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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. |
6358d6e8 NP |
157 | This is a thin archive without a symbol table, which makes it |
158 | unsuitable as a linker input. | |
159 | ||
160 | The scripts/link-vmlinux.sh script later makes an aggregate | |
f49821ee | 161 | built-in.a with "${AR} rcsTP", which creates the thin archive |
6358d6e8 NP |
162 | with a symbol table and an index, making it a valid input for |
163 | the final vmlinux link passes. | |
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164 | |
165 | The order of files in $(obj-y) is significant. Duplicates in | |
166 | the lists are allowed: the first instance will be linked into | |
f49821ee | 167 | built-in.a and succeeding instances will be ignored. |
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168 | |
169 | Link order is significant, because certain functions | |
170 | (module_init() / __initcall) will be called during boot in the | |
171 | order they appear. So keep in mind that changing the link | |
a07f6033 JE |
172 | order may e.g. change the order in which your SCSI |
173 | controllers are detected, and thus your disks are renumbered. | |
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174 | |
175 | Example: | |
176 | #drivers/isdn/i4l/Makefile | |
177 | # Makefile for the kernel ISDN subsystem and device drivers. | |
178 | # Each configuration option enables a list of files. | |
2f5a2f81 | 179 | obj-$(CONFIG_ISDN_I4L) += isdn.o |
1da177e4 LT |
180 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o |
181 | ||
182 | --- 3.3 Loadable module goals - obj-m | |
183 | ||
39fed701 | 184 | $(obj-m) specifies object files which are built as loadable |
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185 | kernel modules. |
186 | ||
187 | A module may be built from one source file or several source | |
188 | files. In the case of one source file, the kbuild makefile | |
189 | simply adds the file to $(obj-m). | |
190 | ||
191 | Example: | |
192 | #drivers/isdn/i4l/Makefile | |
193 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o | |
194 | ||
195 | Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm' | |
196 | ||
197 | If a kernel module is built from several source files, you specify | |
4f827280 MM |
198 | that you want to build a module in the same way as above; however, |
199 | kbuild needs to know which object files you want to build your | |
200 | module from, so you have to tell it by setting a $(<module_name>-y) | |
201 | variable. | |
1da177e4 LT |
202 | |
203 | Example: | |
204 | #drivers/isdn/i4l/Makefile | |
4f827280 MM |
205 | obj-$(CONFIG_ISDN_I4L) += isdn.o |
206 | isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o | |
1da177e4 LT |
207 | |
208 | In this example, the module name will be isdn.o. Kbuild will | |
4f827280 | 209 | compile the objects listed in $(isdn-y) and then run |
1da177e4 LT |
210 | "$(LD) -r" on the list of these files to generate isdn.o. |
211 | ||
4f827280 MM |
212 | Due to kbuild recognizing $(<module_name>-y) for composite objects, |
213 | you can use the value of a CONFIG_ symbol to optionally include an | |
214 | object file as part of a composite object. | |
1da177e4 LT |
215 | |
216 | Example: | |
217 | #fs/ext2/Makefile | |
4f827280 MM |
218 | obj-$(CONFIG_EXT2_FS) += ext2.o |
219 | ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \ | |
220 | namei.o super.o symlink.o | |
221 | ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \ | |
222 | xattr_trusted.o | |
223 | ||
224 | In this example, xattr.o, xattr_user.o and xattr_trusted.o are only | |
225 | part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR) | |
226 | evaluates to 'y'. | |
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227 | |
228 | Note: Of course, when you are building objects into the kernel, | |
229 | the syntax above will also work. So, if you have CONFIG_EXT2_FS=y, | |
230 | kbuild will build an ext2.o file for you out of the individual | |
f49821ee | 231 | parts and then link this into built-in.a, as you would expect. |
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232 | |
233 | --- 3.4 Objects which export symbols | |
234 | ||
235 | No special notation is required in the makefiles for | |
236 | modules exporting symbols. | |
237 | ||
238 | --- 3.5 Library file goals - lib-y | |
239 | ||
a07f6033 | 240 | Objects listed with obj-* are used for modules, or |
f49821ee | 241 | combined in a built-in.a for that specific directory. |
1da177e4 LT |
242 | There is also the possibility to list objects that will |
243 | be included in a library, lib.a. | |
244 | All objects listed with lib-y are combined in a single | |
245 | library for that directory. | |
5d3f083d ML |
246 | Objects that are listed in obj-y and additionally listed in |
247 | lib-y will not be included in the library, since they will | |
248 | be accessible anyway. | |
a07f6033 | 249 | For consistency, objects listed in lib-m will be included in lib.a. |
1da177e4 LT |
250 | |
251 | Note that the same kbuild makefile may list files to be built-in | |
252 | and to be part of a library. Therefore the same directory | |
f49821ee | 253 | may contain both a built-in.a and a lib.a file. |
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254 | |
255 | Example: | |
2f5a2f81 MM |
256 | #arch/x86/lib/Makefile |
257 | lib-y := delay.o | |
1da177e4 | 258 | |
2f5a2f81 MM |
259 | This will create a library lib.a based on delay.o. For kbuild to |
260 | actually recognize that there is a lib.a being built, the directory | |
261 | shall be listed in libs-y. | |
052ad274 | 262 | See also "6.4 List directories to visit when descending". |
39e6e9cf | 263 | |
a07f6033 | 264 | Use of lib-y is normally restricted to lib/ and arch/*/lib. |
1da177e4 LT |
265 | |
266 | --- 3.6 Descending down in directories | |
267 | ||
268 | A Makefile is only responsible for building objects in its own | |
269 | directory. Files in subdirectories should be taken care of by | |
270 | Makefiles in these subdirs. The build system will automatically | |
271 | invoke make recursively in subdirectories, provided you let it know of | |
272 | them. | |
273 | ||
a07f6033 | 274 | To do so, obj-y and obj-m are used. |
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275 | ext2 lives in a separate directory, and the Makefile present in fs/ |
276 | tells kbuild to descend down using the following assignment. | |
277 | ||
278 | Example: | |
279 | #fs/Makefile | |
280 | obj-$(CONFIG_EXT2_FS) += ext2/ | |
281 | ||
282 | If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular) | |
283 | the corresponding obj- variable will be set, and kbuild will descend | |
284 | down in the ext2 directory. | |
285 | Kbuild only uses this information to decide that it needs to visit | |
286 | the directory, it is the Makefile in the subdirectory that | |
39fed701 | 287 | specifies what is modular and what is built-in. |
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288 | |
289 | It is good practice to use a CONFIG_ variable when assigning directory | |
290 | names. This allows kbuild to totally skip the directory if the | |
291 | corresponding CONFIG_ option is neither 'y' nor 'm'. | |
292 | ||
293 | --- 3.7 Compilation flags | |
294 | ||
