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1 | Linux Kernel Makefiles |
2 | ||
3 | This document describes the Linux kernel Makefiles. | |
4 | ||
5 | === Table of Contents | |
6 | ||
7 | === 1 Overview | |
8 | === 2 Who does what | |
9 | === 3 The kbuild files | |
10 | --- 3.1 Goal definitions | |
11 | --- 3.2 Built-in object goals - obj-y | |
12 | --- 3.3 Loadable module goals - obj-m | |
13 | --- 3.4 Objects which export symbols | |
14 | --- 3.5 Library file goals - lib-y | |
15 | --- 3.6 Descending down in directories | |
16 | --- 3.7 Compilation flags | |
17 | --- 3.8 Command line dependency | |
18 | --- 3.9 Dependency tracking | |
19 | --- 3.10 Special Rules | |
20a468b5 | 20 | --- 3.11 $(CC) support functions |
691ef3e7 | 21 | --- 3.12 $(LD) support functions |
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22 | |
23 | === 4 Host Program support | |
24 | --- 4.1 Simple Host Program | |
25 | --- 4.2 Composite Host Programs | |
62e22107 MY |
26 | --- 4.3 Using C++ for host programs |
27 | --- 4.4 Controlling compiler options for host programs | |
28 | --- 4.5 When host programs are actually built | |
29 | --- 4.6 Using hostprogs-$(CONFIG_FOO) | |
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30 | |
31 | === 5 Kbuild clean infrastructure | |
32 | ||
33 | === 6 Architecture Makefiles | |
34 | --- 6.1 Set variables to tweak the build to the architecture | |
052ad274 PA |
35 | --- 6.2 Add prerequisites to archheaders: |
36 | --- 6.3 Add prerequisites to archprepare: | |
37 | --- 6.4 List directories to visit when descending | |
38 | --- 6.5 Architecture-specific boot images | |
39 | --- 6.6 Building non-kbuild targets | |
40 | --- 6.7 Commands useful for building a boot image | |
41 | --- 6.8 Custom kbuild commands | |
42 | --- 6.9 Preprocessing linker scripts | |
43 | --- 6.10 Generic header files | |
fbe6e37d | 44 | --- 6.11 Post-link pass |
1da177e4 | 45 | |
c7bb349e | 46 | === 7 Kbuild syntax for exported headers |
fcc8487d | 47 | --- 7.1 no-export-headers |
40f1d4c2 | 48 | --- 7.2 genhdr-y |
bd73a328 ND |
49 | --- 7.3 generic-y |
50 | --- 7.4 generated-y | |
fcc8487d ND |
51 | --- 7.5 mandatory-y |
52 | --- 7.6 subdir-y | |
c7bb349e SR |
53 | |
54 | === 8 Kbuild Variables | |
55 | === 9 Makefile language | |
56 | === 10 Credits | |
57 | === 11 TODO | |
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58 | |
59 | === 1 Overview | |
60 | ||
61 | The Makefiles have five parts: | |
62 | ||
63 | Makefile the top Makefile. | |
64 | .config the kernel configuration file. | |
65 | arch/$(ARCH)/Makefile the arch Makefile. | |
66 | scripts/Makefile.* common rules etc. for all kbuild Makefiles. | |
67 | kbuild Makefiles there are about 500 of these. | |
68 | ||
69 | The top Makefile reads the .config file, which comes from the kernel | |
70 | configuration process. | |
71 | ||
72 | The top Makefile is responsible for building two major products: vmlinux | |
73 | (the resident kernel image) and modules (any module files). | |
74 | It builds these goals by recursively descending into the subdirectories of | |
75 | the kernel source tree. | |
76 | The list of subdirectories which are visited depends upon the kernel | |
77 | configuration. The top Makefile textually includes an arch Makefile | |
78 | with the name arch/$(ARCH)/Makefile. The arch Makefile supplies | |
79 | architecture-specific information to the top Makefile. | |
80 | ||
81 | Each subdirectory has a kbuild Makefile which carries out the commands | |
82 | passed down from above. The kbuild Makefile uses information from the | |
39e6e9cf | 83 | .config file to construct various file lists used by kbuild to build |
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84 | any built-in or modular targets. |
85 | ||
86 | scripts/Makefile.* contains all the definitions/rules etc. that | |
