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1 | Linux Kernel Makefiles |
2 | ||
3 | This 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 | |
20a468b5 | 20 | --- 3.11 $(CC) support functions |
691ef3e7 | 21 | --- 3.12 $(LD) support functions |
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22 | |
23 | === 4 Host Program support | |
24 | --- 4.1 Simple Host Program | |
25 | --- 4.2 Composite Host Programs | |
39e6e9cf | 26 | --- 4.3 Defining shared libraries |
1da177e4 LT |
27 | --- 4.4 Using C++ for host programs |
28 | --- 4.5 Controlling compiler options for host programs | |
29 | --- 4.6 When host programs are actually built | |
30 | --- 4.7 Using hostprogs-$(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 | |
5bb78269 | 36 | --- 6.2 Add prerequisites to archprepare: |
1da177e4 | 37 | --- 6.3 List directories to visit when descending |
5c811e59 | 38 | --- 6.4 Architecture-specific boot images |
1da177e4 LT |
39 | --- 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 | |
1da177e4 | 43 | |
c7bb349e SR |
44 | === 7 Kbuild syntax for exported headers |
45 | --- 7.1 header-y | |
46 | --- 7.2 objhdr-y | |
47 | --- 7.3 destination-y | |
c7bb349e SR |
48 | |
49 | === 8 Kbuild Variables | |
50 | === 9 Makefile language | |
51 | === 10 Credits | |
52 | === 11 TODO | |
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53 | |
54 | === 1 Overview | |
55 | ||
56 | The Makefiles have five parts: | |
57 | ||
58 | Makefile the top Makefile. | |
59 | .config the kernel configuration file. | |
60 | arch/$(ARCH)/Makefile the arch Makefile. | |
61 | scripts/Makefile.* common rules etc. for all kbuild Makefiles. | |
62 | kbuild Makefiles there are about 500 of these. | |
63 | ||
64 | The top Makefile reads the .config file, which comes from the kernel | |
65 | configuration process. | |
66 | ||
67 | The top Makefile is responsible for building two major products: vmlinux | |
68 | (the resident kernel image) and modules (any module files). | |
69 | It builds these goals by recursively descending into the subdirectories of | |
70 | the kernel source tree. | |
71 | The list of subdirectories which are visited depends upon the kernel | |
72 | configuration. The top Makefile textually includes an arch Makefile | |
73 | with the name arch/$(ARCH)/Makefile. The arch Makefile supplies | |
74 | architecture-specific information to the top Makefile. | |
75 | ||
76 | Each subdirectory has a kbuild Makefile which carries out the commands | |
77 | passed down from above. The kbuild Makefile uses information from the | |
39e6e9cf | 78 | .config file to construct various file lists used by kbuild to build |
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79 | any built-in or modular targets. |
80 | ||
81 | scripts/Makefile.* contains all the definitions/rules etc. that | |
82 | are used to build the kernel based on the kbuild makefiles. | |
83 | ||
84 | ||
85 | === 2 Who does what | |
86 | ||
87 | People have four different relationships with the kernel Makefiles. | |
88 | ||
89 | *Users* are people who build kernels. These people type commands such as | |
90 | "make menuconfig" or "make". They usually do not read or edit | |
91 | any kernel Makefiles (or any other source files). | |
92 | ||
93 | *Normal developers* are people who work on features such as device | |
94 | drivers, file systems, and network protocols. These people need to | |
a07f6033 | 95 | maintain the kbuild Makefiles for the subsystem they are |
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96 | working on. In order to do this effectively, they need some overall |
97 | knowledge about the kernel Makefiles, plus detailed knowledge about the | |
98 | public interface for kbuild. | |
99 | ||
100 | *Arch developers* are people who work on an entire architecture, such | |
101 | as sparc or ia64. Arch developers need to know about the arch Makefile | |
102 | as well as kbuild Makefiles. | |
103 | ||
104 | *Kbuild developers* are people who work on the kernel build system itself. | |
105 | These people need to know about all aspects of the kernel Makefiles. | |
106 | ||
107 | This document is aimed towards normal developers and arch developers. | |
108 | ||
109 | ||
110 | === 3 The kbuild files | |
111 | ||
112 | Most Makefiles within the kernel are kbuild Makefiles that use the | |
a07f6033 | 113 | kbuild infrastructure. This chapter introduces the syntax used in the |
1da177e4 | 114 | kbuild makefiles. |
172c3ae3 | 115 | The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can |
a07f6033 | 116 | be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild' |
172c3ae3 | 117 | file will be used. |
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118 | |
119 | Section 3.1 "Goal definitions" is a quick intro, further chapters provide | |
120 | more details, with real examples. | |
121 | ||
122 | --- 3.1 Goal definitions | |
123 | ||
124 | Goal definitions are the main part (heart) of the kbuild Makefile. | |
125 | These lines define the files to be built, any special compilation | |
126 | options, and any subdirectories to be entered recursively. | |
127 | ||
128 | The most simple kbuild makefile contains one line: | |
129 | ||
130 | Example: | |
131 | obj-y += foo.o | |
132 | ||
5c811e59 | 133 | This tells kbuild that there is one object in that directory, named |
1da177e4 LT |
134 | foo.o. foo.o will be built from foo.c or foo.S. |
135 | ||
136 | If foo.o shall be built as a module, the variable obj-m is used. | |
137 | Therefore the following pattern is often used: | |
138 | ||
139 | Example: | |
140 | obj-$(CONFIG_FOO) += foo.o | |
141 | ||
142 | $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module). | |
143 | If CONFIG_FOO is neither y nor m, then the file will not be compiled | |
144 | nor linked. | |
145 | ||
146 | --- 3.2 Built-in object goals - obj-y | |
147 | ||
148 | The kbuild Makefile specifies object files for vmlinux | |
a07f6033 | 149 | in the $(obj-y) lists. These lists depend on the kernel |
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150 | configuration. |
151 | ||
152 | Kbuild compiles all the $(obj-y) files. It then calls | |
153 | "$(LD) -r" to merge these files into one built-in.o file. | |
154 | built-in.o is later linked into vmlinux by the parent Makefile. | |
155 | ||
156 | The order of files in $(obj-y) is significant. Duplicates in | |
157 | the lists are allowed: the first instance will be linked into | |
158 | built-in.o and succeeding instances will be ignored. | |
159 | ||
160 | Link order is significant, because certain functions | |
161 | (module_init() / __initcall) will be called during boot in the | |
162 | order they appear. So keep in mind that changing the link | |
a07f6033 JE |
163 | order may e.g. change the order in which your SCSI |
164 | controllers are detected, and thus your disks are renumbered. | |
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165 | |
166 | Example: | |
167 | #drivers/isdn/i4l/Makefile | |
168 | # Makefile for the kernel ISDN subsystem and device drivers. | |
169 | # Each configuration option enables a list of files. | |
2f5a2f81 | 170 | obj-$(CONFIG_ISDN_I4L) += isdn.o |
1da177e4 LT |
171 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o |
172 | ||
173 | --- 3.3 Loadable module goals - obj-m | |
174 | ||
175 | $(obj-m) specify object files which are built as loadable | |
176 | kernel modules. | |
177 | ||
178 | A module may be built from one source file or several source | |
179 | files. In the case of one source file, the kbuild makefile | |
180 | simply adds the file to $(obj-m). | |
