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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 | ||
5410ecc0 MF |
386 | $(kecho) |
387 | echoing information to user in a rule is often a good practice | |
388 | but when execution "make -s" one does not expect to see any output | |
389 | except for warnings/errors. | |
390 | To support this kbuild define $(kecho) which will echo out the | |
391 | text following $(kecho) to stdout except if "make -s" is used. | |
392 | ||
393 | Example: | |
394 | #arch/blackfin/boot/Makefile | |
395 | $(obj)/vmImage: $(obj)/vmlinux.gz | |
396 | $(call if_changed,uimage) | |
397 | @$(kecho) 'Kernel: $@ is ready' | |
398 | ||
399 | ||
20a468b5 SR |
400 | --- 3.11 $(CC) support functions |
401 | ||
a07f6033 | 402 | The kernel may be built with several different versions of |
20a468b5 SR |
403 | $(CC), each supporting a unique set of features and options. |
404 | kbuild provide basic support to check for valid options for $(CC). | |
e95be9a5 | 405 | $(CC) is usually the gcc compiler, but other alternatives are |
20a468b5 SR |
406 | available. |
407 | ||
408 | as-option | |
a07f6033 JE |
409 | as-option is used to check if $(CC) -- when used to compile |
410 | assembler (*.S) files -- supports the given option. An optional | |
411 | second option may be specified if the first option is not supported. | |
20a468b5 SR |
412 | |
413 | Example: | |
414 | #arch/sh/Makefile | |
415 | cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) | |
416 | ||
a07f6033 | 417 | In the above example, cflags-y will be assigned the option |
20a468b5 SR |
418 | -Wa$(comma)-isa=$(isa-y) 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 | ||
0b0bf7a3 | 422 | ld-option |
39e6e9cf | 423 | ld-option is used to check if $(CC) when used to link object files |
0b0bf7a3 RM |
424 | supports the given option. An optional second option may be |
425 | specified if first option are not supported. | |
426 | ||
427 | Example: | |
428 | #arch/i386/kernel/Makefile | |
429 | vsyscall-flags += $(call ld-option, -Wl$(comma)--hash-style=sysv) | |
430 | ||
5c811e59 | 431 | In the above example, vsyscall-flags will be assigned the option |
0b0bf7a3 RM |
432 | -Wl$(comma)--hash-style=sysv if it is supported by $(CC). |
433 | The second argument is optional, and if supplied will be used | |
434 | if first argument is not supported. | |
435 | ||
e2414910 AK |
436 | as-instr |
437 | as-instr checks if the assembler reports a specific instruction | |
438 | and then outputs either option1 or option2 | |
439 | C escapes are supported in the test instruction | |
222d394d | 440 | Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options |
e2414910 | 441 | |
20a468b5 | 442 | cc-option |
a07f6033 | 443 | cc-option is used to check if $(CC) supports a given option, and not |
20a468b5 SR |
444 | supported to use an optional second option. |
445 | ||
446 | Example: | |
447 | #arch/i386/Makefile | |
448 | cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) | |
449 | ||
5c811e59 | 450 | In the above example, cflags-y will be assigned the option |
a07f6033 JE |
451 | -march=pentium-mmx if supported by $(CC), otherwise -march=i586. |
452 | The second argument to cc-option is optional, and if omitted, | |
20a468b5 | 453 | cflags-y will be assigned no value if first option is not supported. |
a0f97e06 | 454 | Note: cc-option uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
455 | |
456 | cc-option-yn | |
39e6e9cf | 457 | cc-option-yn is used to check if gcc supports a given option |
20a468b5 SR |
458 | and return 'y' if supported, otherwise 'n'. |
459 | ||
460 | Example: | |
461 | #arch/ppc/Makefile | |
462 | biarch := $(call cc-option-yn, -m32) | |
463 | aflags-$(biarch) += -a32 | |
464 | cflags-$(biarch) += -m32 | |
39e6e9cf | 465 | |
a07f6033 JE |
466 | In the above example, $(biarch) is set to y if $(CC) supports the -m32 |
467 | option. When $(biarch) equals 'y', the expanded variables $(aflags-y) | |
468 | and $(cflags-y) will be assigned the values -a32 and -m32, | |
469 | respectively. | |
a0f97e06 | 470 | Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
471 | |
472 | cc-option-align | |
a07f6033 JE |
473 | gcc versions >= 3.0 changed the type of options used to specify |
474 | alignment of functions, loops etc. $(cc-option-align), when used | |
475 | as prefix to the align options, will select the right prefix: | |
20a468b5 SR |
476 | gcc < 3.00 |
477 | cc-option-align = -malign | |
478 | gcc >= 3.00 | |
479 | cc-option-align = -falign | |
39e6e9cf | 480 | |
20a468b5 | 481 | Example: |
a0f97e06 | 482 | KBUILD_CFLAGS += $(cc-option-align)-functions=4 |
20a468b5 | 483 | |
a07f6033 JE |
484 | In the above example, the option -falign-functions=4 is used for |
485 | gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used. | |
