6032006-07-27 22:14:29 +02001103 In this example, extra-y is used to list object files that
Linus Torvalds1da177e2005-04-16 15:20:36 -07001104 shall be built, but shall not be linked as part of built-in.o.
1105
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001106
H. Peter Anvin052ad272011-11-17 13:17:35 -08001107--- 6.7 Commands useful for building a boot image
Linus Torvalds1da177e2005-04-16 15:20:36 -07001108
1109 Kbuild provides a few macros that are useful when building a
1110 boot image.
1111
1112 if_changed
1113
1114 if_changed is the infrastructure used for the following commands.
1115
1116 Usage:
1117 target: source(s) FORCE
1118 $(call if_changed,ld/objcopy/gzip)
1119
Jan Engelhardta07f6032006-07-27 22:14:29 +02001120 When the rule is evaluated, it is checked to see if any files
Randy Dunlap5c811e52007-02-17 20:03:14 +01001121 need an update, or the command line has changed since the last
Linus Torvalds1da177e2005-04-16 15:20:36 -07001122 invocation. The latter will force a rebuild if any options
1123 to the executable have changed.
1124 Any target that utilises if_changed must be listed in $(targets),
1125 otherwise the command line check will fail, and the target will
1126 always be built.
1127 Assignments to $(targets) are without $(obj)/ prefix.
1128 if_changed may be used in conjunction with custom commands as
H. Peter Anvin052ad272011-11-17 13:17:35 -08001129 defined in 6.8 "Custom kbuild commands".
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +02001130
Linus Torvalds1da177e2005-04-16 15:20:36 -07001131 Note: It is a typical mistake to forget the FORCE prerequisite.
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +02001132 Another common pitfall is that whitespace is sometimes
1133 significant; for instance, the below will fail (note the extra space
1134 after the comma):
1135 target: source(s) FORCE
1136 #WRONG!# $(call if_changed, ld/objcopy/gzip)
Linus Torvalds1da177e2005-04-16 15:20:36 -07001137
1138 ld
Jan Engelhardta07f6032006-07-27 22:14:29 +02001139 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001140
Linus Torvalds1da177e2005-04-16 15:20:36 -07001141 objcopy
1142 Copy binary. Uses OBJCOPYFLAGS usually specified in
1143 arch/$(ARCH)/Makefile.
1144 OBJCOPYFLAGS_$@ may be used to set additional options.
1145
1146 gzip
1147 Compress target. Use maximum compression to compress target.
1148
1149 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001150 #arch/x86/boot/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001151 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1152 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
1153
1154 targets += setup setup.o bootsect bootsect.o
1155 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1156 $(call if_changed,ld)
1157
Jan Engelhardta07f6032006-07-27 22:14:29 +02001158 In this example, there are two possible targets, requiring different
1159 options to the linker. The linker options are specified using the
Linus Torvalds1da177e2005-04-16 15:20:36 -07001160 LDFLAGS_$@ syntax - one for each potential target.
Matt LaPlante5d3f0832006-11-30 05:21:10 +01001161 $(targets) are assigned all potential targets, by which kbuild knows
Linus Torvalds1da177e2005-04-16 15:20:36 -07001162 the targets and will:
1163 1) check for commandline changes
1164 2) delete target during make clean
1165
1166 The ": %: %.o" part of the prerequisite is a shorthand that
1167 free us from listing the setup.o and bootsect.o files.
1168 Note: It is a common mistake to forget the "target :=" assignment,
1169 resulting in the target file being recompiled for no
1170 obvious reason.
1171
Dirk Brandewieaab94332010-12-22 11:57:26 -08001172 dtc
1173 Create flattend device tree blob object suitable for linking
1174 into vmlinux. Device tree blobs linked into vmlinux are placed
1175 in an init section in the image. Platform code *must* copy the
1176 blob to non-init memory prior to calling unflatten_device_tree().
1177
1178 Example:
1179 #arch/x86/platform/ce4100/Makefile
1180 clean-files := *dtb.S
1181
1182 DTC_FLAGS := -p 1024
1183 obj-y += foo.dtb.o
1184
1185 $(obj)/%.dtb: $(src)/%.dts
1186 $(call cmd,dtc)
Linus Torvalds1da177e2005-04-16 15:20:36 -07001187
H. Peter Anvin052ad272011-11-17 13:17:35 -08001188--- 6.8 Custom kbuild commands
Linus Torvalds1da177e2005-04-16 15:20:36 -07001189
Jan Engelhardta07f6032006-07-27 22:14:29 +02001190 When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
Linus Torvalds1da177e2005-04-16 15:20:36 -07001191 of a command is normally displayed.