181 | ||
182 | Example: | |
183 | #drivers/isdn/i4l/Makefile | |
184 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o | |
185 | ||
186 | Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm' | |
187 | ||
188 | If a kernel module is built from several source files, you specify | |
4f827280 MM |
189 | that you want to build a module in the same way as above; however, |
190 | kbuild needs to know which object files you want to build your | |
191 | module from, so you have to tell it by setting a $(<module_name>-y) | |
192 | variable. | |
1da177e4 LT |
193 | |
194 | Example: | |
195 | #drivers/isdn/i4l/Makefile | |
4f827280 MM |
196 | obj-$(CONFIG_ISDN_I4L) += isdn.o |
197 | isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o | |
1da177e4 LT |
198 | |
199 | In this example, the module name will be isdn.o. Kbuild will | |
4f827280 | 200 | compile the objects listed in $(isdn-y) and then run |
1da177e4 LT |
201 | "$(LD) -r" on the list of these files to generate isdn.o. |
202 | ||
4f827280 MM |
203 | Due to kbuild recognizing $(<module_name>-y) for composite objects, |
204 | you can use the value of a CONFIG_ symbol to optionally include an | |
205 | object file as part of a composite object. | |
1da177e4 LT |
206 | |
207 | Example: | |
208 | #fs/ext2/Makefile | |
4f827280 MM |
209 | obj-$(CONFIG_EXT2_FS) += ext2.o |
210 | ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \ | |
211 | namei.o super.o symlink.o | |
212 | ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \ | |
213 | xattr_trusted.o | |
214 | ||
215 | In this example, xattr.o, xattr_user.o and xattr_trusted.o are only | |
216 | part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR) | |
217 | evaluates to 'y'. | |
1da177e4 LT |
218 | |
219 | Note: Of course, when you are building objects into the kernel, | |
220 | the syntax above will also work. So, if you have CONFIG_EXT2_FS=y, | |
221 | kbuild will build an ext2.o file for you out of the individual | |
222 | parts and then link this into built-in.o, as you would expect. | |
223 | ||
224 | --- 3.4 Objects which export symbols | |
225 | ||
226 | No special notation is required in the makefiles for | |
227 | modules exporting symbols. | |
228 | ||
229 | --- 3.5 Library file goals - lib-y | |
230 | ||
a07f6033 | 231 | Objects listed with obj-* are used for modules, or |
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232 | combined in a built-in.o for that specific directory. |
233 | There is also the possibility to list objects that will | |
234 | be included in a library, lib.a. | |
235 | All objects listed with lib-y are combined in a single | |
236 | library for that directory. | |
5d3f083d ML |
237 | Objects that are listed in obj-y and additionally listed in |
238 | lib-y will not be included in the library, since they will | |
239 | be accessible anyway. | |
a07f6033 | 240 | For consistency, objects listed in lib-m will be included in lib.a. |
1da177e4 LT |
241 | |
242 | Note that the same kbuild makefile may list files to be built-in | |
243 | and to be part of a library. Therefore the same directory | |
244 | may contain both a built-in.o and a lib.a file. | |
245 | ||
246 | Example: | |
2f5a2f81 MM |
247 | #arch/x86/lib/Makefile |
248 | lib-y := delay.o | |
1da177e4 | 249 | |
2f5a2f81 MM |
250 | This will create a library lib.a based on delay.o. For kbuild to |
251 | actually recognize that there is a lib.a being built, the directory | |
252 | shall be listed in libs-y. | |
1da177e4 | 253 | See also "6.3 List directories to visit when descending". |
39e6e9cf | 254 | |
a07f6033 | 255 | Use of lib-y is normally restricted to lib/ and arch/*/lib. |
1da177e4 LT |
256 | |
257 | --- 3.6 Descending down in directories | |
258 | ||
259 | A Makefile is only responsible for building objects in its own | |
260 | directory. Files in subdirectories should be taken care of by | |
261 | Makefiles in these subdirs. The build system will automatically | |
262 | invoke make recursively in subdirectories, provided you let it know of | |
263 | them. | |
264 | ||
a07f6033 | 265 | To do so, obj-y and obj-m are used. |
1da177e4 LT |
266 | ext2 lives in a separate directory, and the Makefile present in fs/ |
267 | tells kbuild to descend down using the following assignment. | |
268 | ||
269 | Example: | |
270 | #fs/Makefile | |
271 | obj-$(CONFIG_EXT2_FS) += ext2/ | |
272 | ||
273 | If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular) | |
274 | the corresponding obj- variable will be set, and kbuild will descend | |
275 | down in the ext2 directory. | |
276 | Kbuild only uses this information to decide that it needs to visit | |
277 | the directory, it is the Makefile in the subdirectory that | |
278 | specifies what is modules and what is built-in. | |
279 | ||
280 | It is good practice to use a CONFIG_ variable when assigning directory | |
281 | names. This allows kbuild to totally skip the directory if the | |
282 | corresponding CONFIG_ option is neither 'y' nor 'm'. | |
283 | ||
284 | --- 3.7 Compilation flags | |
285 | ||
f77bf014 | 286 | ccflags-y, asflags-y and ldflags-y |
c95940f2 NK |
287 | These three flags apply only to the kbuild makefile in which they |
288 | are assigned. They are used for all the normal cc, as and ld | |
289 | invocations happening during a recursive build. | |
f77bf014 | 290 | Note: Flags with the same behaviour were previously named: |
c95940f2 NK |
291 | EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS. |
292 | They are still supported but their usage is deprecated. | |
1da177e4 | 293 | |
eb07e1b4 | 294 | ccflags-y specifies options for compiling with $(CC). |
1da177e4 LT |
295 | |
296 | Example: | |
eb07e1b4 MM |
297 | # drivers/acpi/Makefile |
298 | ccflags-y := -Os | |
299 | ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT | |
1da177e4 LT |
300 | |
301 | This variable is necessary because the top Makefile owns the | |
a0f97e06 | 302 | variable $(KBUILD_CFLAGS) and uses it for compilation flags for the |
1da177e4 LT |
303 | entire tree. |
304 | ||
eb07e1b4 | 305 | asflags-y specifies options for assembling with $(AS). |
1da177e4 LT |
306 | |
307 | Example: | |
eb07e1b4 MM |
308 | #arch/sparc/kernel/Makefile |
309 | asflags-y := -ansi | |
1da177e4 | 310 | |
eb07e1b4 | 311 | ldflags-y specifies options for linking with $(LD). |
1da177e4 LT |
312 | |
313 | Example: | |
eb07e1b4 MM |
314 | #arch/cris/boot/compressed/Makefile |
315 | ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds | |
1da177e4 | 316 | |
720097d8 | 317 | subdir-ccflags-y, subdir-asflags-y |
eb07e1b4 | 318 | The two flags listed above are similar to ccflags-y and asflags-y. |
c95940f2 NK |
319 | The difference is that the subdir- variants have effect for the kbuild |
320 | file where they are present and all subdirectories. | |
321 | Options specified using subdir-* are added to the commandline before | |
322 | the options specified using the non-subdir variants. | |
720097d8 SR |
323 | |
324 | Example: | |
325 | subdir-ccflags-y := -Werror | |
326 | ||
1da177e4 LT |
327 | CFLAGS_$@, AFLAGS_$@ |
328 | ||
329 | CFLAGS_$@ and AFLAGS_$@ only apply to commands in current | |
330 | kbuild makefile. | |
331 | ||
332 | $(CFLAGS_$@) specifies per-file options for $(CC). The $@ | |
333 | part has a literal value which specifies the file that it is for. | |
334 | ||
335 | Example: | |
336 | # drivers/scsi/Makefile | |
337 | CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF | |
338 | CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \ | |
339 | -DGDTH_STATISTICS | |
1da177e4 | 340 | |
