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1 | Linux Kernel Makefiles |
2 | ||
3 | This document describes the Linux kernel Makefiles. | |
4 | ||
5 | === Table of Contents | |
6 | ||
7 | === 1 Overview | |
8 | === 2 Who does what | |
9 | === 3 The kbuild files | |
10 | --- 3.1 Goal definitions | |
11 | --- 3.2 Built-in object goals - obj-y | |
12 | --- 3.3 Loadable module goals - obj-m | |
13 | --- 3.4 Objects which export symbols | |
14 | --- 3.5 Library file goals - lib-y | |
15 | --- 3.6 Descending down in directories | |
16 | --- 3.7 Compilation flags | |
17 | --- 3.8 Command line dependency | |
18 | --- 3.9 Dependency tracking | |
19 | --- 3.10 Special Rules | |
20a468b5 | 20 | --- 3.11 $(CC) support functions |
691ef3e7 | 21 | --- 3.12 $(LD) support functions |
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22 | |
23 | === 4 Host Program support | |
24 | --- 4.1 Simple Host Program | |
25 | --- 4.2 Composite Host Programs | |
39e6e9cf | 26 | --- 4.3 Defining shared libraries |
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27 | --- 4.4 Using C++ for host programs |
28 | --- 4.5 Controlling compiler options for host programs | |
29 | --- 4.6 When host programs are actually built | |
30 | --- 4.7 Using hostprogs-$(CONFIG_FOO) | |
31 | ||
32 | === 5 Kbuild clean infrastructure | |
33 | ||
34 | === 6 Architecture Makefiles | |
35 | --- 6.1 Set variables to tweak the build to the architecture | |
5bb78269 | 36 | --- 6.2 Add prerequisites to archprepare: |
1da177e4 | 37 | --- 6.3 List directories to visit when descending |
5c811e59 | 38 | --- 6.4 Architecture-specific boot images |
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39 | --- 6.5 Building non-kbuild targets |
40 | --- 6.6 Commands useful for building a boot image | |
41 | --- 6.7 Custom kbuild commands | |
42 | --- 6.8 Preprocessing linker scripts | |
1da177e4 | 43 | |
c7bb349e SR |
44 | === 7 Kbuild syntax for exported headers |
45 | --- 7.1 header-y | |
46 | --- 7.2 objhdr-y | |
47 | --- 7.3 destination-y | |
48 | --- 7.4 unifdef-y (deprecated) | |
49 | ||
50 | === 8 Kbuild Variables | |
51 | === 9 Makefile language | |
52 | === 10 Credits | |
53 | === 11 TODO | |
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54 | |
55 | === 1 Overview | |
56 | ||
57 | The Makefiles have five parts: | |
58 | ||
59 | Makefile the top Makefile. | |
60 | .config the kernel configuration file. | |
61 | arch/$(ARCH)/Makefile the arch Makefile. | |
62 | scripts/Makefile.* common rules etc. for all kbuild Makefiles. | |
63 | kbuild Makefiles there are about 500 of these. | |
64 | ||
65 | The top Makefile reads the .config file, which comes from the kernel | |
66 | configuration process. | |
67 | ||
68 | The top Makefile is responsible for building two major products: vmlinux | |
69 | (the resident kernel image) and modules (any module files). | |
70 | It builds these goals by recursively descending into the subdirectories of | |
71 | the kernel source tree. | |
72 | The list of subdirectories which are visited depends upon the kernel | |
73 | configuration. The top Makefile textually includes an arch Makefile | |
74 | with the name arch/$(ARCH)/Makefile. The arch Makefile supplies | |
75 | architecture-specific information to the top Makefile. | |
76 | ||
77 | Each subdirectory has a kbuild Makefile which carries out the commands | |
78 | passed down from above. The kbuild Makefile uses information from the | |
39e6e9cf | 79 | .config file to construct various file lists used by kbuild to build |
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80 | any built-in or modular targets. |
81 | ||
82 | scripts/Makefile.* contains all the definitions/rules etc. that | |
83 | are used to build the kernel based on the kbuild makefiles. | |
84 | ||
85 | ||
86 | === 2 Who does what | |
87 | ||
88 | People have four different relationships with the kernel Makefiles. | |
89 | ||
90 | *Users* are people who build kernels. These people type commands such as | |
91 | "make menuconfig" or "make". They usually do not read or edit | |
92 | any kernel Makefiles (or any other source files). | |
93 | ||
94 | *Normal developers* are people who work on features such as device | |
95 | drivers, file systems, and network protocols. These people need to | |
a07f6033 | 96 | maintain the kbuild Makefiles for the subsystem they are |
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97 | working on. In order to do this effectively, they need some overall |
98 | knowledge about the kernel Makefiles, plus detailed knowledge about the | |
99 | public interface for kbuild. | |
100 | ||
101 | *Arch developers* are people who work on an entire architecture, such | |
102 | as sparc or ia64. Arch developers need to know about the arch Makefile | |
103 | as well as kbuild Makefiles. | |
104 | ||
105 | *Kbuild developers* are people who work on the kernel build system itself. | |
106 | These people need to know about all aspects of the kernel Makefiles. | |
107 | ||
108 | This document is aimed towards normal developers and arch developers. | |
109 | ||
110 | ||
111 | === 3 The kbuild files | |
112 | ||
113 | Most Makefiles within the kernel are kbuild Makefiles that use the | |
a07f6033 | 114 | kbuild infrastructure. This chapter introduces the syntax used in the |
1da177e4 | 115 | kbuild makefiles. |
172c3ae3 | 116 | The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can |
a07f6033 | 117 | be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild' |
172c3ae3 | 118 | file will be used. |
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119 | |
120 | Section 3.1 "Goal definitions" is a quick intro, further chapters provide | |
121 | more details, with real examples. | |
122 | ||
123 | --- 3.1 Goal definitions | |
124 | ||
125 | Goal definitions are the main part (heart) of the kbuild Makefile. | |
126 | These lines define the files to be built, any special compilation | |
127 | options, and any subdirectories to be entered recursively. | |
128 | ||
129 | The most simple kbuild makefile contains one line: | |
130 | ||
131 | Example: | |
132 | obj-y += foo.o | |
133 | ||
5c811e59 | 134 | This tells kbuild that there is one object in that directory, named |
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135 | foo.o. foo.o will be built from foo.c or foo.S. |
136 | ||
137 | If foo.o shall be built as a module, the variable obj-m is used. | |
138 | Therefore the following pattern is often used: | |
139 | ||
140 | Example: | |
141 | obj-$(CONFIG_FOO) += foo.o | |
142 | ||
143 | $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module). | |
144 | If CONFIG_FOO is neither y nor m, then the file will not be compiled | |
145 | nor linked. | |
146 | ||
147 | --- 3.2 Built-in object goals - obj-y | |
148 | ||
149 | The kbuild Makefile specifies object files for vmlinux | |
a07f6033 | 150 | in the $(obj-y) lists. These lists depend on the kernel |
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151 | configuration. |
152 | ||
153 | Kbuild compiles all the $(obj-y) files. It then calls | |
154 | "$(LD) -r" to merge these files into one built-in.o file. | |
155 | built-in.o is later linked into vmlinux by the parent Makefile. | |
156 | ||
157 | The order of files in $(obj-y) is significant. Duplicates in | |
158 | the lists are allowed: the first instance will be linked into | |
159 | built-in.o and succeeding instances will be ignored. | |
160 | ||
161 | Link order is significant, because certain functions | |