f77bf014 | 295 | ccflags-y, asflags-y and ldflags-y |
c95940f2 NK |
296 | These three flags apply only to the kbuild makefile in which they |
297 | are assigned. They are used for all the normal cc, as and ld | |
298 | invocations happening during a recursive build. | |
f77bf014 | 299 | Note: Flags with the same behaviour were previously named: |
c95940f2 NK |
300 | EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS. |
301 | They are still supported but their usage is deprecated. | |
1da177e4 | 302 | |
eb07e1b4 | 303 | ccflags-y specifies options for compiling with $(CC). |
1da177e4 LT |
304 | |
305 | Example: | |
3d91a353 SD |
306 | # drivers/acpi/acpica/Makefile |
307 | ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA | |
308 | ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT | |
1da177e4 LT |
309 | |
310 | This variable is necessary because the top Makefile owns the | |
a0f97e06 | 311 | variable $(KBUILD_CFLAGS) and uses it for compilation flags for the |
1da177e4 LT |
312 | entire tree. |
313 | ||
eb07e1b4 | 314 | asflags-y specifies options for assembling with $(AS). |
1da177e4 LT |
315 | |
316 | Example: | |
eb07e1b4 MM |
317 | #arch/sparc/kernel/Makefile |
318 | asflags-y := -ansi | |
1da177e4 | 319 | |
eb07e1b4 | 320 | ldflags-y specifies options for linking with $(LD). |
1da177e4 LT |
321 | |
322 | Example: | |
eb07e1b4 MM |
323 | #arch/cris/boot/compressed/Makefile |
324 | ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds | |
1da177e4 | 325 | |
720097d8 | 326 | subdir-ccflags-y, subdir-asflags-y |
eb07e1b4 | 327 | The two flags listed above are similar to ccflags-y and asflags-y. |
c95940f2 NK |
328 | The difference is that the subdir- variants have effect for the kbuild |
329 | file where they are present and all subdirectories. | |
330 | Options specified using subdir-* are added to the commandline before | |
331 | the options specified using the non-subdir variants. | |
720097d8 SR |
332 | |
333 | Example: | |
334 | subdir-ccflags-y := -Werror | |
335 | ||
1da177e4 LT |
336 | CFLAGS_$@, AFLAGS_$@ |
337 | ||
338 | CFLAGS_$@ and AFLAGS_$@ only apply to commands in current | |
339 | kbuild makefile. | |
340 | ||
341 | $(CFLAGS_$@) specifies per-file options for $(CC). The $@ | |
342 | part has a literal value which specifies the file that it is for. | |
343 | ||
344 | Example: | |
345 | # drivers/scsi/Makefile | |
346 | CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF | |
347 | CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \ | |
348 | -DGDTH_STATISTICS | |
1da177e4 | 349 | |
eb07e1b4 | 350 | These two lines specify compilation flags for aha152x.o and gdth.o. |
1da177e4 LT |
351 | |
352 | $(AFLAGS_$@) is a similar feature for source files in assembly | |
353 | languages. | |
354 | ||
355 | Example: | |
356 | # arch/arm/kernel/Makefile | |
eb07e1b4 MM |
357 | AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET) |
358 | AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312 | |
359 | AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt | |
360 | ||
1da177e4 LT |
361 | |
362 | --- 3.9 Dependency tracking | |
363 | ||
364 | Kbuild tracks dependencies on the following: | |
365 | 1) All prerequisite files (both *.c and *.h) | |
366 | 2) CONFIG_ options used in all prerequisite files | |
367 | 3) Command-line used to compile target | |
368 | ||
369 | Thus, if you change an option to $(CC) all affected files will | |
370 | be re-compiled. | |
371 | ||
372 | --- 3.10 Special Rules | |
373 | ||
374 | Special rules are used when the kbuild infrastructure does | |
375 | not provide the required support. A typical example is | |
376 | header files generated during the build process. | |
5c811e59 | 377 | Another example are the architecture-specific Makefiles which |
a07f6033 | 378 | need special rules to prepare boot images etc. |
1da177e4 LT |
379 | |
380 | Special rules are written as normal Make rules. | |
381 | Kbuild is not executing in the directory where the Makefile is | |
382 | located, so all special rules shall provide a relative | |
383 | path to prerequisite files and target files. | |
384 | ||
385 | Two variables are used when defining special rules: | |
386 | ||
387 | $(src) | |
388 | $(src) is a relative path which points to the directory | |
389 | where the Makefile is located. Always use $(src) when | |
390 | referring to files located in the src tree. | |
391 | ||
392 | $(obj) | |
393 | $(obj) is a relative path which points to the directory | |
394 | where the target is saved. Always use $(obj) when | |
395 | referring to generated files. | |
396 | ||
397 | Example: | |
398 | #drivers/scsi/Makefile | |
399 | $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl | |
400 | $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl | |
401 | ||
402 | This is a special rule, following the normal syntax | |
403 | required by make. | |
404 | The target file depends on two prerequisite files. References | |
405 | to the target file are prefixed with $(obj), references | |
406 | to prerequisites are referenced with $(src) (because they are not | |
407 | generated files). | |
408 | ||
5410ecc0 MF |
409 | $(kecho) |
410 | echoing information to user in a rule is often a good practice | |
411 | but when execution "make -s" one does not expect to see any output | |
412 | except for warnings/errors. | |
39fed701 | 413 | To support this kbuild defines $(kecho) which will echo out the |
5410ecc0 MF |
414 | text following $(kecho) to stdout except if "make -s" is used. |
415 | ||
416 | Example: | |
417 | #arch/blackfin/boot/Makefile | |
418 | $(obj)/vmImage: $(obj)/vmlinux.gz | |
419 | $(call if_changed,uimage) | |
420 | @$(kecho) 'Kernel: $@ is ready' | |
421 | ||
422 | ||
20a468b5 SR |
423 | --- 3.11 $(CC) support functions |
424 | ||
a07f6033 | 425 | The kernel may be built with several different versions of |
20a468b5 | 426 | $(CC), each supporting a unique set of features and options. |
39fed701 | 427 | kbuild provides basic support to check for valid options for $(CC). |
e95be9a5 | 428 | $(CC) is usually the gcc compiler, but other alternatives are |
20a468b5 SR |
429 | available. |
430 | ||
431 | as-option | |
a07f6033 JE |
432 | as-option is used to check if $(CC) -- when used to compile |
433 | assembler (*.S) files -- supports the given option. An optional | |
434 | second option may be specified if the first option is not supported. | |
20a468b5 SR |
435 | |
436 | Example: | |
437 | #arch/sh/Makefile | |
438 | cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) | |
439 | ||
a07f6033 | 440 | In the above example, cflags-y will be assigned the option |
20a468b5 SR |
441 | -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). |
442 | The second argument is optional, and if supplied will be used | |
443 | if first argument is not supported. | |
444 | ||
f86fd306 SR |
445 | cc-ldoption |
446 | cc-ldoption is used to check if $(CC) when used to link object files | |
0b0bf7a3 RM |
447 | supports the given option. An optional second option may be |
448 | specified if first option are not supported. | |
449 | ||
450 | Example: | |
25eb650a | 451 | #arch/x86/kernel/Makefile |
f86fd306 | 452 | vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) |
0b0bf7a3 | 453 | |
5c811e59 | 454 | In the above example, vsyscall-flags will be assigned the option |
