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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Linux Kernel Makefiles
2
3This document describes the Linux kernel Makefiles.
4
5=== Table of Contents
6
7 === 1 Overview
8 === 2 Who does what
9 === 3 The kbuild files
10 --- 3.1 Goal definitions
11 --- 3.2 Built-in object goals - obj-y
12 --- 3.3 Loadable module goals - obj-m
13 --- 3.4 Objects which export symbols
14 --- 3.5 Library file goals - lib-y
15 --- 3.6 Descending down in directories
16 --- 3.7 Compilation flags
17 --- 3.8 Command line dependency
18 --- 3.9 Dependency tracking
19 --- 3.10 Special Rules
Sam Ravnborg20a468b2006-01-22 13:34:15 +010020 --- 3.11 $(CC) support functions
Sam Ravnborg691ef3e2009-09-19 10:31:45 +020021 --- 3.12 $(LD) support functions
Linus Torvalds1da177e2005-04-16 15:20:36 -070022
23 === 4 Host Program support
24 --- 4.1 Simple Host Program
25 --- 4.2 Composite Host Programs
Bryce Harrington39e6e9c2006-09-20 12:43:37 -070026 --- 4.3 Defining shared libraries
Linus Torvalds1da177e2005-04-16 15:20:36 -070027 --- 4.4 Using C++ for host programs
28 --- 4.5 Controlling compiler options for host programs
29 --- 4.6 When host programs are actually built
30 --- 4.7 Using hostprogs-$(CONFIG_FOO)
31
32 === 5 Kbuild clean infrastructure
33
34 === 6 Architecture Makefiles
35 --- 6.1 Set variables to tweak the build to the architecture
Sam Ravnborg5bb78262005-09-11 22:30:22 +020036 --- 6.2 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -070037 --- 6.3 List directories to visit when descending
Randy Dunlap5c811e52007-02-17 20:03:14 +010038 --- 6.4 Architecture-specific boot images
Linus Torvalds1da177e2005-04-16 15:20:36 -070039 --- 6.5 Building non-kbuild targets
40 --- 6.6 Commands useful for building a boot image
41 --- 6.7 Custom kbuild commands
42 --- 6.8 Preprocessing linker scripts
Sam Ravnborgd8ecc5c2011-04-27 22:29:49 +020043 --- 6.9 Generic header files
Linus Torvalds1da177e2005-04-16 15:20:36 -070044
Sam Ravnborgc7bb3492009-04-10 08:52:43 +020045 === 7 Kbuild syntax for exported headers
46 --- 7.1 header-y
47 --- 7.2 objhdr-y
48 --- 7.3 destination-y
Sam Ravnborgd8ecc5c2011-04-27 22:29:49 +020049 --- 7.4 generic-y
Sam Ravnborgc7bb3492009-04-10 08:52:43 +020050
51 === 8 Kbuild Variables
52 === 9 Makefile language
53 === 10 Credits
54 === 11 TODO
Linus Torvalds1da177e2005-04-16 15:20:36 -070055
56=== 1 Overview
57
58The Makefiles have five parts:
59
60 Makefile the top Makefile.
61 .config the kernel configuration file.
62 arch/$(ARCH)/Makefile the arch Makefile.
63 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
64 kbuild Makefiles there are about 500 of these.
65
66The top Makefile reads the .config file, which comes from the kernel
67configuration process.
68
69The top Makefile is responsible for building two major products: vmlinux
70(the resident kernel image) and modules (any module files).
71It builds these goals by recursively descending into the subdirectories of
72the kernel source tree.
73The list of subdirectories which are visited depends upon the kernel
74configuration. The top Makefile textually includes an arch Makefile
75with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
76architecture-specific information to the top Makefile.
77
78Each subdirectory has a kbuild Makefile which carries out the commands
79passed down from above. The kbuild Makefile uses information from the
Bryce Harrington39e6e9c2006-09-20 12:43:37 -070080.config file to construct various file lists used by kbuild to build
Linus Torvalds1da177e2005-04-16 15:20:36 -070081any built-in or modular targets.
82
83scripts/Makefile.* contains all the definitions/rules etc. that
84are used to build the kernel based on the kbuild makefiles.
85
86
87=== 2 Who does what
88
89People have four different relationships with the kernel Makefiles.
90
91*Users* are people who build kernels. These people type commands such as
92"make menuconfig" or "make". They usually do not read or edit
93any kernel Makefiles (or any other source files).
94
95*Normal developers* are people who work on features such as device
96drivers, file systems, and network protocols. These people need to
Jan Engelhardta07f6032006-07-27 22:14:29 +020097maintain the kbuild Makefiles for the subsystem they are
Linus Torvalds1da177e2005-04-16 15:20:36 -070098working on. In order to do this effectively, they need some overall
99knowledge about the kernel Makefiles, plus detailed knowledge about the
100public interface for kbuild.
101
102*Arch developers* are people who work on an entire architecture, such
103as sparc or ia64. Arch developers need to know about the arch Makefile
104as well as kbuild Makefiles.
105
106*Kbuild developers* are people who work on the kernel build system itself.
107These people need to know about all aspects of the kernel Makefiles.
108
109This document is aimed towards normal developers and arch developers.
110
111
112=== 3 The kbuild files
113
114Most Makefiles within the kernel are kbuild Makefiles that use the
Jan Engelhardta07f6032006-07-27 22:14:29 +0200115kbuild infrastructure. This chapter introduces the syntax used in the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700116kbuild makefiles.
Sam Ravnborg172c3ae2006-03-10 00:23:32 +0100117The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
Jan Engelhardta07f6032006-07-27 22:14:29 +0200118be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
Sam Ravnborg172c3ae2006-03-10 00:23:32 +0100119file will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700120
121Section 3.1 "Goal definitions" is a quick intro, further chapters provide
122more details, with real examples.
123
124--- 3.1 Goal definitions
125
126 Goal definitions are the main part (heart) of the kbuild Makefile.
127 These lines define the files to be built, any special compilation
128 options, and any subdirectories to be entered recursively.
129
130 The most simple kbuild makefile contains one line:
131
132 Example:
133 obj-y += foo.o
134
Randy Dunlap5c811e52007-02-17 20:03:14 +0100135 This tells kbuild that there is one object in that directory, named
Linus Torvalds1da177e2005-04-16 15:20:36 -0700136 foo.o. foo.o will be built from foo.c or foo.S.
137
138 If foo.o shall be built as a module, the variable obj-m is used.
139 Therefore the following pattern is often used:
140
141 Example:
142 obj-$(CONFIG_FOO) += foo.o
143
144 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
145 If CONFIG_FOO is neither y nor m, then the file will not be compiled
146 nor linked.
147
148--- 3.2 Built-in object goals - obj-y
149
150 The kbuild Makefile specifies object files for vmlinux
Jan Engelhardta07f6032006-07-27 22:14:29 +0200151 in the $(obj-y) lists. These lists depend on the kernel
Linus Torvalds1da177e2005-04-16 15:20:36 -0700152 configuration.
153
154 Kbuild compiles all the $(obj-y) files. It then calls
155 "$(LD) -r" to merge these files into one built-in.o file.