162 | (module_init() / __initcall) will be called during boot in the | |
163 | order they appear. So keep in mind that changing the link | |
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164 | order may e.g. change the order in which your SCSI |
165 | controllers are detected, and thus your disks are renumbered. | |
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166 | |
167 | Example: | |
168 | #drivers/isdn/i4l/Makefile | |
169 | # Makefile for the kernel ISDN subsystem and device drivers. | |
170 | # Each configuration option enables a list of files. | |
171 | obj-$(CONFIG_ISDN) += isdn.o | |
172 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o | |
173 | ||
174 | --- 3.3 Loadable module goals - obj-m | |
175 | ||
176 | $(obj-m) specify object files which are built as loadable | |
177 | kernel modules. | |
178 | ||
179 | A module may be built from one source file or several source | |
180 | files. In the case of one source file, the kbuild makefile | |
181 | simply adds the file to $(obj-m). | |
182 | ||
183 | Example: | |
184 | #drivers/isdn/i4l/Makefile | |
185 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o | |
186 | ||
187 | Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm' | |
188 | ||
189 | If a kernel module is built from several source files, you specify | |
190 | that you want to build a module in the same way as above. | |
191 | ||
192 | Kbuild needs to know which the parts that you want to build your | |
193 | module from, so you have to tell it by setting an | |
194 | $(<module_name>-objs) variable. | |
195 | ||
196 | Example: | |
197 | #drivers/isdn/i4l/Makefile | |
198 | obj-$(CONFIG_ISDN) += isdn.o | |
199 | isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o | |
200 | ||
201 | In this example, the module name will be isdn.o. Kbuild will | |
202 | compile the objects listed in $(isdn-objs) and then run | |
203 | "$(LD) -r" on the list of these files to generate isdn.o. | |
204 | ||
205 | Kbuild recognises objects used for composite objects by the suffix | |
206 | -objs, and the suffix -y. This allows the Makefiles to use | |
207 | the value of a CONFIG_ symbol to determine if an object is part | |
208 | of a composite object. | |
209 | ||
210 | Example: | |
211 | #fs/ext2/Makefile | |
212 | obj-$(CONFIG_EXT2_FS) += ext2.o | |
39e6e9cf | 213 | ext2-y := balloc.o bitmap.o |
1da177e4 | 214 | ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o |
39e6e9cf | 215 | |
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216 | In this example, xattr.o is only part of the composite object |
217 | ext2.o if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'. | |
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218 | |
219 | Note: Of course, when you are building objects into the kernel, | |
220 | the syntax above will also work. So, if you have CONFIG_EXT2_FS=y, | |
221 | kbuild will build an ext2.o file for you out of the individual | |
222 | parts and then link this into built-in.o, as you would expect. | |
223 | ||
224 | --- 3.4 Objects which export symbols | |
225 | ||
226 | No special notation is required in the makefiles for | |
227 | modules exporting symbols. | |
228 | ||
229 | --- 3.5 Library file goals - lib-y | |
230 | ||
a07f6033 | 231 | Objects listed with obj-* are used for modules, or |
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232 | combined in a built-in.o for that specific directory. |
233 | There is also the possibility to list objects that will | |
234 | be included in a library, lib.a. | |
235 | All objects listed with lib-y are combined in a single | |
236 | library for that directory. | |
5d3f083d ML |
237 | Objects that are listed in obj-y and additionally listed in |
238 | lib-y will not be included in the library, since they will | |
239 | be accessible anyway. | |
a07f6033 | 240 | For consistency, objects listed in lib-m will be included in lib.a. |
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241 | |
242 | Note that the same kbuild makefile may list files to be built-in | |
243 | and to be part of a library. Therefore the same directory | |
244 | may contain both a built-in.o and a lib.a file. | |
245 | ||
246 | Example: | |
247 | #arch/i386/lib/Makefile | |
248 | lib-y := checksum.o delay.o | |
249 | ||
250 | This will create a library lib.a based on checksum.o and delay.o. | |
a07f6033 | 251 | For kbuild to actually recognize that there is a lib.a being built, |
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252 | the directory shall be listed in libs-y. |
253 | See also "6.3 List directories to visit when descending". | |
39e6e9cf | 254 | |
a07f6033 | 255 | Use of lib-y is normally restricted to lib/ and arch/*/lib. |
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256 | |
257 | --- 3.6 Descending down in directories | |
258 | ||
259 | A Makefile is only responsible for building objects in its own | |
260 | directory. Files in subdirectories should be taken care of by | |
261 | Makefiles in these subdirs. The build system will automatically | |
262 | invoke make recursively in subdirectories, provided you let it know of | |
263 | them. | |
264 | ||
a07f6033 | 265 | To do so, obj-y and obj-m are used. |
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266 | ext2 lives in a separate directory, and the Makefile present in fs/ |
267 | tells kbuild to descend down using the following assignment. | |
268 | ||
269 | Example: | |
270 | #fs/Makefile | |
271 | obj-$(CONFIG_EXT2_FS) += ext2/ | |
272 | ||
273 | If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular) | |
274 | the corresponding obj- variable will be set, and kbuild will descend | |
275 | down in the ext2 directory. | |
276 | Kbuild only uses this information to decide that it needs to visit | |
277 | the directory, it is the Makefile in the subdirectory that | |
278 | specifies what is modules and what is built-in. | |
279 | ||
280 | It is good practice to use a CONFIG_ variable when assigning directory | |
281 | names. This allows kbuild to totally skip the directory if the | |
282 | corresponding CONFIG_ option is neither 'y' nor 'm'. | |
283 | ||
284 | --- 3.7 Compilation flags | |
285 | ||
f77bf014 SR |
286 | ccflags-y, asflags-y and ldflags-y |
287 | The three flags listed above applies only to the kbuild makefile | |
288 | where they are assigned. They are used for all the normal | |
289 | cc, as and ld invocation happenign during a recursive build. | |
290 | Note: Flags with the same behaviour were previously named: | |
291 | EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS. | |
292 | They are yet supported but their use are deprecated. | |
1da177e4 | 293 | |
f77bf014 | 294 | ccflags-y specifies options for compiling C files with $(CC). |
1da177e4 LT |
295 | |
296 | Example: | |
297 | # drivers/sound/emu10k1/Makefile | |
f77bf014 SR |
298 | ccflags-y += -I$(obj) |
299 | ccflags-$(DEBUG) += -DEMU10K1_DEBUG | |
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300 | |
301 | ||
302 | This variable is necessary because the top Makefile owns the | |
a0f97e06 | 303 | variable $(KBUILD_CFLAGS) and uses it for compilation flags for the |
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304 | entire tree. |
305 | ||
f77bf014 | 306 | asflags-y is a similar string for per-directory options |
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307 | when compiling assembly language source. |
308 | ||
309 | Example: | |
310 | #arch/x86_64/kernel/Makefile | |
f77bf014 | 311 | asflags-y := -traditional |
