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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 $(LD) support functions |
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22 | |
23 | === 4 Host Program support | |
24 | --- 4.1 Simple Host Program | |
25 | --- 4.2 Composite Host Programs | |
62e22107 MY |
26 | --- 4.3 Using C++ for host programs |
27 | --- 4.4 Controlling compiler options for host programs | |
28 | --- 4.5 When host programs are actually built | |
29 | --- 4.6 Using hostprogs-$(CONFIG_FOO) | |
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30 | |
31 | === 5 Kbuild clean infrastructure | |
32 | ||
33 | === 6 Architecture Makefiles | |
34 | --- 6.1 Set variables to tweak the build to the architecture | |
052ad274 PA |
35 | --- 6.2 Add prerequisites to archheaders: |
36 | --- 6.3 Add prerequisites to archprepare: | |
37 | --- 6.4 List directories to visit when descending | |
38 | --- 6.5 Architecture-specific boot images | |
39 | --- 6.6 Building non-kbuild targets | |
40 | --- 6.7 Commands useful for building a boot image | |
41 | --- 6.8 Custom kbuild commands | |
42 | --- 6.9 Preprocessing linker scripts | |
43 | --- 6.10 Generic header files | |
fbe6e37d | 44 | --- 6.11 Post-link pass |
1da177e4 | 45 | |
c7bb349e | 46 | === 7 Kbuild syntax for exported headers |
fcc8487d | 47 | --- 7.1 no-export-headers |
ae3f4151 MY |
48 | --- 7.2 generic-y |
49 | --- 7.3 generated-y | |
50 | --- 7.4 mandatory-y | |
c7bb349e SR |
51 | |
52 | === 8 Kbuild Variables | |
53 | === 9 Makefile language | |
54 | === 10 Credits | |
55 | === 11 TODO | |
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56 | |
57 | === 1 Overview | |
58 | ||
59 | The Makefiles have five parts: | |
60 | ||
61 | Makefile the top Makefile. | |
62 | .config the kernel configuration file. | |
63 | arch/$(ARCH)/Makefile the arch Makefile. | |
64 | scripts/Makefile.* common rules etc. for all kbuild Makefiles. | |
65 | kbuild Makefiles there are about 500 of these. | |
66 | ||
67 | The top Makefile reads the .config file, which comes from the kernel | |
68 | configuration process. | |
69 | ||
70 | The top Makefile is responsible for building two major products: vmlinux | |
71 | (the resident kernel image) and modules (any module files). | |
72 | It builds these goals by recursively descending into the subdirectories of | |
73 | the kernel source tree. | |
74 | The list of subdirectories which are visited depends upon the kernel | |
75 | configuration. The top Makefile textually includes an arch Makefile | |
76 | with the name arch/$(ARCH)/Makefile. The arch Makefile supplies | |
77 | architecture-specific information to the top Makefile. | |
78 | ||
79 | Each subdirectory has a kbuild Makefile which carries out the commands | |
80 | passed down from above. The kbuild Makefile uses information from the | |
39e6e9cf | 81 | .config file to construct various file lists used by kbuild to build |
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82 | any built-in or modular targets. |
83 | ||
84 | scripts/Makefile.* contains all the definitions/rules etc. that | |
85 | are used to build the kernel based on the kbuild makefiles. | |
86 | ||
87 | ||
88 | === 2 Who does what | |
89 | ||
90 | People have four different relationships with the kernel Makefiles. | |
91 | ||
92 | *Users* are people who build kernels. These people type commands such as | |
93 | "make menuconfig" or "make". They usually do not read or edit | |
94 | any kernel Makefiles (or any other source files). | |
95 | ||
96 | *Normal developers* are people who work on features such as device | |
97 | drivers, file systems, and network protocols. These people need to | |
a07f6033 | 98 | maintain the kbuild Makefiles for the subsystem they are |
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99 | working on. In order to do this effectively, they need some overall |
100 | knowledge about the kernel Makefiles, plus detailed knowledge about the | |
101 | public interface for kbuild. | |
102 | ||
103 | *Arch developers* are people who work on an entire architecture, such | |
104 | as sparc or ia64. Arch developers need to know about the arch Makefile | |
105 | as well as kbuild Makefiles. | |
106 | ||
107 | *Kbuild developers* are people who work on the kernel build system itself. | |
108 | These people need to know about all aspects of the kernel Makefiles. | |
109 | ||
110 | This document is aimed towards normal developers and arch developers. | |
111 | ||
112 | ||
113 | === 3 The kbuild files | |
114 | ||
115 | Most Makefiles within the kernel are kbuild Makefiles that use the | |
a07f6033 | 116 | kbuild infrastructure. This chapter introduces the syntax used in the |
1da177e4 | 117 | kbuild makefiles. |
172c3ae3 | 118 | The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can |
a07f6033 | 119 | be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild' |
172c3ae3 | 120 | file will be used. |
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121 | |
122 | Section 3.1 "Goal definitions" is a quick intro, further chapters provide | |
123 | more details, with real examples. | |
124 | ||
125 | --- 3.1 Goal definitions | |
126 | ||
127 | Goal definitions are the main part (heart) of the kbuild Makefile. | |
128 | These lines define the files to be built, any special compilation | |
129 | options, and any subdirectories to be entered recursively. | |
130 | ||
131 | The most simple kbuild makefile contains one line: | |
132 | ||
133 | Example: | |
134 | obj-y += foo.o | |
135 | ||
5c811e59 | 136 | This tells kbuild that there is one object in that directory, named |
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137 | foo.o. foo.o will be built from foo.c or foo.S. |
138 | ||
139 | If foo.o shall be built as a module, the variable obj-m is used. | |
140 | Therefore the following pattern is often used: | |
141 | ||
142 | Example: | |
143 | obj-$(CONFIG_FOO) += foo.o | |
144 | ||
145 | $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module). | |
146 | If CONFIG_FOO is neither y nor m, then the file will not be compiled | |
147 | nor linked. | |
148 | ||
149 | --- 3.2 Built-in object goals - obj-y | |
150 | ||
151 | The kbuild Makefile specifies object files for vmlinux | |
a07f6033 | 152 | in the $(obj-y) lists. These lists depend on the kernel |
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153 | configuration. |
154 | ||
155 | Kbuild compiles all the $(obj-y) files. It then calls | |
f49821ee | 156 | "$(AR) rcSTP" to merge these files into one built-in.a file. |
dee94953 MY |
157 | This is a thin archive without a symbol table. It will be later |
158 | linked into vmlinux by scripts/link-vmlinux.sh | |
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159 | |
160 | The order of files in $(obj-y) is significant. Duplicates in | |
161 | the lists are allowed: the first instance will be linked into | |
f49821ee | 162 | built-in.a and succeeding instances will be ignored. |
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163 | |
164 | Link order is significant, because certain functions | |
165 | (module_init() / __initcall) will be called during boot in the | |
166 | order they appear. So keep in mind that changing the link | |
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167 | order may e.g. change the order in which your SCSI |
168 | controllers are detected, and thus your disks are renumbered. | |
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169 | |
170 | Example: | |
171 | #drivers/isdn/i4l/Makefile | |
172 | # Makefile for the kernel ISDN subsystem and device drivers. | |
173 | # Each configuration option enables a list of files. | |
2f5a2f81 | 174 | obj-$(CONFIG_ISDN_I4L) += isdn.o |
1da177e4 LT |
175 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o |
176 | ||
177 | --- 3.3 Loadable module goals - obj-m | |
178 | ||
39fed701 | 179 | $(obj-m) specifies object files which are built as loadable |