0b0bf7a3 RM |
455 | -Wl$(comma)--hash-style=sysv if it is supported by $(CC). |
456 | The second argument is optional, and if supplied will be used | |
457 | if first argument is not supported. | |
458 | ||
e2414910 AK |
459 | as-instr |
460 | as-instr checks if the assembler reports a specific instruction | |
461 | and then outputs either option1 or option2 | |
462 | C escapes are supported in the test instruction | |
222d394d | 463 | Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options |
e2414910 | 464 | |
20a468b5 | 465 | cc-option |
39fed701 GU |
466 | cc-option is used to check if $(CC) supports a given option, and if |
467 | not supported to use an optional second option. | |
20a468b5 SR |
468 | |
469 | Example: | |
25eb650a | 470 | #arch/x86/Makefile |
20a468b5 SR |
471 | cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) |
472 | ||
5c811e59 | 473 | In the above example, cflags-y will be assigned the option |
a07f6033 JE |
474 | -march=pentium-mmx if supported by $(CC), otherwise -march=i586. |
475 | The second argument to cc-option is optional, and if omitted, | |
20a468b5 | 476 | cflags-y will be assigned no value if first option is not supported. |
a0f97e06 | 477 | Note: cc-option uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
478 | |
479 | cc-option-yn | |
39e6e9cf | 480 | cc-option-yn is used to check if gcc supports a given option |
20a468b5 SR |
481 | and return 'y' if supported, otherwise 'n'. |
482 | ||
483 | Example: | |
484 | #arch/ppc/Makefile | |
485 | biarch := $(call cc-option-yn, -m32) | |
486 | aflags-$(biarch) += -a32 | |
487 | cflags-$(biarch) += -m32 | |
39e6e9cf | 488 | |
a07f6033 JE |
489 | In the above example, $(biarch) is set to y if $(CC) supports the -m32 |
490 | option. When $(biarch) equals 'y', the expanded variables $(aflags-y) | |
491 | and $(cflags-y) will be assigned the values -a32 and -m32, | |
492 | respectively. | |
a0f97e06 | 493 | Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options |
20a468b5 | 494 | |
8417da6f MM |
495 | cc-disable-warning |
496 | cc-disable-warning checks if gcc supports a given warning and returns | |
497 | the commandline switch to disable it. This special function is needed, | |
498 | because gcc 4.4 and later accept any unknown -Wno-* option and only | |
499 | warn about it if there is another warning in the source file. | |
500 | ||
501 | Example: | |
502 | KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable) | |
503 | ||
504 | In the above example, -Wno-unused-but-set-variable will be added to | |
505 | KBUILD_CFLAGS only if gcc really accepts it. | |
506 | ||
20a468b5 | 507 | cc-version |
a07f6033 | 508 | cc-version returns a numerical version of the $(CC) compiler version. |
20a468b5 SR |
509 | The format is <major><minor> where both are two digits. So for example |
510 | gcc 3.41 would return 0341. | |
511 | cc-version is useful when a specific $(CC) version is faulty in one | |
a07f6033 | 512 | area, for example -mregparm=3 was broken in some gcc versions |
20a468b5 SR |
513 | even though the option was accepted by gcc. |
514 | ||
515 | Example: | |
25eb650a | 516 | #arch/x86/Makefile |
20a468b5 | 517 | cflags-y += $(shell \ |
665d92e3 | 518 | if [ $(cc-version) -ge 0300 ] ; then \ |
20a468b5 SR |
519 | echo "-mregparm=3"; fi ;) |
520 | ||
a07f6033 | 521 | In the above example, -mregparm=3 is only used for gcc version greater |
20a468b5 SR |
522 | than or equal to gcc 3.0. |
523 | ||
524 | cc-ifversion | |
6dcb4e5e MY |
525 | cc-ifversion tests the version of $(CC) and equals the fourth parameter |
526 | if version expression is true, or the fifth (if given) if the version | |
527 | expression is false. | |
20a468b5 SR |
528 | |
529 | Example: | |
530 | #fs/reiserfs/Makefile | |
f77bf014 | 531 | ccflags-y := $(call cc-ifversion, -lt, 0402, -O1) |
20a468b5 | 532 | |
f77bf014 | 533 | In this example, ccflags-y will be assigned the value -O1 if the |
20a468b5 | 534 | $(CC) version is less than 4.2. |
39e6e9cf | 535 | cc-ifversion takes all the shell operators: |
20a468b5 SR |
536 | -eq, -ne, -lt, -le, -gt, and -ge |
537 | The third parameter may be a text as in this example, but it may also | |
538 | be an expanded variable or a macro. | |
539 | ||
7015030f SR |
540 | cc-fullversion |
541 | cc-fullversion is useful when the exact version of gcc is needed. | |
542 | One typical use-case is when a specific GCC version is broken. | |
543 | cc-fullversion points out a more specific version than cc-version does. | |
544 | ||
545 | Example: | |
546 | #arch/powerpc/Makefile | |
665d92e3 | 547 | $(Q)if test "$(cc-fullversion)" = "040200" ; then \ |
7015030f SR |
548 | echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \ |
549 | false ; \ | |
550 | fi | |
551 | ||
39fed701 GU |
552 | In this example for a specific GCC version the build will error out |
553 | explaining to the user why it stops. | |
1da177e4 | 554 | |
910b4046 | 555 | cc-cross-prefix |
631bcfbb | 556 | cc-cross-prefix is used to check if there exists a $(CC) in path with |
910b4046 SR |
557 | one of the listed prefixes. The first prefix where there exist a |
558 | prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found | |
559 | then nothing is returned. | |
560 | Additional prefixes are separated by a single space in the | |
561 | call of cc-cross-prefix. | |
631bcfbb GU |
562 | This functionality is useful for architecture Makefiles that try |
563 | to set CROSS_COMPILE to well-known values but may have several | |
910b4046 | 564 | values to select between. |
631bcfbb GU |
565 | It is recommended only to try to set CROSS_COMPILE if it is a cross |
566 | build (host arch is different from target arch). And if CROSS_COMPILE | |
910b4046 SR |
567 | is already set then leave it with the old value. |
568 | ||
569 | Example: | |
570 | #arch/m68k/Makefile | |
571 | ifneq ($(SUBARCH),$(ARCH)) | |
572 | ifeq ($(CROSS_COMPILE),) | |
573 | CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) | |
574 | endif | |
575 | endif | |
576 | ||
691ef3e7 SR |
577 | --- 3.12 $(LD) support functions |
578 | ||
579 | ld-option | |
580 | ld-option is used to check if $(LD) supports the supplied option. | |
581 | ld-option takes two options as arguments. | |
582 | The second argument is an optional option that can be used if the | |
583 | first option is not supported by $(LD). | |
584 | ||
585 | Example: | |
586 | #Makefile | |
5b83df2b | 587 | LDFLAGS_vmlinux += $(call ld-option, -X) |
691ef3e7 SR |
588 | |
589 | ||
1da177e4 LT |
590 | === 4 Host Program support |
591 | ||
592 | Kbuild supports building executables on the host for use during the | |
593 | compilation stage. | |
594 | Two steps are required in order to use a host executable. | |
595 | ||
596 | The first step is to tell kbuild that a host program exists. This is | |
597 | done utilising the variable hostprogs-y. | |
598 | ||
599 | The second step is to add an explicit dependency to the executable. | |
39e6e9cf | 600 | This can be done in two ways. Either add the dependency in a rule, |
1da177e4 LT |
601 | or utilise the variable $(always). |
602 | Both possibilities are described in the following. | |
603 | ||
604 | --- 4.1 Simple Host Program | |
605 | ||
606 | In some cases there is a need to compile and run a program on the | |