eb07e1b4 | 341 | These two lines specify compilation flags for aha152x.o and gdth.o. |
1da177e4 LT |
342 | |
343 | $(AFLAGS_$@) is a similar feature for source files in assembly | |
344 | languages. | |
345 | ||
346 | Example: | |
347 | # arch/arm/kernel/Makefile | |
eb07e1b4 MM |
348 | AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET) |
349 | AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312 | |
350 | AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt | |
351 | ||
1da177e4 LT |
352 | |
353 | --- 3.9 Dependency tracking | |
354 | ||
355 | Kbuild tracks dependencies on the following: | |
356 | 1) All prerequisite files (both *.c and *.h) | |
357 | 2) CONFIG_ options used in all prerequisite files | |
358 | 3) Command-line used to compile target | |
359 | ||
360 | Thus, if you change an option to $(CC) all affected files will | |
361 | be re-compiled. | |
362 | ||
363 | --- 3.10 Special Rules | |
364 | ||
365 | Special rules are used when the kbuild infrastructure does | |
366 | not provide the required support. A typical example is | |
367 | header files generated during the build process. | |
5c811e59 | 368 | Another example are the architecture-specific Makefiles which |
a07f6033 | 369 | need special rules to prepare boot images etc. |
1da177e4 LT |
370 | |
371 | Special rules are written as normal Make rules. | |
372 | Kbuild is not executing in the directory where the Makefile is | |
373 | located, so all special rules shall provide a relative | |
374 | path to prerequisite files and target files. | |
375 | ||
376 | Two variables are used when defining special rules: | |
377 | ||
378 | $(src) | |
379 | $(src) is a relative path which points to the directory | |
380 | where the Makefile is located. Always use $(src) when | |
381 | referring to files located in the src tree. | |
382 | ||
383 | $(obj) | |
384 | $(obj) is a relative path which points to the directory | |
385 | where the target is saved. Always use $(obj) when | |
386 | referring to generated files. | |
387 | ||
388 | Example: | |
389 | #drivers/scsi/Makefile | |
390 | $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl | |
391 | $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl | |
392 | ||
393 | This is a special rule, following the normal syntax | |
394 | required by make. | |
395 | The target file depends on two prerequisite files. References | |
396 | to the target file are prefixed with $(obj), references | |
397 | to prerequisites are referenced with $(src) (because they are not | |
398 | generated files). | |
399 | ||
5410ecc0 MF |
400 | $(kecho) |
401 | echoing information to user in a rule is often a good practice | |
402 | but when execution "make -s" one does not expect to see any output | |
403 | except for warnings/errors. | |
404 | To support this kbuild define $(kecho) which will echo out the | |
405 | text following $(kecho) to stdout except if "make -s" is used. | |
406 | ||
407 | Example: | |
408 | #arch/blackfin/boot/Makefile | |
409 | $(obj)/vmImage: $(obj)/vmlinux.gz | |
410 | $(call if_changed,uimage) | |
411 | @$(kecho) 'Kernel: $@ is ready' | |
412 | ||
413 | ||
20a468b5 SR |
414 | --- 3.11 $(CC) support functions |
415 | ||
a07f6033 | 416 | The kernel may be built with several different versions of |
20a468b5 SR |
417 | $(CC), each supporting a unique set of features and options. |
418 | kbuild provide basic support to check for valid options for $(CC). | |
e95be9a5 | 419 | $(CC) is usually the gcc compiler, but other alternatives are |
20a468b5 SR |
420 | available. |
421 | ||
422 | as-option | |
a07f6033 JE |
423 | as-option is used to check if $(CC) -- when used to compile |
424 | assembler (*.S) files -- supports the given option. An optional | |
425 | second option may be specified if the first option is not supported. | |
20a468b5 SR |
426 | |
427 | Example: | |
428 | #arch/sh/Makefile | |
429 | cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) | |
430 | ||
a07f6033 | 431 | In the above example, cflags-y will be assigned the option |
20a468b5 SR |
432 | -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). |
433 | The second argument is optional, and if supplied will be used | |
434 | if first argument is not supported. | |
435 | ||
f86fd306 SR |
436 | cc-ldoption |
437 | cc-ldoption is used to check if $(CC) when used to link object files | |
0b0bf7a3 RM |
438 | supports the given option. An optional second option may be |
439 | specified if first option are not supported. | |
440 | ||
441 | Example: | |
442 | #arch/i386/kernel/Makefile | |
f86fd306 | 443 | vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) |
0b0bf7a3 | 444 | |
5c811e59 | 445 | In the above example, vsyscall-flags will be assigned the option |
0b0bf7a3 RM |
446 | -Wl$(comma)--hash-style=sysv if it is supported by $(CC). |
447 | The second argument is optional, and if supplied will be used | |
448 | if first argument is not supported. | |
449 | ||
e2414910 AK |
450 | as-instr |
451 | as-instr checks if the assembler reports a specific instruction | |
452 | and then outputs either option1 or option2 | |
453 | C escapes are supported in the test instruction | |
222d394d | 454 | Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options |
e2414910 | 455 | |
20a468b5 | 456 | cc-option |
a07f6033 | 457 | cc-option is used to check if $(CC) supports a given option, and not |
20a468b5 SR |
458 | supported to use an optional second option. |
459 | ||
460 | Example: | |
461 | #arch/i386/Makefile | |
462 | cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) | |
463 | ||
5c811e59 | 464 | In the above example, cflags-y will be assigned the option |
a07f6033 JE |
465 | -march=pentium-mmx if supported by $(CC), otherwise -march=i586. |
466 | The second argument to cc-option is optional, and if omitted, | |
20a468b5 | 467 | cflags-y will be assigned no value if first option is not supported. |
a0f97e06 | 468 | Note: cc-option uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
469 | |
470 | cc-option-yn | |
39e6e9cf | 471 | cc-option-yn is used to check if gcc supports a given option |
20a468b5 SR |
472 | and return 'y' if supported, otherwise 'n'. |
473 | ||
474 | Example: | |
475 | #arch/ppc/Makefile | |
476 | biarch := $(call cc-option-yn, -m32) | |
477 | aflags-$(biarch) += -a32 | |
478 | cflags-$(biarch) += -m32 | |
39e6e9cf | 479 | |
a07f6033 JE |
480 | In the above example, $(biarch) is set to y if $(CC) supports the -m32 |
481 | option. When $(biarch) equals 'y', the expanded variables $(aflags-y) | |
482 | and $(cflags-y) will be assigned the values -a32 and -m32, | |
483 | respectively. | |
a0f97e06 | 484 | Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
485 | |
486 | cc-option-align | |
a07f6033 JE |
487 | gcc versions >= 3.0 changed the type of options used to specify |
488 | alignment of functions, loops etc. $(cc-option-align), when used | |
489 | as prefix to the align options, will select the right prefix: | |
20a468b5 SR |
490 | gcc < 3.00 |
491 | cc-option-align = -malign | |
492 | gcc >= 3.00 | |
493 | cc-option-align = -falign | |
39e6e9cf | 494 | |
20a468b5 | 495 | Example: |
a0f97e06 | 496 | KBUILD_CFLAGS += $(cc-option-align)-functions=4 |
20a468b5 | 497 | |
a07f6033 JE |
498 | In the above example, the option -falign-functions=4 is used for |
499 | gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used. | |
a0f97e06 | 500 | Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options |
39e6e9cf | 501 | |
20a468b5 | 502 | cc-version |
a07f6033 | 503 | cc-version returns a numerical version of the $(CC) compiler version. |
20a468b5 SR |