87 | are used to build the kernel based on the kbuild makefiles. | |
88 | ||
89 | ||
90 | === 2 Who does what | |
91 | ||
92 | People have four different relationships with the kernel Makefiles. | |
93 | ||
94 | *Users* are people who build kernels. These people type commands such as | |
95 | "make menuconfig" or "make". They usually do not read or edit | |
96 | any kernel Makefiles (or any other source files). | |
97 | ||
98 | *Normal developers* are people who work on features such as device | |
99 | drivers, file systems, and network protocols. These people need to | |
a07f6033 | 100 | maintain the kbuild Makefiles for the subsystem they are |
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101 | working on. In order to do this effectively, they need some overall |
102 | knowledge about the kernel Makefiles, plus detailed knowledge about the | |
103 | public interface for kbuild. | |
104 | ||
105 | *Arch developers* are people who work on an entire architecture, such | |
106 | as sparc or ia64. Arch developers need to know about the arch Makefile | |
107 | as well as kbuild Makefiles. | |
108 | ||
109 | *Kbuild developers* are people who work on the kernel build system itself. | |
110 | These people need to know about all aspects of the kernel Makefiles. | |
111 | ||
112 | This document is aimed towards normal developers and arch developers. | |
113 | ||
114 | ||
115 | === 3 The kbuild files | |
116 | ||
117 | Most Makefiles within the kernel are kbuild Makefiles that use the | |
a07f6033 | 118 | kbuild infrastructure. This chapter introduces the syntax used in the |
1da177e4 | 119 | kbuild makefiles. |
172c3ae3 | 120 | The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can |
a07f6033 | 121 | be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild' |
172c3ae3 | 122 | file will be used. |
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123 | |
124 | Section 3.1 "Goal definitions" is a quick intro, further chapters provide | |
125 | more details, with real examples. | |
126 | ||
127 | --- 3.1 Goal definitions | |
128 | ||
129 | Goal definitions are the main part (heart) of the kbuild Makefile. | |
130 | These lines define the files to be built, any special compilation | |
131 | options, and any subdirectories to be entered recursively. | |
132 | ||
133 | The most simple kbuild makefile contains one line: | |
134 | ||
135 | Example: | |
136 | obj-y += foo.o | |
137 | ||
5c811e59 | 138 | This tells kbuild that there is one object in that directory, named |
1da177e4 LT |
139 | foo.o. foo.o will be built from foo.c or foo.S. |
140 | ||
141 | If foo.o shall be built as a module, the variable obj-m is used. | |
142 | Therefore the following pattern is often used: | |
143 | ||
144 | Example: | |
145 | obj-$(CONFIG_FOO) += foo.o | |
146 | ||
147 | $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module). | |
148 | If CONFIG_FOO is neither y nor m, then the file will not be compiled | |
149 | nor linked. | |
150 | ||
151 | --- 3.2 Built-in object goals - obj-y | |
152 | ||
153 | The kbuild Makefile specifies object files for vmlinux | |
a07f6033 | 154 | in the $(obj-y) lists. These lists depend on the kernel |
1da177e4 LT |
155 | configuration. |
156 | ||
157 | Kbuild compiles all the $(obj-y) files. It then calls | |
158 | "$(LD) -r" to merge these files into one built-in.o file. | |
159 | built-in.o is later linked into vmlinux by the parent Makefile. | |
160 | ||
161 | The order of files in $(obj-y) is significant. Duplicates in | |
162 | the lists are allowed: the first instance will be linked into | |
163 | built-in.o and succeeding instances will be ignored. | |
164 | ||
165 | Link order is significant, because certain functions | |
166 | (module_init() / __initcall) will be called during boot in the | |
167 | order they appear. So keep in mind that changing the link | |