607 | computer where the build is running. | |
608 | The following line tells kbuild that the program bin2hex shall be | |
609 | built on the build host. | |
610 | ||
611 | Example: | |
612 | hostprogs-y := bin2hex | |
613 | ||
614 | Kbuild assumes in the above example that bin2hex is made from a single | |
615 | c-source file named bin2hex.c located in the same directory as | |
616 | the Makefile. | |
39e6e9cf | 617 | |
1da177e4 LT |
618 | --- 4.2 Composite Host Programs |
619 | ||
620 | Host programs can be made up based on composite objects. | |
621 | The syntax used to define composite objects for host programs is | |
622 | similar to the syntax used for kernel objects. | |
5d3f083d | 623 | $(<executable>-objs) lists all objects used to link the final |
1da177e4 LT |
624 | executable. |
625 | ||
626 | Example: | |
627 | #scripts/lxdialog/Makefile | |
39e6e9cf | 628 | hostprogs-y := lxdialog |
1da177e4 LT |
629 | lxdialog-objs := checklist.o lxdialog.o |
630 | ||
631 | Objects with extension .o are compiled from the corresponding .c | |
a07f6033 | 632 | files. In the above example, checklist.c is compiled to checklist.o |
1da177e4 | 633 | and lxdialog.c is compiled to lxdialog.o. |
a07f6033 | 634 | Finally, the two .o files are linked to the executable, lxdialog. |
1da177e4 LT |
635 | Note: The syntax <executable>-y is not permitted for host-programs. |
636 | ||
62e22107 | 637 | --- 4.3 Using C++ for host programs |
1da177e4 LT |
638 | |
639 | kbuild offers support for host programs written in C++. This was | |
640 | introduced solely to support kconfig, and is not recommended | |
641 | for general use. | |
642 | ||
643 | Example: | |
644 | #scripts/kconfig/Makefile | |
645 | hostprogs-y := qconf | |
646 | qconf-cxxobjs := qconf.o | |
647 | ||
648 | In the example above the executable is composed of the C++ file | |
649 | qconf.cc - identified by $(qconf-cxxobjs). | |
39e6e9cf | 650 | |
39fed701 | 651 | If qconf is composed of a mixture of .c and .cc files, then an |
1da177e4 LT |
652 | additional line can be used to identify this. |
653 | ||
654 | Example: | |
655 | #scripts/kconfig/Makefile | |
656 | hostprogs-y := qconf | |
657 | qconf-cxxobjs := qconf.o | |
658 | qconf-objs := check.o | |
39e6e9cf | 659 | |
62e22107 | 660 | --- 4.4 Controlling compiler options for host programs |
1da177e4 LT |
661 | |
662 | When compiling host programs, it is possible to set specific flags. | |
663 | The programs will always be compiled utilising $(HOSTCC) passed | |
664 | the options specified in $(HOSTCFLAGS). | |
665 | To set flags that will take effect for all host programs created | |
a07f6033 | 666 | in that Makefile, use the variable HOST_EXTRACFLAGS. |
1da177e4 LT |
667 | |
668 | Example: | |
669 | #scripts/lxdialog/Makefile | |
670 | HOST_EXTRACFLAGS += -I/usr/include/ncurses | |
39e6e9cf | 671 | |
1da177e4 LT |
672 | To set specific flags for a single file the following construction |
673 | is used: | |
674 | ||
675 | Example: | |
676 | #arch/ppc64/boot/Makefile | |
677 | HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) | |
39e6e9cf | 678 | |
1da177e4 | 679 | It is also possible to specify additional options to the linker. |
39e6e9cf | 680 | |
1da177e4 LT |
681 | Example: |
682 | #scripts/kconfig/Makefile | |
683 | HOSTLOADLIBES_qconf := -L$(QTDIR)/lib | |
684 | ||
a07f6033 JE |
685 | When linking qconf, it will be passed the extra option |
686 | "-L$(QTDIR)/lib". | |
39e6e9cf | 687 | |
62e22107 | 688 | --- 4.5 When host programs are actually built |
1da177e4 LT |
689 | |
690 | Kbuild will only build host-programs when they are referenced | |
691 | as a prerequisite. | |
692 | This is possible in two ways: | |
693 | ||
694 | (1) List the prerequisite explicitly in a special rule. | |
695 | ||
696 | Example: | |
697 | #drivers/pci/Makefile | |
698 | hostprogs-y := gen-devlist | |
699 | $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist | |
700 | ( cd $(obj); ./gen-devlist ) < $< | |
701 | ||
39e6e9cf | 702 | The target $(obj)/devlist.h will not be built before |
1da177e4 LT |
703 | $(obj)/gen-devlist is updated. Note that references to |
704 | the host programs in special rules must be prefixed with $(obj). | |
705 | ||
706 | (2) Use $(always) | |
707 | When there is no suitable special rule, and the host program | |
708 | shall be built when a makefile is entered, the $(always) | |
709 | variable shall be used. | |
710 | ||
711 | Example: | |
712 | #scripts/lxdialog/Makefile | |
713 | hostprogs-y := lxdialog | |
714 | always := $(hostprogs-y) | |
715 | ||
716 | This will tell kbuild to build lxdialog even if not referenced in | |
717 | any rule. | |
718 | ||
62e22107 | 719 | --- 4.6 Using hostprogs-$(CONFIG_FOO) |
1da177e4 | 720 | |
39e6e9cf | 721 | A typical pattern in a Kbuild file looks like this: |
1da177e4 LT |
722 | |
723 | Example: | |
724 | #scripts/Makefile | |
725 | hostprogs-$(CONFIG_KALLSYMS) += kallsyms | |
726 | ||
727 | Kbuild knows about both 'y' for built-in and 'm' for module. | |
39fed701 | 728 | So if a config symbol evaluates to 'm', kbuild will still build |
a07f6033 JE |
729 | the binary. In other words, Kbuild handles hostprogs-m exactly |
730 | like hostprogs-y. But only hostprogs-y is recommended to be used | |
731 | when no CONFIG symbols are involved. | |
1da177e4 LT |
732 | |
733 | === 5 Kbuild clean infrastructure | |
734 | ||
a07f6033 | 735 | "make clean" deletes most generated files in the obj tree where the kernel |
1da177e4 LT |
736 | is compiled. This includes generated files such as host programs. |
737 | Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always), | |
738 | $(extra-y) and $(targets). They are all deleted during "make clean". | |
739 | Files matching the patterns "*.[oas]", "*.ko", plus some additional files | |
740 | generated by kbuild are deleted all over the kernel src tree when | |
741 | "make clean" is executed. | |
742 | ||
743 | Additional files can be specified in kbuild makefiles by use of $(clean-files). | |
744 | ||
745 | Example: | |
97659181 MM |
746 | #lib/Makefile |
747 | clean-files := crc32table.h | |
1da177e4 | 748 | |
bd55daf4 JR |
749 | When executing "make clean", the file "crc32table.h" will be deleted. |
750 | Kbuild will assume files to be in the same relative directory as the | |
97659181 | 751 | Makefile, except if prefixed with $(objtree). |
1da177e4 | 752 | |
39e6e9cf BH |
753 | To delete a directory hierarchy use: |
754 | ||
1da177e4 LT |
755 | Example: |
756 | #scripts/package/Makefile | |
757 | clean-dirs := $(objtree)/debian/ | |
758 | ||
97659181 MM |
759 | This will delete the directory debian in the toplevel directory, including all |
760 | subdirectories. | |
1da177e4 | 761 | |
ef8ff89b MM |
762 | To exclude certain files from make clean, use the $(no-clean-files) variable. |
763 | This is only a special case used in the top level Kbuild file: | |
764 | ||
765 | Example: | |
766 | #Kbuild | |
767 | no-clean-files := $(bounds-file) $(offsets-file) | |
768 | ||
1da177e4 LT |
769 | Usually kbuild descends down in subdirectories due to "obj-* := dir/", |
770 | but in the architecture makefiles where the kbuild infrastructure | |
771 | is not sufficient this sometimes needs to be explicit. | |
772 | ||
773 | Example: | |
25eb650a | 774 | #arch/x86/boot/Makefile |