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180 | kernel modules. |
181 | ||
182 | A module may be built from one source file or several source | |
183 | files. In the case of one source file, the kbuild makefile | |
184 | simply adds the file to $(obj-m). | |
185 | ||
186 | Example: | |
187 | #drivers/isdn/i4l/Makefile | |
188 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o | |
189 | ||
190 | Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm' | |
191 | ||
192 | If a kernel module is built from several source files, you specify | |
4f827280 MM |
193 | that you want to build a module in the same way as above; however, |
194 | kbuild needs to know which object files you want to build your | |
195 | module from, so you have to tell it by setting a $(<module_name>-y) | |
196 | variable. | |
1da177e4 LT |
197 | |
198 | Example: | |
199 | #drivers/isdn/i4l/Makefile | |
4f827280 MM |
200 | obj-$(CONFIG_ISDN_I4L) += isdn.o |
201 | isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o | |
1da177e4 LT |
202 | |
203 | In this example, the module name will be isdn.o. Kbuild will | |
4f827280 | 204 | compile the objects listed in $(isdn-y) and then run |
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205 | "$(LD) -r" on the list of these files to generate isdn.o. |
206 | ||
4f827280 MM |
207 | Due to kbuild recognizing $(<module_name>-y) for composite objects, |
208 | you can use the value of a CONFIG_ symbol to optionally include an | |
209 | object file as part of a composite object. | |
1da177e4 LT |
210 | |
211 | Example: | |
212 | #fs/ext2/Makefile | |
4f827280 MM |
213 | obj-$(CONFIG_EXT2_FS) += ext2.o |
214 | ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \ | |
215 | namei.o super.o symlink.o | |
216 | ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \ | |
217 | xattr_trusted.o | |
218 | ||
219 | In this example, xattr.o, xattr_user.o and xattr_trusted.o are only | |
220 | part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR) | |
221 | evaluates to 'y'. | |
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222 | |
223 | Note: Of course, when you are building objects into the kernel, | |
224 | the syntax above will also work. So, if you have CONFIG_EXT2_FS=y, | |
225 | kbuild will build an ext2.o file for you out of the individual | |
f49821ee | 226 | parts and then link this into built-in.a, as you would expect. |
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227 | |
228 | --- 3.4 Objects which export symbols | |
229 | ||
230 | No special notation is required in the makefiles for | |
231 | modules exporting symbols. | |
232 | ||
233 | --- 3.5 Library file goals - lib-y | |
234 | ||
a07f6033 | 235 | Objects listed with obj-* are used for modules, or |
f49821ee | 236 | combined in a built-in.a for that specific directory. |
1da177e4 LT |
237 | There is also the possibility to list objects that will |
238 | be included in a library, lib.a. | |
239 | All objects listed with lib-y are combined in a single | |
240 | library for that directory. | |
5d3f083d ML |
241 | Objects that are listed in obj-y and additionally listed in |
242 | lib-y will not be included in the library, since they will | |
243 | be accessible anyway. | |
a07f6033 | 244 | For consistency, objects listed in lib-m will be included in lib.a. |
1da177e4 LT |
245 | |
246 | Note that the same kbuild makefile may list files to be built-in | |
247 | and to be part of a library. Therefore the same directory | |
f49821ee | 248 | may contain both a built-in.a and a lib.a file. |
1da177e4 LT |
249 | |
250 | Example: | |
2f5a2f81 MM |
251 | #arch/x86/lib/Makefile |
252 | lib-y := delay.o | |
1da177e4 | 253 | |
2f5a2f81 MM |
254 | This will create a library lib.a based on delay.o. For kbuild to |
255 | actually recognize that there is a lib.a being built, the directory | |
256 | shall be listed in libs-y. | |
052ad274 | 257 | See also "6.4 List directories to visit when descending". |
39e6e9cf | 258 | |
a07f6033 | 259 | Use of lib-y is normally restricted to lib/ and arch/*/lib. |
1da177e4 LT |
260 | |
261 | --- 3.6 Descending down in directories | |
262 | ||
263 | A Makefile is only responsible for building objects in its own | |
264 | directory. Files in subdirectories should be taken care of by | |
265 | Makefiles in these subdirs. The build system will automatically | |
266 | invoke make recursively in subdirectories, provided you let it know of | |
267 | them. | |
268 | ||
a07f6033 | 269 | To do so, obj-y and obj-m are used. |
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270 | ext2 lives in a separate directory, and the Makefile present in fs/ |
271 | tells kbuild to descend down using the following assignment. | |
272 | ||
273 | Example: | |
274 | #fs/Makefile | |
275 | obj-$(CONFIG_EXT2_FS) += ext2/ | |
276 | ||
277 | If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular) | |
278 | the corresponding obj- variable will be set, and kbuild will descend | |
279 | down in the ext2 directory. | |
280 | Kbuild only uses this information to decide that it needs to visit | |
281 | the directory, it is the Makefile in the subdirectory that | |
39fed701 | 282 | specifies what is modular and what is built-in. |
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283 | |
284 | It is good practice to use a CONFIG_ variable when assigning directory | |
285 | names. This allows kbuild to totally skip the directory if the | |
286 | corresponding CONFIG_ option is neither 'y' nor 'm'. | |
287 | ||
288 | --- 3.7 Compilation flags | |
289 | ||
f77bf014 | 290 | ccflags-y, asflags-y and ldflags-y |
c95940f2 NK |
291 | These three flags apply only to the kbuild makefile in which they |
292 | are assigned. They are used for all the normal cc, as and ld | |
293 | invocations happening during a recursive build. | |
f77bf014 | 294 | Note: Flags with the same behaviour were previously named: |
c95940f2 NK |
295 | EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS. |
296 | They are still supported but their usage is deprecated. | |
1da177e4 | 297 | |
eb07e1b4 | 298 | ccflags-y specifies options for compiling with $(CC). |
1da177e4 LT |
299 | |
300 | Example: | |
3d91a353 SD |
301 | # drivers/acpi/acpica/Makefile |
302 | ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA | |
303 | ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT | |
1da177e4 LT |
304 | |
305 | This variable is necessary because the top Makefile owns the | |
a0f97e06 | 306 | variable $(KBUILD_CFLAGS) and uses it for compilation flags for the |
1da177e4 LT |
307 | entire tree. |
308 | ||
eb07e1b4 | 309 | asflags-y specifies options for assembling with $(AS). |
1da177e4 LT |
310 | |
311 | Example: | |
eb07e1b4 MM |
312 | #arch/sparc/kernel/Makefile |
313 | asflags-y := -ansi | |
1da177e4 | 314 | |
eb07e1b4 | 315 | ldflags-y specifies options for linking with $(LD). |
1da177e4 LT |
316 | |
317 | Example: | |
eb07e1b4 MM |
318 | #arch/cris/boot/compressed/Makefile |
319 | ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds | |
1da177e4 | 320 | |
720097d8 | 321 | subdir-ccflags-y, subdir-asflags-y |
eb07e1b4 | 322 | The two flags listed above are similar to ccflags-y and asflags-y. |
c95940f2 NK |
323 | The difference is that the subdir- variants have effect for the kbuild |
324 | file where they are present and all subdirectories. | |
325 | Options specified using subdir-* are added to the commandline before | |
326 | the options specified using the non-subdir variants. | |
720097d8 SR |
327 | |
328 | Example: | |
329 | subdir-ccflags-y := -Werror | |
330 | ||
1da177e4 LT |
331 | CFLAGS_$@, AFLAGS_$@ |
332 | ||
333 | CFLAGS_$@ and AFLAGS_$@ only apply to commands in current | |
334 | kbuild makefile. | |
335 | ||
336 | $(CFLAGS_$@) specifies per-file options for $(CC). The $@ | |
337 | part has a literal value which specifies the file that it is for. | |
338 | ||
339 | Example: | |