a0f97e06 | 486 | Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options |
39e6e9cf | 487 | |
20a468b5 | 488 | cc-version |
a07f6033 | 489 | cc-version returns a numerical version of the $(CC) compiler version. |
20a468b5 SR |
490 | The format is <major><minor> where both are two digits. So for example |
491 | gcc 3.41 would return 0341. | |
492 | cc-version is useful when a specific $(CC) version is faulty in one | |
a07f6033 | 493 | area, for example -mregparm=3 was broken in some gcc versions |
20a468b5 SR |
494 | even though the option was accepted by gcc. |
495 | ||
496 | Example: | |
497 | #arch/i386/Makefile | |
498 | cflags-y += $(shell \ | |
499 | if [ $(call cc-version) -ge 0300 ] ; then \ | |
500 | echo "-mregparm=3"; fi ;) | |
501 | ||
a07f6033 | 502 | In the above example, -mregparm=3 is only used for gcc version greater |
20a468b5 SR |
503 | than or equal to gcc 3.0. |
504 | ||
505 | cc-ifversion | |
a07f6033 | 506 | cc-ifversion tests the version of $(CC) and equals last argument if |
20a468b5 SR |
507 | version expression is true. |
508 | ||
509 | Example: | |
510 | #fs/reiserfs/Makefile | |
f77bf014 | 511 | ccflags-y := $(call cc-ifversion, -lt, 0402, -O1) |
20a468b5 | 512 | |
f77bf014 | 513 | In this example, ccflags-y will be assigned the value -O1 if the |
20a468b5 | 514 | $(CC) version is less than 4.2. |
39e6e9cf | 515 | cc-ifversion takes all the shell operators: |
20a468b5 SR |
516 | -eq, -ne, -lt, -le, -gt, and -ge |
517 | The third parameter may be a text as in this example, but it may also | |
518 | be an expanded variable or a macro. | |
519 | ||
7015030f SR |
520 | cc-fullversion |
521 | cc-fullversion is useful when the exact version of gcc is needed. | |
522 | One typical use-case is when a specific GCC version is broken. | |
523 | cc-fullversion points out a more specific version than cc-version does. | |
524 | ||
525 | Example: | |
526 | #arch/powerpc/Makefile | |
527 | $(Q)if test "$(call cc-fullversion)" = "040200" ; then \ | |
528 | echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \ | |
529 | false ; \ | |
530 | fi | |
531 | ||
532 | In this example for a specific GCC version the build will error out explaining | |
533 | to the user why it stops. | |
1da177e4 | 534 | |
910b4046 | 535 | cc-cross-prefix |
631bcfbb | 536 | cc-cross-prefix is used to check if there exists a $(CC) in path with |
910b4046 SR |
537 | one of the listed prefixes. The first prefix where there exist a |
538 | prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found | |
539 | then nothing is returned. | |
540 | Additional prefixes are separated by a single space in the | |
541 | call of cc-cross-prefix. | |
631bcfbb GU |
542 | This functionality is useful for architecture Makefiles that try |
543 | to set CROSS_COMPILE to well-known values but may have several | |
910b4046 | 544 | values to select between. |
631bcfbb GU |
545 | It is recommended only to try to set CROSS_COMPILE if it is a cross |
546 | build (host arch is different from target arch). And if CROSS_COMPILE | |
910b4046 SR |
547 | is already set then leave it with the old value. |
548 | ||
549 | Example: | |
550 | #arch/m68k/Makefile | |
551 | ifneq ($(SUBARCH),$(ARCH)) | |
552 | ifeq ($(CROSS_COMPILE),) | |
553 | CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) | |
554 | endif | |
555 | endif | |
556 | ||
1da177e4 LT |
557 | === 4 Host Program support |
558 | ||
559 | Kbuild supports building executables on the host for use during the | |
560 | compilation stage. | |
561 | Two steps are required in order to use a host executable. | |
562 | ||
563 | The first step is to tell kbuild that a host program exists. This is | |
564 | done utilising the variable hostprogs-y. | |
565 | ||
566 | The second step is to add an explicit dependency to the executable. | |
39e6e9cf | 567 | This can be done in two ways. Either add the dependency in a rule, |
1da177e4 LT |
568 | or utilise the variable $(always). |
569 | Both possibilities are described in the following. | |
570 | ||
571 | --- 4.1 Simple Host Program | |
572 | ||
573 | In some cases there is a need to compile and run a program on the | |
574 | computer where the build is running. | |
575 | The following line tells kbuild that the program bin2hex shall be | |
576 | built on the build host. | |
577 | ||
578 | Example: | |
579 | hostprogs-y := bin2hex | |
580 | ||
581 | Kbuild assumes in the above example that bin2hex is made from a single | |
582 | c-source file named bin2hex.c located in the same directory as | |
583 | the Makefile. | |
39e6e9cf | 584 | |
1da177e4 LT |
585 | --- 4.2 Composite Host Programs |
586 | ||
587 | Host programs can be made up based on composite objects. | |
588 | The syntax used to define composite objects for host programs is | |