156 built-in.o is later linked into vmlinux by the parent Makefile.
157
158 The order of files in $(obj-y) is significant. Duplicates in
159 the lists are allowed: the first instance will be linked into
160 built-in.o and succeeding instances will be ignored.
161
162 Link order is significant, because certain functions
163 (module_init() / __initcall) will be called during boot in the
164 order they appear. So keep in mind that changing the link
Jan Engelhardta07f6032006-07-27 22:14:29 +0200165 order may e.g. change the order in which your SCSI
166 controllers are detected, and thus your disks are renumbered.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700167
168 Example:
169 #drivers/isdn/i4l/Makefile
170 # Makefile for the kernel ISDN subsystem and device drivers.
171 # Each configuration option enables a list of files.
Matt Mooney2f5a2f82010-08-05 11:23:11 -0700172 obj-$(CONFIG_ISDN_I4L) += isdn.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700173 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
174
175--- 3.3 Loadable module goals - obj-m
176
177 $(obj-m) specify object files which are built as loadable
178 kernel modules.
179
180 A module may be built from one source file or several source
181 files. In the case of one source file, the kbuild makefile
182 simply adds the file to $(obj-m).
183
184 Example:
185 #drivers/isdn/i4l/Makefile
186 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
187
188 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
189
190 If a kernel module is built from several source files, you specify
Matt Mooney4f827282010-08-05 11:23:11 -0700191 that you want to build a module in the same way as above; however,
192 kbuild needs to know which object files you want to build your
193 module from, so you have to tell it by setting a $(<module_name>-y)
194 variable.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700195
196 Example:
197 #drivers/isdn/i4l/Makefile
Matt Mooney4f827282010-08-05 11:23:11 -0700198 obj-$(CONFIG_ISDN_I4L) += isdn.o
199 isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700200
201 In this example, the module name will be isdn.o. Kbuild will
Matt Mooney4f827282010-08-05 11:23:11 -0700202 compile the objects listed in $(isdn-y) and then run
Linus Torvalds1da177e2005-04-16 15:20:36 -0700203 "$(LD) -r" on the list of these files to generate isdn.o.
204
Matt Mooney4f827282010-08-05 11:23:11 -0700205 Due to kbuild recognizing $(<module_name>-y) for composite objects,
206 you can use the value of a CONFIG_ symbol to optionally include an
207 object file as part of a composite object.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700208
209 Example:
210 #fs/ext2/Makefile
Matt Mooney4f827282010-08-05 11:23:11 -0700211 obj-$(CONFIG_EXT2_FS) += ext2.o
212 ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
213 namei.o super.o symlink.o
214 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
215 xattr_trusted.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700216
Matt Mooney4f827282010-08-05 11:23:11 -0700217 In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
218 part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
219 evaluates to 'y'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700220
221 Note: Of course, when you are building objects into the kernel,
222 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
223 kbuild will build an ext2.o file for you out of the individual
224 parts and then link this into built-in.o, as you would expect.
225
226--- 3.4 Objects which export symbols
227
228 No special notation is required in the makefiles for
229 modules exporting symbols.
230
231--- 3.5 Library file goals - lib-y
232
Jan Engelhardta07f6032006-07-27 22:14:29 +0200233 Objects listed with obj-* are used for modules, or
Linus Torvalds1da177e2005-04-16 15:20:36 -0700234 combined in a built-in.o for that specific directory.
235 There is also the possibility to list objects that will
236 be included in a library, lib.a.
237 All objects listed with lib-y are combined in a single
238 library for that directory.
Matt LaPlante5d3f0832006-11-30 05:21:10 +0100239 Objects that are listed in obj-y and additionally listed in
240 lib-y will not be included in the library, since they will
241 be accessible anyway.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200242 For consistency, objects listed in lib-m will be included in lib.a.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700243
244 Note that the same kbuild makefile may list files to be built-in
245 and to be part of a library. Therefore the same directory
246 may contain both a built-in.o and a lib.a file.
247
248 Example:
Matt Mooney2f5a2f82010-08-05 11:23:11 -0700249 #arch/x86/lib/Makefile
250 lib-y := delay.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700251
Matt Mooney2f5a2f82010-08-05 11:23:11 -0700252 This will create a library lib.a based on delay.o. For kbuild to
253 actually recognize that there is a lib.a being built, the directory
254 shall be listed in libs-y.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700255 See also "6.3 List directories to visit when descending".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700256
Jan Engelhardta07f6032006-07-27 22:14:29 +0200257 Use of lib-y is normally restricted to lib/ and arch/*/lib.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700258
259--- 3.6 Descending down in directories
260
261 A Makefile is only responsible for building objects in its own
262 directory. Files in subdirectories should be taken care of by
263 Makefiles in these subdirs. The build system will automatically
264 invoke make recursively in subdirectories, provided you let it know of
265 them.
266
Jan Engelhardta07f6032006-07-27 22:14:29 +0200267 To do so, obj-y and obj-m are used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700268 ext2 lives in a separate directory, and the Makefile present in fs/
269 tells kbuild to descend down using the following assignment.
270
271 Example:
272 #fs/Makefile
273 obj-$(CONFIG_EXT2_FS) += ext2/
274
275 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
276 the corresponding obj- variable will be set, and kbuild will descend
277 down in the ext2 directory.
278 Kbuild only uses this information to decide that it needs to visit
279 the directory, it is the Makefile in the subdirectory that
280 specifies what is modules and what is built-in.
281
282 It is good practice to use a CONFIG_ variable when assigning directory
283 names. This allows kbuild to totally skip the directory if the
284 corresponding CONFIG_ option is neither 'y' nor 'm'.
285
286--- 3.7 Compilation flags
287
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200288 ccflags-y, asflags-y and ldflags-y
Nicolas Kaiserc95940f2010-08-05 11:23:11 -0700289 These three flags apply only to the kbuild makefile in which they
290 are assigned. They are used for all the normal cc, as and ld
291 invocations happening during a recursive build.
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200292 Note: Flags with the same behaviour were previously named:
Nicolas Kaiserc95940f2010-08-05 11:23:11 -0700293 EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
294 They are still supported but their usage is deprecated.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700295
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700296 ccflags-y specifies options for compiling with $(CC).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700297
298 Example:
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700299 # drivers/acpi/Makefile
300 ccflags-y := -Os
301 ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT
Linus Torvalds1da177e2005-04-16 15:20:36 -0700302
303 This variable is necessary because the top Makefile owns the
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200304 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700305 entire tree.
306
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700307 asflags-y specifies options for assembling with $(AS).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700308
309 Example:
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700310 #arch/sparc/kernel/Makefile
311 asflags-y := -ansi
Linus Torvalds1da177e2005-04-16 15:20:36 -0700312
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700313 ldflags-y specifies options for linking with $(LD).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700314
315 Example:
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700316 #arch/cris/boot/compressed/Makefile
317 ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
Linus Torvalds1da177e2005-04-16 15:20:36 -0700318
Sam Ravnborg720097d2009-04-19 11:04:26 +0200319 subdir-ccflags-y, subdir-asflags-y
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700320 The two flags listed above are similar to ccflags-y and asflags-y.