1da177e4 LT |
775 | subdir- := compressed/ |
776 | ||
777 | The above assignment instructs kbuild to descend down in the | |
778 | directory compressed/ when "make clean" is executed. | |
779 | ||
39fed701 | 780 | To support the clean infrastructure in the Makefiles that build the |
1da177e4 LT |
781 | final bootimage there is an optional target named archclean: |
782 | ||
783 | Example: | |
25eb650a | 784 | #arch/x86/Makefile |
1da177e4 | 785 | archclean: |
25eb650a | 786 | $(Q)$(MAKE) $(clean)=arch/x86/boot |
1da177e4 | 787 | |
25eb650a WG |
788 | When "make clean" is executed, make will descend down in arch/x86/boot, |
789 | and clean as usual. The Makefile located in arch/x86/boot/ may use | |
1da177e4 LT |
790 | the subdir- trick to descend further down. |
791 | ||
792 | Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is | |
793 | included in the top level makefile, and the kbuild infrastructure | |
794 | is not operational at that point. | |
795 | ||
796 | Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will | |
797 | be visited during "make clean". | |
798 | ||
799 | === 6 Architecture Makefiles | |
800 | ||
801 | The top level Makefile sets up the environment and does the preparation, | |
802 | before starting to descend down in the individual directories. | |
a07f6033 JE |
803 | The top level makefile contains the generic part, whereas |
804 | arch/$(ARCH)/Makefile contains what is required to set up kbuild | |
805 | for said architecture. | |
806 | To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines | |
1da177e4 LT |
807 | a few targets. |
808 | ||
a07f6033 JE |
809 | When kbuild executes, the following steps are followed (roughly): |
810 | 1) Configuration of the kernel => produce .config | |
1da177e4 | 811 | 2) Store kernel version in include/linux/version.h |
b22ae40e | 812 | 3) Updating all other prerequisites to the target prepare: |
1da177e4 | 813 | - Additional prerequisites are specified in arch/$(ARCH)/Makefile |
b22ae40e | 814 | 4) Recursively descend down in all directories listed in |
1da177e4 | 815 | init-* core* drivers-* net-* libs-* and build all targets. |
a07f6033 | 816 | - The values of the above variables are expanded in arch/$(ARCH)/Makefile. |
b22ae40e | 817 | 5) All object files are then linked and the resulting file vmlinux is |
a07f6033 | 818 | located at the root of the obj tree. |
1da177e4 LT |
819 | The very first objects linked are listed in head-y, assigned by |
820 | arch/$(ARCH)/Makefile. | |
b22ae40e | 821 | 6) Finally, the architecture-specific part does any required post processing |
1da177e4 LT |
822 | and builds the final bootimage. |
823 | - This includes building boot records | |
5c811e59 | 824 | - Preparing initrd images and the like |
1da177e4 LT |
825 | |
826 | ||
827 | --- 6.1 Set variables to tweak the build to the architecture | |
828 | ||
829 | LDFLAGS Generic $(LD) options | |
830 | ||
831 | Flags used for all invocations of the linker. | |
832 | Often specifying the emulation is sufficient. | |
833 | ||
834 | Example: | |
835 | #arch/s390/Makefile | |
836 | LDFLAGS := -m elf_s390 | |
f77bf014 | 837 | Note: ldflags-y can be used to further customise |
a9af3305 | 838 | the flags used. See chapter 3.7. |
39e6e9cf | 839 | |
1da177e4 LT |
840 | LDFLAGS_vmlinux Options for $(LD) when linking vmlinux |
841 | ||
842 | LDFLAGS_vmlinux is used to specify additional flags to pass to | |
a07f6033 | 843 | the linker when linking the final vmlinux image. |
1da177e4 LT |
844 | LDFLAGS_vmlinux uses the LDFLAGS_$@ support. |
845 | ||
846 | Example: | |
25eb650a | 847 | #arch/x86/Makefile |
1da177e4 LT |
848 | LDFLAGS_vmlinux := -e stext |
849 | ||
850 | OBJCOPYFLAGS objcopy flags | |
851 | ||
852 | When $(call if_changed,objcopy) is used to translate a .o file, | |
a07f6033 | 853 | the flags specified in OBJCOPYFLAGS will be used. |
1da177e4 LT |
854 | $(call if_changed,objcopy) is often used to generate raw binaries on |
855 | vmlinux. | |
856 | ||
857 | Example: | |
858 | #arch/s390/Makefile | |
859 | OBJCOPYFLAGS := -O binary | |
860 | ||
861 | #arch/s390/boot/Makefile | |
862 | $(obj)/image: vmlinux FORCE | |
863 | $(call if_changed,objcopy) | |
864 | ||
a07f6033 | 865 | In this example, the binary $(obj)/image is a binary version of |
1da177e4 LT |
866 | vmlinux. The usage of $(call if_changed,xxx) will be described later. |
867 | ||
222d394d | 868 | KBUILD_AFLAGS $(AS) assembler flags |
1da177e4 LT |
869 | |
870 | Default value - see top level Makefile | |
871 | Append or modify as required per architecture. | |
872 | ||
873 | Example: | |
874 | #arch/sparc64/Makefile | |
222d394d | 875 | KBUILD_AFLAGS += -m64 -mcpu=ultrasparc |
1da177e4 | 876 | |
a0f97e06 | 877 | KBUILD_CFLAGS $(CC) compiler flags |
1da177e4 LT |
878 | |
879 | Default value - see top level Makefile | |
880 | Append or modify as required per architecture. | |
881 | ||
a0f97e06 | 882 | Often, the KBUILD_CFLAGS variable depends on the configuration. |
1da177e4 LT |
883 | |
884 | Example: | |
ff4eb04c PB |
885 | #arch/x86/boot/compressed/Makefile |
886 | cflags-$(CONFIG_X86_32) := -march=i386 | |
887 | cflags-$(CONFIG_X86_64) := -mcmodel=small | |
a0f97e06 | 888 | KBUILD_CFLAGS += $(cflags-y) |
1da177e4 LT |
889 | |
890 | Many arch Makefiles dynamically run the target C compiler to | |
891 | probe supported options: | |
892 | ||
25eb650a | 893 | #arch/x86/Makefile |
1da177e4 LT |
894 | |
895 | ... | |
896 | cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ | |
897 | -march=pentium2,-march=i686) | |
898 | ... | |
899 | # Disable unit-at-a-time mode ... | |
a0f97e06 | 900 | KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) |
1da177e4 LT |
901 | ... |
902 | ||
903 | ||
a07f6033 | 904 | The first example utilises the trick that a config option expands |
1da177e4 LT |
905 | to 'y' when selected. |
906 | ||
80c00ba9 | 907 | KBUILD_AFLAGS_KERNEL $(AS) options specific for built-in |
1da177e4 | 908 | |
80c00ba9 | 909 | $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile |
1da177e4 LT |
910 | resident kernel code. |
911 | ||
6588169d | 912 | KBUILD_AFLAGS_MODULE Options for $(AS) when building modules |
1da177e4 | 913 | |
39fed701 | 914 | $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that |
6588169d SR |
915 | are used for $(AS). |
916 | From commandline AFLAGS_MODULE shall be used (see kbuild.txt). | |
1da177e4 | 917 | |
80c00ba9 SR |
918 | KBUILD_CFLAGS_KERNEL $(CC) options specific for built-in |
919 | ||
920 | $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile | |
921 | resident kernel code. | |
922 | ||
6588169d SR |
923 | KBUILD_CFLAGS_MODULE Options for $(CC) when building modules |
924 | ||
39fed701 | 925 | $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that |
6588169d SR |
926 | are used for $(CC). |
927 | From commandline CFLAGS_MODULE shall be used (see kbuild.txt). | |
928 | ||
929 | KBUILD_LDFLAGS_MODULE Options for $(LD) when linking modules | |
930 | ||
39fed701 | 931 | $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options |
6588169d SR |
932 | used when linking modules. This is often a linker script. |
933 | From commandline LDFLAGS_MODULE shall be used (see kbuild.txt). | |
39e6e9cf | 934 | |
40df759e MM |
935 | KBUILD_ARFLAGS Options for $(AR) when creating archives |