340 | # drivers/scsi/Makefile | |
341 | CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF | |
342 | CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \ | |
343 | -DGDTH_STATISTICS | |
1da177e4 | 344 | |
eb07e1b4 | 345 | These two lines specify compilation flags for aha152x.o and gdth.o. |
1da177e4 LT |
346 | |
347 | $(AFLAGS_$@) is a similar feature for source files in assembly | |
348 | languages. | |
349 | ||
350 | Example: | |
351 | # arch/arm/kernel/Makefile | |
eb07e1b4 MM |
352 | AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET) |
353 | AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312 | |
354 | AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt | |
355 | ||
1da177e4 LT |
356 | |
357 | --- 3.9 Dependency tracking | |
358 | ||
359 | Kbuild tracks dependencies on the following: | |
360 | 1) All prerequisite files (both *.c and *.h) | |
361 | 2) CONFIG_ options used in all prerequisite files | |
362 | 3) Command-line used to compile target | |
363 | ||
364 | Thus, if you change an option to $(CC) all affected files will | |
365 | be re-compiled. | |
366 | ||
367 | --- 3.10 Special Rules | |
368 | ||
369 | Special rules are used when the kbuild infrastructure does | |
370 | not provide the required support. A typical example is | |
371 | header files generated during the build process. | |
5c811e59 | 372 | Another example are the architecture-specific Makefiles which |
a07f6033 | 373 | need special rules to prepare boot images etc. |
1da177e4 LT |
374 | |
375 | Special rules are written as normal Make rules. | |
376 | Kbuild is not executing in the directory where the Makefile is | |
377 | located, so all special rules shall provide a relative | |
378 | path to prerequisite files and target files. | |
379 | ||
380 | Two variables are used when defining special rules: | |
381 | ||
382 | $(src) | |
383 | $(src) is a relative path which points to the directory | |
384 | where the Makefile is located. Always use $(src) when | |
385 | referring to files located in the src tree. | |
386 | ||
387 | $(obj) | |
388 | $(obj) is a relative path which points to the directory | |
389 | where the target is saved. Always use $(obj) when | |
390 | referring to generated files. | |
391 | ||
392 | Example: | |
393 | #drivers/scsi/Makefile | |
394 | $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl | |
395 | $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl | |
396 | ||
397 | This is a special rule, following the normal syntax | |
398 | required by make. | |
399 | The target file depends on two prerequisite files. References | |
400 | to the target file are prefixed with $(obj), references | |
401 | to prerequisites are referenced with $(src) (because they are not | |
402 | generated files). | |
403 | ||
5410ecc0 MF |
404 | $(kecho) |
405 | echoing information to user in a rule is often a good practice | |
406 | but when execution "make -s" one does not expect to see any output | |
407 | except for warnings/errors. | |
39fed701 | 408 | To support this kbuild defines $(kecho) which will echo out the |
5410ecc0 MF |
409 | text following $(kecho) to stdout except if "make -s" is used. |
410 | ||
411 | Example: | |
412 | #arch/blackfin/boot/Makefile | |
413 | $(obj)/vmImage: $(obj)/vmlinux.gz | |
414 | $(call if_changed,uimage) | |
415 | @$(kecho) 'Kernel: $@ is ready' | |
416 | ||
417 | ||
20a468b5 SR |
418 | --- 3.11 $(CC) support functions |
419 | ||
a07f6033 | 420 | The kernel may be built with several different versions of |
20a468b5 | 421 | $(CC), each supporting a unique set of features and options. |
39fed701 | 422 | kbuild provides basic support to check for valid options for $(CC). |
e95be9a5 | 423 | $(CC) is usually the gcc compiler, but other alternatives are |
20a468b5 SR |
424 | available. |
425 | ||
426 | as-option | |
a07f6033 JE |
427 | as-option is used to check if $(CC) -- when used to compile |
428 | assembler (*.S) files -- supports the given option. An optional | |
429 | second option may be specified if the first option is not supported. | |
20a468b5 SR |
430 | |
431 | Example: | |
432 | #arch/sh/Makefile | |
433 | cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) | |
434 | ||
a07f6033 | 435 | In the above example, cflags-y will be assigned the option |
20a468b5 SR |
436 | -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). |
437 | The second argument is optional, and if supplied will be used | |
438 | if first argument is not supported. | |
439 | ||
f86fd306 SR |
440 | cc-ldoption |
441 | cc-ldoption is used to check if $(CC) when used to link object files | |
0b0bf7a3 RM |
442 | supports the given option. An optional second option may be |
443 | specified if first option are not supported. | |
444 | ||
445 | Example: | |
25eb650a | 446 | #arch/x86/kernel/Makefile |
f86fd306 | 447 | vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) |
0b0bf7a3 | 448 | |
5c811e59 | 449 | In the above example, vsyscall-flags will be assigned the option |
0b0bf7a3 RM |
450 | -Wl$(comma)--hash-style=sysv if it is supported by $(CC). |
451 | The second argument is optional, and if supplied will be used | |
452 | if first argument is not supported. | |
453 | ||
e2414910 AK |
454 | as-instr |
455 | as-instr checks if the assembler reports a specific instruction | |
456 | and then outputs either option1 or option2 | |
457 | C escapes are supported in the test instruction | |
222d394d | 458 | Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options |
e2414910 | 459 | |
20a468b5 | 460 | cc-option |
39fed701 GU |
461 | cc-option is used to check if $(CC) supports a given option, and if |
462 | not supported to use an optional second option. | |
20a468b5 SR |
463 | |
464 | Example: | |
25eb650a | 465 | #arch/x86/Makefile |
20a468b5 SR |
466 | cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) |
467 | ||
5c811e59 | 468 | In the above example, cflags-y will be assigned the option |
a07f6033 JE |
469 | -march=pentium-mmx if supported by $(CC), otherwise -march=i586. |
470 | The second argument to cc-option is optional, and if omitted, | |
20a468b5 | 471 | cflags-y will be assigned no value if first option is not supported. |
a0f97e06 | 472 | Note: cc-option uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
473 | |
474 | cc-option-yn | |
39e6e9cf | 475 | cc-option-yn is used to check if gcc supports a given option |
20a468b5 SR |
476 | and return 'y' if supported, otherwise 'n'. |
477 | ||
478 | Example: | |
479 | #arch/ppc/Makefile | |
480 | biarch := $(call cc-option-yn, -m32) | |
481 | aflags-$(biarch) += -a32 | |
482 | cflags-$(biarch) += -m32 | |
39e6e9cf | 483 | |
a07f6033 JE |
484 | In the above example, $(biarch) is set to y if $(CC) supports the -m32 |
485 | option. When $(biarch) equals 'y', the expanded variables $(aflags-y) | |
486 | and $(cflags-y) will be assigned the values -a32 and -m32, | |
487 | respectively. | |
a0f97e06 | 488 | Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options |
20a468b5 | 489 | |
8417da6f MM |
490 | cc-disable-warning |
491 | cc-disable-warning checks if gcc supports a given warning and returns | |
492 | the commandline switch to disable it. This special function is needed, | |
493 | because gcc 4.4 and later accept any unknown -Wno-* option and only | |
494 | warn about it if there is another warning in the source file. | |
495 | ||
496 | Example: | |
497 | KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable) | |
498 | ||
499 | In the above example, -Wno-unused-but-set-variable will be added to | |
500 | KBUILD_CFLAGS only if gcc really accepts it. | |
501 | ||
20a468b5 | 502 | cc-ifversion |
6dcb4e5e MY |
503 | cc-ifversion tests the version of $(CC) and equals the fourth parameter |
504 | if version expression is true, or the fifth (if given) if the version | |
505 | expression is false. | |
20a468b5 SR |
506 | |
507 | Example: | |
508 | #fs/reiserfs/Makefile | |