Nicolas Kaiserc95940f2010-08-05 11:23:11 -0700321 The difference is that the subdir- variants have effect for the kbuild
322 file where they are present and all subdirectories.
323 Options specified using subdir-* are added to the commandline before
324 the options specified using the non-subdir variants.
Sam Ravnborg720097d2009-04-19 11:04:26 +0200325
326 Example:
327 subdir-ccflags-y := -Werror
328
Linus Torvalds1da177e2005-04-16 15:20:36 -0700329 CFLAGS_$@, 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
Linus Torvalds1da177e2005-04-16 15:20:36 -0700342
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700343 These two lines specify compilation flags for aha152x.o and gdth.o.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700344
345 $(AFLAGS_$@) is a similar feature for source files in assembly
346 languages.
347
348 Example:
349 # arch/arm/kernel/Makefile
Matt Mooneyeb07e1b2010-08-05 11:23:11 -0700350 AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
351 AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
352 AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt
353
Linus Torvalds1da177e2005-04-16 15:20:36 -0700354
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
Sam Ravnborgf86fd302009-09-19 10:14:33 +0200438 cc-ldoption
439 cc-ldoption 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:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800444 #arch/x86/kernel/Makefile
Sam Ravnborgf86fd302009-09-19 10:14:33 +0200445 vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
Roland McGrath0b0bf7a2006-07-30 03:04:06 -0700446
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:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800463 #arch/x86/Makefile
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100464 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
Michal Marek8417da62011-05-02 12:51:15 +0200504 cc-disable-warning
505 cc-disable-warning checks if gcc supports a given warning and returns
506 the commandline switch to disable it. This special function is needed,
507 because gcc 4.4 and later accept any unknown -Wno-* option and only
508 warn about it if there is another warning in the source file.
509
510 Example:
511 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
512
513 In the above example, -Wno-unused-but-set-variable will be added to
514 KBUILD_CFLAGS only if gcc really accepts it.
515
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100516 cc-version
Jan Engelhardta07f6032006-07-27 22:14:29 +0200517 cc-version returns a numerical version of the $(CC) compiler version.
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100518 The format is <major><minor> where both are two digits. So for example
519 gcc 3.41 would return 0341.
520 cc-version is useful when a specific $(CC) version is faulty in one
Jan Engelhardta07f6032006-07-27 22:14:29 +0200521 area, for example -mregparm=3 was broken in some gcc versions
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100522 even though the option was accepted by gcc.
523
524 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800525 #arch/x86/Makefile
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100526 cflags-y += $(shell \
527 if [ $(call cc-version) -ge 0300 ] ; then \
528 echo "-mregparm=3"; fi ;)
529
Jan Engelhardta07f6032006-07-27 22:14:29 +0200530 In the above example, -mregparm=3 is only used for gcc version greater
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100531 than or equal to gcc 3.0.
532
533 cc-ifversion
Jan Engelhardta07f6032006-07-27 22:14:29 +0200534 cc-ifversion tests the version of $(CC) and equals last argument if
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100535 version expression is true.
536
537 Example:
538 #fs/reiserfs/Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200539 ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100540
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200541 In this example, ccflags-y will be assigned the value -O1 if the
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100542 $(CC) version is less than 4.2.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700543 cc-ifversion takes all the shell operators:
Sam Ravnborg20a468b2006-01-22 13:34:15 +0100544 -eq, -ne, -lt, -le, -gt, and -ge
545 The third parameter may be a text as in this example, but it may also
546 be an expanded variable or a macro.
547
Sam Ravnborg70150302007-05-28 22:26:55 +0200548 cc-fullversion
549 cc-fullversion is useful when the exact version of gcc is needed.
550 One typical use-case is when a specific GCC version is broken.
551 cc-fullversion points out a more specific version than cc-version does.
552
553 Example:
554 #arch/powerpc/Makefile
555 $(Q)if test "$(call cc-fullversion)" = "040200" ; then \
556 echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \
557 false ; \
558 fi
559
560 In this example for a specific GCC version the build will error out explaining
561 to the user why it stops.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700562
Sam Ravnborg910b4042007-10-19 21:46:01 +0200563 cc-cross-prefix
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200564 cc-cross-prefix is used to check if there exists a $(CC) in path with
Sam Ravnborg910b4042007-10-19 21:46:01 +0200565 one of the listed prefixes. The first prefix where there exist a
566 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
567 then nothing is returned.
568 Additional prefixes are separated by a single space in the
569 call of cc-cross-prefix.
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200570 This functionality is useful for architecture Makefiles that try
571 to set CROSS_COMPILE to well-known values but may have several
Sam Ravnborg910b4042007-10-19 21:46:01 +0200572 values to select between.
Geert Uytterhoeven631bcfb2007-10-21 19:55:00 +0200573 It is recommended only to try to set CROSS_COMPILE if it is a cross
574 build (host arch is different from target arch). And if CROSS_COMPILE
Sam Ravnborg910b4042007-10-19 21:46:01 +0200575 is already set then leave it with the old value.
576
577 Example:
578 #arch/m68k/Makefile
579 ifneq ($(SUBARCH),$(ARCH))
580 ifeq ($(CROSS_COMPILE),)
581 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
582 endif
583 endif
584
Sam Ravnborg691ef3e2009-09-19 10:31:45 +0200585--- 3.12 $(LD) support functions
586
587 ld-option
588 ld-option is used to check if $(LD) supports the supplied option.
589 ld-option takes two options as arguments.
590 The second argument is an optional option that can be used if the
591 first option is not supported by $(LD).
592
593 Example:
594 #Makefile
595 LDFLAGS_vmlinux += $(call really-ld-option, -X)
596
597
Linus Torvalds1da177e2005-04-16 15:20:36 -0700598=== 4 Host Program support
599
600Kbuild supports building executables on the host for use during the
601compilation stage.
602Two steps are required in order to use a host executable.
603
604The first step is to tell kbuild that a host program exists. This is
605done utilising the variable hostprogs-y.
606
607The second step is to add an explicit dependency to the executable.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700608This can be done in two ways. Either add the dependency in a rule,
Linus Torvalds1da177e2005-04-16 15:20:36 -0700609or utilise the variable $(always).
610Both possibilities are described in the following.
611
612--- 4.1 Simple Host Program
613
614 In some cases there is a need to compile and run a program on the
615 computer where the build is running.
616 The following line tells kbuild that the program bin2hex shall be
617 built on the build host.
618
619 Example:
620 hostprogs-y := bin2hex
621
622 Kbuild assumes in the above example that bin2hex is made from a single
623 c-source file named bin2hex.c located in the same directory as
624 the Makefile.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700625
Linus Torvalds1da177e2005-04-16 15:20:36 -0700626--- 4.2 Composite Host Programs
627
628 Host programs can be made up based on composite objects.
629 The syntax used to define composite objects for host programs is
630 similar to the syntax used for kernel objects.
Matt LaPlante5d3f0832006-11-30 05:21:10 +0100631 $(<executable>-objs) lists all objects used to link the final
Linus Torvalds1da177e2005-04-16 15:20:36 -0700632 executable.