589 | similar to the syntax used for kernel objects. | |
5d3f083d | 590 | $(<executable>-objs) lists all objects used to link the final |
1da177e4 LT |
591 | executable. |
592 | ||
593 | Example: | |
594 | #scripts/lxdialog/Makefile | |
39e6e9cf | 595 | hostprogs-y := lxdialog |
1da177e4 LT |
596 | lxdialog-objs := checklist.o lxdialog.o |
597 | ||
598 | Objects with extension .o are compiled from the corresponding .c | |
a07f6033 | 599 | files. In the above example, checklist.c is compiled to checklist.o |
1da177e4 | 600 | and lxdialog.c is compiled to lxdialog.o. |
a07f6033 | 601 | Finally, the two .o files are linked to the executable, lxdialog. |
1da177e4 LT |
602 | Note: The syntax <executable>-y is not permitted for host-programs. |
603 | ||
39e6e9cf BH |
604 | --- 4.3 Defining shared libraries |
605 | ||
1da177e4 LT |
606 | Objects with extension .so are considered shared libraries, and |
607 | will be compiled as position independent objects. | |
608 | Kbuild provides support for shared libraries, but the usage | |
609 | shall be restricted. | |
610 | In the following example the libkconfig.so shared library is used | |
611 | to link the executable conf. | |
612 | ||
613 | Example: | |
614 | #scripts/kconfig/Makefile | |
615 | hostprogs-y := conf | |
616 | conf-objs := conf.o libkconfig.so | |
617 | libkconfig-objs := expr.o type.o | |
39e6e9cf | 618 | |
1da177e4 LT |
619 | Shared libraries always require a corresponding -objs line, and |
620 | in the example above the shared library libkconfig is composed by | |
621 | the two objects expr.o and type.o. | |
622 | expr.o and type.o will be built as position independent code and | |
623 | linked as a shared library libkconfig.so. C++ is not supported for | |
624 | shared libraries. | |
625 | ||
626 | --- 4.4 Using C++ for host programs | |
627 | ||
628 | kbuild offers support for host programs written in C++. This was | |
629 | introduced solely to support kconfig, and is not recommended | |
630 | for general use. | |
631 | ||
632 | Example: | |
633 | #scripts/kconfig/Makefile | |
634 | hostprogs-y := qconf | |
635 | qconf-cxxobjs := qconf.o | |
636 | ||
637 | In the example above the executable is composed of the C++ file | |
638 | qconf.cc - identified by $(qconf-cxxobjs). | |
39e6e9cf | 639 | |
1da177e4 LT |
640 | If qconf is composed by a mixture of .c and .cc files, then an |
641 | additional line can be used to identify this. | |
642 | ||
643 | Example: | |
644 | #scripts/kconfig/Makefile | |
645 | hostprogs-y := qconf | |
646 | qconf-cxxobjs := qconf.o | |
647 | qconf-objs := check.o | |
39e6e9cf | 648 | |
1da177e4 LT |
649 | --- 4.5 Controlling compiler options for host programs |
650 | ||
651 | When compiling host programs, it is possible to set specific flags. | |
652 | The programs will always be compiled utilising $(HOSTCC) passed | |
653 | the options specified in $(HOSTCFLAGS). | |
654 | To set flags that will take effect for all host programs created | |
a07f6033 | 655 | in that Makefile, use the variable HOST_EXTRACFLAGS. |
1da177e4 LT |
656 | |
657 | Example: | |
658 | #scripts/lxdialog/Makefile | |
659 | HOST_EXTRACFLAGS += -I/usr/include/ncurses | |
39e6e9cf | 660 | |
1da177e4 LT |
661 | To set specific flags for a single file the following construction |
662 | is used: | |
663 | ||
664 | Example: | |
665 | #arch/ppc64/boot/Makefile | |
666 | HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) | |
39e6e9cf | 667 | |
1da177e4 | 668 | It is also possible to specify additional options to the linker. |
39e6e9cf | 669 | |
1da177e4 LT |
670 | Example: |
671 | #scripts/kconfig/Makefile | |
672 | HOSTLOADLIBES_qconf := -L$(QTDIR)/lib | |
673 | ||
a07f6033 JE |
674 | When linking qconf, it will be passed the extra option |
675 | "-L$(QTDIR)/lib". | |
39e6e9cf | 676 | |
1da177e4 LT |
677 | --- 4.6 When host programs are actually built |
678 | ||
679 | Kbuild will only build host-programs when they are referenced | |
680 | as a prerequisite. | |
681 | This is possible in two ways: | |
682 | ||
683 | (1) List the prerequisite explicitly in a special rule. | |
684 | ||
685 | Example: | |
686 | #drivers/pci/Makefile | |
687 | hostprogs-y := gen-devlist | |
688 | $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist | |
689 | ( cd $(obj); ./gen-devlist ) < $< | |
690 | ||
39e6e9cf | 691 | The target $(obj)/devlist.h will not be built before |
1da177e4 LT |
692 | $(obj)/gen-devlist is updated. Note that references to |
693 | the host programs in special rules must be prefixed with $(obj). | |
694 | ||
695 | (2) Use $(always) | |
696 | When there is no suitable special rule, and the host program | |
697 | shall be built when a makefile is entered, the $(always) | |
698 | variable shall be used. | |
699 | ||