1192 To enable this behaviour for custom commands kbuild requires
1193 two variables to be set:
1194 quiet_cmd_<command> - what shall be echoed
1195 cmd_<command> - the command to execute
1196
1197 Example:
1198 #
1199 quiet_cmd_image = BUILD $@
1200 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
1201 $(obj)/vmlinux.bin > $@
1202
1203 targets += bzImage
1204 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
1205 $(call if_changed,image)
1206 @echo 'Kernel: $@ is ready'
1207
Jan Engelhardta07f6032006-07-27 22:14:29 +02001208 When updating the $(obj)/bzImage target, the line
Linus Torvalds1da177e2005-04-16 15:20:36 -07001209
Wanlong Gao25eb6502011-06-13 17:53:53 +08001210 BUILD arch/x86/boot/bzImage
Linus Torvalds1da177e2005-04-16 15:20:36 -07001211
1212 will be displayed with "make KBUILD_VERBOSE=0".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001213
Linus Torvalds1da177e2005-04-16 15:20:36 -07001214
H. Peter Anvin052ad272011-11-17 13:17:35 -08001215--- 6.9 Preprocessing linker scripts
Linus Torvalds1da177e2005-04-16 15:20:36 -07001216
Jan Engelhardta07f6032006-07-27 22:14:29 +02001217 When the vmlinux image is built, the linker script
Linus Torvalds1da177e2005-04-16 15:20:36 -07001218 arch/$(ARCH)/kernel/vmlinux.lds is used.
1219 The script is a preprocessed variant of the file vmlinux.lds.S
1220 located in the same directory.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001221 kbuild knows .lds files and includes a rule *lds.S -> *lds.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001222
Linus Torvalds1da177e2005-04-16 15:20:36 -07001223 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001224 #arch/x86/kernel/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001225 always := vmlinux.lds
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001226
Linus Torvalds1da177e2005-04-16 15:20:36 -07001227 #Makefile
1228 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001229
1230 The assignment to $(always) is used to tell kbuild to build the
Jan Engelhardta07f6032006-07-27 22:14:29 +02001231 target vmlinux.lds.
1232 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
Linus Torvalds1da177e2005-04-16 15:20:36 -07001233 specified options when building the target vmlinux.lds.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001234
Jan Engelhardta07f6032006-07-27 22:14:29 +02001235 When building the *.lds target, kbuild uses the variables:
Sam Ravnborg06c50402007-10-15 22:17:25 +02001236 KBUILD_CPPFLAGS : Set in top-level Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +02001237 cppflags-y : May be set in the kbuild makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001238 CPPFLAGS_$(@F) : Target specific flags.
1239 Note that the full filename is used in this
1240 assignment.
1241
1242 The kbuild infrastructure for *lds file are used in several
Randy Dunlap5c811e52007-02-17 20:03:14 +01001243 architecture-specific files.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001244
H. Peter Anvin052ad272011-11-17 13:17:35 -08001245--- 6.10 Generic header files
Sam Ravnborgd8ecc5c2011-04-27 22:29:49 +02001246
1247 The directory include/asm-generic contains the header files
1248 that may be shared between individual architectures.
1249 The recommended approach how to use a generic header file is
1250 to list the file in the Kbuild file.
1251 See "7.4 generic-y" for further info on syntax etc.
1252
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001253=== 7 Kbuild syntax for exported headers
Linus Torvalds1da177e2005-04-16 15:20:36 -07001254
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001255The kernel include a set of headers that is exported to userspace.
Nicolas Kaiserc95940f2010-08-05 11:23:11 -07001256Many headers can be exported as-is but other headers require a
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001257minimal pre-processing before they are ready for user-space.
1258The pre-processing does:
1259- drop kernel specific annotations
1260- drop include of compiler.h
Nicolas Kaiserc95940f2010-08-05 11:23:11 -07001261- drop all sections that are kernel internal (guarded by ifdef __KERNEL__)
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001262
Nicolas Kaiserc95940f2010-08-05 11:23:11 -07001263Each relevant directory contains a file name "Kbuild" which specifies the
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001264headers to be exported.
1265See subsequent chapter for the syntax of the Kbuild file.
1266
1267 --- 7.1 header-y
1268
1269 header-y specify header files to be exported.
1270
1271 Example:
1272 #include/linux/Kbuild
1273 header-y += usb/
1274 header-y += aio_abi.h
1275
1276 The convention is to list one file per line and
1277 preferably in alphabetic order.
1278
1279 header-y also specify which subdirectories to visit.
1280 A subdirectory is identified by a trailing '/' which
1281 can be seen in the example above for the usb subdirectory.
1282
1283 Subdirectories are visited before their parent directories.
1284
1285 --- 7.2 objhdr-y
1286
1287 objhdr-y specifies generated files to be exported.
1288 Generated files are special as they need to be looked
1289 up in another directory when doing 'make O=...' builds.
1290
1291 Example:
1292 #include/linux/Kbuild
1293 objhdr-y += version.h
1294
1295 --- 7.3 destination-y
1296
1297 When an architecture have a set of exported headers that needs to be
1298 exported to a different directory destination-y is used.
1299 destination-y specify the destination directory for all exported
1300 headers in the file where it is present.