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312 | |
313 | ||
f77bf014 | 314 | ldflags-y is a string for per-directory options to $(LD). |
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315 | |
316 | Example: | |
317 | #arch/m68k/fpsp040/Makefile | |
f77bf014 | 318 | ldflags-y := -x |
1da177e4 | 319 | |
720097d8 SR |
320 | subdir-ccflags-y, subdir-asflags-y |
321 | The two flags listed above are similar to ccflags-y and as-falgs-y. | |
322 | The difference is that the subdir- variants has effect for the kbuild | |
e8d400a9 | 323 | file where they are present and all subdirectories. |
720097d8 SR |
324 | Options specified using subdir-* are added to the commandline before |
325 | the options specified using the non-subdir variants. | |
326 | ||
327 | Example: | |
328 | subdir-ccflags-y := -Werror | |
329 | ||
1da177e4 LT |
330 | CFLAGS_$@, AFLAGS_$@ |
331 | ||
332 | CFLAGS_$@ and AFLAGS_$@ only apply to commands in current | |
333 | kbuild makefile. | |
334 | ||
335 | $(CFLAGS_$@) specifies per-file options for $(CC). The $@ | |
336 | part has a literal value which specifies the file that it is for. | |
337 | ||
338 | Example: | |
339 | # drivers/scsi/Makefile | |
340 | CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF | |
341 | CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \ | |
342 | -DGDTH_STATISTICS | |
343 | CFLAGS_seagate.o = -DARBITRATE -DPARITY -DSEAGATE_USE_ASM | |
344 | ||
345 | These three lines specify compilation flags for aha152x.o, | |
346 | gdth.o, and seagate.o | |
347 | ||
348 | $(AFLAGS_$@) is a similar feature for source files in assembly | |
349 | languages. | |
350 | ||
351 | Example: | |
352 | # arch/arm/kernel/Makefile | |
353 | AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional | |
354 | AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional | |
355 | ||
356 | --- 3.9 Dependency tracking | |
357 | ||
358 | Kbuild tracks dependencies on the following: | |
359 | 1) All prerequisite files (both *.c and *.h) | |
360 | 2) CONFIG_ options used in all prerequisite files | |
361 | 3) Command-line used to compile target | |
362 | ||
363 | Thus, if you change an option to $(CC) all affected files will | |
364 | be re-compiled. | |
365 | ||
366 | --- 3.10 Special Rules | |
367 | ||
368 | Special rules are used when the kbuild infrastructure does | |
369 | not provide the required support. A typical example is | |
370 | header files generated during the build process. | |
5c811e59 | 371 | Another example are the architecture-specific Makefiles which |
a07f6033 | 372 | need special rules to prepare boot images etc. |
1da177e4 LT |
373 | |
374 | Special rules are written as normal Make rules. | |
375 | Kbuild is not executing in the directory where the Makefile is | |
376 | located, so all special rules shall provide a relative | |
377 | path to prerequisite files and target files. | |
378 | ||
379 | Two variables are used when defining special rules: | |
380 | ||
381 | $(src) | |
382 | $(src) is a relative path which points to the directory | |
383 | where the Makefile is located. Always use $(src) when | |
384 | referring to files located in the src tree. | |
385 | ||
386 | $(obj) | |
387 | $(obj) is a relative path which points to the directory | |
388 | where the target is saved. Always use $(obj) when | |
389 | referring to generated files. | |
390 | ||
391 | Example: | |
392 | #drivers/scsi/Makefile | |
393 | $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl | |
394 | $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl | |
395 | ||
396 | This is a special rule, following the normal syntax | |
397 | required by make. | |
398 | The target file depends on two prerequisite files. References | |
399 | to the target file are prefixed with $(obj), references | |
400 | to prerequisites are referenced with $(src) (because they are not | |
401 | generated files). | |
402 | ||
5410ecc0 MF |
403 | $(kecho) |
404 | echoing information to user in a rule is often a good practice | |
405 | but when execution "make -s" one does not expect to see any output | |
406 | except for warnings/errors. | |
407 | To support this kbuild define $(kecho) which will echo out the | |
408 | text following $(kecho) to stdout except if "make -s" is used. | |
409 | ||
410 | Example: | |
411 | #arch/blackfin/boot/Makefile | |
412 | $(obj)/vmImage: $(obj)/vmlinux.gz | |
413 | $(call if_changed,uimage) | |
414 | @$(kecho) 'Kernel: $@ is ready' | |
415 | ||
416 | ||
20a468b5 SR |
417 | --- 3.11 $(CC) support functions |
418 | ||
a07f6033 | 419 | The kernel may be built with several different versions of |
20a468b5 SR |
420 | $(CC), each supporting a unique set of features and options. |
421 | kbuild provide basic support to check for valid options for $(CC). | |
e95be9a5 | 422 | $(CC) is usually the gcc compiler, but other alternatives are |
20a468b5 SR |
423 | available. |
424 | ||
425 | as-option | |
a07f6033 JE |
426 | as-option is used to check if $(CC) -- when used to compile |
427 | assembler (*.S) files -- supports the given option. An optional | |
428 | second option may be specified if the first option is not supported. | |
20a468b5 SR |
429 | |
430 | Example: | |
431 | #arch/sh/Makefile | |
432 | cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) | |
433 | ||
a07f6033 | 434 | In the above example, cflags-y will be assigned the option |
20a468b5 SR |
435 | -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). |
436 | The second argument is optional, and if supplied will be used | |
437 | if first argument is not supported. | |
438 | ||
f86fd306 SR |
439 | cc-ldoption |
440 | cc-ldoption is used to check if $(CC) when used to link object files | |
0b0bf7a3 RM |
441 | supports the given option. An optional second option may be |
442 | specified if first option are not supported. | |
443 | ||
444 | Example: | |
445 | #arch/i386/kernel/Makefile | |
f86fd306 | 446 | vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) |
0b0bf7a3 | 447 | |
5c811e59 | 448 | In the above example, vsyscall-flags will be assigned the option |
0b0bf7a3 RM |
449 | -Wl$(comma)--hash-style=sysv if it is supported by $(CC). |
450 | The second argument is optional, and if supplied will be used | |
451 | if first argument is not supported. | |
452 | ||
e2414910 AK |
453 | as-instr |
454 | as-instr checks if the assembler reports a specific instruction | |
455 | and then outputs either option1 or option2 | |
456 | C escapes are supported in the test instruction | |
222d394d | 457 | Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options |
e2414910 | 458 | |
20a468b5 | 459 | cc-option |
a07f6033 | 460 | cc-option is used to check if $(CC) supports a given option, and not |
20a468b5 SR |
461 | supported to use an optional second option. |
462 | ||
463 | Example: | |
464 | #arch/i386/Makefile | |
465 | cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) | |
466 | ||
5c811e59 | 467 | In the above example, cflags-y will be assigned the option |
a07f6033 JE |
468 | -march=pentium-mmx if supported by $(CC), otherwise -march=i586. |
469 | The second argument to cc-option is optional, and if omitted, | |
20a468b5 | 470 | cflags-y will be assigned no value if first option is not supported. |
a0f97e06 | 471 | Note: cc-option uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
472 | |
473 | cc-option-yn | |
39e6e9cf | 474 | cc-option-yn is used to check if gcc supports a given option |
20a468b5 SR |
475 | and return 'y' if supported, otherwise 'n'. |
476 | ||
477 | Example: | |
478 | #arch/ppc/Makefile | |