700 | Example: | |
701 | #scripts/lxdialog/Makefile | |
702 | hostprogs-y := lxdialog | |
703 | always := $(hostprogs-y) | |
704 | ||
705 | This will tell kbuild to build lxdialog even if not referenced in | |
706 | any rule. | |
707 | ||
708 | --- 4.7 Using hostprogs-$(CONFIG_FOO) | |
709 | ||
39e6e9cf | 710 | A typical pattern in a Kbuild file looks like this: |
1da177e4 LT |
711 | |
712 | Example: | |
713 | #scripts/Makefile | |
714 | hostprogs-$(CONFIG_KALLSYMS) += kallsyms | |
715 | ||
716 | Kbuild knows about both 'y' for built-in and 'm' for module. | |
717 | So if a config symbol evaluate to 'm', kbuild will still build | |
a07f6033 JE |
718 | the binary. In other words, Kbuild handles hostprogs-m exactly |
719 | like hostprogs-y. But only hostprogs-y is recommended to be used | |
720 | when no CONFIG symbols are involved. | |
1da177e4 LT |
721 | |
722 | === 5 Kbuild clean infrastructure | |
723 | ||
a07f6033 | 724 | "make clean" deletes most generated files in the obj tree where the kernel |
1da177e4 LT |
725 | is compiled. This includes generated files such as host programs. |
726 | Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always), | |
727 | $(extra-y) and $(targets). They are all deleted during "make clean". | |
728 | Files matching the patterns "*.[oas]", "*.ko", plus some additional files | |
729 | generated by kbuild are deleted all over the kernel src tree when | |
730 | "make clean" is executed. | |
731 | ||
732 | Additional files can be specified in kbuild makefiles by use of $(clean-files). | |
733 | ||
734 | Example: | |
735 | #drivers/pci/Makefile | |
736 | clean-files := devlist.h classlist.h | |
737 | ||
738 | When executing "make clean", the two files "devlist.h classlist.h" will | |
739 | be deleted. Kbuild will assume files to be in same relative directory as the | |
740 | Makefile except if an absolute path is specified (path starting with '/'). | |
741 | ||
39e6e9cf BH |
742 | To delete a directory hierarchy use: |
743 | ||
1da177e4 LT |
744 | Example: |
745 | #scripts/package/Makefile | |
746 | clean-dirs := $(objtree)/debian/ | |
747 | ||
748 | This will delete the directory debian, including all subdirectories. | |
749 | Kbuild will assume the directories to be in the same relative path as the | |
750 | Makefile if no absolute path is specified (path does not start with '/'). | |
751 | ||
752 | Usually kbuild descends down in subdirectories due to "obj-* := dir/", | |
753 | but in the architecture makefiles where the kbuild infrastructure | |
754 | is not sufficient this sometimes needs to be explicit. | |
755 | ||
756 | Example: | |
757 | #arch/i386/boot/Makefile | |
758 | subdir- := compressed/ | |
759 | ||
760 | The above assignment instructs kbuild to descend down in the | |
761 | directory compressed/ when "make clean" is executed. | |
762 | ||
763 | To support the clean infrastructure in the Makefiles that builds the | |
764 | final bootimage there is an optional target named archclean: | |
765 | ||
766 | Example: | |
767 | #arch/i386/Makefile | |
768 | archclean: | |
769 | $(Q)$(MAKE) $(clean)=arch/i386/boot | |
770 | ||
771 | When "make clean" is executed, make will descend down in arch/i386/boot, | |
772 | and clean as usual. The Makefile located in arch/i386/boot/ may use | |
773 | the subdir- trick to descend further down. | |
774 | ||
775 | Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is | |
776 | included in the top level makefile, and the kbuild infrastructure | |
777 | is not operational at that point. | |
778 | ||
779 | Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will | |
780 | be visited during "make clean". | |
781 | ||
782 | === 6 Architecture Makefiles | |
783 | ||
784 | The top level Makefile sets up the environment and does the preparation, | |
785 | before starting to descend down in the individual directories. | |
a07f6033 JE |
786 | The top level makefile contains the generic part, whereas |
787 | arch/$(ARCH)/Makefile contains what is required to set up kbuild | |
788 | for said architecture. | |
789 | To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines | |
1da177e4 LT |
790 | a few targets. |
791 | ||
a07f6033 JE |
792 | When kbuild executes, the following steps are followed (roughly): |
793 | 1) Configuration of the kernel => produce .config | |
1da177e4 LT |
794 | 2) Store kernel version in include/linux/version.h |
795 | 3) Symlink include/asm to include/asm-$(ARCH) | |
796 | 4) Updating all other prerequisites to the target prepare: | |
797 | - Additional prerequisites are specified in arch/$(ARCH)/Makefile | |
798 | 5) Recursively descend down in all directories listed in | |
799 | init-* core* drivers-* net-* libs-* and build all targets. | |
a07f6033 | 800 | - The values of the above variables are expanded in arch/$(ARCH)/Makefile. |