a07f6033 JE |
168 | order may e.g. change the order in which your SCSI |
169 | controllers are detected, and thus your disks are renumbered. | |
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170 | |
171 | Example: | |
172 | #drivers/isdn/i4l/Makefile | |
173 | # Makefile for the kernel ISDN subsystem and device drivers. | |
174 | # Each configuration option enables a list of files. | |
2f5a2f81 | 175 | obj-$(CONFIG_ISDN_I4L) += isdn.o |
1da177e4 LT |
176 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o |
177 | ||
178 | --- 3.3 Loadable module goals - obj-m | |
179 | ||
39fed701 | 180 | $(obj-m) specifies object files which are built as loadable |
1da177e4 LT |
181 | kernel modules. |
182 | ||
183 | A module may be built from one source file or several source | |
184 | files. In the case of one source file, the kbuild makefile | |
185 | simply adds the file to $(obj-m). | |
186 | ||
187 | Example: | |
188 | #drivers/isdn/i4l/Makefile | |
189 | obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o | |
190 | ||
191 | Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm' | |
192 | ||
193 | If a kernel module is built from several source files, you specify | |
4f827280 MM |
194 | that you want to build a module in the same way as above; however, |
195 | kbuild needs to know which object files you want to build your | |
196 | module from, so you have to tell it by setting a $(<module_name>-y) | |
197 | variable. | |
1da177e4 LT |
198 | |
199 | Example: | |
200 | #drivers/isdn/i4l/Makefile | |
4f827280 MM |
201 | obj-$(CONFIG_ISDN_I4L) += isdn.o |
202 | isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o | |
1da177e4 LT |
203 | |
204 | In this example, the module name will be isdn.o. Kbuild will | |
4f827280 | 205 | compile the objects listed in $(isdn-y) and then run |
1da177e4 LT |
206 | "$(LD) -r" on the list of these files to generate isdn.o. |
207 | ||
4f827280 MM |
208 | Due to kbuild recognizing $(<module_name>-y) for composite objects, |
209 | you can use the value of a CONFIG_ symbol to optionally include an | |
210 | object file as part of a composite object. | |
1da177e4 LT |
211 | |
212 | Example: | |
213 | #fs/ext2/Makefile | |
4f827280 MM |
214 | obj-$(CONFIG_EXT2_FS) += ext2.o |
215 | ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \ | |
216 | namei.o super.o symlink.o | |
217 | ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \ | |
218 | xattr_trusted.o | |
219 | ||
220 | In this example, xattr.o, xattr_user.o and xattr_trusted.o are only | |
221 | part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR) | |
222 | evaluates to 'y'. | |
1da177e4 LT |
223 | |
224 | Note: Of course, when you are building objects into the kernel, | |
225 | the syntax above will also work. So, if you have CONFIG_EXT2_FS=y, | |
226 | kbuild will build an ext2.o file for you out of the individual | |
227 | parts and then link this into built-in.o, as you would expect. | |
228 | ||
229 | --- 3.4 Objects which export symbols | |
230 | ||
231 | No special notation is required in the makefiles for | |
232 | modules exporting symbols. | |
233 | ||
234 | --- 3.5 Library file goals - lib-y | |
235 | ||
a07f6033 | 236 | Objects listed with obj-* are used for modules, or |
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237 | combined in a built-in.o for that specific directory. |
238 | There is also the possibility to list objects that will | |
239 | be included in a library, lib.a. | |
240 | All objects listed with lib-y are combined in a single | |
241 | library for that directory. | |
5d3f083d ML |
242 | Objects that are listed in obj-y and additionally listed in |
243 | lib-y will not be included in the library, since they will | |
244 | be accessible anyway. | |
a07f6033 | 245 | For consistency, objects listed in lib-m will be included in lib.a. |