f77bf014 | 509 | ccflags-y := $(call cc-ifversion, -lt, 0402, -O1) |
20a468b5 | 510 | |
f77bf014 | 511 | In this example, ccflags-y will be assigned the value -O1 if the |
20a468b5 | 512 | $(CC) version is less than 4.2. |
39e6e9cf | 513 | cc-ifversion takes all the shell operators: |
20a468b5 SR |
514 | -eq, -ne, -lt, -le, -gt, and -ge |
515 | The third parameter may be a text as in this example, but it may also | |
516 | be an expanded variable or a macro. | |
517 | ||
910b4046 | 518 | cc-cross-prefix |
631bcfbb | 519 | cc-cross-prefix is used to check if there exists a $(CC) in path with |
910b4046 SR |
520 | one of the listed prefixes. The first prefix where there exist a |
521 | prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found | |
522 | then nothing is returned. | |
523 | Additional prefixes are separated by a single space in the | |
524 | call of cc-cross-prefix. | |
631bcfbb GU |
525 | This functionality is useful for architecture Makefiles that try |
526 | to set CROSS_COMPILE to well-known values but may have several | |
910b4046 | 527 | values to select between. |
631bcfbb GU |
528 | It is recommended only to try to set CROSS_COMPILE if it is a cross |
529 | build (host arch is different from target arch). And if CROSS_COMPILE | |
910b4046 SR |
530 | is already set then leave it with the old value. |
531 | ||
532 | Example: | |
533 | #arch/m68k/Makefile | |
534 | ifneq ($(SUBARCH),$(ARCH)) | |
535 | ifeq ($(CROSS_COMPILE),) | |
536 | CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) | |
537 | endif | |
538 | endif | |
539 | ||
691ef3e7 SR |
540 | --- 3.12 $(LD) support functions |
541 | ||
542 | ld-option | |
543 | ld-option is used to check if $(LD) supports the supplied option. | |
544 | ld-option takes two options as arguments. | |
545 | The second argument is an optional option that can be used if the | |
546 | first option is not supported by $(LD). | |
547 | ||
548 | Example: | |
549 | #Makefile | |
5b83df2b | 550 | LDFLAGS_vmlinux += $(call ld-option, -X) |
691ef3e7 SR |
551 | |
552 | ||
1da177e4 LT |
553 | === 4 Host Program support |
554 | ||
555 | Kbuild supports building executables on the host for use during the | |
556 | compilation stage. | |
557 | Two steps are required in order to use a host executable. | |
558 | ||
559 | The first step is to tell kbuild that a host program exists. This is | |
560 | done utilising the variable hostprogs-y. | |
561 | ||
562 | The second step is to add an explicit dependency to the executable. | |
39e6e9cf | 563 | This can be done in two ways. Either add the dependency in a rule, |
1da177e4 LT |
564 | or utilise the variable $(always). |
565 | Both possibilities are described in the following. | |
566 | ||
567 | --- 4.1 Simple Host Program | |
568 | ||
569 | In some cases there is a need to compile and run a program on the | |
570 | computer where the build is running. | |
571 | The following line tells kbuild that the program bin2hex shall be | |
572 | built on the build host. | |
573 | ||
574 | Example: | |
575 | hostprogs-y := bin2hex | |
576 | ||
577 | Kbuild assumes in the above example that bin2hex is made from a single | |
578 | c-source file named bin2hex.c located in the same directory as | |
579 | the Makefile. | |
39e6e9cf | 580 | |
1da177e4 LT |
581 | --- 4.2 Composite Host Programs |
582 | ||
583 | Host programs can be made up based on composite objects. | |
584 | The syntax used to define composite objects for host programs is | |
585 | similar to the syntax used for kernel objects. | |
5d3f083d | 586 | $(<executable>-objs) lists all objects used to link the final |
1da177e4 LT |
587 | executable. |
588 | ||
589 | Example: | |
590 | #scripts/lxdialog/Makefile | |
39e6e9cf | 591 | hostprogs-y := lxdialog |
1da177e4 LT |
592 | lxdialog-objs := checklist.o lxdialog.o |
593 | ||
594 | Objects with extension .o are compiled from the corresponding .c | |
a07f6033 | 595 | files. In the above example, checklist.c is compiled to checklist.o |
1da177e4 | 596 | and lxdialog.c is compiled to lxdialog.o. |
a07f6033 | 597 | Finally, the two .o files are linked to the executable, lxdialog. |
1da177e4 LT |
598 | Note: The syntax <executable>-y is not permitted for host-programs. |
599 | ||
62e22107 | 600 | --- 4.3 Using C++ for host programs |
1da177e4 LT |
601 | |
602 | kbuild offers support for host programs written in C++. This was | |
603 | introduced solely to support kconfig, and is not recommended | |
604 | for general use. | |
605 | ||
606 | Example: | |
607 | #scripts/kconfig/Makefile | |
608 | hostprogs-y := qconf | |
609 | qconf-cxxobjs := qconf.o | |
610 | ||
611 | In the example above the executable is composed of the C++ file | |
612 | qconf.cc - identified by $(qconf-cxxobjs). | |
39e6e9cf | 613 | |
39fed701 | 614 | If qconf is composed of a mixture of .c and .cc files, then an |
1da177e4 LT |
615 | additional line can be used to identify this. |
616 | ||
617 | Example: | |
618 | #scripts/kconfig/Makefile | |
619 | hostprogs-y := qconf | |
620 | qconf-cxxobjs := qconf.o | |
621 | qconf-objs := check.o | |
39e6e9cf | 622 | |
62e22107 | 623 | --- 4.4 Controlling compiler options for host programs |
1da177e4 LT |
624 | |
625 | When compiling host programs, it is possible to set specific flags. | |
626 | The programs will always be compiled utilising $(HOSTCC) passed | |
96f14fe7 | 627 | the options specified in $(KBUILD_HOSTCFLAGS). |
1da177e4 | 628 | To set flags that will take effect for all host programs created |
a07f6033 | 629 | in that Makefile, use the variable HOST_EXTRACFLAGS. |
1da177e4 LT |
630 | |
631 | Example: | |
632 | #scripts/lxdialog/Makefile | |
633 | HOST_EXTRACFLAGS += -I/usr/include/ncurses | |
39e6e9cf | 634 | |
1da177e4 LT |
635 | To set specific flags for a single file the following construction |
636 | is used: | |
637 | ||
638 | Example: | |
639 | #arch/ppc64/boot/Makefile | |
640 | HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) | |
39e6e9cf | 641 | |
1da177e4 | 642 | It is also possible to specify additional options to the linker. |
39e6e9cf | 643 | |
1da177e4 LT |
644 | Example: |
645 | #scripts/kconfig/Makefile | |
c86b1f93 | 646 | HOSTLDLIBS_qconf := -L$(QTDIR)/lib |
1da177e4 | 647 | |
a07f6033 JE |
648 | When linking qconf, it will be passed the extra option |
649 | "-L$(QTDIR)/lib". | |
39e6e9cf | 650 | |
62e22107 | 651 | --- 4.5 When host programs are actually built |
1da177e4 LT |
652 | |
653 | Kbuild will only build host-programs when they are referenced | |
654 | as a prerequisite. | |
655 | This is possible in two ways: | |
656 | ||
657 | (1) List the prerequisite explicitly in a special rule. | |
658 | ||
659 | Example: | |
660 | #drivers/pci/Makefile | |
661 | hostprogs-y := gen-devlist | |
662 | $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist | |
663 | ( cd $(obj); ./gen-devlist ) < $< | |
664 | ||
39e6e9cf | 665 | The target $(obj)/devlist.h will not be built before |
1da177e4 LT |
666 | $(obj)/gen-devlist is updated. Note that references to |
667 | the host programs in special rules must be prefixed with $(obj). | |
668 | ||
669 | (2) Use $(always) | |
670 | When there is no suitable special rule, and the host program | |
671 | shall be built when a makefile is entered, the $(always) | |
672 | variable shall be used. | |
673 | ||
674 | Example: | |
675 | #scripts/lxdialog/Makefile | |
676 | hostprogs-y := lxdialog | |
677 | always := $(hostprogs-y) | |
678 | ||
679 | This will tell kbuild to build lxdialog even if not referenced in | |
680 | any rule. | |
681 | ||
62e22107 | 682 | --- 4.6 Using hostprogs-$(CONFIG_FOO) |
1da177e4 | 683 | |
39e6e9cf | 684 | A typical pattern in a Kbuild file looks like this: |
1da177e4 LT |
685 | |
686 | Example: | |
687 | #scripts/Makefile | |
688 | hostprogs-$(CONFIG_KALLSYMS) += kallsyms | |
689 | ||
690 | Kbuild knows about both 'y' for built-in and 'm' for module. | |
39fed701 | 691 | So if a config symbol evaluates to 'm', kbuild will still build |