39e6e9cf | 801 | 6) All object files are then linked and the resulting file vmlinux is |
a07f6033 | 802 | located at the root of the obj tree. |
1da177e4 LT |
803 | The very first objects linked are listed in head-y, assigned by |
804 | arch/$(ARCH)/Makefile. | |
5c811e59 | 805 | 7) Finally, the architecture-specific part does any required post processing |
1da177e4 LT |
806 | and builds the final bootimage. |
807 | - This includes building boot records | |
5c811e59 | 808 | - Preparing initrd images and the like |
1da177e4 LT |
809 | |
810 | ||
811 | --- 6.1 Set variables to tweak the build to the architecture | |
812 | ||
813 | LDFLAGS Generic $(LD) options | |
814 | ||
815 | Flags used for all invocations of the linker. | |
816 | Often specifying the emulation is sufficient. | |
817 | ||
818 | Example: | |
819 | #arch/s390/Makefile | |
820 | LDFLAGS := -m elf_s390 | |
f77bf014 | 821 | Note: ldflags-y can be used to further customise |
a9af3305 | 822 | the flags used. See chapter 3.7. |
39e6e9cf | 823 | |
1da177e4 LT |
824 | LDFLAGS_MODULE Options for $(LD) when linking modules |
825 | ||
826 | LDFLAGS_MODULE is used to set specific flags for $(LD) when | |
827 | linking the .ko files used for modules. | |
828 | Default is "-r", for relocatable output. | |
829 | ||
830 | LDFLAGS_vmlinux Options for $(LD) when linking vmlinux | |
831 | ||
832 | LDFLAGS_vmlinux is used to specify additional flags to pass to | |
a07f6033 | 833 | the linker when linking the final vmlinux image. |
1da177e4 LT |
834 | LDFLAGS_vmlinux uses the LDFLAGS_$@ support. |
835 | ||
836 | Example: | |
837 | #arch/i386/Makefile | |
838 | LDFLAGS_vmlinux := -e stext | |
839 | ||
840 | OBJCOPYFLAGS objcopy flags | |
841 | ||
842 | When $(call if_changed,objcopy) is used to translate a .o file, | |
a07f6033 | 843 | the flags specified in OBJCOPYFLAGS will be used. |
1da177e4 LT |
844 | $(call if_changed,objcopy) is often used to generate raw binaries on |
845 | vmlinux. | |
846 | ||
847 | Example: | |
848 | #arch/s390/Makefile | |
849 | OBJCOPYFLAGS := -O binary | |
850 | ||
851 | #arch/s390/boot/Makefile | |
852 | $(obj)/image: vmlinux FORCE | |
853 | $(call if_changed,objcopy) | |
854 | ||
a07f6033 | 855 | In this example, the binary $(obj)/image is a binary version of |
1da177e4 LT |
856 | vmlinux. The usage of $(call if_changed,xxx) will be described later. |
857 | ||
222d394d | 858 | KBUILD_AFLAGS $(AS) assembler flags |
1da177e4 LT |
859 | |
860 | Default value - see top level Makefile | |
861 | Append or modify as required per architecture. | |
862 | ||
863 | Example: | |
864 | #arch/sparc64/Makefile | |
222d394d | 865 | KBUILD_AFLAGS += -m64 -mcpu=ultrasparc |
1da177e4 | 866 | |
a0f97e06 | 867 | KBUILD_CFLAGS $(CC) compiler flags |
1da177e4 LT |
868 | |
869 | Default value - see top level Makefile | |
870 | Append or modify as required per architecture. | |
871 | ||
a0f97e06 | 872 | Often, the KBUILD_CFLAGS variable depends on the configuration. |
1da177e4 LT |
873 | |
874 | Example: | |
875 | #arch/i386/Makefile | |
876 | cflags-$(CONFIG_M386) += -march=i386 | |
a0f97e06 | 877 | KBUILD_CFLAGS += $(cflags-y) |
1da177e4 LT |
878 | |
879 | Many arch Makefiles dynamically run the target C compiler to | |
880 | probe supported options: | |
881 | ||
882 | #arch/i386/Makefile | |
883 | ||
884 | ... | |
885 | cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ | |
886 | -march=pentium2,-march=i686) | |
887 | ... | |
888 | # Disable unit-at-a-time mode ... | |
a0f97e06 | 889 | KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) |
1da177e4 LT |
890 | ... |
891 | ||
892 | ||
a07f6033 | 893 | The first example utilises the trick that a config option expands |
1da177e4 LT |
894 | to 'y' when selected. |
895 | ||
896 | CFLAGS_KERNEL $(CC) options specific for built-in | |
897 | ||
898 | $(CFLAGS_KERNEL) contains extra C compiler flags used to compile | |
899 | resident kernel code. | |
900 | ||
901 | CFLAGS_MODULE $(CC) options specific for modules | |
902 | ||
903 | $(CFLAGS_MODULE) contains extra C compiler flags used to compile code | |
904 | for loadable kernel modules. | |
905 | ||
39e6e9cf | 906 | |
5bb78269 | 907 | --- 6.2 Add prerequisites to archprepare: |
1da177e4 | 908 | |
a07f6033 | 909 | The archprepare: rule is used to list prerequisites that need to be |
1da177e4 | 910 | built before starting to descend down in the subdirectories. |
a07f6033 | 911 | This is usually used for header files containing assembler constants. |
1da177e4 LT |
912 | |
913 | Example: | |
5bb78269 SR |
914 | #arch/arm/Makefile |
915 | archprepare: maketools | |
1da177e4 | 916 | |
a07f6033 | 917 | In this example, the file target maketools will be processed |
5bb78269 | 918 | before descending down in the subdirectories. |