1da177e4 LT |
246 | |
247 | Note that the same kbuild makefile may list files to be built-in | |
248 | and to be part of a library. Therefore the same directory | |
249 | may contain both a built-in.o and a lib.a file. | |
250 | ||
251 | Example: | |
2f5a2f81 MM |
252 | #arch/x86/lib/Makefile |
253 | lib-y := delay.o | |
1da177e4 | 254 | |
2f5a2f81 MM |
255 | This will create a library lib.a based on delay.o. For kbuild to |
256 | actually recognize that there is a lib.a being built, the directory | |
257 | shall be listed in libs-y. | |
052ad274 | 258 | See also "6.4 List directories to visit when descending". |
39e6e9cf | 259 | |
a07f6033 | 260 | Use of lib-y is normally restricted to lib/ and arch/*/lib. |
1da177e4 LT |
261 | |
262 | --- 3.6 Descending down in directories | |
263 | ||
264 | A Makefile is only responsible for building objects in its own | |
265 | directory. Files in subdirectories should be taken care of by | |
266 | Makefiles in these subdirs. The build system will automatically | |
267 | invoke make recursively in subdirectories, provided you let it know of | |
268 | them. | |
269 | ||
a07f6033 | 270 | To do so, obj-y and obj-m are used. |
1da177e4 LT |
271 | ext2 lives in a separate directory, and the Makefile present in fs/ |
272 | tells kbuild to descend down using the following assignment. | |
273 | ||
274 | Example: | |
275 | #fs/Makefile | |
276 | obj-$(CONFIG_EXT2_FS) += ext2/ | |
277 | ||
278 | If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular) | |
279 | the corresponding obj- variable will be set, and kbuild will descend | |
280 | down in the ext2 directory. | |
281 | Kbuild only uses this information to decide that it needs to visit | |
282 | the directory, it is the Makefile in the subdirectory that | |
39fed701 | 283 | specifies what is modular and what is built-in. |
1da177e4 LT |
284 | |
285 | It is good practice to use a CONFIG_ variable when assigning directory | |
286 | names. This allows kbuild to totally skip the directory if the | |
287 | corresponding CONFIG_ option is neither 'y' nor 'm'. | |
288 | ||
289 | --- 3.7 Compilation flags | |
290 | ||
f77bf014 | 291 | ccflags-y, asflags-y and ldflags-y |
c95940f2 NK |
292 | These three flags apply only to the kbuild makefile in which they |
293 | are assigned. They are used for all the normal cc, as and ld | |
294 | invocations happening during a recursive build. | |
f77bf014 | 295 | Note: Flags with the same behaviour were previously named: |
c95940f2 NK |
296 | EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS. |
297 | They are still supported but their usage is deprecated. | |
1da177e4 | 298 | |
eb07e1b4 | 299 | ccflags-y specifies options for compiling with $(CC). |
1da177e4 LT |
300 | |
301 | Example: | |
eb07e1b4 MM |
302 | # drivers/acpi/Makefile |
303 | ccflags-y := -Os | |
304 | ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT | |
1da177e4 LT |
305 | |
306 | This variable is necessary because the top Makefile owns the | |
a0f97e06 | 307 | variable $(KBUILD_CFLAGS) and uses it for compilation flags for the |
1da177e4 LT |
308 | entire tree. |
309 | ||
eb07e1b4 | 310 | asflags-y specifies options for assembling with $(AS). |
1da177e4 LT |
311 | |
312 | Example: | |
eb07e1b4 MM |
313 | #arch/sparc/kernel/Makefile |
314 | asflags-y := -ansi | |
1da177e4 | 315 | |
eb07e1b4 | 316 | ldflags-y specifies options for linking with $(LD). |
1da177e4 LT |
317 | |
318 | Example: | |
eb07e1b4 MM |
319 | #arch/cris/boot/compressed/Makefile |
320 | ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds | |
1da177e4 | 321 | |
720097d8 | 322 | subdir-ccflags-y, subdir-asflags-y |
eb07e1b4 | 323 | The two flags listed above are similar to ccflags-y and asflags-y. |
c95940f2 NK |
324 | The difference is that the subdir- variants have effect for the kbuild |