a07f6033 JE |
692 | the binary. In other words, Kbuild handles hostprogs-m exactly |
693 | like hostprogs-y. But only hostprogs-y is recommended to be used | |
694 | when no CONFIG symbols are involved. | |
1da177e4 LT |
695 | |
696 | === 5 Kbuild clean infrastructure | |
697 | ||
a07f6033 | 698 | "make clean" deletes most generated files in the obj tree where the kernel |
1da177e4 LT |
699 | is compiled. This includes generated files such as host programs. |
700 | Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always), | |
701 | $(extra-y) and $(targets). They are all deleted during "make clean". | |
702 | Files matching the patterns "*.[oas]", "*.ko", plus some additional files | |
703 | generated by kbuild are deleted all over the kernel src tree when | |
704 | "make clean" is executed. | |
705 | ||
706 | Additional files can be specified in kbuild makefiles by use of $(clean-files). | |
707 | ||
708 | Example: | |
97659181 MM |
709 | #lib/Makefile |
710 | clean-files := crc32table.h | |
1da177e4 | 711 | |
bd55daf4 JR |
712 | When executing "make clean", the file "crc32table.h" will be deleted. |
713 | Kbuild will assume files to be in the same relative directory as the | |
97659181 | 714 | Makefile, except if prefixed with $(objtree). |
1da177e4 | 715 | |
39e6e9cf BH |
716 | To delete a directory hierarchy use: |
717 | ||
1da177e4 LT |
718 | Example: |
719 | #scripts/package/Makefile | |
720 | clean-dirs := $(objtree)/debian/ | |
721 | ||
97659181 MM |
722 | This will delete the directory debian in the toplevel directory, including all |
723 | subdirectories. | |
1da177e4 | 724 | |
ef8ff89b MM |
725 | To exclude certain files from make clean, use the $(no-clean-files) variable. |
726 | This is only a special case used in the top level Kbuild file: | |
727 | ||
728 | Example: | |
729 | #Kbuild | |
730 | no-clean-files := $(bounds-file) $(offsets-file) | |
731 | ||
1da177e4 LT |
732 | Usually kbuild descends down in subdirectories due to "obj-* := dir/", |
733 | but in the architecture makefiles where the kbuild infrastructure | |
734 | is not sufficient this sometimes needs to be explicit. | |
735 | ||
736 | Example: | |
25eb650a | 737 | #arch/x86/boot/Makefile |
1da177e4 LT |
738 | subdir- := compressed/ |
739 | ||
740 | The above assignment instructs kbuild to descend down in the | |
741 | directory compressed/ when "make clean" is executed. | |
742 | ||
39fed701 | 743 | To support the clean infrastructure in the Makefiles that build the |
1da177e4 LT |
744 | final bootimage there is an optional target named archclean: |
745 | ||
746 | Example: | |
25eb650a | 747 | #arch/x86/Makefile |
1da177e4 | 748 | archclean: |
25eb650a | 749 | $(Q)$(MAKE) $(clean)=arch/x86/boot |
1da177e4 | 750 | |
25eb650a WG |
751 | When "make clean" is executed, make will descend down in arch/x86/boot, |
752 | and clean as usual. The Makefile located in arch/x86/boot/ may use | |
1da177e4 LT |
753 | the subdir- trick to descend further down. |
754 | ||
755 | Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is | |
756 | included in the top level makefile, and the kbuild infrastructure | |
757 | is not operational at that point. | |
758 | ||
759 | Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will | |
760 | be visited during "make clean". | |
761 | ||
762 | === 6 Architecture Makefiles | |
763 | ||
764 | The top level Makefile sets up the environment and does the preparation, | |
765 | before starting to descend down in the individual directories. | |
a07f6033 JE |
766 | The top level makefile contains the generic part, whereas |
767 | arch/$(ARCH)/Makefile contains what is required to set up kbuild | |
768 | for said architecture. | |
769 | To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines | |
1da177e4 LT |
770 | a few targets. |
771 | ||
a07f6033 JE |
772 | When kbuild executes, the following steps are followed (roughly): |
773 | 1) Configuration of the kernel => produce .config | |
1da177e4 | 774 | 2) Store kernel version in include/linux/version.h |
b22ae40e | 775 | 3) Updating all other prerequisites to the target prepare: |
1da177e4 | 776 | - Additional prerequisites are specified in arch/$(ARCH)/Makefile |
b22ae40e | 777 | 4) Recursively descend down in all directories listed in |
1da177e4 | 778 | init-* core* drivers-* net-* libs-* and build all targets. |
a07f6033 | 779 | - The values of the above variables are expanded in arch/$(ARCH)/Makefile. |
b22ae40e | 780 | 5) All object files are then linked and the resulting file vmlinux is |
a07f6033 | 781 | located at the root of the obj tree. |
1da177e4 LT |
782 | The very first objects linked are listed in head-y, assigned by |
783 | arch/$(ARCH)/Makefile. | |
b22ae40e | 784 | 6) Finally, the architecture-specific part does any required post processing |
1da177e4 LT |
785 | and builds the final bootimage. |
786 | - This includes building boot records | |
5c811e59 | 787 | - Preparing initrd images and the like |
1da177e4 LT |
788 | |
789 | ||
790 | --- 6.1 Set variables to tweak the build to the architecture | |
791 | ||
792 | LDFLAGS Generic $(LD) options | |
793 | ||
794 | Flags used for all invocations of the linker. | |
795 | Often specifying the emulation is sufficient. | |
796 | ||
797 | Example: | |
798 | #arch/s390/Makefile | |
799 | LDFLAGS := -m elf_s390 | |
f77bf014 | 800 | Note: ldflags-y can be used to further customise |
a9af3305 | 801 | the flags used. See chapter 3.7. |
39e6e9cf | 802 | |
1da177e4 LT |
803 | LDFLAGS_vmlinux Options for $(LD) when linking vmlinux |
804 | ||
805 | LDFLAGS_vmlinux is used to specify additional flags to pass to | |
a07f6033 | 806 | the linker when linking the final vmlinux image. |
1da177e4 LT |
807 | LDFLAGS_vmlinux uses the LDFLAGS_$@ support. |
808 | ||
809 | Example: | |
25eb650a | 810 | #arch/x86/Makefile |
1da177e4 LT |
811 | LDFLAGS_vmlinux := -e stext |
812 | ||
813 | OBJCOPYFLAGS objcopy flags | |
814 | ||
815 | When $(call if_changed,objcopy) is used to translate a .o file, | |
a07f6033 | 816 | the flags specified in OBJCOPYFLAGS will be used. |
1da177e4 LT |
817 | $(call if_changed,objcopy) is often used to generate raw binaries on |
818 | vmlinux. | |
819 | ||
820 | Example: | |
821 | #arch/s390/Makefile | |
822 | OBJCOPYFLAGS := -O binary | |
823 | ||
824 | #arch/s390/boot/Makefile | |
825 | $(obj)/image: vmlinux FORCE | |
826 | $(call if_changed,objcopy) | |
827 | ||
a07f6033 | 828 | In this example, the binary $(obj)/image is a binary version of |
1da177e4 LT |
829 | vmlinux. The usage of $(call if_changed,xxx) will be described later. |
830 | ||
222d394d | 831 | KBUILD_AFLAGS $(AS) assembler flags |
1da177e4 LT |
832 | |
833 | Default value - see top level Makefile | |
834 | Append or modify as required per architecture. | |
835 | ||
836 | Example: | |
837 | #arch/sparc64/Makefile | |
222d394d | 838 | KBUILD_AFLAGS += -m64 -mcpu=ultrasparc |
1da177e4 | 839 | |
a0f97e06 | 840 | KBUILD_CFLAGS $(CC) compiler flags |
1da177e4 LT |
841 | |
842 | Default value - see top level Makefile | |
843 | Append or modify as required per architecture. | |
844 | ||
a0f97e06 | 845 | Often, the KBUILD_CFLAGS variable depends on the configuration. |
1da177e4 LT |
846 | |
847 | Example: | |
ff4eb04c PB |
848 | #arch/x86/boot/compressed/Makefile |
849 | cflags-$(CONFIG_X86_32) := -march=i386 | |
850 | cflags-$(CONFIG_X86_64) := -mcmodel=small | |
a0f97e06 | 851 | KBUILD_CFLAGS += $(cflags-y) |
1da177e4 LT |
852 | |
853 | Many arch Makefiles dynamically run the target C compiler to | |
854 | probe supported options: | |
855 | ||
25eb650a | 856 | #arch/x86/Makefile |
1da177e4 LT |
857 | |
858 | ... | |
859 | cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ | |
860 | -march=pentium2,-march=i686) | |
861 | ... | |
862 | # Disable unit-at-a-time mode ... | |
a0f97e06 | 863 | KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) |
1da177e4 LT |
864 | ... |
865 | ||