1301
1302 Example:
1303 #arch/xtensa/platforms/s6105/include/platform/Kbuild
1304 destination-y := include/linux
1305
1306 In the example above all exported headers in the Kbuild file
1307 will be located in the directory "include/linux" when exported.
1308
Sam Ravnborgd8ecc5c2011-04-27 22:29:49 +02001309 --- 7.4 generic-y
1310
1311 If an architecture uses a verbatim copy of a header from
1312 include/asm-generic then this is listed in the file
1313 arch/$(ARCH)/include/asm/Kbuild like this:
1314
1315 Example:
1316 #arch/x86/include/asm/Kbuild
1317 generic-y += termios.h
1318 generic-y += rtc.h
1319
1320 During the prepare phase of the build a wrapper include
1321 file is generated in the directory:
1322
1323 arch/$(ARCH)/include/generated/asm
1324
1325 When a header is exported where the architecture uses
1326 the generic header a similar wrapper is generated as part
1327 of the set of exported headers in the directory:
1328
1329 usr/include/asm
1330
1331 The generated wrapper will in both cases look like the following:
1332
1333 Example: termios.h
1334 #include <asm-generic/termios.h>
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001335
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001336=== 8 Kbuild Variables
Linus Torvalds1da177e2005-04-16 15:20:36 -07001337
1338The top Makefile exports the following variables:
1339
1340 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1341
1342 These variables define the current kernel version. A few arch
1343 Makefiles actually use these values directly; they should use
1344 $(KERNELRELEASE) instead.
1345
1346 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1347 three-part version number, such as "2", "4", and "0". These three
1348 values are always numeric.
1349
1350 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1351 or additional patches. It is usually some non-numeric string
1352 such as "-pre4", and is often blank.
1353
1354 KERNELRELEASE
1355
1356 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1357 for constructing installation directory names or showing in
1358 version strings. Some arch Makefiles use it for this purpose.
1359
1360 ARCH
1361
1362 This variable defines the target architecture, such as "i386",
1363 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1364 determine which files to compile.
1365
1366 By default, the top Makefile sets $(ARCH) to be the same as the
1367 host system architecture. For a cross build, a user may
1368 override the value of $(ARCH) on the command line:
1369
1370 make ARCH=m68k ...
1371
1372
1373 INSTALL_PATH
1374
1375 This variable defines a place for the arch Makefiles to install
1376 the resident kernel image and System.map file.
Randy Dunlap5c811e52007-02-17 20:03:14 +01001377 Use this for architecture-specific install targets.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001378
1379 INSTALL_MOD_PATH, MODLIB
1380
1381 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1382 installation. This variable is not defined in the Makefile but
1383 may be passed in by the user if desired.
1384
1385 $(MODLIB) specifies the directory for module installation.
1386 The top Makefile defines $(MODLIB) to
1387 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1388 override this value on the command line if desired.
1389
Theodore Ts'oac031f22006-06-21 20:53:09 -04001390 INSTALL_MOD_STRIP
1391
1392 If this variable is specified, will cause modules to be stripped
1393 after they are installed. If INSTALL_MOD_STRIP is '1', then the
1394 default option --strip-debug will be used. Otherwise,
Gilles Espinasse177b2412011-01-09 08:59:49 +01001395 INSTALL_MOD_STRIP value will be used as the option(s) to the strip
1396 command.
Theodore Ts'oac031f22006-06-21 20:53:09 -04001397
1398
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001399=== 9 Makefile language
Linus Torvalds1da177e2005-04-16 15:20:36 -07001400
Jan Engelhardta07f6032006-07-27 22:14:29 +02001401The kernel Makefiles are designed to be run with GNU Make. The Makefiles
Linus Torvalds1da177e2005-04-16 15:20:36 -07001402use only the documented features of GNU Make, but they do use many
1403GNU extensions.
1404
1405GNU Make supports elementary list-processing functions. The kernel
1406Makefiles use a novel style of list building and manipulation with few
1407"if" statements.
1408
1409GNU Make has two assignment operators, ":=" and "=". ":=" performs
1410immediate evaluation of the right-hand side and stores an actual string
1411into the left-hand side. "=" is like a formula definition; it stores the
1412right-hand side in an unevaluated form and then evaluates this form each
1413time the left-hand side is used.
1414
1415There are some cases where "=" is appropriate. Usually, though, ":="
1416is the right choice.
1417
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001418=== 10 Credits
Linus Torvalds1da177e2005-04-16 15:20:36 -07001419
1420Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1421Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1422Updates by Sam Ravnborg <sam@ravnborg.org>
Jan Engelhardta07f6032006-07-27 22:14:29 +02001423Language QA by Jan Engelhardt <jengelh@gmx.de>
Linus Torvalds1da177e2005-04-16 15:20:36 -07001424
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001425=== 11 TODO
Linus Torvalds1da177e2005-04-16 15:20:36 -07001426
Jan Engelhardta07f6032006-07-27 22:14:29 +02001427- Describe how kbuild supports shipped files with _shipped.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001428- Generating offset header files.
1429- Add more variables to section 7?
1430
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001431
1432