504 | The format is <major><minor> where both are two digits. So for example |
505 | gcc 3.41 would return 0341. | |
506 | cc-version is useful when a specific $(CC) version is faulty in one | |
a07f6033 | 507 | area, for example -mregparm=3 was broken in some gcc versions |
20a468b5 SR |
508 | even though the option was accepted by gcc. |
509 | ||
510 | Example: | |
511 | #arch/i386/Makefile | |
512 | cflags-y += $(shell \ | |
513 | if [ $(call cc-version) -ge 0300 ] ; then \ | |
514 | echo "-mregparm=3"; fi ;) | |
515 | ||
a07f6033 | 516 | In the above example, -mregparm=3 is only used for gcc version greater |
20a468b5 SR |
517 | than or equal to gcc 3.0. |
518 | ||
519 | cc-ifversion | |
a07f6033 | 520 | cc-ifversion tests the version of $(CC) and equals last argument if |
20a468b5 SR |
521 | version expression is true. |
522 | ||
523 | Example: | |
524 | #fs/reiserfs/Makefile | |
f77bf014 | 525 | ccflags-y := $(call cc-ifversion, -lt, 0402, -O1) |
20a468b5 | 526 | |
f77bf014 | 527 | In this example, ccflags-y will be assigned the value -O1 if the |
20a468b5 | 528 | $(CC) version is less than 4.2. |
39e6e9cf | 529 | cc-ifversion takes all the shell operators: |
20a468b5 SR |
530 | -eq, -ne, -lt, -le, -gt, and -ge |
531 | The third parameter may be a text as in this example, but it may also | |
532 | be an expanded variable or a macro. | |
533 | ||
7015030f SR |
534 | cc-fullversion |
535 | cc-fullversion is useful when the exact version of gcc is needed. | |
536 | One typical use-case is when a specific GCC version is broken. | |
537 | cc-fullversion points out a more specific version than cc-version does. | |
538 | ||
539 | Example: | |
540 | #arch/powerpc/Makefile | |
541 | $(Q)if test "$(call cc-fullversion)" = "040200" ; then \ | |
542 | echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \ | |
543 | false ; \ | |
544 | fi | |
545 | ||
546 | In this example for a specific GCC version the build will error out explaining | |
547 | to the user why it stops. | |
1da177e4 | 548 | |
910b4046 | 549 | cc-cross-prefix |
631bcfbb | 550 | cc-cross-prefix is used to check if there exists a $(CC) in path with |
910b4046 SR |
551 | one of the listed prefixes. The first prefix where there exist a |
552 | prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found | |
553 | then nothing is returned. | |
554 | Additional prefixes are separated by a single space in the | |
555 | call of cc-cross-prefix. | |
631bcfbb GU |
556 | This functionality is useful for architecture Makefiles that try |
557 | to set CROSS_COMPILE to well-known values but may have several | |
910b4046 | 558 | values to select between. |
631bcfbb GU |
559 | It is recommended only to try to set CROSS_COMPILE if it is a cross |
560 | build (host arch is different from target arch). And if CROSS_COMPILE | |
910b4046 SR |
561 | is already set then leave it with the old value. |
562 | ||
563 | Example: | |
564 | #arch/m68k/Makefile | |
565 | ifneq ($(SUBARCH),$(ARCH)) | |
566 | ifeq ($(CROSS_COMPILE),) | |
567 | CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) | |
568 | endif | |
569 | endif | |
570 | ||
691ef3e7 SR |
571 | --- 3.12 $(LD) support functions |
572 | ||
573 | ld-option | |
574 | ld-option is used to check if $(LD) supports the supplied option. | |
575 | ld-option takes two options as arguments. | |
576 | The second argument is an optional option that can be used if the | |
577 | first option is not supported by $(LD). | |
578 | ||
579 | Example: | |
580 | #Makefile | |
581 | LDFLAGS_vmlinux += $(call really-ld-option, -X) | |
582 | ||
583 | ||
1da177e4 LT |
584 | === 4 Host Program support |
585 | ||
586 | Kbuild supports building executables on the host for use during the | |
587 | compilation stage. | |
588 | Two steps are required in order to use a host executable. | |
589 | ||
590 | The first step is to tell kbuild that a host program exists. This is | |
591 | done utilising the variable hostprogs-y. | |
592 | ||
593 | The second step is to add an explicit dependency to the executable. | |
39e6e9cf | 594 | This can be done in two ways. Either add the dependency in a rule, |
1da177e4 LT |
595 | or utilise the variable $(always). |
596 | Both possibilities are described in the following. | |
597 | ||
598 | --- 4.1 Simple Host Program | |
599 | ||
600 | In some cases there is a need to compile and run a program on the | |
601 | computer where the build is running. | |
602 | The following line tells kbuild that the program bin2hex shall be | |
603 | built on the build host. | |
604 | ||
605 | Example: | |
606 | hostprogs-y := bin2hex | |
607 | ||
608 | Kbuild assumes in the above example that bin2hex is made from a single | |
609 | c-source file named bin2hex.c located in the same directory as | |
610 | the Makefile. | |
39e6e9cf | 611 | |
1da177e4 LT |
612 | --- 4.2 Composite Host Programs |
613 | ||
614 | Host programs can be made up based on composite objects. | |
615 | The syntax used to define composite objects for host programs is | |
616 | similar to the syntax used for kernel objects. | |
5d3f083d | 617 | $(<executable>-objs) lists all objects used to link the final |
1da177e4 LT |
618 | executable. |
619 | ||
620 | Example: | |
621 | #scripts/lxdialog/Makefile | |
39e6e9cf | 622 | hostprogs-y := lxdialog |
1da177e4 LT |
623 | lxdialog-objs := checklist.o lxdialog.o |
624 | ||
625 | Objects with extension .o are compiled from the corresponding .c | |
a07f6033 | 626 | files. In the above example, checklist.c is compiled to checklist.o |
1da177e4 | 627 | and lxdialog.c is compiled to lxdialog.o. |
a07f6033 | 628 | Finally, the two .o files are linked to the executable, lxdialog. |
1da177e4 LT |
629 | Note: The syntax <executable>-y is not permitted for host-programs. |
630 | ||
39e6e9cf BH |
631 | --- 4.3 Defining shared libraries |
632 | ||
1da177e4 LT |
633 | Objects with extension .so are considered shared libraries, and |
634 | will be compiled as position independent objects. | |
635 | Kbuild provides support for shared libraries, but the usage | |
636 | shall be restricted. | |
637 | In the following example the libkconfig.so shared library is used | |
638 | to link the executable conf. | |
639 | ||
640 | Example: | |
641 | #scripts/kconfig/Makefile | |
642 | hostprogs-y := conf | |
643 | conf-objs := conf.o libkconfig.so | |
644 | libkconfig-objs := expr.o type.o | |
39e6e9cf | 645 | |
1da177e4 LT |
646 | Shared libraries always require a corresponding -objs line, and |
647 | in the example above the shared library libkconfig is composed by | |
648 | the two objects expr.o and type.o. | |
649 | expr.o and type.o will be built as position independent code and | |
650 | linked as a shared library libkconfig.so. C++ is not supported for | |
651 | shared libraries. | |
652 | ||
653 | --- 4.4 Using C++ for host programs | |
654 | ||
655 | kbuild offers support for host programs written in C++. This was | |
656 | introduced solely to support kconfig, and is not recommended | |
657 | for general use. | |
658 | ||
659 | Example: | |
660 | #scripts/kconfig/Makefile | |
661 | hostprogs-y := qconf | |
662 | qconf-cxxobjs := qconf.o | |
663 | ||
664 | In the example above the executable is composed of the C++ file | |
665 | qconf.cc - identified by $(qconf-cxxobjs). | |
39e6e9cf | 666 | |
1da177e4 LT |
667 | If qconf is composed by a mixture of .c and .cc files, then an |
668 | additional line can be used to identify this. | |
669 | ||
670 | Example: | |
671 | #scripts/kconfig/Makefile | |
672 | hostprogs-y := qconf | |
673 | qconf-cxxobjs := qconf.o | |
674 | qconf-objs := check.o | |