479 | biarch := $(call cc-option-yn, -m32) | |
480 | aflags-$(biarch) += -a32 | |
481 | cflags-$(biarch) += -m32 | |
39e6e9cf | 482 | |
a07f6033 JE |
483 | In the above example, $(biarch) is set to y if $(CC) supports the -m32 |
484 | option. When $(biarch) equals 'y', the expanded variables $(aflags-y) | |
485 | and $(cflags-y) will be assigned the values -a32 and -m32, | |
486 | respectively. | |
a0f97e06 | 487 | Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
488 | |
489 | cc-option-align | |
a07f6033 JE |
490 | gcc versions >= 3.0 changed the type of options used to specify |
491 | alignment of functions, loops etc. $(cc-option-align), when used | |
492 | as prefix to the align options, will select the right prefix: | |
20a468b5 SR |
493 | gcc < 3.00 |
494 | cc-option-align = -malign | |
495 | gcc >= 3.00 | |
496 | cc-option-align = -falign | |
39e6e9cf | 497 | |
20a468b5 | 498 | Example: |
a0f97e06 | 499 | KBUILD_CFLAGS += $(cc-option-align)-functions=4 |
20a468b5 | 500 | |
a07f6033 JE |
501 | In the above example, the option -falign-functions=4 is used for |
502 | gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used. | |
a0f97e06 | 503 | Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options |
39e6e9cf | 504 | |
20a468b5 | 505 | cc-version |
a07f6033 | 506 | cc-version returns a numerical version of the $(CC) compiler version. |
20a468b5 SR |
507 | The format is <major><minor> where both are two digits. So for example |
508 | gcc 3.41 would return 0341. | |
509 | cc-version is useful when a specific $(CC) version is faulty in one | |
a07f6033 | 510 | area, for example -mregparm=3 was broken in some gcc versions |
20a468b5 SR |
511 | even though the option was accepted by gcc. |
512 | ||
513 | Example: | |
514 | #arch/i386/Makefile | |
515 | cflags-y += $(shell \ | |
516 | if [ $(call cc-version) -ge 0300 ] ; then \ | |
517 | echo "-mregparm=3"; fi ;) | |
518 | ||
a07f6033 | 519 | In the above example, -mregparm=3 is only used for gcc version greater |
20a468b5 SR |
520 | than or equal to gcc 3.0. |
521 | ||
522 | cc-ifversion | |
a07f6033 | 523 | cc-ifversion tests the version of $(CC) and equals last argument if |
20a468b5 SR |
524 | version expression is true. |
525 | ||
526 | Example: | |
527 | #fs/reiserfs/Makefile | |
f77bf014 | 528 | ccflags-y := $(call cc-ifversion, -lt, 0402, -O1) |
20a468b5 | 529 | |
f77bf014 | 530 | In this example, ccflags-y will be assigned the value -O1 if the |
20a468b5 | 531 | $(CC) version is less than 4.2. |
39e6e9cf | 532 | cc-ifversion takes all the shell operators: |
20a468b5 SR |
533 | -eq, -ne, -lt, -le, -gt, and -ge |
534 | The third parameter may be a text as in this example, but it may also | |
535 | be an expanded variable or a macro. | |
536 | ||
7015030f SR |
537 | cc-fullversion |
538 | cc-fullversion is useful when the exact version of gcc is needed. | |
539 | One typical use-case is when a specific GCC version is broken. | |
540 | cc-fullversion points out a more specific version than cc-version does. | |
541 | ||
542 | Example: | |
543 | #arch/powerpc/Makefile | |
544 | $(Q)if test "$(call cc-fullversion)" = "040200" ; then \ | |
545 | echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \ | |
546 | false ; \ | |
547 | fi | |
548 | ||
549 | In this example for a specific GCC version the build will error out explaining | |
550 | to the user why it stops. | |
1da177e4 | 551 | |
910b4046 | 552 | cc-cross-prefix |
631bcfbb | 553 | cc-cross-prefix is used to check if there exists a $(CC) in path with |
910b4046 SR |
554 | one of the listed prefixes. The first prefix where there exist a |
555 | prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found | |
556 | then nothing is returned. | |
557 | Additional prefixes are separated by a single space in the | |
558 | call of cc-cross-prefix. | |
631bcfbb GU |
559 | This functionality is useful for architecture Makefiles that try |
560 | to set CROSS_COMPILE to well-known values but may have several | |
910b4046 | 561 | values to select between. |
631bcfbb GU |
562 | It is recommended only to try to set CROSS_COMPILE if it is a cross |
563 | build (host arch is different from target arch). And if CROSS_COMPILE | |
910b4046 SR |
564 | is already set then leave it with the old value. |
565 | ||
566 | Example: | |
567 | #arch/m68k/Makefile | |
568 | ifneq ($(SUBARCH),$(ARCH)) | |
569 | ifeq ($(CROSS_COMPILE),) | |
570 | CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) | |
571 | endif | |
572 | endif | |
573 | ||
691ef3e7 SR |
574 | --- 3.12 $(LD) support functions |
575 | ||
576 | ld-option | |
577 | ld-option is used to check if $(LD) supports the supplied option. | |
578 | ld-option takes two options as arguments. | |
579 | The second argument is an optional option that can be used if the | |
580 | first option is not supported by $(LD). | |
581 | ||
582 | Example: | |
583 | #Makefile | |
584 | LDFLAGS_vmlinux += $(call really-ld-option, -X) | |
585 | ||
586 | ||
1da177e4 LT |
587 | === 4 Host Program support |
588 | ||
589 | Kbuild supports building executables on the host for use during the | |
590 | compilation stage. | |
591 | Two steps are required in order to use a host executable. | |
592 | ||
593 | The first step is to tell kbuild that a host program exists. This is | |
594 | done utilising the variable hostprogs-y. | |
595 | ||
596 | The second step is to add an explicit dependency to the executable. | |
39e6e9cf | 597 | This can be done in two ways. Either add the dependency in a rule, |
1da177e4 LT |
598 | or utilise the variable $(always). |
599 | Both possibilities are described in the following. | |
600 | ||
601 | --- 4.1 Simple Host Program | |
602 | ||
603 | In some cases there is a need to compile and run a program on the | |
604 | computer where the build is running. | |
605 | The following line tells kbuild that the program bin2hex shall be | |
606 | built on the build host. | |
607 | ||
608 | Example: | |
609 | hostprogs-y := bin2hex | |
610 | ||
611 | Kbuild assumes in the above example that bin2hex is made from a single | |
612 | c-source file named bin2hex.c located in the same directory as | |
613 | the Makefile. | |
39e6e9cf | 614 | |
1da177e4 LT |
615 | --- 4.2 Composite Host Programs |
616 | ||
617 | Host programs can be made up based on composite objects. | |
618 | The syntax used to define composite objects for host programs is | |
619 | similar to the syntax used for kernel objects. | |
5d3f083d | 620 | $(<executable>-objs) lists all objects used to link the final |
1da177e4 LT |
621 | executable. |
622 | ||
623 | Example: | |
624 | #scripts/lxdialog/Makefile | |
39e6e9cf | 625 | hostprogs-y := lxdialog |
1da177e4 LT |
626 | lxdialog-objs := checklist.o lxdialog.o |
627 | ||
628 | Objects with extension .o are compiled from the corresponding .c | |
a07f6033 | 629 | files. In the above example, checklist.c is compiled to checklist.o |
1da177e4 | 630 | and lxdialog.c is compiled to lxdialog.o. |
a07f6033 | 631 | Finally, the two .o files are linked to the executable, lxdialog. |
1da177e4 LT |
632 | Note: The syntax <executable>-y is not permitted for host-programs. |
633 | ||
39e6e9cf BH |
634 | --- 4.3 Defining shared libraries |
635 | ||
1da177e4 LT |
636 | Objects with extension .so are considered shared libraries, and |
637 | will be compiled as position independent objects. | |
638 | Kbuild provides support for shared libraries, but the usage | |
639 | shall be restricted. | |
640 | In the following example the libkconfig.so shared library is used | |