633
634 Example:
635 #scripts/lxdialog/Makefile
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700636 hostprogs-y := lxdialog
Linus Torvalds1da177e2005-04-16 15:20:36 -0700637 lxdialog-objs := checklist.o lxdialog.o
638
639 Objects with extension .o are compiled from the corresponding .c
Jan Engelhardta07f6032006-07-27 22:14:29 +0200640 files. In the above example, checklist.c is compiled to checklist.o
Linus Torvalds1da177e2005-04-16 15:20:36 -0700641 and lxdialog.c is compiled to lxdialog.o.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200642 Finally, the two .o files are linked to the executable, lxdialog.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700643 Note: The syntax <executable>-y is not permitted for host-programs.
644
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700645--- 4.3 Defining shared libraries
646
Linus Torvalds1da177e2005-04-16 15:20:36 -0700647 Objects with extension .so are considered shared libraries, and
648 will be compiled as position independent objects.
649 Kbuild provides support for shared libraries, but the usage
650 shall be restricted.
651 In the following example the libkconfig.so shared library is used
652 to link the executable conf.
653
654 Example:
655 #scripts/kconfig/Makefile
656 hostprogs-y := conf
657 conf-objs := conf.o libkconfig.so
658 libkconfig-objs := expr.o type.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700659
Linus Torvalds1da177e2005-04-16 15:20:36 -0700660 Shared libraries always require a corresponding -objs line, and
661 in the example above the shared library libkconfig is composed by
662 the two objects expr.o and type.o.
663 expr.o and type.o will be built as position independent code and
664 linked as a shared library libkconfig.so. C++ is not supported for
665 shared libraries.
666
667--- 4.4 Using C++ for host programs
668
669 kbuild offers support for host programs written in C++. This was
670 introduced solely to support kconfig, and is not recommended
671 for general use.
672
673 Example:
674 #scripts/kconfig/Makefile
675 hostprogs-y := qconf
676 qconf-cxxobjs := qconf.o
677
678 In the example above the executable is composed of the C++ file
679 qconf.cc - identified by $(qconf-cxxobjs).
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700680
Linus Torvalds1da177e2005-04-16 15:20:36 -0700681 If qconf is composed by a mixture of .c and .cc files, then an
682 additional line can be used to identify this.
683
684 Example:
685 #scripts/kconfig/Makefile
686 hostprogs-y := qconf
687 qconf-cxxobjs := qconf.o
688 qconf-objs := check.o
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700689
Linus Torvalds1da177e2005-04-16 15:20:36 -0700690--- 4.5 Controlling compiler options for host programs
691
692 When compiling host programs, it is possible to set specific flags.
693 The programs will always be compiled utilising $(HOSTCC) passed
694 the options specified in $(HOSTCFLAGS).
695 To set flags that will take effect for all host programs created
Jan Engelhardta07f6032006-07-27 22:14:29 +0200696 in that Makefile, use the variable HOST_EXTRACFLAGS.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700697
698 Example:
699 #scripts/lxdialog/Makefile
700 HOST_EXTRACFLAGS += -I/usr/include/ncurses
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700701
Linus Torvalds1da177e2005-04-16 15:20:36 -0700702 To set specific flags for a single file the following construction
703 is used:
704
705 Example:
706 #arch/ppc64/boot/Makefile
707 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700708
Linus Torvalds1da177e2005-04-16 15:20:36 -0700709 It is also possible to specify additional options to the linker.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700710
Linus Torvalds1da177e2005-04-16 15:20:36 -0700711 Example:
712 #scripts/kconfig/Makefile
713 HOSTLOADLIBES_qconf := -L$(QTDIR)/lib
714
Jan Engelhardta07f6032006-07-27 22:14:29 +0200715 When linking qconf, it will be passed the extra option
716 "-L$(QTDIR)/lib".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700717
Linus Torvalds1da177e2005-04-16 15:20:36 -0700718--- 4.6 When host programs are actually built
719
720 Kbuild will only build host-programs when they are referenced
721 as a prerequisite.
722 This is possible in two ways:
723
724 (1) List the prerequisite explicitly in a special rule.
725
726 Example:
727 #drivers/pci/Makefile
728 hostprogs-y := gen-devlist
729 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
730 ( cd $(obj); ./gen-devlist ) < $<
731
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700732 The target $(obj)/devlist.h will not be built before
Linus Torvalds1da177e2005-04-16 15:20:36 -0700733 $(obj)/gen-devlist is updated. Note that references to
734 the host programs in special rules must be prefixed with $(obj).
735
736 (2) Use $(always)
737 When there is no suitable special rule, and the host program
738 shall be built when a makefile is entered, the $(always)
739 variable shall be used.
740
741 Example:
742 #scripts/lxdialog/Makefile
743 hostprogs-y := lxdialog
744 always := $(hostprogs-y)
745
746 This will tell kbuild to build lxdialog even if not referenced in
747 any rule.
748
749--- 4.7 Using hostprogs-$(CONFIG_FOO)
750
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700751 A typical pattern in a Kbuild file looks like this:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700752
753 Example:
754 #scripts/Makefile
755 hostprogs-$(CONFIG_KALLSYMS) += kallsyms
756
757 Kbuild knows about both 'y' for built-in and 'm' for module.
758 So if a config symbol evaluate to 'm', kbuild will still build
Jan Engelhardta07f6032006-07-27 22:14:29 +0200759 the binary. In other words, Kbuild handles hostprogs-m exactly
760 like hostprogs-y. But only hostprogs-y is recommended to be used
761 when no CONFIG symbols are involved.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700762
763=== 5 Kbuild clean infrastructure
764
Jan Engelhardta07f6032006-07-27 22:14:29 +0200765"make clean" deletes most generated files in the obj tree where the kernel
Linus Torvalds1da177e2005-04-16 15:20:36 -0700766is compiled. This includes generated files such as host programs.
767Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
768$(extra-y) and $(targets). They are all deleted during "make clean".
769Files matching the patterns "*.[oas]", "*.ko", plus some additional files
770generated by kbuild are deleted all over the kernel src tree when
771"make clean" is executed.
772
773Additional files can be specified in kbuild makefiles by use of $(clean-files).
774
775 Example:
776 #drivers/pci/Makefile
777 clean-files := devlist.h classlist.h
778
779When executing "make clean", the two files "devlist.h classlist.h" will
780be deleted. Kbuild will assume files to be in same relative directory as the
781Makefile except if an absolute path is specified (path starting with '/').
782
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700783To delete a directory hierarchy use:
784
Linus Torvalds1da177e2005-04-16 15:20:36 -0700785 Example:
786 #scripts/package/Makefile
787 clean-dirs := $(objtree)/debian/
788
789This will delete the directory debian, including all subdirectories.
790Kbuild will assume the directories to be in the same relative path as the
791Makefile if no absolute path is specified (path does not start with '/').
792
Michal Marekef8ff892010-03-09 16:00:20 +0100793To exclude certain files from make clean, use the $(no-clean-files) variable.