866 | ||
a07f6033 | 867 | The first example utilises the trick that a config option expands |
1da177e4 LT |
868 | to 'y' when selected. |
869 | ||
80c00ba9 | 870 | KBUILD_AFLAGS_KERNEL $(AS) options specific for built-in |
1da177e4 | 871 | |
80c00ba9 | 872 | $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile |
1da177e4 LT |
873 | resident kernel code. |
874 | ||
6588169d | 875 | KBUILD_AFLAGS_MODULE Options for $(AS) when building modules |
1da177e4 | 876 | |
39fed701 | 877 | $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that |
6588169d SR |
878 | are used for $(AS). |
879 | From commandline AFLAGS_MODULE shall be used (see kbuild.txt). | |
1da177e4 | 880 | |
80c00ba9 SR |
881 | KBUILD_CFLAGS_KERNEL $(CC) options specific for built-in |
882 | ||
883 | $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile | |
884 | resident kernel code. | |
885 | ||
6588169d SR |
886 | KBUILD_CFLAGS_MODULE Options for $(CC) when building modules |
887 | ||
39fed701 | 888 | $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that |
6588169d SR |
889 | are used for $(CC). |
890 | From commandline CFLAGS_MODULE shall be used (see kbuild.txt). | |
891 | ||
892 | KBUILD_LDFLAGS_MODULE Options for $(LD) when linking modules | |
893 | ||
39fed701 | 894 | $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options |
6588169d SR |
895 | used when linking modules. This is often a linker script. |
896 | From commandline LDFLAGS_MODULE shall be used (see kbuild.txt). | |
39e6e9cf | 897 | |
40df759e MM |
898 | KBUILD_ARFLAGS Options for $(AR) when creating archives |
899 | ||
900 | $(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic | |
901 | mode) if this option is supported by $(AR). | |
902 | ||
61754c18 MM |
903 | ARCH_CPPFLAGS, ARCH_AFLAGS, ARCH_CFLAGS Overrides the kbuild defaults |
904 | ||
905 | These variables are appended to the KBUILD_CPPFLAGS, | |
906 | KBUILD_AFLAGS, and KBUILD_CFLAGS, respectively, after the | |
907 | top-level Makefile has set any other flags. This provides a | |
908 | means for an architecture to override the defaults. | |
909 | ||
910 | ||
052ad274 PA |
911 | --- 6.2 Add prerequisites to archheaders: |
912 | ||
913 | The archheaders: rule is used to generate header files that | |
914 | may be installed into user space by "make header_install" or | |
915 | "make headers_install_all". In order to support | |
916 | "make headers_install_all", this target has to be able to run | |
917 | on an unconfigured tree, or a tree configured for another | |
918 | architecture. | |
919 | ||
920 | It is run before "make archprepare" when run on the | |
921 | architecture itself. | |
922 | ||
923 | ||
924 | --- 6.3 Add prerequisites to archprepare: | |
1da177e4 | 925 | |
a07f6033 | 926 | The archprepare: rule is used to list prerequisites that need to be |
1da177e4 | 927 | built before starting to descend down in the subdirectories. |
a07f6033 | 928 | This is usually used for header files containing assembler constants. |
1da177e4 LT |
929 | |
930 | Example: | |
5bb78269 SR |
931 | #arch/arm/Makefile |
932 | archprepare: maketools | |
1da177e4 | 933 | |
a07f6033 | 934 | In this example, the file target maketools will be processed |
5bb78269 | 935 | before descending down in the subdirectories. |
1da177e4 LT |
936 | See also chapter XXX-TODO that describe how kbuild supports |
937 | generating offset header files. | |
938 | ||
939 | ||
052ad274 | 940 | --- 6.4 List directories to visit when descending |
1da177e4 LT |
941 | |
942 | An arch Makefile cooperates with the top Makefile to define variables | |
943 | which specify how to build the vmlinux file. Note that there is no | |
944 | corresponding arch-specific section for modules; the module-building | |
945 | machinery is all architecture-independent. | |
946 | ||
39e6e9cf | 947 | |
1da177e4 LT |
948 | head-y, init-y, core-y, libs-y, drivers-y, net-y |
949 | ||
a07f6033 JE |
950 | $(head-y) lists objects to be linked first in vmlinux. |
951 | $(libs-y) lists directories where a lib.a archive can be located. | |
f49821ee | 952 | The rest list directories where a built-in.a object file can be |
a07f6033 | 953 | located. |
1da177e4 LT |
954 | |
955 | $(init-y) objects will be located after $(head-y). | |
956 | Then the rest follows in this order: | |
957 | $(core-y), $(libs-y), $(drivers-y) and $(net-y). | |
958 | ||
a07f6033 | 959 | The top level Makefile defines values for all generic directories, |
5c811e59 | 960 | and arch/$(ARCH)/Makefile only adds architecture-specific directories. |
1da177e4 LT |
961 | |
962 | Example: | |
963 | #arch/sparc64/Makefile | |
964 | core-y += arch/sparc64/kernel/ | |
965 | libs-y += arch/sparc64/prom/ arch/sparc64/lib/ | |
966 | drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ | |
967 | ||
968 | ||
052ad274 | 969 | --- 6.5 Architecture-specific boot images |
1da177e4 LT |
970 | |
971 | An arch Makefile specifies goals that take the vmlinux file, compress | |
972 | it, wrap it in bootstrapping code, and copy the resulting files | |
973 | somewhere. This includes various kinds of installation commands. | |
974 | The actual goals are not standardized across architectures. | |
975 | ||
976 | It is common to locate any additional processing in a boot/ | |
977 | directory below arch/$(ARCH)/. | |
978 | ||
979 | Kbuild does not provide any smart way to support building a | |
980 | target specified in boot/. Therefore arch/$(ARCH)/Makefile shall | |
981 | call make manually to build a target in boot/. | |
982 | ||
983 | The recommended approach is to include shortcuts in | |
984 | arch/$(ARCH)/Makefile, and use the full path when calling down | |
985 | into the arch/$(ARCH)/boot/Makefile. | |
986 | ||
987 | Example: | |
25eb650a WG |
988 | #arch/x86/Makefile |
989 | boot := arch/x86/boot | |
1da177e4 LT |
990 | bzImage: vmlinux |
991 | $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ | |
992 | ||
993 | "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke | |
994 | make in a subdirectory. | |
995 | ||
5c811e59 | 996 | There are no rules for naming architecture-specific targets, |
1da177e4 | 997 | but executing "make help" will list all relevant targets. |
a07f6033 | 998 | To support this, $(archhelp) must be defined. |
1da177e4 LT |
999 | |
1000 | Example: | |
25eb650a | 1001 | #arch/x86/Makefile |
1da177e4 LT |
1002 | define archhelp |
1003 | echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' | |
39e6e9cf | 1004 | endif |
1da177e4 LT |
1005 | |
1006 | When make is executed without arguments, the first goal encountered | |
1007 | will be built. In the top level Makefile the first goal present | |
1008 | is all:. | |
a07f6033 JE |
1009 | An architecture shall always, per default, build a bootable image. |
1010 | In "make help", the default goal is highlighted with a '*'. | |
1da177e4 LT |
1011 | Add a new prerequisite to all: to select a default goal different |
1012 | from vmlinux. | |
1013 | ||
1014 | Example: | |
25eb650a | 1015 | #arch/x86/Makefile |
39e6e9cf | 1016 | all: bzImage |
1da177e4 LT |
1017 | |
1018 | When "make" is executed without arguments, bzImage will be built. | |
1019 | ||
052ad274 | 1020 | --- 6.6 Building non-kbuild targets |
1da177e4 LT |
1021 | |
1022 | extra-y | |
1023 | ||
39fed701 | 1024 | extra-y specifies additional targets created in the current |
1da177e4 LT |
1025 | directory, in addition to any targets specified by obj-*. |
1026 | ||
1027 | Listing all targets in extra-y is required for two purposes: | |
1028 | 1) Enable kbuild to check changes in command lines | |
1029 | - When $(call if_changed,xxx) is used | |
1030 | 2) kbuild knows what files to delete during "make clean" | |
1031 | ||
1032 | Example: | |
25eb650a | 1033 | #arch/x86/kernel/Makefile |
1da177e4 LT |
1034 | extra-y := head.o init_task.o |
1035 | ||
a07f6033 | 1036 | In this example, extra-y is used to list object files that |
f49821ee | 1037 | shall be built, but shall not be linked as part of built-in.a. |