936 | ||
937 | $(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic | |
938 | mode) if this option is supported by $(AR). | |
939 | ||
61754c18 MM |
940 | ARCH_CPPFLAGS, ARCH_AFLAGS, ARCH_CFLAGS Overrides the kbuild defaults |
941 | ||
942 | These variables are appended to the KBUILD_CPPFLAGS, | |
943 | KBUILD_AFLAGS, and KBUILD_CFLAGS, respectively, after the | |
944 | top-level Makefile has set any other flags. This provides a | |
945 | means for an architecture to override the defaults. | |
946 | ||
947 | ||
052ad274 PA |
948 | --- 6.2 Add prerequisites to archheaders: |
949 | ||
950 | The archheaders: rule is used to generate header files that | |
951 | may be installed into user space by "make header_install" or | |
952 | "make headers_install_all". In order to support | |
953 | "make headers_install_all", this target has to be able to run | |
954 | on an unconfigured tree, or a tree configured for another | |
955 | architecture. | |
956 | ||
957 | It is run before "make archprepare" when run on the | |
958 | architecture itself. | |
959 | ||
960 | ||
961 | --- 6.3 Add prerequisites to archprepare: | |
1da177e4 | 962 | |
a07f6033 | 963 | The archprepare: rule is used to list prerequisites that need to be |
1da177e4 | 964 | built before starting to descend down in the subdirectories. |
a07f6033 | 965 | This is usually used for header files containing assembler constants. |
1da177e4 LT |
966 | |
967 | Example: | |
5bb78269 SR |
968 | #arch/arm/Makefile |
969 | archprepare: maketools | |
1da177e4 | 970 | |
a07f6033 | 971 | In this example, the file target maketools will be processed |
5bb78269 | 972 | before descending down in the subdirectories. |
1da177e4 LT |
973 | See also chapter XXX-TODO that describe how kbuild supports |
974 | generating offset header files. | |
975 | ||
976 | ||
052ad274 | 977 | --- 6.4 List directories to visit when descending |
1da177e4 LT |
978 | |
979 | An arch Makefile cooperates with the top Makefile to define variables | |
980 | which specify how to build the vmlinux file. Note that there is no | |
981 | corresponding arch-specific section for modules; the module-building | |
982 | machinery is all architecture-independent. | |
983 | ||
39e6e9cf | 984 | |
1da177e4 LT |
985 | head-y, init-y, core-y, libs-y, drivers-y, net-y |
986 | ||
a07f6033 JE |
987 | $(head-y) lists objects to be linked first in vmlinux. |
988 | $(libs-y) lists directories where a lib.a archive can be located. | |
f49821ee | 989 | The rest list directories where a built-in.a object file can be |
a07f6033 | 990 | located. |
1da177e4 LT |
991 | |
992 | $(init-y) objects will be located after $(head-y). | |
993 | Then the rest follows in this order: | |
994 | $(core-y), $(libs-y), $(drivers-y) and $(net-y). | |
995 | ||
a07f6033 | 996 | The top level Makefile defines values for all generic directories, |
5c811e59 | 997 | and arch/$(ARCH)/Makefile only adds architecture-specific directories. |
1da177e4 LT |
998 | |
999 | Example: | |
1000 | #arch/sparc64/Makefile | |
1001 | core-y += arch/sparc64/kernel/ | |
1002 | libs-y += arch/sparc64/prom/ arch/sparc64/lib/ | |
1003 | drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ | |
1004 | ||
1005 | ||
052ad274 | 1006 | --- 6.5 Architecture-specific boot images |
1da177e4 LT |
1007 | |
1008 | An arch Makefile specifies goals that take the vmlinux file, compress | |
1009 | it, wrap it in bootstrapping code, and copy the resulting files | |
1010 | somewhere. This includes various kinds of installation commands. | |
1011 | The actual goals are not standardized across architectures. | |
1012 | ||
1013 | It is common to locate any additional processing in a boot/ | |
1014 | directory below arch/$(ARCH)/. | |
1015 | ||
1016 | Kbuild does not provide any smart way to support building a | |
1017 | target specified in boot/. Therefore arch/$(ARCH)/Makefile shall | |
1018 | call make manually to build a target in boot/. | |
1019 | ||
1020 | The recommended approach is to include shortcuts in | |
1021 | arch/$(ARCH)/Makefile, and use the full path when calling down | |
1022 | into the arch/$(ARCH)/boot/Makefile. | |
1023 | ||
1024 | Example: | |
25eb650a WG |
1025 | #arch/x86/Makefile |
1026 | boot := arch/x86/boot | |
1da177e4 LT |
1027 | bzImage: vmlinux |
1028 | $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ | |
1029 | ||
1030 | "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke | |
1031 | make in a subdirectory. | |
1032 | ||
5c811e59 | 1033 | There are no rules for naming architecture-specific targets, |
1da177e4 | 1034 | but executing "make help" will list all relevant targets. |
a07f6033 | 1035 | To support this, $(archhelp) must be defined. |
1da177e4 LT |
1036 | |
1037 | Example: | |
25eb650a | 1038 | #arch/x86/Makefile |
1da177e4 LT |
1039 | define archhelp |
1040 | echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' | |
39e6e9cf | 1041 | endif |
1da177e4 LT |
1042 | |
1043 | When make is executed without arguments, the first goal encountered | |
1044 | will be built. In the top level Makefile the first goal present | |
1045 | is all:. | |
a07f6033 JE |
1046 | An architecture shall always, per default, build a bootable image. |
1047 | In "make help", the default goal is highlighted with a '*'. | |
1da177e4 LT |
1048 | Add a new prerequisite to all: to select a default goal different |
1049 | from vmlinux. | |
1050 | ||
1051 | Example: | |
25eb650a | 1052 | #arch/x86/Makefile |
39e6e9cf | 1053 | all: bzImage |
1da177e4 LT |
1054 | |
1055 | When "make" is executed without arguments, bzImage will be built. | |
1056 | ||
052ad274 | 1057 | --- 6.6 Building non-kbuild targets |
1da177e4 LT |
1058 | |
1059 | extra-y | |
1060 | ||
39fed701 | 1061 | extra-y specifies additional targets created in the current |
1da177e4 LT |
1062 | directory, in addition to any targets specified by obj-*. |
1063 | ||
1064 | Listing all targets in extra-y is required for two purposes: | |
1065 | 1) Enable kbuild to check changes in command lines | |
1066 | - When $(call if_changed,xxx) is used | |
1067 | 2) kbuild knows what files to delete during "make clean" | |
1068 | ||
1069 | Example: | |
25eb650a | 1070 | #arch/x86/kernel/Makefile |
1da177e4 LT |
1071 | extra-y := head.o init_task.o |
1072 | ||
a07f6033 | 1073 | In this example, extra-y is used to list object files that |
f49821ee | 1074 | shall be built, but shall not be linked as part of built-in.a. |
1da177e4 | 1075 | |
39e6e9cf | 1076 | |
052ad274 | 1077 | --- 6.7 Commands useful for building a boot image |
1da177e4 LT |
1078 | |
1079 | Kbuild provides a few macros that are useful when building a | |
1080 | boot image. | |
1081 | ||
1082 | if_changed | |
1083 | ||
1084 | if_changed is the infrastructure used for the following commands. | |
1085 | ||
1086 | Usage: | |
1087 | target: source(s) FORCE | |
ef80f0a1 | 1088 | $(call if_changed,ld/objcopy/gzip/...) |
1da177e4 | 1089 | |
a07f6033 | 1090 | When the rule is evaluated, it is checked to see if any files |
5c811e59 | 1091 | need an update, or the command line has changed since the last |
1da177e4 LT |
1092 | invocation. The latter will force a rebuild if any options |
1093 | to the executable have changed. | |
1094 | Any target that utilises if_changed must be listed in $(targets), | |