794This is only a special case used in the top level Kbuild file:
795
796 Example:
797 #Kbuild
798 no-clean-files := $(bounds-file) $(offsets-file)
799
Linus Torvalds1da177e2005-04-16 15:20:36 -0700800Usually kbuild descends down in subdirectories due to "obj-* := dir/",
801but in the architecture makefiles where the kbuild infrastructure
802is not sufficient this sometimes needs to be explicit.
803
804 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800805 #arch/x86/boot/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700806 subdir- := compressed/
807
808The above assignment instructs kbuild to descend down in the
809directory compressed/ when "make clean" is executed.
810
811To support the clean infrastructure in the Makefiles that builds the
812final bootimage there is an optional target named archclean:
813
814 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800815 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700816 archclean:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800817 $(Q)$(MAKE) $(clean)=arch/x86/boot
Linus Torvalds1da177e2005-04-16 15:20:36 -0700818
Wanlong Gao25eb6502011-06-13 17:53:53 +0800819When "make clean" is executed, make will descend down in arch/x86/boot,
820and clean as usual. The Makefile located in arch/x86/boot/ may use
Linus Torvalds1da177e2005-04-16 15:20:36 -0700821the subdir- trick to descend further down.
822
823Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
824included in the top level makefile, and the kbuild infrastructure
825is not operational at that point.
826
827Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
828be visited during "make clean".
829
830=== 6 Architecture Makefiles
831
832The top level Makefile sets up the environment and does the preparation,
833before starting to descend down in the individual directories.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200834The top level makefile contains the generic part, whereas
835arch/$(ARCH)/Makefile contains what is required to set up kbuild
836for said architecture.
837To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
Linus Torvalds1da177e2005-04-16 15:20:36 -0700838a few targets.
839
Jan Engelhardta07f6032006-07-27 22:14:29 +0200840When kbuild executes, the following steps are followed (roughly):
8411) Configuration of the kernel => produce .config
Linus Torvalds1da177e2005-04-16 15:20:36 -07008422) Store kernel version in include/linux/version.h
8433) Symlink include/asm to include/asm-$(ARCH)
8444) Updating all other prerequisites to the target prepare:
845 - Additional prerequisites are specified in arch/$(ARCH)/Makefile
8465) Recursively descend down in all directories listed in
847 init-* core* drivers-* net-* libs-* and build all targets.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200848 - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07008496) All object files are then linked and the resulting file vmlinux is
Jan Engelhardta07f6032006-07-27 22:14:29 +0200850 located at the root of the obj tree.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700851 The very first objects linked are listed in head-y, assigned by
852 arch/$(ARCH)/Makefile.
Randy Dunlap5c811e52007-02-17 20:03:14 +01008537) Finally, the architecture-specific part does any required post processing
Linus Torvalds1da177e2005-04-16 15:20:36 -0700854 and builds the final bootimage.
855 - This includes building boot records
Randy Dunlap5c811e52007-02-17 20:03:14 +0100856 - Preparing initrd images and the like
Linus Torvalds1da177e2005-04-16 15:20:36 -0700857
858
859--- 6.1 Set variables to tweak the build to the architecture
860
861 LDFLAGS Generic $(LD) options
862
863 Flags used for all invocations of the linker.
864 Often specifying the emulation is sufficient.
865
866 Example:
867 #arch/s390/Makefile
868 LDFLAGS := -m elf_s390
Sam Ravnborgf77bf012007-10-15 22:25:06 +0200869 Note: ldflags-y can be used to further customise
Sam Ravnborga9af3302007-09-30 20:08:25 +0200870 the flags used. See chapter 3.7.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700871
Linus Torvalds1da177e2005-04-16 15:20:36 -0700872 LDFLAGS_MODULE Options for $(LD) when linking modules
873
874 LDFLAGS_MODULE is used to set specific flags for $(LD) when
875 linking the .ko files used for modules.
876 Default is "-r", for relocatable output.
877
878 LDFLAGS_vmlinux Options for $(LD) when linking vmlinux
879
880 LDFLAGS_vmlinux is used to specify additional flags to pass to
Jan Engelhardta07f6032006-07-27 22:14:29 +0200881 the linker when linking the final vmlinux image.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700882 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
883
884 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800885 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700886 LDFLAGS_vmlinux := -e stext
887
888 OBJCOPYFLAGS objcopy flags
889
890 When $(call if_changed,objcopy) is used to translate a .o file,
Jan Engelhardta07f6032006-07-27 22:14:29 +0200891 the flags specified in OBJCOPYFLAGS will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700892 $(call if_changed,objcopy) is often used to generate raw binaries on
893 vmlinux.
894
895 Example:
896 #arch/s390/Makefile
897 OBJCOPYFLAGS := -O binary
898
899 #arch/s390/boot/Makefile
900 $(obj)/image: vmlinux FORCE
901 $(call if_changed,objcopy)
902
Jan Engelhardta07f6032006-07-27 22:14:29 +0200903 In this example, the binary $(obj)/image is a binary version of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700904 vmlinux. The usage of $(call if_changed,xxx) will be described later.
905
Sam Ravnborg222d3942007-10-15 21:59:31 +0200906 KBUILD_AFLAGS $(AS) assembler flags
Linus Torvalds1da177e2005-04-16 15:20:36 -0700907
908 Default value - see top level Makefile
909 Append or modify as required per architecture.
910
911 Example:
912 #arch/sparc64/Makefile
Sam Ravnborg222d3942007-10-15 21:59:31 +0200913 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
Linus Torvalds1da177e2005-04-16 15:20:36 -0700914
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200915 KBUILD_CFLAGS $(CC) compiler flags
Linus Torvalds1da177e2005-04-16 15:20:36 -0700916
917 Default value - see top level Makefile
918 Append or modify as required per architecture.
919
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200920 Often, the KBUILD_CFLAGS variable depends on the configuration.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700921
922 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +0800923 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700924 cflags-$(CONFIG_M386) += -march=i386
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200925 KBUILD_CFLAGS += $(cflags-y)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700926
927 Many arch Makefiles dynamically run the target C compiler to
928 probe supported options:
929
Wanlong Gao25eb6502011-06-13 17:53:53 +0800930 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700931
932 ...
933 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
934 -march=pentium2,-march=i686)
935 ...
936 # Disable unit-at-a-time mode ...
Sam Ravnborga0f97e02007-10-14 22:21:35 +0200937 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700938 ...
939
940
Jan Engelhardta07f6032006-07-27 22:14:29 +0200941 The first example utilises the trick that a config option expands
Linus Torvalds1da177e2005-04-16 15:20:36 -0700942 to 'y' when selected.
943
Sam Ravnborg80c00ba2010-07-28 19:11:27 +0200944 KBUILD_AFLAGS_KERNEL $(AS) options specific for built-in
Linus Torvalds1da177e2005-04-16 15:20:36 -0700945
Sam Ravnborg80c00ba2010-07-28 19:11:27 +0200946 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
Linus Torvalds1da177e2005-04-16 15:20:36 -0700947 resident kernel code.
948
Sam Ravnborg65881692010-07-28 17:33:09 +0200949 KBUILD_AFLAGS_MODULE Options for $(AS) when building modules
Linus Torvalds1da177e2005-04-16 15:20:36 -0700950
Sam Ravnborg65881692010-07-28 17:33:09 +0200951 $(KBUILD_AFLAGS_MODULE) is used to add arch specific options that
952 are used for $(AS).