325 | file where they are present and all subdirectories. | |
326 | Options specified using subdir-* are added to the commandline before | |
327 | the options specified using the non-subdir variants. | |
720097d8 SR |
328 | |
329 | Example: | |
330 | subdir-ccflags-y := -Werror | |
331 | ||
1da177e4 LT |
332 | CFLAGS_$@, AFLAGS_$@ |
333 | ||
334 | CFLAGS_$@ and AFLAGS_$@ only apply to commands in current | |
335 | kbuild makefile. | |
336 | ||
337 | $(CFLAGS_$@) specifies per-file options for $(CC). The $@ | |
338 | part has a literal value which specifies the file that it is for. | |
339 | ||
340 | Example: | |
341 | # drivers/scsi/Makefile | |
342 | CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF | |
343 | CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \ | |
344 | -DGDTH_STATISTICS | |
1da177e4 | 345 | |
eb07e1b4 | 346 | These two lines specify compilation flags for aha152x.o and gdth.o. |
1da177e4 LT |
347 | |
348 | $(AFLAGS_$@) is a similar feature for source files in assembly | |
349 | languages. | |
350 | ||
351 | Example: | |
352 | # arch/arm/kernel/Makefile | |
eb07e1b4 MM |
353 | AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET) |
354 | AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312 | |
355 | AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt | |
356 | ||
1da177e4 LT |
357 | |
358 | --- 3.9 Dependency tracking | |
359 | ||
360 | Kbuild tracks dependencies on the following: | |
361 | 1) All prerequisite files (both *.c and *.h) | |
362 | 2) CONFIG_ options used in all prerequisite files | |
363 | 3) Command-line used to compile target | |
364 | ||
365 | Thus, if you change an option to $(CC) all affected files will | |
366 | be re-compiled. | |
367 | ||
368 | --- 3.10 Special Rules | |
369 | ||
370 | Special rules are used when the kbuild infrastructure does | |
371 | not provide the required support. A typical example is | |
372 | header files generated during the build process. | |
5c811e59 | 373 | Another example are the architecture-specific Makefiles which |
a07f6033 | 374 | need special rules to prepare boot images etc. |
1da177e4 LT |
375 | |
376 | Special rules are written as normal Make rules. | |
377 | Kbuild is not executing in the directory where the Makefile is | |
378 | located, so all special rules shall provide a relative | |
379 | path to prerequisite files and target files. | |
380 | ||
381 | Two variables are used when defining special rules: | |
382 | ||
383 | $(src) | |
384 | $(src) is a relative path which points to the directory | |
385 | where the Makefile is located. Always use $(src) when | |
386 | referring to files located in the src tree. | |
387 | ||
388 | $(obj) | |
389 | $(obj) is a relative path which points to the directory | |
390 | where the target is saved. Always use $(obj) when | |
391 | referring to generated files. | |
392 | ||
393 | Example: | |
394 | #drivers/scsi/Makefile | |
395 | $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl | |
396 | $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl | |
397 | ||
398 | This is a special rule, following the normal syntax | |
399 | required by make. | |
400 | The target file depends on two prerequisite files. References | |
401 | to the target file are prefixed with $(obj), references | |
402 | to prerequisites are referenced with $(src) (because they are not | |
403 | generated files). | |
404 | ||
5410ecc0 MF |
405 | $(kecho) |
406 | echoing information to user in a rule is often a good practice | |
407 | but when execution "make -s" one does not expect to see any output | |
408 | except for warnings/errors. | |
39fed701 | 409 | To support this kbuild defines $(kecho) which will echo out the |
5410ecc0 MF |
410 | text following $(kecho) to stdout except if "make -s" is used. |
411 | ||
412 | Example: | |
413 | #arch/blackfin/boot/Makefile | |