1da177e4 LT |
919 | See also chapter XXX-TODO that describe how kbuild supports |
920 | generating offset header files. | |
921 | ||
922 | ||
923 | --- 6.3 List directories to visit when descending | |
924 | ||
925 | An arch Makefile cooperates with the top Makefile to define variables | |
926 | which specify how to build the vmlinux file. Note that there is no | |
927 | corresponding arch-specific section for modules; the module-building | |
928 | machinery is all architecture-independent. | |
929 | ||
39e6e9cf | 930 | |
1da177e4 LT |
931 | head-y, init-y, core-y, libs-y, drivers-y, net-y |
932 | ||
a07f6033 JE |
933 | $(head-y) lists objects to be linked first in vmlinux. |
934 | $(libs-y) lists directories where a lib.a archive can be located. | |
5c811e59 | 935 | The rest list directories where a built-in.o object file can be |
a07f6033 | 936 | located. |
1da177e4 LT |
937 | |
938 | $(init-y) objects will be located after $(head-y). | |
939 | Then the rest follows in this order: | |
940 | $(core-y), $(libs-y), $(drivers-y) and $(net-y). | |
941 | ||
a07f6033 | 942 | The top level Makefile defines values for all generic directories, |
5c811e59 | 943 | and arch/$(ARCH)/Makefile only adds architecture-specific directories. |
1da177e4 LT |
944 | |
945 | Example: | |
946 | #arch/sparc64/Makefile | |
947 | core-y += arch/sparc64/kernel/ | |
948 | libs-y += arch/sparc64/prom/ arch/sparc64/lib/ | |
949 | drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ | |
950 | ||
951 | ||
5c811e59 | 952 | --- 6.4 Architecture-specific boot images |
1da177e4 LT |
953 | |
954 | An arch Makefile specifies goals that take the vmlinux file, compress | |
955 | it, wrap it in bootstrapping code, and copy the resulting files | |
956 | somewhere. This includes various kinds of installation commands. | |
957 | The actual goals are not standardized across architectures. | |
958 | ||
959 | It is common to locate any additional processing in a boot/ | |
960 | directory below arch/$(ARCH)/. | |
961 | ||
962 | Kbuild does not provide any smart way to support building a | |
963 | target specified in boot/. Therefore arch/$(ARCH)/Makefile shall | |
964 | call make manually to build a target in boot/. | |
965 | ||
966 | The recommended approach is to include shortcuts in | |
967 | arch/$(ARCH)/Makefile, and use the full path when calling down | |
968 | into the arch/$(ARCH)/boot/Makefile. | |
969 | ||
970 | Example: | |
971 | #arch/i386/Makefile | |
972 | boot := arch/i386/boot | |
973 | bzImage: vmlinux | |
974 | $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ | |
975 | ||
976 | "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke | |
977 | make in a subdirectory. | |
978 | ||
5c811e59 | 979 | There are no rules for naming architecture-specific targets, |
1da177e4 | 980 | but executing "make help" will list all relevant targets. |
a07f6033 | 981 | To support this, $(archhelp) must be defined. |
1da177e4 LT |
982 | |
983 | Example: | |
984 | #arch/i386/Makefile | |
985 | define archhelp | |
986 | echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' | |
39e6e9cf | 987 | endif |
1da177e4 LT |
988 | |
989 | When make is executed without arguments, the first goal encountered | |
990 | will be built. In the top level Makefile the first goal present | |
991 | is all:. | |
a07f6033 JE |
992 | An architecture shall always, per default, build a bootable image. |
993 | In "make help", the default goal is highlighted with a '*'. | |
1da177e4 LT |
994 | Add a new prerequisite to all: to select a default goal different |
995 | from vmlinux. | |
996 | ||
997 | Example: | |
998 | #arch/i386/Makefile | |
39e6e9cf | 999 | all: bzImage |
1da177e4 LT |
1000 | |
1001 | When "make" is executed without arguments, bzImage will be built. | |
1002 | ||
1003 | --- 6.5 Building non-kbuild targets | |
1004 | ||
1005 | extra-y | |
1006 | ||
1007 | extra-y specify additional targets created in the current | |
1008 | directory, in addition to any targets specified by obj-*. | |
1009 | ||
1010 | Listing all targets in extra-y is required for two purposes: | |
1011 | 1) Enable kbuild to check changes in command lines | |
1012 | - When $(call if_changed,xxx) is used | |
1013 | 2) kbuild knows what files to delete during "make clean" | |
1014 | ||
1015 | Example: | |
1016 | #arch/i386/kernel/Makefile | |
1017 | extra-y := head.o init_task.o | |
1018 | ||
a07f6033 | 1019 | In this example, extra-y is used to list object files that |
1da177e4 LT |
1020 | shall be built, but shall not be linked as part of built-in.o. |
1021 | ||
39e6e9cf | 1022 | |
1da177e4 LT |
1023 | --- 6.6 Commands useful for building a boot image |
1024 | ||
1025 | Kbuild provides a few macros that are useful when building a | |
1026 | boot image. | |
1027 | ||