39e6e9cf | 675 | |
1da177e4 LT |
676 | --- 4.5 Controlling compiler options for host programs |
677 | ||
678 | When compiling host programs, it is possible to set specific flags. | |
679 | The programs will always be compiled utilising $(HOSTCC) passed | |
680 | the options specified in $(HOSTCFLAGS). | |
681 | To set flags that will take effect for all host programs created | |
a07f6033 | 682 | in that Makefile, use the variable HOST_EXTRACFLAGS. |
1da177e4 LT |
683 | |
684 | Example: | |
685 | #scripts/lxdialog/Makefile | |
686 | HOST_EXTRACFLAGS += -I/usr/include/ncurses | |
39e6e9cf | 687 | |
1da177e4 LT |
688 | To set specific flags for a single file the following construction |
689 | is used: | |
690 | ||
691 | Example: | |
692 | #arch/ppc64/boot/Makefile | |
693 | HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) | |
39e6e9cf | 694 | |
1da177e4 | 695 | It is also possible to specify additional options to the linker. |
39e6e9cf | 696 | |
1da177e4 LT |
697 | Example: |
698 | #scripts/kconfig/Makefile | |
699 | HOSTLOADLIBES_qconf := -L$(QTDIR)/lib | |
700 | ||
a07f6033 JE |
701 | When linking qconf, it will be passed the extra option |
702 | "-L$(QTDIR)/lib". | |
39e6e9cf | 703 | |
1da177e4 LT |
704 | --- 4.6 When host programs are actually built |
705 | ||
706 | Kbuild will only build host-programs when they are referenced | |
707 | as a prerequisite. | |
708 | This is possible in two ways: | |
709 | ||
710 | (1) List the prerequisite explicitly in a special rule. | |
711 | ||
712 | Example: | |
713 | #drivers/pci/Makefile | |
714 | hostprogs-y := gen-devlist | |
715 | $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist | |
716 | ( cd $(obj); ./gen-devlist ) < $< | |
717 | ||
39e6e9cf | 718 | The target $(obj)/devlist.h will not be built before |
1da177e4 LT |
719 | $(obj)/gen-devlist is updated. Note that references to |
720 | the host programs in special rules must be prefixed with $(obj). | |
721 | ||
722 | (2) Use $(always) | |
723 | When there is no suitable special rule, and the host program | |
724 | shall be built when a makefile is entered, the $(always) | |
725 | variable shall be used. | |
726 | ||
727 | Example: | |
728 | #scripts/lxdialog/Makefile | |
729 | hostprogs-y := lxdialog | |
730 | always := $(hostprogs-y) | |
731 | ||
732 | This will tell kbuild to build lxdialog even if not referenced in | |
733 | any rule. | |
734 | ||
735 | --- 4.7 Using hostprogs-$(CONFIG_FOO) | |
736 | ||
39e6e9cf | 737 | A typical pattern in a Kbuild file looks like this: |
1da177e4 LT |
738 | |
739 | Example: | |
740 | #scripts/Makefile | |
741 | hostprogs-$(CONFIG_KALLSYMS) += kallsyms | |
742 | ||
743 | Kbuild knows about both 'y' for built-in and 'm' for module. | |
744 | So if a config symbol evaluate to 'm', kbuild will still build | |
a07f6033 JE |
745 | the binary. In other words, Kbuild handles hostprogs-m exactly |
746 | like hostprogs-y. But only hostprogs-y is recommended to be used | |
747 | when no CONFIG symbols are involved. | |
1da177e4 LT |
748 | |
749 | === 5 Kbuild clean infrastructure | |
750 | ||
a07f6033 | 751 | "make clean" deletes most generated files in the obj tree where the kernel |
1da177e4 LT |
752 | is compiled. This includes generated files such as host programs. |
753 | Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always), | |
754 | $(extra-y) and $(targets). They are all deleted during "make clean". | |
755 | Files matching the patterns "*.[oas]", "*.ko", plus some additional files | |
756 | generated by kbuild are deleted all over the kernel src tree when | |
757 | "make clean" is executed. | |
758 | ||
759 | Additional files can be specified in kbuild makefiles by use of $(clean-files). | |
760 | ||
761 | Example: | |
762 | #drivers/pci/Makefile | |
763 | clean-files := devlist.h classlist.h | |
764 | ||
765 | When executing "make clean", the two files "devlist.h classlist.h" will | |
766 | be deleted. Kbuild will assume files to be in same relative directory as the | |
767 | Makefile except if an absolute path is specified (path starting with '/'). | |
768 | ||
39e6e9cf BH |
769 | To delete a directory hierarchy use: |
770 | ||
1da177e4 LT |
771 | Example: |
772 | #scripts/package/Makefile | |
773 | clean-dirs := $(objtree)/debian/ | |
774 | ||
775 | This will delete the directory debian, including all subdirectories. | |
776 | Kbuild will assume the directories to be in the same relative path as the | |
777 | Makefile if no absolute path is specified (path does not start with '/'). | |
778 | ||
ef8ff89b MM |
779 | To exclude certain files from make clean, use the $(no-clean-files) variable. |
780 | This is only a special case used in the top level Kbuild file: | |
781 | ||
782 | Example: | |
783 | #Kbuild | |
784 | no-clean-files := $(bounds-file) $(offsets-file) | |
785 | ||
1da177e4 LT |
786 | Usually kbuild descends down in subdirectories due to "obj-* := dir/", |
787 | but in the architecture makefiles where the kbuild infrastructure | |
788 | is not sufficient this sometimes needs to be explicit. | |
789 | ||
790 | Example: | |
791 | #arch/i386/boot/Makefile | |
792 | subdir- := compressed/ | |
793 | ||
794 | The above assignment instructs kbuild to descend down in the | |
795 | directory compressed/ when "make clean" is executed. | |
796 | ||
797 | To support the clean infrastructure in the Makefiles that builds the | |
798 | final bootimage there is an optional target named archclean: | |
799 | ||
800 | Example: | |
801 | #arch/i386/Makefile | |
802 | archclean: | |
803 | $(Q)$(MAKE) $(clean)=arch/i386/boot | |
804 | ||
805 | When "make clean" is executed, make will descend down in arch/i386/boot, | |
806 | and clean as usual. The Makefile located in arch/i386/boot/ may use | |
807 | the subdir- trick to descend further down. | |
808 | ||
809 | Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is | |
810 | included in the top level makefile, and the kbuild infrastructure | |
811 | is not operational at that point. | |
812 | ||
813 | Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will | |
814 | be visited during "make clean". | |
815 | ||
816 | === 6 Architecture Makefiles | |
817 | ||
818 | The top level Makefile sets up the environment and does the preparation, | |
819 | before starting to descend down in the individual directories. | |
a07f6033 JE |
820 | The top level makefile contains the generic part, whereas |
821 | arch/$(ARCH)/Makefile contains what is required to set up kbuild | |
822 | for said architecture. | |
823 | To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines | |
1da177e4 LT |
824 | a few targets. |
825 | ||
a07f6033 JE |
826 | When kbuild executes, the following steps are followed (roughly): |
827 | 1) Configuration of the kernel => produce .config | |
1da177e4 LT |
828 | 2) Store kernel version in include/linux/version.h |
829 | 3) Symlink include/asm to include/asm-$(ARCH) | |
830 | 4) Updating all other prerequisites to the target prepare: | |
831 | - Additional prerequisites are specified in arch/$(ARCH)/Makefile | |
832 | 5) Recursively descend down in all directories listed in | |
833 | init-* core* drivers-* net-* libs-* and build all targets. | |
a07f6033 | 834 | - The values of the above variables are expanded in arch/$(ARCH)/Makefile. |
39e6e9cf | 835 | 6) All object files are then linked and the resulting file vmlinux is |
a07f6033 | 836 | located at the root of the obj tree. |
1da177e4 LT |
837 | The very first objects linked are listed in head-y, assigned by |
838 | arch/$(ARCH)/Makefile. | |