1095 | otherwise the command line check will fail, and the target will | |
1096 | always be built. | |
1097 | Assignments to $(targets) are without $(obj)/ prefix. | |
1098 | if_changed may be used in conjunction with custom commands as | |
052ad274 | 1099 | defined in 6.8 "Custom kbuild commands". |
49490571 | 1100 | |
1da177e4 | 1101 | Note: It is a typical mistake to forget the FORCE prerequisite. |
49490571 PBG |
1102 | Another common pitfall is that whitespace is sometimes |
1103 | significant; for instance, the below will fail (note the extra space | |
1104 | after the comma): | |
1105 | target: source(s) FORCE | |
ef80f0a1 | 1106 | #WRONG!# $(call if_changed, ld/objcopy/gzip/...) |
1da177e4 LT |
1107 | |
1108 | ld | |
a07f6033 | 1109 | Link target. Often, LDFLAGS_$@ is used to set specific options to ld. |
39e6e9cf | 1110 | |
1da177e4 | 1111 | Example: |
25eb650a | 1112 | #arch/x86/boot/Makefile |
1da177e4 LT |
1113 | LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary |
1114 | LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext | |
1115 | ||
1116 | targets += setup setup.o bootsect bootsect.o | |
1117 | $(obj)/setup $(obj)/bootsect: %: %.o FORCE | |
1118 | $(call if_changed,ld) | |
1119 | ||
a07f6033 JE |
1120 | In this example, there are two possible targets, requiring different |
1121 | options to the linker. The linker options are specified using the | |
1da177e4 | 1122 | LDFLAGS_$@ syntax - one for each potential target. |
5d3f083d | 1123 | $(targets) are assigned all potential targets, by which kbuild knows |
1da177e4 LT |
1124 | the targets and will: |
1125 | 1) check for commandline changes | |
1126 | 2) delete target during make clean | |
1127 | ||
1128 | The ": %: %.o" part of the prerequisite is a shorthand that | |
39fed701 | 1129 | frees us from listing the setup.o and bootsect.o files. |
ef80f0a1 | 1130 | Note: It is a common mistake to forget the "targets :=" assignment, |
1da177e4 LT |
1131 | resulting in the target file being recompiled for no |
1132 | obvious reason. | |
1133 | ||
d87e47e1 C |
1134 | objcopy |
1135 | Copy binary. Uses OBJCOPYFLAGS usually specified in | |
1136 | arch/$(ARCH)/Makefile. | |
1137 | OBJCOPYFLAGS_$@ may be used to set additional options. | |
1138 | ||
1139 | gzip | |
1140 | Compress target. Use maximum compression to compress target. | |
1141 | ||
1142 | Example: | |
1143 | #arch/x86/boot/compressed/Makefile | |
1144 | $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE | |
1145 | $(call if_changed,gzip) | |
1146 | ||
aab94339 | 1147 | dtc |
c1410562 | 1148 | Create flattened device tree blob object suitable for linking |
aab94339 DB |
1149 | into vmlinux. Device tree blobs linked into vmlinux are placed |
1150 | in an init section in the image. Platform code *must* copy the | |
1151 | blob to non-init memory prior to calling unflatten_device_tree(). | |
1152 | ||
90b335fb SW |
1153 | To use this command, simply add *.dtb into obj-y or targets, or make |
1154 | some other target depend on %.dtb | |
aab94339 | 1155 | |
90b335fb SW |
1156 | A central rule exists to create $(obj)/%.dtb from $(src)/%.dts; |
1157 | architecture Makefiles do no need to explicitly write out that rule. | |
aab94339 | 1158 | |
90b335fb SW |
1159 | Example: |
1160 | targets += $(dtb-y) | |
90b335fb | 1161 | DTC_FLAGS ?= -p 1024 |
1da177e4 | 1162 | |
052ad274 | 1163 | --- 6.8 Custom kbuild commands |
1da177e4 | 1164 | |
a07f6033 | 1165 | When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand |
1da177e4 LT |
1166 | of a command is normally displayed. |
1167 | To enable this behaviour for custom commands kbuild requires | |
1168 | two variables to be set: | |
1169 | quiet_cmd_<command> - what shall be echoed | |
1170 | cmd_<command> - the command to execute | |
1171 | ||
1172 | Example: | |
1173 | # | |
1174 | quiet_cmd_image = BUILD $@ | |
1175 | cmd_image = $(obj)/tools/build $(BUILDFLAGS) \ | |
1176 | $(obj)/vmlinux.bin > $@ | |
1177 | ||
1178 | targets += bzImage | |
1179 | $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE | |
1180 | $(call if_changed,image) | |
1181 | @echo 'Kernel: $@ is ready' | |
1182 | ||
a07f6033 | 1183 | When updating the $(obj)/bzImage target, the line |
1da177e4 | 1184 | |
25eb650a | 1185 | BUILD arch/x86/boot/bzImage |
1da177e4 LT |
1186 | |
1187 | will be displayed with "make KBUILD_VERBOSE=0". | |
39e6e9cf | 1188 | |
1da177e4 | 1189 | |
052ad274 | 1190 | --- 6.9 Preprocessing linker scripts |
1da177e4 | 1191 | |
a07f6033 | 1192 | When the vmlinux image is built, the linker script |
1da177e4 LT |
1193 | arch/$(ARCH)/kernel/vmlinux.lds is used. |
1194 | The script is a preprocessed variant of the file vmlinux.lds.S | |
1195 | located in the same directory. | |
a07f6033 | 1196 | kbuild knows .lds files and includes a rule *lds.S -> *lds. |
39e6e9cf | 1197 | |
1da177e4 | 1198 | Example: |
25eb650a | 1199 | #arch/x86/kernel/Makefile |
1da177e4 | 1200 | always := vmlinux.lds |
39e6e9cf | 1201 | |
1da177e4 LT |
1202 | #Makefile |
1203 | export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) | |
39e6e9cf BH |
1204 | |
1205 | The assignment to $(always) is used to tell kbuild to build the | |
a07f6033 JE |
1206 | target vmlinux.lds. |
1207 | The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the | |
1da177e4 | 1208 | specified options when building the target vmlinux.lds. |
39e6e9cf | 1209 | |
a07f6033 | 1210 | When building the *.lds target, kbuild uses the variables: |
06c5040c | 1211 | KBUILD_CPPFLAGS : Set in top-level Makefile |
f77bf014 | 1212 | cppflags-y : May be set in the kbuild makefile |
39fed701 | 1213 | CPPFLAGS_$(@F) : Target-specific flags. |
1da177e4 LT |
1214 | Note that the full filename is used in this |
1215 | assignment. | |
1216 | ||
39fed701 | 1217 | The kbuild infrastructure for *lds files is used in several |
5c811e59 | 1218 | architecture-specific files. |
1da177e4 | 1219 | |
052ad274 | 1220 | --- 6.10 Generic header files |
d8ecc5cd SR |
1221 | |
1222 | The directory include/asm-generic contains the header files | |
1223 | that may be shared between individual architectures. | |
1224 | The recommended approach how to use a generic header file is | |
1225 | to list the file in the Kbuild file. | |
d87e47e1 | 1226 | See "7.2 generic-y" for further info on syntax etc. |
d8ecc5cd | 1227 | |
fbe6e37d NP |
1228 | --- 6.11 Post-link pass |
1229 | ||
1230 | If the file arch/xxx/Makefile.postlink exists, this makefile | |
1231 | will be invoked for post-link objects (vmlinux and modules.ko) | |
1232 | for architectures to run post-link passes on. Must also handle | |
1233 | the clean target. | |
1234 | ||
1235 | This pass runs after kallsyms generation. If the architecture | |
1236 | needs to modify symbol locations, rather than manipulate the | |
1237 | kallsyms, it may be easier to add another postlink target for | |
1238 | .tmp_vmlinux? targets to be called from link-vmlinux.sh. | |
1239 | ||
1240 | For example, powerpc uses this to check relocation sanity of | |
1241 | the linked vmlinux file. | |
1242 | ||
c7bb349e SR |
1243 | === 7 Kbuild syntax for exported headers |
1244 | ||
39fed701 | 1245 | The kernel includes a set of headers that is exported to userspace. |
c95940f2 | 1246 | Many headers can be exported as-is but other headers require a |
c7bb349e SR |