953 From commandline AFLAGS_MODULE shall be used (see kbuild.txt).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700954
Sam Ravnborg80c00ba2010-07-28 19:11:27 +0200955 KBUILD_CFLAGS_KERNEL $(CC) options specific for built-in
956
957 $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
958 resident kernel code.
959
Sam Ravnborg65881692010-07-28 17:33:09 +0200960 KBUILD_CFLAGS_MODULE Options for $(CC) when building modules
961
962 $(KBUILD_CFLAGS_MODULE) is used to add arch specific options that
963 are used for $(CC).
964 From commandline CFLAGS_MODULE shall be used (see kbuild.txt).
965
966 KBUILD_LDFLAGS_MODULE Options for $(LD) when linking modules
967
968 $(KBUILD_LDFLAGS_MODULE) is used to add arch specific options
969 used when linking modules. This is often a linker script.
970 From commandline LDFLAGS_MODULE shall be used (see kbuild.txt).
Bryce Harrington39e6e9c2006-09-20 12:43:37 -0700971
Michal Marek40df7592011-04-20 13:45:30 +0200972 KBUILD_ARFLAGS Options for $(AR) when creating archives
973
974 $(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic
975 mode) if this option is supported by $(AR).
976
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200977--- 6.2 Add prerequisites to archprepare:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700978
Jan Engelhardta07f6032006-07-27 22:14:29 +0200979 The archprepare: rule is used to list prerequisites that need to be
Linus Torvalds1da177e2005-04-16 15:20:36 -0700980 built before starting to descend down in the subdirectories.
Jan Engelhardta07f6032006-07-27 22:14:29 +0200981 This is usually used for header files containing assembler constants.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700982
983 Example:
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200984 #arch/arm/Makefile
985 archprepare: maketools
Linus Torvalds1da177e2005-04-16 15:20:36 -0700986
Jan Engelhardta07f6032006-07-27 22:14:29 +0200987 In this example, the file target maketools will be processed
Sam Ravnborg5bb78262005-09-11 22:30:22 +0200988 before descending down in the subdirectories.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700989 See also chapter XXX-TODO that describe how kbuild supports
990 generating offset header files.
991
992
993--- 6.3 List directories to visit when descending
994
995 An arch Makefile cooperates with the top Makefile to define variables
996 which specify how to build the vmlinux file. Note that there is no
997 corresponding arch-specific section for modules; the module-building
998 machinery is all architecture-independent.
999
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001000
Linus Torvalds1da177e2005-04-16 15:20:36 -07001001 head-y, init-y, core-y, libs-y, drivers-y, net-y
1002
Jan Engelhardta07f6032006-07-27 22:14:29 +02001003 $(head-y) lists objects to be linked first in vmlinux.
1004 $(libs-y) lists directories where a lib.a archive can be located.
Randy Dunlap5c811e52007-02-17 20:03:14 +01001005 The rest list directories where a built-in.o object file can be
Jan Engelhardta07f6032006-07-27 22:14:29 +02001006 located.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001007
1008 $(init-y) objects will be located after $(head-y).
1009 Then the rest follows in this order:
1010 $(core-y), $(libs-y), $(drivers-y) and $(net-y).
1011
Jan Engelhardta07f6032006-07-27 22:14:29 +02001012 The top level Makefile defines values for all generic directories,
Randy Dunlap5c811e52007-02-17 20:03:14 +01001013 and arch/$(ARCH)/Makefile only adds architecture-specific directories.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001014
1015 Example:
1016 #arch/sparc64/Makefile
1017 core-y += arch/sparc64/kernel/
1018 libs-y += arch/sparc64/prom/ arch/sparc64/lib/
1019 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
1020
1021
Randy Dunlap5c811e52007-02-17 20:03:14 +01001022--- 6.4 Architecture-specific boot images
Linus Torvalds1da177e2005-04-16 15:20:36 -07001023
1024 An arch Makefile specifies goals that take the vmlinux file, compress
1025 it, wrap it in bootstrapping code, and copy the resulting files
1026 somewhere. This includes various kinds of installation commands.
1027 The actual goals are not standardized across architectures.
1028
1029 It is common to locate any additional processing in a boot/
1030 directory below arch/$(ARCH)/.
1031
1032 Kbuild does not provide any smart way to support building a
1033 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
1034 call make manually to build a target in boot/.
1035
1036 The recommended approach is to include shortcuts in
1037 arch/$(ARCH)/Makefile, and use the full path when calling down
1038 into the arch/$(ARCH)/boot/Makefile.
1039
1040 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001041 #arch/x86/Makefile
1042 boot := arch/x86/boot
Linus Torvalds1da177e2005-04-16 15:20:36 -07001043 bzImage: vmlinux
1044 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
1045
1046 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
1047 make in a subdirectory.
1048
Randy Dunlap5c811e52007-02-17 20:03:14 +01001049 There are no rules for naming architecture-specific targets,
Linus Torvalds1da177e2005-04-16 15:20:36 -07001050 but executing "make help" will list all relevant targets.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001051 To support this, $(archhelp) must be defined.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001052
1053 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001054 #arch/x86/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001055 define archhelp
1056 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001057 endif
Linus Torvalds1da177e2005-04-16 15:20:36 -07001058
1059 When make is executed without arguments, the first goal encountered
1060 will be built. In the top level Makefile the first goal present
1061 is all:.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001062 An architecture shall always, per default, build a bootable image.
1063 In "make help", the default goal is highlighted with a '*'.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001064 Add a new prerequisite to all: to select a default goal different
1065 from vmlinux.
1066
1067 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001068 #arch/x86/Makefile
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001069 all: bzImage
Linus Torvalds1da177e2005-04-16 15:20:36 -07001070
1071 When "make" is executed without arguments, bzImage will be built.
1072
1073--- 6.5 Building non-kbuild targets
1074
1075 extra-y
1076
1077 extra-y specify additional targets created in the current
1078 directory, in addition to any targets specified by obj-*.
1079
1080 Listing all targets in extra-y is required for two purposes:
1081 1) Enable kbuild to check changes in command lines
1082 - When $(call if_changed,xxx) is used
1083 2) kbuild knows what files to delete during "make clean"
1084
1085 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001086 #arch/x86/kernel/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001087 extra-y := head.o init_task.o
1088
Jan Engelhardta07f6032006-07-27 22:14:29 +02001089 In this example, extra-y is used to list object files that
Linus Torvalds1da177e2005-04-16 15:20:36 -07001090 shall be built, but shall not be linked as part of built-in.o.
1091
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001092
Linus Torvalds1da177e2005-04-16 15:20:36 -07001093--- 6.6 Commands useful for building a boot image
1094
1095 Kbuild provides a few macros that are useful when building a
1096 boot image.
1097
1098 if_changed
1099
1100 if_changed is the infrastructure used for the following commands.