414 | $(obj)/vmImage: $(obj)/vmlinux.gz | |
415 | $(call if_changed,uimage) | |
416 | @$(kecho) 'Kernel: $@ is ready' | |
417 | ||
418 | ||
20a468b5 SR |
419 | --- 3.11 $(CC) support functions |
420 | ||
a07f6033 | 421 | The kernel may be built with several different versions of |
20a468b5 | 422 | $(CC), each supporting a unique set of features and options. |
39fed701 | 423 | kbuild provides basic support to check for valid options for $(CC). |
e95be9a5 | 424 | $(CC) is usually the gcc compiler, but other alternatives are |
20a468b5 SR |
425 | available. |
426 | ||
427 | as-option | |
a07f6033 JE |
428 | as-option is used to check if $(CC) -- when used to compile |
429 | assembler (*.S) files -- supports the given option. An optional | |
430 | second option may be specified if the first option is not supported. | |
20a468b5 SR |
431 | |
432 | Example: | |
433 | #arch/sh/Makefile | |
434 | cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) | |
435 | ||
a07f6033 | 436 | In the above example, cflags-y will be assigned the option |
20a468b5 SR |
437 | -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). |
438 | The second argument is optional, and if supplied will be used | |
439 | if first argument is not supported. | |
440 | ||
f86fd306 SR |
441 | cc-ldoption |
442 | cc-ldoption is used to check if $(CC) when used to link object files | |
0b0bf7a3 RM |
443 | supports the given option. An optional second option may be |
444 | specified if first option are not supported. | |
445 | ||
446 | Example: | |
25eb650a | 447 | #arch/x86/kernel/Makefile |
f86fd306 | 448 | vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) |
0b0bf7a3 | 449 | |
5c811e59 | 450 | In the above example, vsyscall-flags will be assigned the option |
0b0bf7a3 RM |
451 | -Wl$(comma)--hash-style=sysv if it is supported by $(CC). |
452 | The second argument is optional, and if supplied will be used | |
453 | if first argument is not supported. | |
454 | ||
e2414910 AK |
455 | as-instr |
456 | as-instr checks if the assembler reports a specific instruction | |
457 | and then outputs either option1 or option2 | |
458 | C escapes are supported in the test instruction | |
222d394d | 459 | Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options |
e2414910 | 460 | |
20a468b5 | 461 | cc-option |
39fed701 GU |
462 | cc-option is used to check if $(CC) supports a given option, and if |
463 | not supported to use an optional second option. | |
20a468b5 SR |
464 | |
465 | Example: | |
25eb650a | 466 | #arch/x86/Makefile |
20a468b5 SR |
467 | cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) |
468 | ||
5c811e59 | 469 | In the above example, cflags-y will be assigned the option |
a07f6033 JE |
470 | -march=pentium-mmx if supported by $(CC), otherwise -march=i586. |
471 | The second argument to cc-option is optional, and if omitted, | |
20a468b5 | 472 | cflags-y will be assigned no value if first option is not supported. |
a0f97e06 | 473 | Note: cc-option uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
474 | |
475 | cc-option-yn | |
39e6e9cf | 476 | cc-option-yn is used to check if gcc supports a given option |
20a468b5 SR |
477 | and return 'y' if supported, otherwise 'n'. |
478 | ||
479 | Example: | |
480 | #arch/ppc/Makefile | |
481 | biarch := $(call cc-option-yn, -m32) | |
482 | aflags-$(biarch) += -a32 | |
483 | cflags-$(biarch) += -m32 | |
39e6e9cf | 484 | |
a07f6033 JE |
485 | In the above example, $(biarch) is set to y if $(CC) supports the -m32 |
486 | option. When $(biarch) equals 'y', the expanded variables $(aflags-y) | |
487 | and $(cflags-y) will be assigned the values -a32 and -m32, | |
488 | respectively. | |
a0f97e06 | 489 | Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options |
20a468b5 SR |
490 | |
491 | cc-option-align | |
a07f6033 JE |