641 | to link the executable conf. | |
642 | ||
643 | Example: | |
644 | #scripts/kconfig/Makefile | |
645 | hostprogs-y := conf | |
646 | conf-objs := conf.o libkconfig.so | |
647 | libkconfig-objs := expr.o type.o | |
39e6e9cf | 648 | |
1da177e4 LT |
649 | Shared libraries always require a corresponding -objs line, and |
650 | in the example above the shared library libkconfig is composed by | |
651 | the two objects expr.o and type.o. | |
652 | expr.o and type.o will be built as position independent code and | |
653 | linked as a shared library libkconfig.so. C++ is not supported for | |
654 | shared libraries. | |
655 | ||
656 | --- 4.4 Using C++ for host programs | |
657 | ||
658 | kbuild offers support for host programs written in C++. This was | |
659 | introduced solely to support kconfig, and is not recommended | |
660 | for general use. | |
661 | ||
662 | Example: | |
663 | #scripts/kconfig/Makefile | |
664 | hostprogs-y := qconf | |
665 | qconf-cxxobjs := qconf.o | |
666 | ||
667 | In the example above the executable is composed of the C++ file | |
668 | qconf.cc - identified by $(qconf-cxxobjs). | |
39e6e9cf | 669 | |
1da177e4 LT |
670 | If qconf is composed by a mixture of .c and .cc files, then an |
671 | additional line can be used to identify this. | |
672 | ||
673 | Example: | |
674 | #scripts/kconfig/Makefile | |
675 | hostprogs-y := qconf | |
676 | qconf-cxxobjs := qconf.o | |
677 | qconf-objs := check.o | |
39e6e9cf | 678 | |
1da177e4 LT |
679 | --- 4.5 Controlling compiler options for host programs |
680 | ||
681 | When compiling host programs, it is possible to set specific flags. | |
682 | The programs will always be compiled utilising $(HOSTCC) passed | |
683 | the options specified in $(HOSTCFLAGS). | |
684 | To set flags that will take effect for all host programs created | |
a07f6033 | 685 | in that Makefile, use the variable HOST_EXTRACFLAGS. |
1da177e4 LT |
686 | |
687 | Example: | |
688 | #scripts/lxdialog/Makefile | |
689 | HOST_EXTRACFLAGS += -I/usr/include/ncurses | |
39e6e9cf | 690 | |
1da177e4 LT |
691 | To set specific flags for a single file the following construction |
692 | is used: | |
693 | ||
694 | Example: | |
695 | #arch/ppc64/boot/Makefile | |
696 | HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) | |
39e6e9cf | 697 | |
1da177e4 | 698 | It is also possible to specify additional options to the linker. |
39e6e9cf | 699 | |
1da177e4 LT |
700 | Example: |
701 | #scripts/kconfig/Makefile | |
702 | HOSTLOADLIBES_qconf := -L$(QTDIR)/lib | |
703 | ||
a07f6033 JE |
704 | When linking qconf, it will be passed the extra option |
705 | "-L$(QTDIR)/lib". | |
39e6e9cf | 706 | |
1da177e4 LT |
707 | --- 4.6 When host programs are actually built |
708 | ||
709 | Kbuild will only build host-programs when they are referenced | |
710 | as a prerequisite. | |
711 | This is possible in two ways: | |
712 | ||
713 | (1) List the prerequisite explicitly in a special rule. | |
714 | ||
715 | Example: | |
716 | #drivers/pci/Makefile | |
717 | hostprogs-y := gen-devlist | |
718 | $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist | |
719 | ( cd $(obj); ./gen-devlist ) < $< | |
720 | ||
39e6e9cf | 721 | The target $(obj)/devlist.h will not be built before |
1da177e4 LT |
722 | $(obj)/gen-devlist is updated. Note that references to |
723 | the host programs in special rules must be prefixed with $(obj). | |
724 | ||
725 | (2) Use $(always) | |
726 | When there is no suitable special rule, and the host program | |
727 | shall be built when a makefile is entered, the $(always) | |
728 | variable shall be used. | |
729 | ||
730 | Example: | |
731 | #scripts/lxdialog/Makefile | |
732 | hostprogs-y := lxdialog | |
733 | always := $(hostprogs-y) | |
734 | ||
735 | This will tell kbuild to build lxdialog even if not referenced in | |
736 | any rule. | |
737 | ||
738 | --- 4.7 Using hostprogs-$(CONFIG_FOO) | |
739 | ||
39e6e9cf | 740 | A typical pattern in a Kbuild file looks like this: |
1da177e4 LT |
741 | |
742 | Example: | |
743 | #scripts/Makefile | |
744 | hostprogs-$(CONFIG_KALLSYMS) += kallsyms | |
745 | ||
746 | Kbuild knows about both 'y' for built-in and 'm' for module. | |
747 | So if a config symbol evaluate to 'm', kbuild will still build | |
a07f6033 JE |
748 | the binary. In other words, Kbuild handles hostprogs-m exactly |
749 | like hostprogs-y. But only hostprogs-y is recommended to be used | |
750 | when no CONFIG symbols are involved. | |
1da177e4 LT |
751 | |
752 | === 5 Kbuild clean infrastructure | |
753 | ||
a07f6033 | 754 | "make clean" deletes most generated files in the obj tree where the kernel |
1da177e4 LT |
755 | is compiled. This includes generated files such as host programs. |
756 | Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always), | |
757 | $(extra-y) and $(targets). They are all deleted during "make clean". | |
758 | Files matching the patterns "*.[oas]", "*.ko", plus some additional files | |
759 | generated by kbuild are deleted all over the kernel src tree when | |
760 | "make clean" is executed. | |
761 | ||
762 | Additional files can be specified in kbuild makefiles by use of $(clean-files). | |
763 | ||
764 | Example: | |
765 | #drivers/pci/Makefile | |
766 | clean-files := devlist.h classlist.h | |
767 | ||
768 | When executing "make clean", the two files "devlist.h classlist.h" will | |
769 | be deleted. Kbuild will assume files to be in same relative directory as the | |
770 | Makefile except if an absolute path is specified (path starting with '/'). | |
771 | ||
39e6e9cf BH |
772 | To delete a directory hierarchy use: |
773 | ||
1da177e4 LT |
774 | Example: |
775 | #scripts/package/Makefile | |
776 | clean-dirs := $(objtree)/debian/ | |
777 | ||
778 | This will delete the directory debian, including all subdirectories. | |
779 | Kbuild will assume the directories to be in the same relative path as the | |
780 | Makefile if no absolute path is specified (path does not start with '/'). | |
781 | ||
782 | Usually kbuild descends down in subdirectories due to "obj-* := dir/", | |
783 | but in the architecture makefiles where the kbuild infrastructure | |
784 | is not sufficient this sometimes needs to be explicit. | |
785 | ||
786 | Example: | |
787 | #arch/i386/boot/Makefile | |
788 | subdir- := compressed/ | |
789 | ||
790 | The above assignment instructs kbuild to descend down in the | |
791 | directory compressed/ when "make clean" is executed. | |
792 | ||
793 | To support the clean infrastructure in the Makefiles that builds the | |
794 | final bootimage there is an optional target named archclean: | |
795 | ||
796 | Example: | |
797 | #arch/i386/Makefile | |
798 | archclean: | |
799 | $(Q)$(MAKE) $(clean)=arch/i386/boot | |
800 | ||
801 | When "make clean" is executed, make will descend down in arch/i386/boot, | |
802 | and clean as usual. The Makefile located in arch/i386/boot/ may use | |
803 | the subdir- trick to descend further down. | |
804 | ||
805 | Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is | |
806 | included in the top level makefile, and the kbuild infrastructure | |
807 | is not operational at that point. | |
808 | ||
809 | Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will | |
810 | be visited during "make clean". | |
811 | ||
812 | === 6 Architecture Makefiles | |
813 | ||
814 | The top level Makefile sets up the environment and does the preparation, | |
815 | before starting to descend down in the individual directories. | |
a07f6033 JE |