1101
1102 Usage:
1103 target: source(s) FORCE
1104 $(call if_changed,ld/objcopy/gzip)
1105
Jan Engelhardta07f6032006-07-27 22:14:29 +02001106 When the rule is evaluated, it is checked to see if any files
Randy Dunlap5c811e52007-02-17 20:03:14 +01001107 need an update, or the command line has changed since the last
Linus Torvalds1da177e2005-04-16 15:20:36 -07001108 invocation. The latter will force a rebuild if any options
1109 to the executable have changed.
1110 Any target that utilises if_changed must be listed in $(targets),
1111 otherwise the command line check will fail, and the target will
1112 always be built.
1113 Assignments to $(targets) are without $(obj)/ prefix.
1114 if_changed may be used in conjunction with custom commands as
1115 defined in 6.7 "Custom kbuild commands".
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +02001116
Linus Torvalds1da177e2005-04-16 15:20:36 -07001117 Note: It is a typical mistake to forget the FORCE prerequisite.
Paolo 'Blaisorblade' Giarrusso49490572005-07-28 17:56:17 +02001118 Another common pitfall is that whitespace is sometimes
1119 significant; for instance, the below will fail (note the extra space
1120 after the comma):
1121 target: source(s) FORCE
1122 #WRONG!# $(call if_changed, ld/objcopy/gzip)
Linus Torvalds1da177e2005-04-16 15:20:36 -07001123
1124 ld
Jan Engelhardta07f6032006-07-27 22:14:29 +02001125 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001126
Linus Torvalds1da177e2005-04-16 15:20:36 -07001127 objcopy
1128 Copy binary. Uses OBJCOPYFLAGS usually specified in
1129 arch/$(ARCH)/Makefile.
1130 OBJCOPYFLAGS_$@ may be used to set additional options.
1131
1132 gzip
1133 Compress target. Use maximum compression to compress target.
1134
1135 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001136 #arch/x86/boot/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001137 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1138 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
1139
1140 targets += setup setup.o bootsect bootsect.o
1141 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1142 $(call if_changed,ld)
1143
Jan Engelhardta07f6032006-07-27 22:14:29 +02001144 In this example, there are two possible targets, requiring different
1145 options to the linker. The linker options are specified using the
Linus Torvalds1da177e2005-04-16 15:20:36 -07001146 LDFLAGS_$@ syntax - one for each potential target.
Matt LaPlante5d3f0832006-11-30 05:21:10 +01001147 $(targets) are assigned all potential targets, by which kbuild knows
Linus Torvalds1da177e2005-04-16 15:20:36 -07001148 the targets and will:
1149 1) check for commandline changes
1150 2) delete target during make clean
1151
1152 The ": %: %.o" part of the prerequisite is a shorthand that
1153 free us from listing the setup.o and bootsect.o files.
1154 Note: It is a common mistake to forget the "target :=" assignment,
1155 resulting in the target file being recompiled for no
1156 obvious reason.
1157
Dirk Brandewieaab94332010-12-22 11:57:26 -08001158 dtc
1159 Create flattend device tree blob object suitable for linking
1160 into vmlinux. Device tree blobs linked into vmlinux are placed
1161 in an init section in the image. Platform code *must* copy the
1162 blob to non-init memory prior to calling unflatten_device_tree().
1163
1164 Example:
1165 #arch/x86/platform/ce4100/Makefile
1166 clean-files := *dtb.S
1167
1168 DTC_FLAGS := -p 1024
1169 obj-y += foo.dtb.o
1170
1171 $(obj)/%.dtb: $(src)/%.dts
1172 $(call cmd,dtc)
Linus Torvalds1da177e2005-04-16 15:20:36 -07001173
1174--- 6.7 Custom kbuild commands
1175
Jan Engelhardta07f6032006-07-27 22:14:29 +02001176 When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
Linus Torvalds1da177e2005-04-16 15:20:36 -07001177 of a command is normally displayed.
1178 To enable this behaviour for custom commands kbuild requires
1179 two variables to be set:
1180 quiet_cmd_<command> - what shall be echoed
1181 cmd_<command> - the command to execute
1182
1183 Example:
1184 #
1185 quiet_cmd_image = BUILD $@
1186 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
1187 $(obj)/vmlinux.bin > $@
1188
1189 targets += bzImage
1190 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
1191 $(call if_changed,image)
1192 @echo 'Kernel: $@ is ready'
1193
Jan Engelhardta07f6032006-07-27 22:14:29 +02001194 When updating the $(obj)/bzImage target, the line
Linus Torvalds1da177e2005-04-16 15:20:36 -07001195
Wanlong Gao25eb6502011-06-13 17:53:53 +08001196 BUILD arch/x86/boot/bzImage
Linus Torvalds1da177e2005-04-16 15:20:36 -07001197
1198 will be displayed with "make KBUILD_VERBOSE=0".
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001199
Linus Torvalds1da177e2005-04-16 15:20:36 -07001200
1201--- 6.8 Preprocessing linker scripts
1202
Jan Engelhardta07f6032006-07-27 22:14:29 +02001203 When the vmlinux image is built, the linker script
Linus Torvalds1da177e2005-04-16 15:20:36 -07001204 arch/$(ARCH)/kernel/vmlinux.lds is used.
1205 The script is a preprocessed variant of the file vmlinux.lds.S
1206 located in the same directory.
Jan Engelhardta07f6032006-07-27 22:14:29 +02001207 kbuild knows .lds files and includes a rule *lds.S -> *lds.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001208
Linus Torvalds1da177e2005-04-16 15:20:36 -07001209 Example:
Wanlong Gao25eb6502011-06-13 17:53:53 +08001210 #arch/x86/kernel/Makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001211 always := vmlinux.lds
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001212
Linus Torvalds1da177e2005-04-16 15:20:36 -07001213 #Makefile
1214 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001215
1216 The assignment to $(always) is used to tell kbuild to build the
Jan Engelhardta07f6032006-07-27 22:14:29 +02001217 target vmlinux.lds.
1218 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
Linus Torvalds1da177e2005-04-16 15:20:36 -07001219 specified options when building the target vmlinux.lds.
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001220
Jan Engelhardta07f6032006-07-27 22:14:29 +02001221 When building the *.lds target, kbuild uses the variables:
Sam Ravnborg06c50402007-10-15 22:17:25 +02001222 KBUILD_CPPFLAGS : Set in top-level Makefile
Sam Ravnborgf77bf012007-10-15 22:25:06 +02001223 cppflags-y : May be set in the kbuild makefile
Linus Torvalds1da177e2005-04-16 15:20:36 -07001224 CPPFLAGS_$(@F) : Target specific flags.
1225 Note that the full filename is used in this
1226 assignment.
1227
1228 The kbuild infrastructure for *lds file are used in several
Randy Dunlap5c811e52007-02-17 20:03:14 +01001229 architecture-specific files.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001230
Sam Ravnborgd8ecc5c2011-04-27 22:29:49 +02001231--- 6.9 Generic header files
1232
1233 The directory include/asm-generic contains the header files
1234 that may be shared between individual architectures.
1235 The recommended approach how to use a generic header file is
1236 to list the file in the Kbuild file.