5c811e59 | 839 | 7) Finally, the architecture-specific part does any required post processing |
1da177e4 LT |
840 | and builds the final bootimage. |
841 | - This includes building boot records | |
5c811e59 | 842 | - Preparing initrd images and the like |
1da177e4 LT |
843 | |
844 | ||
845 | --- 6.1 Set variables to tweak the build to the architecture | |
846 | ||
847 | LDFLAGS Generic $(LD) options | |
848 | ||
849 | Flags used for all invocations of the linker. | |
850 | Often specifying the emulation is sufficient. | |
851 | ||
852 | Example: | |
853 | #arch/s390/Makefile | |
854 | LDFLAGS := -m elf_s390 | |
f77bf014 | 855 | Note: ldflags-y can be used to further customise |
a9af3305 | 856 | the flags used. See chapter 3.7. |
39e6e9cf | 857 | |
1da177e4 LT |
858 | LDFLAGS_MODULE Options for $(LD) when linking modules |
859 | ||
860 | LDFLAGS_MODULE is used to set specific flags for $(LD) when | |
861 | linking the .ko files used for modules. | |
862 | Default is "-r", for relocatable output. | |
863 | ||
864 | LDFLAGS_vmlinux Options for $(LD) when linking vmlinux | |
865 | ||
866 | LDFLAGS_vmlinux is used to specify additional flags to pass to | |
a07f6033 | 867 | the linker when linking the final vmlinux image. |
1da177e4 LT |
868 | LDFLAGS_vmlinux uses the LDFLAGS_$@ support. |
869 | ||
870 | Example: | |
871 | #arch/i386/Makefile | |
872 | LDFLAGS_vmlinux := -e stext | |
873 | ||
874 | OBJCOPYFLAGS objcopy flags | |
875 | ||
876 | When $(call if_changed,objcopy) is used to translate a .o file, | |
a07f6033 | 877 | the flags specified in OBJCOPYFLAGS will be used. |
1da177e4 LT |
878 | $(call if_changed,objcopy) is often used to generate raw binaries on |
879 | vmlinux. | |
880 | ||
881 | Example: | |
882 | #arch/s390/Makefile | |
883 | OBJCOPYFLAGS := -O binary | |
884 | ||
885 | #arch/s390/boot/Makefile | |
886 | $(obj)/image: vmlinux FORCE | |
887 | $(call if_changed,objcopy) | |
888 | ||
a07f6033 | 889 | In this example, the binary $(obj)/image is a binary version of |
1da177e4 LT |
890 | vmlinux. The usage of $(call if_changed,xxx) will be described later. |
891 | ||
222d394d | 892 | KBUILD_AFLAGS $(AS) assembler flags |
1da177e4 LT |
893 | |
894 | Default value - see top level Makefile | |
895 | Append or modify as required per architecture. | |
896 | ||
897 | Example: | |
898 | #arch/sparc64/Makefile | |
222d394d | 899 | KBUILD_AFLAGS += -m64 -mcpu=ultrasparc |
1da177e4 | 900 | |
a0f97e06 | 901 | KBUILD_CFLAGS $(CC) compiler flags |
1da177e4 LT |
902 | |
903 | Default value - see top level Makefile | |
904 | Append or modify as required per architecture. | |
905 | ||
a0f97e06 | 906 | Often, the KBUILD_CFLAGS variable depends on the configuration. |
1da177e4 LT |
907 | |
908 | Example: | |
909 | #arch/i386/Makefile | |
910 | cflags-$(CONFIG_M386) += -march=i386 | |
a0f97e06 | 911 | KBUILD_CFLAGS += $(cflags-y) |
1da177e4 LT |
912 | |
913 | Many arch Makefiles dynamically run the target C compiler to | |
914 | probe supported options: | |
915 | ||
916 | #arch/i386/Makefile | |
917 | ||
918 | ... | |
919 | cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ | |
920 | -march=pentium2,-march=i686) | |
921 | ... | |
922 | # Disable unit-at-a-time mode ... | |
a0f97e06 | 923 | KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) |
1da177e4 LT |
924 | ... |
925 | ||
926 | ||
a07f6033 | 927 | The first example utilises the trick that a config option expands |
1da177e4 LT |
928 | to 'y' when selected. |
929 | ||
80c00ba9 | 930 | KBUILD_AFLAGS_KERNEL $(AS) options specific for built-in |
1da177e4 | 931 | |
80c00ba9 | 932 | $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile |
1da177e4 LT |
933 | resident kernel code. |
934 | ||
6588169d | 935 | KBUILD_AFLAGS_MODULE Options for $(AS) when building modules |
1da177e4 | 936 | |
6588169d SR |
937 | $(KBUILD_AFLAGS_MODULE) is used to add arch specific options that |
938 | are used for $(AS). | |
939 | From commandline AFLAGS_MODULE shall be used (see kbuild.txt). | |
1da177e4 | 940 | |
80c00ba9 SR |
941 | KBUILD_CFLAGS_KERNEL $(CC) options specific for built-in |
942 | ||
943 | $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile | |
944 | resident kernel code. | |
945 | ||
6588169d SR |
946 | KBUILD_CFLAGS_MODULE Options for $(CC) when building modules |
947 | ||
948 | $(KBUILD_CFLAGS_MODULE) is used to add arch specific options that | |
949 | are used for $(CC). | |
950 | From commandline CFLAGS_MODULE shall be used (see kbuild.txt). | |
951 | ||
952 | KBUILD_LDFLAGS_MODULE Options for $(LD) when linking modules | |
953 | ||
954 | $(KBUILD_LDFLAGS_MODULE) is used to add arch specific options | |
955 | used when linking modules. This is often a linker script. | |
956 | From commandline LDFLAGS_MODULE shall be used (see kbuild.txt). | |
39e6e9cf | 957 | |
5bb78269 | 958 | --- 6.2 Add prerequisites to archprepare: |
1da177e4 | 959 | |
a07f6033 | 960 | The archprepare: rule is used to list prerequisites that need to be |
1da177e4 | 961 | built before starting to descend down in the subdirectories. |
a07f6033 | 962 | This is usually used for header files containing assembler constants. |
1da177e4 LT |
963 | |
964 | Example: | |
5bb78269 SR |
965 | #arch/arm/Makefile |
966 | archprepare: maketools | |
1da177e4 | 967 | |
a07f6033 | 968 | In this example, the file target maketools will be processed |
5bb78269 | 969 | before descending down in the subdirectories. |
1da177e4 LT |
970 | See also chapter XXX-TODO that describe how kbuild supports |
971 | generating offset header files. | |
972 | ||
973 | ||
974 | --- 6.3 List directories to visit when descending | |
975 | ||
976 | An arch Makefile cooperates with the top Makefile to define variables | |
977 | which specify how to build the vmlinux file. Note that there is no | |
978 | corresponding arch-specific section for modules; the module-building | |
979 | machinery is all architecture-independent. | |
980 | ||
39e6e9cf | 981 | |
1da177e4 LT |
982 | head-y, init-y, core-y, libs-y, drivers-y, net-y |
983 | ||
a07f6033 JE |
984 | $(head-y) lists objects to be linked first in vmlinux. |
985 | $(libs-y) lists directories where a lib.a archive can be located. | |
5c811e59 | 986 | The rest list directories where a built-in.o object file can be |
a07f6033 | 987 | located. |
1da177e4 LT |
988 | |
989 | $(init-y) objects will be located after $(head-y). | |
990 | Then the rest follows in this order: | |
991 | $(core-y), $(libs-y), $(drivers-y) and $(net-y). | |
992 | ||
a07f6033 | 993 | The top level Makefile defines values for all generic directories, |
5c811e59 | 994 | and arch/$(ARCH)/Makefile only adds architecture-specific directories. |
1da177e4 LT |
995 | |
996 | Example: | |
997 | #arch/sparc64/Makefile | |
998 | core-y += arch/sparc64/kernel/ | |
999 | libs-y += arch/sparc64/prom/ arch/sparc64/lib/ | |
1000 | drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ | |
1001 | ||
1002 | ||
5c811e59 | 1003 | --- 6.4 Architecture-specific boot images |
1da177e4 LT |
1004 | |
1005 | An arch Makefile specifies goals that take the vmlinux file, compress | |
1006 | it, wrap it in bootstrapping code, and copy the resulting files | |
1007 | somewhere. This includes various kinds of installation commands. | |
1008 | The actual goals are not standardized across architectures. | |
1009 | ||
1010 | It is common to locate any additional processing in a boot/ | |
1011 | directory below arch/$(ARCH)/. | |
1012 | ||
1013 | Kbuild does not provide any smart way to support building a | |
1014 | target specified in boot/. Therefore arch/$(ARCH)/Makefile shall | |
1015 | call make manually to build a target in boot/. | |