816 | The top level makefile contains the generic part, whereas |
817 | arch/$(ARCH)/Makefile contains what is required to set up kbuild | |
818 | for said architecture. | |
819 | To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines | |
1da177e4 LT |
820 | a few targets. |
821 | ||
a07f6033 JE |
822 | When kbuild executes, the following steps are followed (roughly): |
823 | 1) Configuration of the kernel => produce .config | |
1da177e4 LT |
824 | 2) Store kernel version in include/linux/version.h |
825 | 3) Symlink include/asm to include/asm-$(ARCH) | |
826 | 4) Updating all other prerequisites to the target prepare: | |
827 | - Additional prerequisites are specified in arch/$(ARCH)/Makefile | |
828 | 5) Recursively descend down in all directories listed in | |
829 | init-* core* drivers-* net-* libs-* and build all targets. | |
a07f6033 | 830 | - The values of the above variables are expanded in arch/$(ARCH)/Makefile. |
39e6e9cf | 831 | 6) All object files are then linked and the resulting file vmlinux is |
a07f6033 | 832 | located at the root of the obj tree. |
1da177e4 LT |
833 | The very first objects linked are listed in head-y, assigned by |
834 | arch/$(ARCH)/Makefile. | |
5c811e59 | 835 | 7) Finally, the architecture-specific part does any required post processing |
1da177e4 LT |
836 | and builds the final bootimage. |
837 | - This includes building boot records | |
5c811e59 | 838 | - Preparing initrd images and the like |
1da177e4 LT |
839 | |
840 | ||
841 | --- 6.1 Set variables to tweak the build to the architecture | |
842 | ||
843 | LDFLAGS Generic $(LD) options | |
844 | ||
845 | Flags used for all invocations of the linker. | |
846 | Often specifying the emulation is sufficient. | |
847 | ||
848 | Example: | |
849 | #arch/s390/Makefile | |
850 | LDFLAGS := -m elf_s390 | |
f77bf014 | 851 | Note: ldflags-y can be used to further customise |
a9af3305 | 852 | the flags used. See chapter 3.7. |
39e6e9cf | 853 | |
1da177e4 LT |
854 | LDFLAGS_MODULE Options for $(LD) when linking modules |
855 | ||
856 | LDFLAGS_MODULE is used to set specific flags for $(LD) when | |
857 | linking the .ko files used for modules. | |
858 | Default is "-r", for relocatable output. | |
859 | ||
860 | LDFLAGS_vmlinux Options for $(LD) when linking vmlinux | |
861 | ||
862 | LDFLAGS_vmlinux is used to specify additional flags to pass to | |
a07f6033 | 863 | the linker when linking the final vmlinux image. |
1da177e4 LT |
864 | LDFLAGS_vmlinux uses the LDFLAGS_$@ support. |
865 | ||
866 | Example: | |
867 | #arch/i386/Makefile | |
868 | LDFLAGS_vmlinux := -e stext | |
869 | ||
870 | OBJCOPYFLAGS objcopy flags | |
871 | ||
872 | When $(call if_changed,objcopy) is used to translate a .o file, | |
a07f6033 | 873 | the flags specified in OBJCOPYFLAGS will be used. |
1da177e4 LT |
874 | $(call if_changed,objcopy) is often used to generate raw binaries on |
875 | vmlinux. | |
876 | ||
877 | Example: | |
878 | #arch/s390/Makefile | |
879 | OBJCOPYFLAGS := -O binary | |
880 | ||
881 | #arch/s390/boot/Makefile | |
882 | $(obj)/image: vmlinux FORCE | |
883 | $(call if_changed,objcopy) | |
884 | ||
a07f6033 | 885 | In this example, the binary $(obj)/image is a binary version of |
1da177e4 LT |
886 | vmlinux. The usage of $(call if_changed,xxx) will be described later. |
887 | ||
222d394d | 888 | KBUILD_AFLAGS $(AS) assembler flags |
1da177e4 LT |
889 | |
890 | Default value - see top level Makefile | |
891 | Append or modify as required per architecture. | |
892 | ||
893 | Example: | |
894 | #arch/sparc64/Makefile | |
222d394d | 895 | KBUILD_AFLAGS += -m64 -mcpu=ultrasparc |
1da177e4 | 896 | |
a0f97e06 | 897 | KBUILD_CFLAGS $(CC) compiler flags |
1da177e4 LT |
898 | |
899 | Default value - see top level Makefile | |
900 | Append or modify as required per architecture. | |
901 | ||
a0f97e06 | 902 | Often, the KBUILD_CFLAGS variable depends on the configuration. |
1da177e4 LT |
903 | |
904 | Example: | |
905 | #arch/i386/Makefile | |
906 | cflags-$(CONFIG_M386) += -march=i386 | |
a0f97e06 | 907 | KBUILD_CFLAGS += $(cflags-y) |
1da177e4 LT |
908 | |
909 | Many arch Makefiles dynamically run the target C compiler to | |
910 | probe supported options: | |
911 | ||
912 | #arch/i386/Makefile | |
913 | ||
914 | ... | |
915 | cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ | |
916 | -march=pentium2,-march=i686) | |
917 | ... | |
918 | # Disable unit-at-a-time mode ... | |
a0f97e06 | 919 | KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) |
1da177e4 LT |
920 | ... |
921 | ||
922 | ||
a07f6033 | 923 | The first example utilises the trick that a config option expands |
1da177e4 LT |
924 | to 'y' when selected. |
925 | ||
926 | CFLAGS_KERNEL $(CC) options specific for built-in | |
927 | ||
928 | $(CFLAGS_KERNEL) contains extra C compiler flags used to compile | |
929 | resident kernel code. | |
930 | ||
931 | CFLAGS_MODULE $(CC) options specific for modules | |
932 | ||
933 | $(CFLAGS_MODULE) contains extra C compiler flags used to compile code | |
934 | for loadable kernel modules. | |
935 | ||
39e6e9cf | 936 | |
5bb78269 | 937 | --- 6.2 Add prerequisites to archprepare: |
1da177e4 | 938 | |
a07f6033 | 939 | The archprepare: rule is used to list prerequisites that need to be |
1da177e4 | 940 | built before starting to descend down in the subdirectories. |
a07f6033 | 941 | This is usually used for header files containing assembler constants. |
1da177e4 LT |
942 | |
943 | Example: | |
5bb78269 SR |
944 | #arch/arm/Makefile |
945 | archprepare: maketools | |
1da177e4 | 946 | |
a07f6033 | 947 | In this example, the file target maketools will be processed |
5bb78269 | 948 | before descending down in the subdirectories. |
1da177e4 LT |
949 | See also chapter XXX-TODO that describe how kbuild supports |
950 | generating offset header files. | |
951 | ||
952 | ||
953 | --- 6.3 List directories to visit when descending | |
954 | ||
955 | An arch Makefile cooperates with the top Makefile to define variables | |
956 | which specify how to build the vmlinux file. Note that there is no | |
957 | corresponding arch-specific section for modules; the module-building | |
958 | machinery is all architecture-independent. | |
959 | ||
39e6e9cf | 960 | |
1da177e4 LT |
961 | head-y, init-y, core-y, libs-y, drivers-y, net-y |
962 | ||
a07f6033 JE |
963 | $(head-y) lists objects to be linked first in vmlinux. |
964 | $(libs-y) lists directories where a lib.a archive can be located. | |
5c811e59 | 965 | The rest list directories where a built-in.o object file can be |
a07f6033 | 966 | located. |
1da177e4 LT |
967 | |
968 | $(init-y) objects will be located after $(head-y). | |
969 | Then the rest follows in this order: | |
970 | $(core-y), $(libs-y), $(drivers-y) and $(net-y). | |
971 | ||
a07f6033 | 972 | The top level Makefile defines values for all generic directories, |
5c811e59 | 973 | and arch/$(ARCH)/Makefile only adds architecture-specific directories. |
1da177e4 LT |
974 | |
975 | Example: | |
976 | #arch/sparc64/Makefile | |
977 | core-y += arch/sparc64/kernel/ | |
978 | libs-y += arch/sparc64/prom/ arch/sparc64/lib/ | |
979 | drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ | |
980 | ||
981 | ||
5c811e59 | 982 | --- 6.4 Architecture-specific boot images |
1da177e4 LT |
983 | |
984 | An arch Makefile specifies goals that take the vmlinux file, compress | |
985 | it, wrap it in bootstrapping code, and copy the resulting files | |
986 | somewhere. This includes various kinds of installation commands. | |