492 | gcc versions >= 3.0 changed the type of options used to specify |
493 | alignment of functions, loops etc. $(cc-option-align), when used | |
494 | as prefix to the align options, will select the right prefix: | |
20a468b5 SR |
495 | gcc < 3.00 |
496 | cc-option-align = -malign | |
497 | gcc >= 3.00 | |
498 | cc-option-align = -falign | |
39e6e9cf | 499 | |
20a468b5 | 500 | Example: |
a0f97e06 | 501 | KBUILD_CFLAGS += $(cc-option-align)-functions=4 |
20a468b5 | 502 | |
a07f6033 JE |
503 | In the above example, the option -falign-functions=4 is used for |
504 | gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used. | |
a0f97e06 | 505 | Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options |
39e6e9cf | 506 | |
8417da6f MM |
507 | cc-disable-warning |
508 | cc-disable-warning checks if gcc supports a given warning and returns | |
509 | the commandline switch to disable it. This special function is needed, | |
510 | because gcc 4.4 and later accept any unknown -Wno-* option and only | |
511 | warn about it if there is another warning in the source file. | |
512 | ||
513 | Example: | |
514 | KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable) | |
515 | ||
516 | In the above example, -Wno-unused-but-set-variable will be added to | |
517 | KBUILD_CFLAGS only if gcc really accepts it. | |
518 | ||
20a468b5 | 519 | cc-version |
a07f6033 | 520 | cc-version returns a numerical version of the $(CC) compiler version. |
20a468b5 SR |
521 | The format is <major><minor> where both are two digits. So for example |
522 | gcc 3.41 would return 0341. | |
523 | cc-version is useful when a specific $(CC) version is faulty in one | |
a07f6033 | 524 | area, for example -mregparm=3 was broken in some gcc versions |
20a468b5 SR |
525 | even though the option was accepted by gcc. |
526 | ||
527 | Example: | |
25eb650a | 528 | #arch/x86/Makefile |
20a468b5 | 529 | cflags-y += $(shell \ |
665d92e3 | 530 | if [ $(cc-version) -ge 0300 ] ; then \ |
20a468b5 SR |
531 | echo "-mregparm=3"; fi ;) |
532 | ||
a07f6033 | 533 | In the above example, -mregparm=3 is only used for gcc version greater |
20a468b5 SR |
534 | than or equal to gcc 3.0. |
535 | ||
536 | cc-ifversion | |
6dcb4e5e MY |
537 | cc-ifversion tests the version of $(CC) and equals the fourth parameter |
538 | if version expression is true, or the fifth (if given) if the version | |
539 | expression is false. | |
20a468b5 SR |
540 | |
541 | Example: | |
542 | #fs/reiserfs/Makefile | |
f77bf014 | 543 | ccflags-y := $(call cc-ifversion, -lt, 0402, -O1) |
20a468b5 | 544 | |
f77bf014 | 545 | In this example, ccflags-y will be assigned the value -O1 if the |
20a468b5 | 546 | $(CC) version is less than 4.2. |
39e6e9cf | 547 | cc-ifversion takes all the shell operators: |
20a468b5 SR |
548 | -eq, -ne, -lt, -le, -gt, and -ge |
549 | The third parameter may be a text as in this example, but it may also | |
550 | be an expanded variable or a macro. | |
551 | ||
7015030f SR |
552 | cc-fullversion |
553 | cc-fullversion is useful when the exact version of gcc is needed. | |
554 | One typical use-case is when a specific GCC version is broken. | |
555 | cc-fullversion points out a more specific version than cc-version does. | |
556 | ||
557 | Example: | |
558 | #arch/powerpc/Makefile | |
665d92e3 | 559 | $(Q)if test "$(cc-fullversion)" = "040200" ; then \ |
7015030f SR |
560 | echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \ |
561 | false ; \ | |
562 | fi | |
563 | ||
39fed701 GU |
564 | In this example for a specific GCC version the build will error out |
565 | explaining to the user why it stops. | |
1da177e4 | 566 | |
910b4046 | 567 | cc-cross-prefix |
631bcfbb | 568 | cc-cross-prefix is used to check if there exists a $(CC) in path with |
910b4046 SR |
569 | one of the listed prefixes. The first prefix where there exist a |