1028 | if_changed | |
1029 | ||
1030 | if_changed is the infrastructure used for the following commands. | |
1031 | ||
1032 | Usage: | |
1033 | target: source(s) FORCE | |
1034 | $(call if_changed,ld/objcopy/gzip) | |
1035 | ||
a07f6033 | 1036 | When the rule is evaluated, it is checked to see if any files |
5c811e59 | 1037 | need an update, or the command line has changed since the last |
1da177e4 LT |
1038 | invocation. The latter will force a rebuild if any options |
1039 | to the executable have changed. | |
1040 | Any target that utilises if_changed must be listed in $(targets), | |
1041 | otherwise the command line check will fail, and the target will | |
1042 | always be built. | |
1043 | Assignments to $(targets) are without $(obj)/ prefix. | |
1044 | if_changed may be used in conjunction with custom commands as | |
1045 | defined in 6.7 "Custom kbuild commands". | |
49490571 | 1046 | |
1da177e4 | 1047 | Note: It is a typical mistake to forget the FORCE prerequisite. |
49490571 PBG |
1048 | Another common pitfall is that whitespace is sometimes |
1049 | significant; for instance, the below will fail (note the extra space | |
1050 | after the comma): | |
1051 | target: source(s) FORCE | |
1052 | #WRONG!# $(call if_changed, ld/objcopy/gzip) | |
1da177e4 LT |
1053 | |
1054 | ld | |
a07f6033 | 1055 | Link target. Often, LDFLAGS_$@ is used to set specific options to ld. |
39e6e9cf | 1056 | |
1da177e4 LT |
1057 | objcopy |
1058 | Copy binary. Uses OBJCOPYFLAGS usually specified in | |
1059 | arch/$(ARCH)/Makefile. | |
1060 | OBJCOPYFLAGS_$@ may be used to set additional options. | |
1061 | ||
1062 | gzip | |
1063 | Compress target. Use maximum compression to compress target. | |
1064 | ||
1065 | Example: | |
1066 | #arch/i386/boot/Makefile | |
1067 | LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary | |
1068 | LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext | |
1069 | ||
1070 | targets += setup setup.o bootsect bootsect.o | |
1071 | $(obj)/setup $(obj)/bootsect: %: %.o FORCE | |
1072 | $(call if_changed,ld) | |
1073 | ||
a07f6033 JE |
1074 | In this example, there are two possible targets, requiring different |
1075 | options to the linker. The linker options are specified using the | |
1da177e4 | 1076 | LDFLAGS_$@ syntax - one for each potential target. |
5d3f083d | 1077 | $(targets) are assigned all potential targets, by which kbuild knows |
1da177e4 LT |
1078 | the targets and will: |
1079 | 1) check for commandline changes | |
1080 | 2) delete target during make clean | |
1081 | ||
1082 | The ": %: %.o" part of the prerequisite is a shorthand that | |
1083 | free us from listing the setup.o and bootsect.o files. | |
1084 | Note: It is a common mistake to forget the "target :=" assignment, | |
1085 | resulting in the target file being recompiled for no | |
1086 | obvious reason. | |
1087 | ||
1088 | ||
1089 | --- 6.7 Custom kbuild commands | |
1090 | ||
a07f6033 | 1091 | When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand |
1da177e4 LT |
1092 | of a command is normally displayed. |
1093 | To enable this behaviour for custom commands kbuild requires | |
1094 | two variables to be set: | |
1095 | quiet_cmd_<command> - what shall be echoed | |
1096 | cmd_<command> - the command to execute | |
1097 | ||
1098 | Example: | |
1099 | # | |
1100 | quiet_cmd_image = BUILD $@ | |
1101 | cmd_image = $(obj)/tools/build $(BUILDFLAGS) \ | |
1102 | $(obj)/vmlinux.bin > $@ | |
1103 | ||
1104 | targets += bzImage | |
1105 | $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE | |
1106 | $(call if_changed,image) | |
1107 | @echo 'Kernel: $@ is ready' | |
1108 | ||
a07f6033 | 1109 | When updating the $(obj)/bzImage target, the line |
1da177e4 LT |
1110 | |
1111 | BUILD arch/i386/boot/bzImage | |
1112 | ||
1113 | will be displayed with "make KBUILD_VERBOSE=0". | |
39e6e9cf | 1114 | |
1da177e4 LT |
1115 | |
1116 | --- 6.8 Preprocessing linker scripts | |
1117 | ||
a07f6033 | 1118 | When the vmlinux image is built, the linker script |
1da177e4 LT |
1119 | arch/$(ARCH)/kernel/vmlinux.lds is used. |
1120 | The script is a preprocessed variant of the file vmlinux.lds.S | |
1121 | located in the same directory. | |
a07f6033 | 1122 | kbuild knows .lds files and includes a rule *lds.S -> *lds. |
39e6e9cf | 1123 | |
1da177e4 LT |
1124 | Example: |
1125 | #arch/i386/kernel/Makefile | |
1126 | always := vmlinux.lds | |
39e6e9cf | 1127 | |
1da177e4 LT |
1128 | #Makefile |
1129 | export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) | |
39e6e9cf BH |
1130 | |
1131 | The assignment to $(always) is used to tell kbuild to build the | |
a07f6033 JE |
1132 | target vmlinux.lds. |
1133 | The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the | |
1da177e4 | 1134 | specified options when building the target vmlinux.lds. |