987 | The actual goals are not standardized across architectures. | |
988 | ||
989 | It is common to locate any additional processing in a boot/ | |
990 | directory below arch/$(ARCH)/. | |
991 | ||
992 | Kbuild does not provide any smart way to support building a | |
993 | target specified in boot/. Therefore arch/$(ARCH)/Makefile shall | |
994 | call make manually to build a target in boot/. | |
995 | ||
996 | The recommended approach is to include shortcuts in | |
997 | arch/$(ARCH)/Makefile, and use the full path when calling down | |
998 | into the arch/$(ARCH)/boot/Makefile. | |
999 | ||
1000 | Example: | |
1001 | #arch/i386/Makefile | |
1002 | boot := arch/i386/boot | |
1003 | bzImage: vmlinux | |
1004 | $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ | |
1005 | ||
1006 | "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke | |
1007 | make in a subdirectory. | |
1008 | ||
5c811e59 | 1009 | There are no rules for naming architecture-specific targets, |
1da177e4 | 1010 | but executing "make help" will list all relevant targets. |
a07f6033 | 1011 | To support this, $(archhelp) must be defined. |
1da177e4 LT |
1012 | |
1013 | Example: | |
1014 | #arch/i386/Makefile | |
1015 | define archhelp | |
1016 | echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' | |
39e6e9cf | 1017 | endif |
1da177e4 LT |
1018 | |
1019 | When make is executed without arguments, the first goal encountered | |
1020 | will be built. In the top level Makefile the first goal present | |
1021 | is all:. | |
a07f6033 JE |
1022 | An architecture shall always, per default, build a bootable image. |
1023 | In "make help", the default goal is highlighted with a '*'. | |
1da177e4 LT |
1024 | Add a new prerequisite to all: to select a default goal different |
1025 | from vmlinux. | |
1026 | ||
1027 | Example: | |
1028 | #arch/i386/Makefile | |
39e6e9cf | 1029 | all: bzImage |
1da177e4 LT |
1030 | |
1031 | When "make" is executed without arguments, bzImage will be built. | |
1032 | ||
1033 | --- 6.5 Building non-kbuild targets | |
1034 | ||
1035 | extra-y | |
1036 | ||
1037 | extra-y specify additional targets created in the current | |
1038 | directory, in addition to any targets specified by obj-*. | |
1039 | ||
1040 | Listing all targets in extra-y is required for two purposes: | |
1041 | 1) Enable kbuild to check changes in command lines | |
1042 | - When $(call if_changed,xxx) is used | |
1043 | 2) kbuild knows what files to delete during "make clean" | |
1044 | ||
1045 | Example: | |
1046 | #arch/i386/kernel/Makefile | |
1047 | extra-y := head.o init_task.o | |
1048 | ||
a07f6033 | 1049 | In this example, extra-y is used to list object files that |
1da177e4 LT |
1050 | shall be built, but shall not be linked as part of built-in.o. |
1051 | ||
39e6e9cf | 1052 | |
1da177e4 LT |
1053 | --- 6.6 Commands useful for building a boot image |
1054 | ||
1055 | Kbuild provides a few macros that are useful when building a | |
1056 | boot image. | |
1057 | ||
1058 | if_changed | |
1059 | ||
1060 | if_changed is the infrastructure used for the following commands. | |
1061 | ||
1062 | Usage: | |
1063 | target: source(s) FORCE | |
1064 | $(call if_changed,ld/objcopy/gzip) | |
1065 | ||
a07f6033 | 1066 | When the rule is evaluated, it is checked to see if any files |
5c811e59 | 1067 | need an update, or the command line has changed since the last |
1da177e4 LT |
1068 | invocation. The latter will force a rebuild if any options |
1069 | to the executable have changed. | |
1070 | Any target that utilises if_changed must be listed in $(targets), | |
1071 | otherwise the command line check will fail, and the target will | |
1072 | always be built. | |
1073 | Assignments to $(targets) are without $(obj)/ prefix. | |
1074 | if_changed may be used in conjunction with custom commands as | |
1075 | defined in 6.7 "Custom kbuild commands". | |
49490571 | 1076 | |
1da177e4 | 1077 | Note: It is a typical mistake to forget the FORCE prerequisite. |
49490571 PBG |
1078 | Another common pitfall is that whitespace is sometimes |
1079 | significant; for instance, the below will fail (note the extra space | |
1080 | after the comma): | |
1081 | target: source(s) FORCE | |
1082 | #WRONG!# $(call if_changed, ld/objcopy/gzip) | |
1da177e4 LT |
1083 | |
1084 | ld | |
a07f6033 | 1085 | Link target. Often, LDFLAGS_$@ is used to set specific options to ld. |
39e6e9cf | 1086 | |
1da177e4 LT |
1087 | objcopy |
1088 | Copy binary. Uses OBJCOPYFLAGS usually specified in | |
1089 | arch/$(ARCH)/Makefile. | |
1090 | OBJCOPYFLAGS_$@ may be used to set additional options. | |
1091 | ||
1092 | gzip | |
1093 | Compress target. Use maximum compression to compress target. | |
1094 | ||
1095 | Example: | |
1096 | #arch/i386/boot/Makefile | |
1097 | LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary | |
1098 | LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext | |
1099 | ||
1100 | targets += setup setup.o bootsect bootsect.o | |
1101 | $(obj)/setup $(obj)/bootsect: %: %.o FORCE | |
1102 | $(call if_changed,ld) | |
1103 | ||
a07f6033 JE |
1104 | In this example, there are two possible targets, requiring different |
1105 | options to the linker. The linker options are specified using the | |
1da177e4 | 1106 | LDFLAGS_$@ syntax - one for each potential target. |
5d3f083d | 1107 | $(targets) are assigned all potential targets, by which kbuild knows |
1da177e4 LT |
1108 | the targets and will: |
1109 | 1) check for commandline changes | |
1110 | 2) delete target during make clean | |
1111 | ||
1112 | The ": %: %.o" part of the prerequisite is a shorthand that | |
1113 | free us from listing the setup.o and bootsect.o files. | |
1114 | Note: It is a common mistake to forget the "target :=" assignment, | |
1115 | resulting in the target file being recompiled for no | |
1116 | obvious reason. | |
1117 | ||
1118 | ||
1119 | --- 6.7 Custom kbuild commands | |
1120 | ||
a07f6033 | 1121 | When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand |
1da177e4 LT |
1122 | of a command is normally displayed. |
1123 | To enable this behaviour for custom commands kbuild requires | |
1124 | two variables to be set: | |
1125 | quiet_cmd_<command> - what shall be echoed | |
1126 | cmd_<command> - the command to execute | |
1127 | ||
1128 | Example: | |
1129 | # | |
1130 | quiet_cmd_image = BUILD $@ | |
1131 | cmd_image = $(obj)/tools/build $(BUILDFLAGS) \ | |
1132 | $(obj)/vmlinux.bin > $@ | |
1133 | ||
1134 | targets += bzImage | |
1135 | $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE | |
1136 | $(call if_changed,image) | |
1137 | @echo 'Kernel: $@ is ready' | |
1138 | ||
a07f6033 | 1139 | When updating the $(obj)/bzImage target, the line |
1da177e4 LT |
1140 | |
1141 | BUILD arch/i386/boot/bzImage | |
1142 | ||
1143 | will be displayed with "make KBUILD_VERBOSE=0". | |
39e6e9cf | 1144 | |
1da177e4 LT |
1145 | |
1146 | --- 6.8 Preprocessing linker scripts | |
1147 | ||
a07f6033 | 1148 | When the vmlinux image is built, the linker script |
1da177e4 LT |
1149 | arch/$(ARCH)/kernel/vmlinux.lds is used. |
1150 | The script is a preprocessed variant of the file vmlinux.lds.S | |
1151 | located in the same directory. | |
a07f6033 | 1152 | kbuild knows .lds files and includes a rule *lds.S -> *lds. |
39e6e9cf | 1153 | |
1da177e4 LT |
1154 | Example: |
1155 | #arch/i386/kernel/Makefile | |
1156 | always := vmlinux.lds | |
39e6e9cf | 1157 | |
1da177e4 LT |
1158 | #Makefile |
1159 | export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) | |
39e6e9cf BH |
1160 | |