1016 | ||
1017 | The recommended approach is to include shortcuts in | |
1018 | arch/$(ARCH)/Makefile, and use the full path when calling down | |
1019 | into the arch/$(ARCH)/boot/Makefile. | |
1020 | ||
1021 | Example: | |
1022 | #arch/i386/Makefile | |
1023 | boot := arch/i386/boot | |
1024 | bzImage: vmlinux | |
1025 | $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ | |
1026 | ||
1027 | "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke | |
1028 | make in a subdirectory. | |
1029 | ||
5c811e59 | 1030 | There are no rules for naming architecture-specific targets, |
1da177e4 | 1031 | but executing "make help" will list all relevant targets. |
a07f6033 | 1032 | To support this, $(archhelp) must be defined. |
1da177e4 LT |
1033 | |
1034 | Example: | |
1035 | #arch/i386/Makefile | |
1036 | define archhelp | |
1037 | echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' | |
39e6e9cf | 1038 | endif |
1da177e4 LT |
1039 | |
1040 | When make is executed without arguments, the first goal encountered | |
1041 | will be built. In the top level Makefile the first goal present | |
1042 | is all:. | |
a07f6033 JE |
1043 | An architecture shall always, per default, build a bootable image. |
1044 | In "make help", the default goal is highlighted with a '*'. | |
1da177e4 LT |
1045 | Add a new prerequisite to all: to select a default goal different |
1046 | from vmlinux. | |
1047 | ||
1048 | Example: | |
1049 | #arch/i386/Makefile | |
39e6e9cf | 1050 | all: bzImage |
1da177e4 LT |
1051 | |
1052 | When "make" is executed without arguments, bzImage will be built. | |
1053 | ||
1054 | --- 6.5 Building non-kbuild targets | |
1055 | ||
1056 | extra-y | |
1057 | ||
1058 | extra-y specify additional targets created in the current | |
1059 | directory, in addition to any targets specified by obj-*. | |
1060 | ||
1061 | Listing all targets in extra-y is required for two purposes: | |
1062 | 1) Enable kbuild to check changes in command lines | |
1063 | - When $(call if_changed,xxx) is used | |
1064 | 2) kbuild knows what files to delete during "make clean" | |
1065 | ||
1066 | Example: | |
1067 | #arch/i386/kernel/Makefile | |
1068 | extra-y := head.o init_task.o | |
1069 | ||
a07f6033 | 1070 | In this example, extra-y is used to list object files that |
1da177e4 LT |
1071 | shall be built, but shall not be linked as part of built-in.o. |
1072 | ||
39e6e9cf | 1073 | |
1da177e4 LT |
1074 | --- 6.6 Commands useful for building a boot image |
1075 | ||
1076 | Kbuild provides a few macros that are useful when building a | |
1077 | boot image. | |
1078 | ||
1079 | if_changed | |
1080 | ||
1081 | if_changed is the infrastructure used for the following commands. | |
1082 | ||
1083 | Usage: | |
1084 | target: source(s) FORCE | |
1085 | $(call if_changed,ld/objcopy/gzip) | |
1086 | ||
a07f6033 | 1087 | When the rule is evaluated, it is checked to see if any files |
5c811e59 | 1088 | need an update, or the command line has changed since the last |
1da177e4 LT |
1089 | invocation. The latter will force a rebuild if any options |
1090 | to the executable have changed. | |
1091 | Any target that utilises if_changed must be listed in $(targets), | |
1092 | otherwise the command line check will fail, and the target will | |
1093 | always be built. | |
1094 | Assignments to $(targets) are without $(obj)/ prefix. | |
1095 | if_changed may be used in conjunction with custom commands as | |
1096 | defined in 6.7 "Custom kbuild commands". | |
49490571 | 1097 | |
1da177e4 | 1098 | Note: It is a typical mistake to forget the FORCE prerequisite. |
49490571 PBG |
1099 | Another common pitfall is that whitespace is sometimes |
1100 | significant; for instance, the below will fail (note the extra space | |
1101 | after the comma): | |
1102 | target: source(s) FORCE | |
1103 | #WRONG!# $(call if_changed, ld/objcopy/gzip) | |
1da177e4 LT |
1104 | |
1105 | ld | |
a07f6033 | 1106 | Link target. Often, LDFLAGS_$@ is used to set specific options to ld. |
39e6e9cf | 1107 | |
1da177e4 LT |
1108 | objcopy |
1109 | Copy binary. Uses OBJCOPYFLAGS usually specified in | |
1110 | arch/$(ARCH)/Makefile. | |
1111 | OBJCOPYFLAGS_$@ may be used to set additional options. | |
1112 | ||
1113 | gzip | |
1114 | Compress target. Use maximum compression to compress target. | |
1115 | ||
1116 | Example: | |
1117 | #arch/i386/boot/Makefile | |
1118 | LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary | |
1119 | LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext | |
1120 | ||
1121 | targets += setup setup.o bootsect bootsect.o | |
1122 | $(obj)/setup $(obj)/bootsect: %: %.o FORCE | |
1123 | $(call if_changed,ld) | |
1124 | ||
a07f6033 JE |
1125 | In this example, there are two possible targets, requiring different |
1126 | options to the linker. The linker options are specified using the | |
1da177e4 | 1127 | LDFLAGS_$@ syntax - one for each potential target. |
5d3f083d | 1128 | $(targets) are assigned all potential targets, by which kbuild knows |
1da177e4 LT |
1129 | the targets and will: |
1130 | 1) check for commandline changes | |
1131 | 2) delete target during make clean | |
1132 | ||
1133 | The ": %: %.o" part of the prerequisite is a shorthand that | |
1134 | free us from listing the setup.o and bootsect.o files. | |
1135 | Note: It is a common mistake to forget the "target :=" assignment, | |
1136 | resulting in the target file being recompiled for no | |
1137 | obvious reason. | |
1138 | ||
aab94339 DB |
1139 | dtc |
1140 | Create flattend device tree blob object suitable for linking | |
1141 | into vmlinux. Device tree blobs linked into vmlinux are placed | |
1142 | in an init section in the image. Platform code *must* copy the | |
1143 | blob to non-init memory prior to calling unflatten_device_tree(). | |
1144 | ||
1145 | Example: | |
1146 | #arch/x86/platform/ce4100/Makefile | |
1147 | clean-files := *dtb.S | |
1148 | ||
1149 | DTC_FLAGS := -p 1024 | |
1150 | obj-y += foo.dtb.o | |
1151 | ||
1152 | $(obj)/%.dtb: $(src)/%.dts | |
1153 | $(call cmd,dtc) | |
1da177e4 LT |
1154 | |
1155 | --- 6.7 Custom kbuild commands | |
1156 | ||
a07f6033 | 1157 | When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand |
1da177e4 LT |
1158 | of a command is normally displayed. |
1159 | To enable this behaviour for custom commands kbuild requires | |
1160 | two variables to be set: | |
1161 | quiet_cmd_<command> - what shall be echoed | |
1162 | cmd_<command> - the command to execute | |
1163 | ||
1164 | Example: | |
1165 | # | |
1166 | quiet_cmd_image = BUILD $@ | |
1167 | cmd_image = $(obj)/tools/build $(BUILDFLAGS) \ | |
1168 | $(obj)/vmlinux.bin > $@ | |
1169 | ||
1170 | targets += bzImage | |
1171 | $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE | |
1172 | $(call if_changed,image) | |
1173 | @echo 'Kernel: $@ is ready' | |
1174 | ||
a07f6033 | 1175 | When updating the $(obj)/bzImage target, the line |
1da177e4 LT |
1176 | |
1177 | BUILD arch/i386/boot/bzImage | |
1178 | ||
1179 | will be displayed with "make KBUILD_VERBOSE=0". | |
39e6e9cf | 1180 | |
1da177e4 LT |
1181 | |
1182 | --- 6.8 Preprocessing linker scripts | |
1183 | ||
a07f6033 | 1184 | When the vmlinux image is built, the linker script |
1da177e4 LT |
1185 | arch/$(ARCH)/kernel/vmlinux.lds is used. |
1186 | The script is a preprocessed variant of the file vmlinux.lds.S | |
1187 | located in the same directory. | |
a07f6033 | 1188 | kbuild knows .lds files and includes a rule *lds.S -> *lds. |
39e6e9cf | 1189 | |
1da177e4 LT |
1190 | Example: |
1191 | #arch/i386/kernel/Makefile | |
1192 | always := vmlinux.lds | |