1237 See "7.4 generic-y" for further info on syntax etc.
1238
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001239=== 7 Kbuild syntax for exported headers
Linus Torvalds1da177e2005-04-16 15:20:36 -07001240
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001241The kernel include a set of headers that is exported to userspace.
Nicolas Kaiserc95940f2010-08-05 11:23:11 -07001242Many headers can be exported as-is but other headers require a
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001243minimal pre-processing before they are ready for user-space.
1244The pre-processing does:
1245- drop kernel specific annotations
1246- drop include of compiler.h
Nicolas Kaiserc95940f2010-08-05 11:23:11 -07001247- drop all sections that are kernel internal (guarded by ifdef __KERNEL__)
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001248
Nicolas Kaiserc95940f2010-08-05 11:23:11 -07001249Each relevant directory contains a file name "Kbuild" which specifies the
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001250headers to be exported.
1251See subsequent chapter for the syntax of the Kbuild file.
1252
1253 --- 7.1 header-y
1254
1255 header-y specify header files to be exported.
1256
1257 Example:
1258 #include/linux/Kbuild
1259 header-y += usb/
1260 header-y += aio_abi.h
1261
1262 The convention is to list one file per line and
1263 preferably in alphabetic order.
1264
1265 header-y also specify which subdirectories to visit.
1266 A subdirectory is identified by a trailing '/' which
1267 can be seen in the example above for the usb subdirectory.
1268
1269 Subdirectories are visited before their parent directories.
1270
1271 --- 7.2 objhdr-y
1272
1273 objhdr-y specifies generated files to be exported.
1274 Generated files are special as they need to be looked
1275 up in another directory when doing 'make O=...' builds.
1276
1277 Example:
1278 #include/linux/Kbuild
1279 objhdr-y += version.h
1280
1281 --- 7.3 destination-y
1282
1283 When an architecture have a set of exported headers that needs to be
1284 exported to a different directory destination-y is used.
1285 destination-y specify the destination directory for all exported
1286 headers in the file where it is present.
1287
1288 Example:
1289 #arch/xtensa/platforms/s6105/include/platform/Kbuild
1290 destination-y := include/linux
1291
1292 In the example above all exported headers in the Kbuild file
1293 will be located in the directory "include/linux" when exported.
1294
Sam Ravnborgd8ecc5c2011-04-27 22:29:49 +02001295 --- 7.4 generic-y
1296
1297 If an architecture uses a verbatim copy of a header from
1298 include/asm-generic then this is listed in the file
1299 arch/$(ARCH)/include/asm/Kbuild like this:
1300
1301 Example:
1302 #arch/x86/include/asm/Kbuild
1303 generic-y += termios.h
1304 generic-y += rtc.h
1305
1306 During the prepare phase of the build a wrapper include
1307 file is generated in the directory:
1308
1309 arch/$(ARCH)/include/generated/asm
1310
1311 When a header is exported where the architecture uses
1312 the generic header a similar wrapper is generated as part
1313 of the set of exported headers in the directory:
1314
1315 usr/include/asm
1316
1317 The generated wrapper will in both cases look like the following:
1318
1319 Example: termios.h
1320 #include <asm-generic/termios.h>
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001321
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001322=== 8 Kbuild Variables
Linus Torvalds1da177e2005-04-16 15:20:36 -07001323
1324The top Makefile exports the following variables:
1325
1326 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1327
1328 These variables define the current kernel version. A few arch
1329 Makefiles actually use these values directly; they should use
1330 $(KERNELRELEASE) instead.
1331
1332 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1333 three-part version number, such as "2", "4", and "0". These three
1334 values are always numeric.
1335
1336 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1337 or additional patches. It is usually some non-numeric string
1338 such as "-pre4", and is often blank.
1339
1340 KERNELRELEASE
1341
1342 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1343 for constructing installation directory names or showing in
1344 version strings. Some arch Makefiles use it for this purpose.
1345
1346 ARCH
1347
1348 This variable defines the target architecture, such as "i386",
1349 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1350 determine which files to compile.
1351
1352 By default, the top Makefile sets $(ARCH) to be the same as the
1353 host system architecture. For a cross build, a user may
1354 override the value of $(ARCH) on the command line:
1355
1356 make ARCH=m68k ...
1357
1358
1359 INSTALL_PATH
1360
1361 This variable defines a place for the arch Makefiles to install
1362 the resident kernel image and System.map file.
Randy Dunlap5c811e52007-02-17 20:03:14 +01001363 Use this for architecture-specific install targets.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001364
1365 INSTALL_MOD_PATH, MODLIB
1366
1367 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1368 installation. This variable is not defined in the Makefile but
1369 may be passed in by the user if desired.
1370
1371 $(MODLIB) specifies the directory for module installation.
1372 The top Makefile defines $(MODLIB) to
1373 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1374 override this value on the command line if desired.
1375
Theodore Ts'oac031f22006-06-21 20:53:09 -04001376 INSTALL_MOD_STRIP
1377
1378 If this variable is specified, will cause modules to be stripped
1379 after they are installed. If INSTALL_MOD_STRIP is '1', then the
1380 default option --strip-debug will be used. Otherwise,
Gilles Espinasse177b2412011-01-09 08:59:49 +01001381 INSTALL_MOD_STRIP value will be used as the option(s) to the strip
1382 command.
Theodore Ts'oac031f22006-06-21 20:53:09 -04001383
1384
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001385=== 9 Makefile language
Linus Torvalds1da177e2005-04-16 15:20:36 -07001386
Jan Engelhardta07f6032006-07-27 22:14:29 +02001387The kernel Makefiles are designed to be run with GNU Make. The Makefiles
Linus Torvalds1da177e2005-04-16 15:20:36 -07001388use only the documented features of GNU Make, but they do use many
1389GNU extensions.
1390
1391GNU Make supports elementary list-processing functions. The kernel
1392Makefiles use a novel style of list building and manipulation with few
1393"if" statements.
1394
1395GNU Make has two assignment operators, ":=" and "=". ":=" performs
1396immediate evaluation of the right-hand side and stores an actual string
1397into the left-hand side. "=" is like a formula definition; it stores the
1398right-hand side in an unevaluated form and then evaluates this form each
1399time the left-hand side is used.
1400
1401There are some cases where "=" is appropriate. Usually, though, ":="
1402is the right choice.
1403
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001404=== 10 Credits
Linus Torvalds1da177e2005-04-16 15:20:36 -07001405
1406Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1407Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1408Updates by Sam Ravnborg <sam@ravnborg.org>
Jan Engelhardta07f6032006-07-27 22:14:29 +02001409Language QA by Jan Engelhardt <jengelh@gmx.de>
Linus Torvalds1da177e2005-04-16 15:20:36 -07001410
Sam Ravnborgc7bb3492009-04-10 08:52:43 +02001411=== 11 TODO
Linus Torvalds1da177e2005-04-16 15:20:36 -07001412
Jan Engelhardta07f6032006-07-27 22:14:29 +02001413- Describe how kbuild supports shipped files with _shipped.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001414- Generating offset header files.
1415- Add more variables to section 7?
1416
Bryce Harrington39e6e9c2006-09-20 12:43:37 -07001417
1418