1161 | The assignment to $(always) is used to tell kbuild to build the | |
a07f6033 JE |
1162 | target vmlinux.lds. |
1163 | The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the | |
1da177e4 | 1164 | specified options when building the target vmlinux.lds. |
39e6e9cf | 1165 | |
a07f6033 | 1166 | When building the *.lds target, kbuild uses the variables: |
06c5040c | 1167 | KBUILD_CPPFLAGS : Set in top-level Makefile |
f77bf014 | 1168 | cppflags-y : May be set in the kbuild makefile |
1da177e4 LT |
1169 | CPPFLAGS_$(@F) : Target specific flags. |
1170 | Note that the full filename is used in this | |
1171 | assignment. | |
1172 | ||
1173 | The kbuild infrastructure for *lds file are used in several | |
5c811e59 | 1174 | architecture-specific files. |
1da177e4 | 1175 | |
c7bb349e SR |
1176 | === 7 Kbuild syntax for exported headers |
1177 | ||
1178 | The kernel include a set of headers that is exported to userspace. | |
1179 | Many headers can be exported as-is but other headers requires a | |
1180 | minimal pre-processing before they are ready for user-space. | |
1181 | The pre-processing does: | |
1182 | - drop kernel specific annotations | |
1183 | - drop include of compiler.h | |
1184 | - drop all sections that is kernel internat (guarded by ifdef __KERNEL__) | |
1185 | ||
1186 | Each relevant directory contain a file name "Kbuild" which specify the | |
1187 | headers to be exported. | |
1188 | See subsequent chapter for the syntax of the Kbuild file. | |
1189 | ||
1190 | --- 7.1 header-y | |
1191 | ||
1192 | header-y specify header files to be exported. | |
1193 | ||
1194 | Example: | |
1195 | #include/linux/Kbuild | |
1196 | header-y += usb/ | |
1197 | header-y += aio_abi.h | |
1198 | ||
1199 | The convention is to list one file per line and | |
1200 | preferably in alphabetic order. | |
1201 | ||
1202 | header-y also specify which subdirectories to visit. | |
1203 | A subdirectory is identified by a trailing '/' which | |
1204 | can be seen in the example above for the usb subdirectory. | |
1205 | ||
1206 | Subdirectories are visited before their parent directories. | |
1207 | ||
1208 | --- 7.2 objhdr-y | |
1209 | ||
1210 | objhdr-y specifies generated files to be exported. | |
1211 | Generated files are special as they need to be looked | |
1212 | up in another directory when doing 'make O=...' builds. | |
1213 | ||
1214 | Example: | |
1215 | #include/linux/Kbuild | |
1216 | objhdr-y += version.h | |
1217 | ||
1218 | --- 7.3 destination-y | |
1219 | ||
1220 | When an architecture have a set of exported headers that needs to be | |
1221 | exported to a different directory destination-y is used. | |
1222 | destination-y specify the destination directory for all exported | |
1223 | headers in the file where it is present. | |
1224 | ||
1225 | Example: | |
1226 | #arch/xtensa/platforms/s6105/include/platform/Kbuild | |
1227 | destination-y := include/linux | |
1228 | ||
1229 | In the example above all exported headers in the Kbuild file | |
1230 | will be located in the directory "include/linux" when exported. | |
1231 | ||
1232 | ||
1233 | --- 7.4 unifdef-y (deprecated) | |
1234 | ||
1235 | unifdef-y is deprecated. A direct replacement is header-y. | |
1236 | ||
1da177e4 | 1237 | |
c7bb349e | 1238 | === 8 Kbuild Variables |
1da177e4 LT |
1239 | |
1240 | The top Makefile exports the following variables: | |
1241 | ||
1242 | VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION | |
1243 | ||
1244 | These variables define the current kernel version. A few arch | |
1245 | Makefiles actually use these values directly; they should use | |
1246 | $(KERNELRELEASE) instead. | |
1247 | ||
1248 | $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic | |
1249 | three-part version number, such as "2", "4", and "0". These three | |
1250 | values are always numeric. | |
1251 | ||
1252 | $(EXTRAVERSION) defines an even tinier sublevel for pre-patches | |
1253 | or additional patches. It is usually some non-numeric string | |
1254 | such as "-pre4", and is often blank. | |
1255 | ||
1256 | KERNELRELEASE | |
1257 | ||
1258 | $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable | |
1259 | for constructing installation directory names or showing in | |
1260 | version strings. Some arch Makefiles use it for this purpose. | |
1261 | ||
1262 | ARCH | |
1263 | ||
1264 | This variable defines the target architecture, such as "i386", | |
1265 | "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to | |
1266 | determine which files to compile. | |
1267 | ||
1268 | By default, the top Makefile sets $(ARCH) to be the same as the | |
1269 | host system architecture. For a cross build, a user may | |
1270 | override the value of $(ARCH) on the command line: | |
1271 | ||
1272 | make ARCH=m68k ... | |
1273 | ||
1274 | ||
1275 | INSTALL_PATH | |
1276 | ||
1277 | This variable defines a place for the arch Makefiles to install | |
1278 | the resident kernel image and System.map file. | |
5c811e59 | 1279 | Use this for architecture-specific install targets. |
1da177e4 LT |
1280 | |
1281 | INSTALL_MOD_PATH, MODLIB | |
1282 | ||
1283 | $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module | |
1284 | installation. This variable is not defined in the Makefile but | |
1285 | may be passed in by the user if desired. | |
1286 | ||
1287 | $(MODLIB) specifies the directory for module installation. | |
1288 | The top Makefile defines $(MODLIB) to | |
1289 | $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may | |
1290 | override this value on the command line if desired. | |
1291 | ||
ac031f26 TT |
1292 | INSTALL_MOD_STRIP |
1293 | ||
1294 | If this variable is specified, will cause modules to be stripped | |
1295 | after they are installed. If INSTALL_MOD_STRIP is '1', then the | |
1296 | default option --strip-debug will be used. Otherwise, | |
1297 | INSTALL_MOD_STRIP will used as the option(s) to the strip command. | |
1298 | ||
1299 | ||
c7bb349e | 1300 | === 9 Makefile language |
1da177e4 | 1301 | |
a07f6033 | 1302 | The kernel Makefiles are designed to be run with GNU Make. The Makefiles |
1da177e4 LT |
1303 | use only the documented features of GNU Make, but they do use many |
1304 | GNU extensions. | |
1305 | ||
1306 | GNU Make supports elementary list-processing functions. The kernel | |
1307 | Makefiles use a novel style of list building and manipulation with few | |
1308 | "if" statements. | |
1309 | ||
1310 | GNU Make has two assignment operators, ":=" and "=". ":=" performs | |
1311 | immediate evaluation of the right-hand side and stores an actual string | |
1312 | into the left-hand side. "=" is like a formula definition; it stores the | |
1313 | right-hand side in an unevaluated form and then evaluates this form each | |
1314 | time the left-hand side is used. | |
1315 | ||
1316 | There are some cases where "=" is appropriate. Usually, though, ":=" | |
1317 | is the right choice. | |
1318 | ||
c7bb349e | 1319 | === 10 Credits |
1da177e4 LT |
1320 | |
1321 | Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> | |
1322 | Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> | |
1323 | Updates by Sam Ravnborg <sam@ravnborg.org> | |
a07f6033 | 1324 | Language QA by Jan Engelhardt <jengelh@gmx.de> |
1da177e4 | 1325 | |
c7bb349e | 1326 | === 11 TODO |
1da177e4 | 1327 | |
a07f6033 | 1328 | - Describe how kbuild supports shipped files with _shipped. |
1da177e4 LT |
1329 | - Generating offset header files. |
1330 | - Add more variables to section 7? | |
1331 | ||
39e6e9cf BH |
1332 | |
1333 |