39e6e9cf | 1193 | |
1da177e4 LT |
1194 | #Makefile |
1195 | export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) | |
39e6e9cf BH |
1196 | |
1197 | The assignment to $(always) is used to tell kbuild to build the | |
a07f6033 JE |
1198 | target vmlinux.lds. |
1199 | The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the | |
1da177e4 | 1200 | specified options when building the target vmlinux.lds. |
39e6e9cf | 1201 | |
a07f6033 | 1202 | When building the *.lds target, kbuild uses the variables: |
06c5040c | 1203 | KBUILD_CPPFLAGS : Set in top-level Makefile |
f77bf014 | 1204 | cppflags-y : May be set in the kbuild makefile |
1da177e4 LT |
1205 | CPPFLAGS_$(@F) : Target specific flags. |
1206 | Note that the full filename is used in this | |
1207 | assignment. | |
1208 | ||
1209 | The kbuild infrastructure for *lds file are used in several | |
5c811e59 | 1210 | architecture-specific files. |
1da177e4 | 1211 | |
c7bb349e SR |
1212 | === 7 Kbuild syntax for exported headers |
1213 | ||
1214 | The kernel include a set of headers that is exported to userspace. | |
c95940f2 | 1215 | Many headers can be exported as-is but other headers require a |
c7bb349e SR |
1216 | minimal pre-processing before they are ready for user-space. |
1217 | The pre-processing does: | |
1218 | - drop kernel specific annotations | |
1219 | - drop include of compiler.h | |
c95940f2 | 1220 | - drop all sections that are kernel internal (guarded by ifdef __KERNEL__) |
c7bb349e | 1221 | |
c95940f2 | 1222 | Each relevant directory contains a file name "Kbuild" which specifies the |
c7bb349e SR |
1223 | headers to be exported. |
1224 | See subsequent chapter for the syntax of the Kbuild file. | |
1225 | ||
1226 | --- 7.1 header-y | |
1227 | ||
1228 | header-y specify header files to be exported. | |
1229 | ||
1230 | Example: | |
1231 | #include/linux/Kbuild | |
1232 | header-y += usb/ | |
1233 | header-y += aio_abi.h | |
1234 | ||
1235 | The convention is to list one file per line and | |
1236 | preferably in alphabetic order. | |
1237 | ||
1238 | header-y also specify which subdirectories to visit. | |
1239 | A subdirectory is identified by a trailing '/' which | |
1240 | can be seen in the example above for the usb subdirectory. | |
1241 | ||
1242 | Subdirectories are visited before their parent directories. | |
1243 | ||
1244 | --- 7.2 objhdr-y | |
1245 | ||
1246 | objhdr-y specifies generated files to be exported. | |
1247 | Generated files are special as they need to be looked | |
1248 | up in another directory when doing 'make O=...' builds. | |
1249 | ||
1250 | Example: | |
1251 | #include/linux/Kbuild | |
1252 | objhdr-y += version.h | |
1253 | ||
1254 | --- 7.3 destination-y | |
1255 | ||
1256 | When an architecture have a set of exported headers that needs to be | |
1257 | exported to a different directory destination-y is used. | |
1258 | destination-y specify the destination directory for all exported | |
1259 | headers in the file where it is present. | |
1260 | ||
1261 | Example: | |
1262 | #arch/xtensa/platforms/s6105/include/platform/Kbuild | |
1263 | destination-y := include/linux | |
1264 | ||
1265 | In the example above all exported headers in the Kbuild file | |
1266 | will be located in the directory "include/linux" when exported. | |
1267 | ||
1268 | ||
c7bb349e | 1269 | === 8 Kbuild Variables |
1da177e4 LT |
1270 | |
1271 | The top Makefile exports the following variables: | |
1272 | ||
1273 | VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION | |
1274 | ||
1275 | These variables define the current kernel version. A few arch | |
1276 | Makefiles actually use these values directly; they should use | |
1277 | $(KERNELRELEASE) instead. | |
1278 | ||
1279 | $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic | |
1280 | three-part version number, such as "2", "4", and "0". These three | |
1281 | values are always numeric. | |
1282 | ||
1283 | $(EXTRAVERSION) defines an even tinier sublevel for pre-patches | |
1284 | or additional patches. It is usually some non-numeric string | |
1285 | such as "-pre4", and is often blank. | |
1286 | ||
1287 | KERNELRELEASE | |
1288 | ||
1289 | $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable | |
1290 | for constructing installation directory names or showing in | |
1291 | version strings. Some arch Makefiles use it for this purpose. | |
1292 | ||
1293 | ARCH | |
1294 | ||
1295 | This variable defines the target architecture, such as "i386", | |
1296 | "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to | |
1297 | determine which files to compile. | |
1298 | ||
1299 | By default, the top Makefile sets $(ARCH) to be the same as the | |
1300 | host system architecture. For a cross build, a user may | |
1301 | override the value of $(ARCH) on the command line: | |
1302 | ||
1303 | make ARCH=m68k ... | |
1304 | ||
1305 | ||
1306 | INSTALL_PATH | |
1307 | ||
1308 | This variable defines a place for the arch Makefiles to install | |
1309 | the resident kernel image and System.map file. | |
5c811e59 | 1310 | Use this for architecture-specific install targets. |
1da177e4 LT |
1311 | |
1312 | INSTALL_MOD_PATH, MODLIB | |
1313 | ||
1314 | $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module | |
1315 | installation. This variable is not defined in the Makefile but | |
1316 | may be passed in by the user if desired. | |
1317 | ||
1318 | $(MODLIB) specifies the directory for module installation. | |
1319 | The top Makefile defines $(MODLIB) to | |
1320 | $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may | |
1321 | override this value on the command line if desired. | |
1322 | ||
ac031f26 TT |
1323 | INSTALL_MOD_STRIP |
1324 | ||
1325 | If this variable is specified, will cause modules to be stripped | |
1326 | after they are installed. If INSTALL_MOD_STRIP is '1', then the | |
1327 | default option --strip-debug will be used. Otherwise, | |
1328 | INSTALL_MOD_STRIP will used as the option(s) to the strip command. | |
1329 | ||
1330 | ||
c7bb349e | 1331 | === 9 Makefile language |
1da177e4 | 1332 | |
a07f6033 | 1333 | The kernel Makefiles are designed to be run with GNU Make. The Makefiles |
1da177e4 LT |
1334 | use only the documented features of GNU Make, but they do use many |
1335 | GNU extensions. | |
1336 | ||
1337 | GNU Make supports elementary list-processing functions. The kernel | |
1338 | Makefiles use a novel style of list building and manipulation with few | |
1339 | "if" statements. | |
1340 | ||
1341 | GNU Make has two assignment operators, ":=" and "=". ":=" performs | |
1342 | immediate evaluation of the right-hand side and stores an actual string | |
1343 | into the left-hand side. "=" is like a formula definition; it stores the | |
1344 | right-hand side in an unevaluated form and then evaluates this form each | |
1345 | time the left-hand side is used. | |
1346 | ||
1347 | There are some cases where "=" is appropriate. Usually, though, ":=" | |
1348 | is the right choice. | |
1349 | ||
c7bb349e | 1350 | === 10 Credits |
1da177e4 LT |
1351 | |
1352 | Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> | |
1353 | Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> | |
1354 | Updates by Sam Ravnborg <sam@ravnborg.org> | |
a07f6033 | 1355 | Language QA by Jan Engelhardt <jengelh@gmx.de> |
1da177e4 | 1356 | |
c7bb349e | 1357 | === 11 TODO |
1da177e4 | 1358 | |
a07f6033 | 1359 | - Describe how kbuild supports shipped files with _shipped. |
1da177e4 LT |
1360 | - Generating offset header files. |
1361 | - Add more variables to section 7? | |
1362 | ||
39e6e9cf BH |
1363 | |
1364 |