39e6e9cf | 1135 | |
a07f6033 | 1136 | When building the *.lds target, kbuild uses the variables: |
06c5040c | 1137 | KBUILD_CPPFLAGS : Set in top-level Makefile |
f77bf014 | 1138 | cppflags-y : May be set in the kbuild makefile |
1da177e4 LT |
1139 | CPPFLAGS_$(@F) : Target specific flags. |
1140 | Note that the full filename is used in this | |
1141 | assignment. | |
1142 | ||
1143 | The kbuild infrastructure for *lds file are used in several | |
5c811e59 | 1144 | architecture-specific files. |
1da177e4 LT |
1145 | |
1146 | ||
1da177e4 LT |
1147 | === 7 Kbuild Variables |
1148 | ||
1149 | The top Makefile exports the following variables: | |
1150 | ||
1151 | VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION | |
1152 | ||
1153 | These variables define the current kernel version. A few arch | |
1154 | Makefiles actually use these values directly; they should use | |
1155 | $(KERNELRELEASE) instead. | |
1156 | ||
1157 | $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic | |
1158 | three-part version number, such as "2", "4", and "0". These three | |
1159 | values are always numeric. | |
1160 | ||
1161 | $(EXTRAVERSION) defines an even tinier sublevel for pre-patches | |
1162 | or additional patches. It is usually some non-numeric string | |
1163 | such as "-pre4", and is often blank. | |
1164 | ||
1165 | KERNELRELEASE | |
1166 | ||
1167 | $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable | |
1168 | for constructing installation directory names or showing in | |
1169 | version strings. Some arch Makefiles use it for this purpose. | |
1170 | ||
1171 | ARCH | |
1172 | ||
1173 | This variable defines the target architecture, such as "i386", | |
1174 | "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to | |
1175 | determine which files to compile. | |
1176 | ||
1177 | By default, the top Makefile sets $(ARCH) to be the same as the | |
1178 | host system architecture. For a cross build, a user may | |
1179 | override the value of $(ARCH) on the command line: | |
1180 | ||
1181 | make ARCH=m68k ... | |
1182 | ||
1183 | ||
1184 | INSTALL_PATH | |
1185 | ||
1186 | This variable defines a place for the arch Makefiles to install | |
1187 | the resident kernel image and System.map file. | |
5c811e59 | 1188 | Use this for architecture-specific install targets. |
1da177e4 LT |
1189 | |
1190 | INSTALL_MOD_PATH, MODLIB | |
1191 | ||
1192 | $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module | |
1193 | installation. This variable is not defined in the Makefile but | |
1194 | may be passed in by the user if desired. | |
1195 | ||
1196 | $(MODLIB) specifies the directory for module installation. | |
1197 | The top Makefile defines $(MODLIB) to | |
1198 | $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may | |
1199 | override this value on the command line if desired. | |
1200 | ||
ac031f26 TT |
1201 | INSTALL_MOD_STRIP |
1202 | ||
1203 | If this variable is specified, will cause modules to be stripped | |
1204 | after they are installed. If INSTALL_MOD_STRIP is '1', then the | |
1205 | default option --strip-debug will be used. Otherwise, | |
1206 | INSTALL_MOD_STRIP will used as the option(s) to the strip command. | |
1207 | ||
1208 | ||
1da177e4 LT |
1209 | === 8 Makefile language |
1210 | ||
a07f6033 | 1211 | The kernel Makefiles are designed to be run with GNU Make. The Makefiles |
1da177e4 LT |
1212 | use only the documented features of GNU Make, but they do use many |
1213 | GNU extensions. | |
1214 | ||
1215 | GNU Make supports elementary list-processing functions. The kernel | |
1216 | Makefiles use a novel style of list building and manipulation with few | |
1217 | "if" statements. | |
1218 | ||
1219 | GNU Make has two assignment operators, ":=" and "=". ":=" performs | |
1220 | immediate evaluation of the right-hand side and stores an actual string | |
1221 | into the left-hand side. "=" is like a formula definition; it stores the | |
1222 | right-hand side in an unevaluated form and then evaluates this form each | |
1223 | time the left-hand side is used. | |
1224 | ||
1225 | There are some cases where "=" is appropriate. Usually, though, ":=" | |
1226 | is the right choice. | |
1227 | ||
1228 | === 9 Credits | |
1229 | ||
1230 | Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> | |
1231 | Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> | |
1232 | Updates by Sam Ravnborg <sam@ravnborg.org> | |
a07f6033 | 1233 | Language QA by Jan Engelhardt <jengelh@gmx.de> |
1da177e4 LT |
1234 | |
1235 | === 10 TODO | |
1236 | ||
a07f6033 | 1237 | - Describe how kbuild supports shipped files with _shipped. |
1da177e4 LT |
1238 | - Generating offset header files. |
1239 | - Add more variables to section 7? | |
1240 | ||
39e6e9cf BH |
1241 | |
1242 |