1247 | minimal pre-processing before they are ready for user-space. |
1248 | The pre-processing does: | |
39fed701 | 1249 | - drop kernel-specific annotations |
c7bb349e | 1250 | - drop include of compiler.h |
c95940f2 | 1251 | - drop all sections that are kernel internal (guarded by ifdef __KERNEL__) |
c7bb349e | 1252 | |
fcc8487d | 1253 | All headers under include/uapi/, include/generated/uapi/, |
61562f98 | 1254 | arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/ |
fcc8487d | 1255 | are exported. |
c7bb349e | 1256 | |
fcc8487d ND |
1257 | A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and |
1258 | arch/<arch>/include/asm/ to list asm files coming from asm-generic. | |
1259 | See subsequent chapter for the syntax of the Kbuild file. | |
c7bb349e | 1260 | |
d87e47e1 | 1261 | --- 7.1 no-export-headers |
c7bb349e | 1262 | |
fcc8487d ND |
1263 | no-export-headers is essentially used by include/uapi/linux/Kbuild to |
1264 | avoid exporting specific headers (e.g. kvm.h) on architectures that do | |
1265 | not support it. It should be avoided as much as possible. | |
c7bb349e | 1266 | |
d87e47e1 | 1267 | --- 7.2 generic-y |
d8ecc5cd SR |
1268 | |
1269 | If an architecture uses a verbatim copy of a header from | |
1270 | include/asm-generic then this is listed in the file | |
1271 | arch/$(ARCH)/include/asm/Kbuild like this: | |
1272 | ||
1273 | Example: | |
1274 | #arch/x86/include/asm/Kbuild | |
1275 | generic-y += termios.h | |
1276 | generic-y += rtc.h | |
1277 | ||
1278 | During the prepare phase of the build a wrapper include | |
1279 | file is generated in the directory: | |
1280 | ||
1281 | arch/$(ARCH)/include/generated/asm | |
1282 | ||
1283 | When a header is exported where the architecture uses | |
1284 | the generic header a similar wrapper is generated as part | |
1285 | of the set of exported headers in the directory: | |
1286 | ||
1287 | usr/include/asm | |
1288 | ||
1289 | The generated wrapper will in both cases look like the following: | |
1290 | ||
1291 | Example: termios.h | |
1292 | #include <asm-generic/termios.h> | |
c7bb349e | 1293 | |
d87e47e1 | 1294 | --- 7.3 generated-y |
54b880ca JH |
1295 | |
1296 | If an architecture generates other header files alongside generic-y | |
ae3f4151 | 1297 | wrappers, generated-y specifies them. |
54b880ca JH |
1298 | |
1299 | This prevents them being treated as stale asm-generic wrappers and | |
1300 | removed. | |
1301 | ||
1302 | Example: | |
1303 | #arch/x86/include/asm/Kbuild | |
1304 | generated-y += syscalls_32.h | |
1305 | ||
d87e47e1 | 1306 | --- 7.4 mandatory-y |
fcc8487d ND |
1307 | |
1308 | mandatory-y is essentially used by include/uapi/asm-generic/Kbuild.asm | |
468f8763 | 1309 | to define the minimum set of headers that must be exported in |
fcc8487d ND |
1310 | include/asm. |
1311 | ||
1312 | The convention is to list one subdir per line and | |
1313 | preferably in alphabetic order. | |
1314 | ||
c7bb349e | 1315 | === 8 Kbuild Variables |
1da177e4 LT |
1316 | |
1317 | The top Makefile exports the following variables: | |
1318 | ||
1319 | VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION | |
1320 | ||
1321 | These variables define the current kernel version. A few arch | |
1322 | Makefiles actually use these values directly; they should use | |
1323 | $(KERNELRELEASE) instead. | |
1324 | ||
1325 | $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic | |
1326 | three-part version number, such as "2", "4", and "0". These three | |
1327 | values are always numeric. | |
1328 | ||
1329 | $(EXTRAVERSION) defines an even tinier sublevel for pre-patches | |
1330 | or additional patches. It is usually some non-numeric string | |
1331 | such as "-pre4", and is often blank. | |
1332 | ||
1333 | KERNELRELEASE | |
1334 | ||
1335 | $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable | |
1336 | for constructing installation directory names or showing in | |
1337 | version strings. Some arch Makefiles use it for this purpose. | |
1338 | ||
1339 | ARCH | |
1340 | ||
1341 | This variable defines the target architecture, such as "i386", | |
1342 | "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to | |
1343 | determine which files to compile. | |
1344 | ||
1345 | By default, the top Makefile sets $(ARCH) to be the same as the | |
1346 | host system architecture. For a cross build, a user may | |
1347 | override the value of $(ARCH) on the command line: | |
1348 | ||
1349 | make ARCH=m68k ... | |
1350 | ||
1351 | ||
1352 | INSTALL_PATH | |
1353 | ||
1354 | This variable defines a place for the arch Makefiles to install | |
1355 | the resident kernel image and System.map file. | |
5c811e59 | 1356 | Use this for architecture-specific install targets. |
1da177e4 LT |
1357 | |
1358 | INSTALL_MOD_PATH, MODLIB | |
1359 | ||
1360 | $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module | |
1361 | installation. This variable is not defined in the Makefile but | |
1362 | may be passed in by the user if desired. | |
1363 | ||
1364 | $(MODLIB) specifies the directory for module installation. | |
1365 | The top Makefile defines $(MODLIB) to | |
1366 | $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may | |
1367 | override this value on the command line if desired. | |
1368 | ||
ac031f26 TT |
1369 | INSTALL_MOD_STRIP |
1370 | ||
39fed701 | 1371 | If this variable is specified, it will cause modules to be stripped |
ac031f26 | 1372 | after they are installed. If INSTALL_MOD_STRIP is '1', then the |
39fed701 | 1373 | default option --strip-debug will be used. Otherwise, the |
177b241d GE |
1374 | INSTALL_MOD_STRIP value will be used as the option(s) to the strip |
1375 | command. | |
ac031f26 TT |
1376 | |
1377 | ||
c7bb349e | 1378 | === 9 Makefile language |
1da177e4 | 1379 | |
a07f6033 | 1380 | The kernel Makefiles are designed to be run with GNU Make. The Makefiles |
1da177e4 LT |
1381 | use only the documented features of GNU Make, but they do use many |
1382 | GNU extensions. | |
1383 | ||
1384 | GNU Make supports elementary list-processing functions. The kernel | |
1385 | Makefiles use a novel style of list building and manipulation with few | |
1386 | "if" statements. | |
1387 | ||
1388 | GNU Make has two assignment operators, ":=" and "=". ":=" performs | |
1389 | immediate evaluation of the right-hand side and stores an actual string | |
1390 | into the left-hand side. "=" is like a formula definition; it stores the | |
1391 | right-hand side in an unevaluated form and then evaluates this form each | |
1392 | time the left-hand side is used. | |
1393 | ||
1394 | There are some cases where "=" is appropriate. Usually, though, ":=" | |
1395 | is the right choice. | |
1396 | ||
c7bb349e | 1397 | === 10 Credits |
1da177e4 LT |
1398 | |
1399 | Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> | |
1400 | Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> | |
1401 | Updates by Sam Ravnborg <sam@ravnborg.org> | |
a07f6033 | 1402 | Language QA by Jan Engelhardt <jengelh@gmx.de> |
1da177e4 | 1403 | |
c7bb349e | 1404 | === 11 TODO |
1da177e4 | 1405 | |
a07f6033 | 1406 | - Describe how kbuild supports shipped files with _shipped. |
1da177e4 LT |
1407 | - Generating offset header files. |
1408 | - Add more variables to section 7? | |
1409 | ||
39e6e9cf BH |
1410 | |
1411 |