1da177e4 | 1038 | |
39e6e9cf | 1039 | |
052ad274 | 1040 | --- 6.7 Commands useful for building a boot image |
1da177e4 LT |
1041 | |
1042 | Kbuild provides a few macros that are useful when building a | |
1043 | boot image. | |
1044 | ||
1045 | if_changed | |
1046 | ||
1047 | if_changed is the infrastructure used for the following commands. | |
1048 | ||
1049 | Usage: | |
1050 | target: source(s) FORCE | |
ef80f0a1 | 1051 | $(call if_changed,ld/objcopy/gzip/...) |
1da177e4 | 1052 | |
a07f6033 | 1053 | When the rule is evaluated, it is checked to see if any files |
5c811e59 | 1054 | need an update, or the command line has changed since the last |
1da177e4 LT |
1055 | invocation. The latter will force a rebuild if any options |
1056 | to the executable have changed. | |
1057 | Any target that utilises if_changed must be listed in $(targets), | |
1058 | otherwise the command line check will fail, and the target will | |
1059 | always be built. | |
1060 | Assignments to $(targets) are without $(obj)/ prefix. | |
1061 | if_changed may be used in conjunction with custom commands as | |
052ad274 | 1062 | defined in 6.8 "Custom kbuild commands". |
49490571 | 1063 | |
1da177e4 | 1064 | Note: It is a typical mistake to forget the FORCE prerequisite. |
49490571 PBG |
1065 | Another common pitfall is that whitespace is sometimes |
1066 | significant; for instance, the below will fail (note the extra space | |
1067 | after the comma): | |
1068 | target: source(s) FORCE | |
ef80f0a1 | 1069 | #WRONG!# $(call if_changed, ld/objcopy/gzip/...) |
1da177e4 | 1070 | |
bb81955f DG |
1071 | Note: if_changed should not be used more than once per target. |
1072 | It stores the executed command in a corresponding .cmd | |
1073 | file and multiple calls would result in overwrites and | |
1074 | unwanted results when the target is up to date and only the | |
1075 | tests on changed commands trigger execution of commands. | |
1076 | ||
1da177e4 | 1077 | ld |
a07f6033 | 1078 | Link target. Often, LDFLAGS_$@ is used to set specific options to ld. |
39e6e9cf | 1079 | |
1da177e4 | 1080 | Example: |
25eb650a | 1081 | #arch/x86/boot/Makefile |
1da177e4 LT |
1082 | LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary |
1083 | LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext | |
1084 | ||
1085 | targets += setup setup.o bootsect bootsect.o | |
1086 | $(obj)/setup $(obj)/bootsect: %: %.o FORCE | |
1087 | $(call if_changed,ld) | |
1088 | ||
a07f6033 JE |
1089 | In this example, there are two possible targets, requiring different |
1090 | options to the linker. The linker options are specified using the | |
1da177e4 | 1091 | LDFLAGS_$@ syntax - one for each potential target. |
5d3f083d | 1092 | $(targets) are assigned all potential targets, by which kbuild knows |
1da177e4 LT |
1093 | the targets and will: |
1094 | 1) check for commandline changes | |
1095 | 2) delete target during make clean | |
1096 | ||
1097 | The ": %: %.o" part of the prerequisite is a shorthand that | |
39fed701 | 1098 | frees us from listing the setup.o and bootsect.o files. |
ef80f0a1 | 1099 | Note: It is a common mistake to forget the "targets :=" assignment, |
1da177e4 LT |
1100 | resulting in the target file being recompiled for no |
1101 | obvious reason. | |
1102 | ||
d87e47e1 C |
1103 | objcopy |
1104 | Copy binary. Uses OBJCOPYFLAGS usually specified in | |
1105 | arch/$(ARCH)/Makefile. | |
1106 | OBJCOPYFLAGS_$@ may be used to set additional options. | |
1107 | ||
1108 | gzip | |
1109 | Compress target. Use maximum compression to compress target. | |
1110 | ||
1111 | Example: | |
1112 | #arch/x86/boot/compressed/Makefile | |
1113 | $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE | |
1114 | $(call if_changed,gzip) | |
1115 | ||
aab94339 | 1116 | dtc |
c1410562 | 1117 | Create flattened device tree blob object suitable for linking |
aab94339 DB |
1118 | into vmlinux. Device tree blobs linked into vmlinux are placed |
1119 | in an init section in the image. Platform code *must* copy the | |
1120 | blob to non-init memory prior to calling unflatten_device_tree(). | |
1121 | ||
90b335fb SW |
1122 | To use this command, simply add *.dtb into obj-y or targets, or make |
1123 | some other target depend on %.dtb | |
aab94339 | 1124 | |
90b335fb SW |
1125 | A central rule exists to create $(obj)/%.dtb from $(src)/%.dts; |
1126 | architecture Makefiles do no need to explicitly write out that rule. | |
aab94339 | 1127 | |
90b335fb SW |
1128 | Example: |
1129 | targets += $(dtb-y) | |
90b335fb | 1130 | DTC_FLAGS ?= -p 1024 |
1da177e4 | 1131 | |
052ad274 | 1132 | --- 6.8 Custom kbuild commands |
1da177e4 | 1133 | |
a07f6033 | 1134 | When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand |
1da177e4 LT |
1135 | of a command is normally displayed. |
1136 | To enable this behaviour for custom commands kbuild requires | |
1137 | two variables to be set: | |
1138 | quiet_cmd_<command> - what shall be echoed | |
1139 | cmd_<command> - the command to execute | |
1140 | ||
1141 | Example: | |
1142 | # | |
1143 | quiet_cmd_image = BUILD $@ | |
1144 | cmd_image = $(obj)/tools/build $(BUILDFLAGS) \ | |
1145 | $(obj)/vmlinux.bin > $@ | |
1146 | ||
1147 | targets += bzImage | |
1148 | $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE | |
1149 | $(call if_changed,image) | |
1150 | @echo 'Kernel: $@ is ready' | |
1151 | ||
a07f6033 | 1152 | When updating the $(obj)/bzImage target, the line |
1da177e4 | 1153 | |
25eb650a | 1154 | BUILD arch/x86/boot/bzImage |
1da177e4 LT |
1155 | |
1156 | will be displayed with "make KBUILD_VERBOSE=0". | |
39e6e9cf | 1157 | |
1da177e4 | 1158 | |
052ad274 | 1159 | --- 6.9 Preprocessing linker scripts |
1da177e4 | 1160 | |
a07f6033 | 1161 | When the vmlinux image is built, the linker script |
1da177e4 LT |
1162 | arch/$(ARCH)/kernel/vmlinux.lds is used. |
1163 | The script is a preprocessed variant of the file vmlinux.lds.S | |
1164 | located in the same directory. | |
a07f6033 | 1165 | kbuild knows .lds files and includes a rule *lds.S -> *lds. |
39e6e9cf | 1166 | |
1da177e4 | 1167 | Example: |
25eb650a | 1168 | #arch/x86/kernel/Makefile |
1da177e4 | 1169 | always := vmlinux.lds |
39e6e9cf | 1170 | |
1da177e4 LT |
1171 | #Makefile |
1172 | export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) | |
39e6e9cf BH |
1173 | |
1174 | The assignment to $(always) is used to tell kbuild to build the | |
a07f6033 JE |
1175 | target vmlinux.lds. |
1176 | The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the | |
1da177e4 | 1177 | specified options when building the target vmlinux.lds. |
39e6e9cf | 1178 | |
a07f6033 | 1179 | When building the *.lds target, kbuild uses the variables: |
06c5040c | 1180 | KBUILD_CPPFLAGS : Set in top-level Makefile |
f77bf014 | 1181 | cppflags-y : May be set in the kbuild makefile |
39fed701 | 1182 | CPPFLAGS_$(@F) : Target-specific flags. |
1da177e4 LT |
1183 | Note that the full filename is used in this |
1184 | assignment. | |
1185 | ||
39fed701 | 1186 | The kbuild infrastructure for *lds files is used in several |
5c811e59 | 1187 | architecture-specific files. |
1da177e4 | 1188 | |
052ad274 | 1189 | --- 6.10 Generic header files |
d8ecc5cd SR |
1190 | |
1191 | The directory include/asm-generic contains the header files | |
1192 | that may be shared between individual architectures. | |
1193 | The recommended approach how to use a generic header file is | |
1194 | to list the file in the Kbuild file. | |
d87e47e1 | 1195 | See "7.2 generic-y" for further info on syntax etc. |
d8ecc5cd | 1196 | |
fbe6e37d NP |
1197 | --- 6.11 Post-link pass |
1198 | ||
1199 | If the file arch/xxx/Makefile.postlink exists, this makefile | |
1200 | will be invoked for post-link objects (vmlinux and modules.ko) | |
1201 | for architectures to run post-link passes on. Must also handle | |
1202 | the clean target. | |
1203 | ||
