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