1204 | This pass runs after kallsyms generation. If the architecture | |
1205 | needs to modify symbol locations, rather than manipulate the | |
1206 | kallsyms, it may be easier to add another postlink target for | |
1207 | .tmp_vmlinux? targets to be called from link-vmlinux.sh. | |
1208 | ||
1209 | For example, powerpc uses this to check relocation sanity of | |
1210 | the linked vmlinux file. | |
1211 | ||
c7bb349e SR |
1212 | === 7 Kbuild syntax for exported headers |
1213 | ||
39fed701 | 1214 | The kernel includes 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: | |
39fed701 | 1218 | - drop kernel-specific annotations |
c7bb349e | 1219 | - drop include of compiler.h |
c95940f2 | 1220 | - drop all sections that are kernel internal (guarded by ifdef __KERNEL__) |
c7bb349e | 1221 | |
fcc8487d | 1222 | All headers under include/uapi/, include/generated/uapi/, |
61562f98 | 1223 | arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/ |
fcc8487d | 1224 | are exported. |
c7bb349e | 1225 | |
fcc8487d ND |
1226 | A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and |
1227 | arch/<arch>/include/asm/ to list asm files coming from asm-generic. | |
1228 | See subsequent chapter for the syntax of the Kbuild file. | |
c7bb349e | 1229 | |
d87e47e1 | 1230 | --- 7.1 no-export-headers |
c7bb349e | 1231 | |
fcc8487d ND |
1232 | no-export-headers is essentially used by include/uapi/linux/Kbuild to |
1233 | avoid exporting specific headers (e.g. kvm.h) on architectures that do | |
1234 | not support it. It should be avoided as much as possible. | |
c7bb349e | 1235 | |
d87e47e1 | 1236 | --- 7.2 generic-y |
d8ecc5cd SR |
1237 | |
1238 | If an architecture uses a verbatim copy of a header from | |
1239 | include/asm-generic then this is listed in the file | |
1240 | arch/$(ARCH)/include/asm/Kbuild like this: | |
1241 | ||
1242 | Example: | |
1243 | #arch/x86/include/asm/Kbuild | |
1244 | generic-y += termios.h | |
1245 | generic-y += rtc.h | |
1246 | ||
1247 | During the prepare phase of the build a wrapper include | |
1248 | file is generated in the directory: | |
1249 | ||
1250 | arch/$(ARCH)/include/generated/asm | |
1251 | ||
1252 | When a header is exported where the architecture uses | |
1253 | the generic header a similar wrapper is generated as part | |
1254 | of the set of exported headers in the directory: | |
1255 | ||
1256 | usr/include/asm | |
1257 | ||
1258 | The generated wrapper will in both cases look like the following: | |
1259 | ||
1260 | Example: termios.h | |
1261 | #include <asm-generic/termios.h> | |
c7bb349e | 1262 | |
d87e47e1 | 1263 | --- 7.3 generated-y |
54b880ca JH |
1264 | |
1265 | If an architecture generates other header files alongside generic-y | |
ae3f4151 | 1266 | wrappers, generated-y specifies them. |
54b880ca JH |
1267 | |
1268 | This prevents them being treated as stale asm-generic wrappers and | |
1269 | removed. | |
1270 | ||
1271 | Example: | |
1272 | #arch/x86/include/asm/Kbuild | |
1273 | generated-y += syscalls_32.h | |
1274 | ||
d87e47e1 | 1275 | --- 7.4 mandatory-y |
fcc8487d | 1276 | |
91998731 MY |
1277 | mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild.asm |
1278 | to define the minimum set of ASM headers that all architectures must have. | |
1279 | ||
1280 | This works like optional generic-y. If a mandatory header is missing | |
1281 | in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate | |
1282 | a wrapper of the asm-generic one. | |
fcc8487d ND |
1283 | |
1284 | The convention is to list one subdir per line and | |
1285 | preferably in alphabetic order. | |
1286 | ||
c7bb349e | 1287 | === 8 Kbuild Variables |
1da177e4 LT |
1288 | |
1289 | The top Makefile exports the following variables: | |
1290 | ||
1291 | VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION | |
1292 | ||
1293 | These variables define the current kernel version. A few arch | |
1294 | Makefiles actually use these values directly; they should use | |
1295 | $(KERNELRELEASE) instead. | |
1296 | ||
1297 | $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic | |
1298 | three-part version number, such as "2", "4", and "0". These three | |
1299 | values are always numeric. | |
1300 | ||
1301 | $(EXTRAVERSION) defines an even tinier sublevel for pre-patches | |
1302 | or additional patches. It is usually some non-numeric string | |
1303 | such as "-pre4", and is often blank. | |
1304 | ||
1305 | KERNELRELEASE | |
1306 | ||
1307 | $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable | |
1308 | for constructing installation directory names or showing in | |
1309 | version strings. Some arch Makefiles use it for this purpose. | |
1310 | ||
1311 | ARCH | |
1312 | ||
1313 | This variable defines the target architecture, such as "i386", | |
1314 | "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to | |
1315 | determine which files to compile. | |
1316 | ||
1317 | By default, the top Makefile sets $(ARCH) to be the same as the | |
1318 | host system architecture. For a cross build, a user may | |
1319 | override the value of $(ARCH) on the command line: | |
1320 | ||
1321 | make ARCH=m68k ... | |
1322 | ||
1323 | ||
1324 | INSTALL_PATH | |
1325 | ||
1326 | This variable defines a place for the arch Makefiles to install | |
1327 | the resident kernel image and System.map file. | |
5c811e59 | 1328 | Use this for architecture-specific install targets. |
1da177e4 LT |
1329 | |
1330 | INSTALL_MOD_PATH, MODLIB | |
1331 | ||
1332 | $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module | |
1333 | installation. This variable is not defined in the Makefile but | |
1334 | may be passed in by the user if desired. | |
1335 | ||
1336 | $(MODLIB) specifies the directory for module installation. | |
1337 | The top Makefile defines $(MODLIB) to | |
1338 | $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may | |
1339 | override this value on the command line if desired. | |
1340 | ||
ac031f26 TT |
1341 | INSTALL_MOD_STRIP |
1342 | ||
39fed701 | 1343 | If this variable is specified, it will cause modules to be stripped |
ac031f26 | 1344 | after they are installed. If INSTALL_MOD_STRIP is '1', then the |
39fed701 | 1345 | default option --strip-debug will be used. Otherwise, the |
177b241d GE |
1346 | INSTALL_MOD_STRIP value will be used as the option(s) to the strip |
1347 | command. | |
ac031f26 TT |
1348 | |
1349 | ||
c7bb349e | 1350 | === 9 Makefile language |
1da177e4 | 1351 | |
a07f6033 | 1352 | The kernel Makefiles are designed to be run with GNU Make. The Makefiles |
1da177e4 LT |
1353 | use only the documented features of GNU Make, but they do use many |
1354 | GNU extensions. | |
1355 | ||
1356 | GNU Make supports elementary list-processing functions. The kernel | |
1357 | Makefiles use a novel style of list building and manipulation with few | |
1358 | "if" statements. | |
1359 | ||
1360 | GNU Make has two assignment operators, ":=" and "=". ":=" performs | |
1361 | immediate evaluation of the right-hand side and stores an actual string | |
1362 | into the left-hand side. "=" is like a formula definition; it stores the | |
1363 | right-hand side in an unevaluated form and then evaluates this form each | |
1364 | time the left-hand side is used. | |
1365 | ||
1366 | There are some cases where "=" is appropriate. Usually, though, ":=" | |
1367 | is the right choice. | |
1368 | ||
c7bb349e | 1369 | === 10 Credits |
1da177e4 LT |
1370 | |
1371 | Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> | |
1372 | Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> | |
1373 | Updates by Sam Ravnborg <sam@ravnborg.org> | |
a07f6033 | 1374 | Language QA by Jan Engelhardt <jengelh@gmx.de> |
1da177e4 | 1375 | |
c7bb349e | 1376 | === 11 TODO |
1da177e4 | 1377 | |
a07f6033 | 1378 | - Describe how kbuild supports shipped files with _shipped. |
1da177e4 LT |
1379 | - Generating offset header files. |
1380 | - Add more variables to section 7? | |
1381 | ||
39e6e9cf BH |
1382 | |
1383 |