570 | prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found | |
571 | then nothing is returned. | |
572 | Additional prefixes are separated by a single space in the | |
573 | call of cc-cross-prefix. | |
631bcfbb GU |
574 | This functionality is useful for architecture Makefiles that try |
575 | to set CROSS_COMPILE to well-known values but may have several | |
910b4046 | 576 | values to select between. |
631bcfbb GU |
577 | It is recommended only to try to set CROSS_COMPILE if it is a cross |
578 | build (host arch is different from target arch). And if CROSS_COMPILE | |
910b4046 SR |
579 | is already set then leave it with the old value. |
580 | ||
581 | Example: | |
582 | #arch/m68k/Makefile | |
583 | ifneq ($(SUBARCH),$(ARCH)) | |
584 | ifeq ($(CROSS_COMPILE),) | |
585 | CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) | |
586 | endif | |
587 | endif | |
588 | ||
691ef3e7 SR |
589 | --- 3.12 $(LD) support functions |
590 | ||
591 | ld-option | |
592 | ld-option is used to check if $(LD) supports the supplied option. | |
593 | ld-option takes two options as arguments. | |
594 | The second argument is an optional option that can be used if the | |
595 | first option is not supported by $(LD). | |
596 | ||
597 | Example: | |
598 | #Makefile | |
5b83df2b | 599 | LDFLAGS_vmlinux += $(call ld-option, -X) |
691ef3e7 SR |
600 | |
601 | ||
1da177e4 LT |
602 | === 4 Host Program support |
603 | ||
604 | Kbuild supports building executables on the host for use during the | |
605 | compilation stage. | |
606 | Two steps are required in order to use a host executable. | |
607 | ||
608 | The first step is to tell kbuild that a host program exists. This is | |
609 | done utilising the variable hostprogs-y. | |
610 | ||
611 | The second step is to add an explicit dependency to the executable. | |
39e6e9cf | 612 | This can be done in two ways. Either add the dependency in a rule, |
1da177e4 LT |
613 | or utilise the variable $(always). |
614 | Both possibilities are described in the following. | |
615 | ||
616 | --- 4.1 Simple Host Program | |
617 | ||
618 | In some cases there is a need to compile and run a program on the | |
619 | computer where the build is running. | |
620 | The following line tells kbuild that the program bin2hex shall be | |
621 | built on the build host. | |
622 | ||
623 | Example: | |
624 | hostprogs-y := bin2hex | |
625 | ||
626 | Kbuild assumes in the above example that bin2hex is made from a single | |
627 | c-source file named bin2hex.c located in the same directory as | |
628 | the Makefile. | |
39e6e9cf | 629 | |
1da177e4 LT |
630 | --- 4.2 Composite Host Programs |
631 | ||
632 | Host programs can be made up based on composite objects. | |
633 | The syntax used to define composite objects for host programs is | |
634 | similar to the syntax used for kernel objects. | |
5d3f083d | 635 | $(<executable>-objs) lists all objects used to link the final |
1da177e4 LT |
636 | executable. |
637 | ||
638 | Example: | |
639 | #scripts/lxdialog/Makefile | |
39e6e9cf | 640 | hostprogs-y := lxdialog |
1da177e4 LT |
641 | lxdialog-objs := checklist.o lxdialog.o |
642 | ||
643 | Objects with extension .o are compiled from the corresponding .c | |
a07f6033 | 644 | files. In the above example, checklist.c is compiled to checklist.o |
1da177e4 | 645 | and lxdialog.c is compiled to lxdialog.o. |
a07f6033 | 646 | Finally, the two .o files are linked to the executable, lxdialog. |
1da177e4 LT |
647 | Note: The syntax <executable>-y is not permitted for host-programs. |
648 | ||
62e22107 | 649 | --- 4.3 Using C++ for host programs |
1da177e4 LT |
650 | |
651 | kbuild offers support for host programs written in C++. This was | |
652 | introduced solely to support kconfig, and is not recommended | |
653 | for general use. | |
654 | ||
655 | Example: | |
656 | #scripts/kconfig/Makefile | |
657 | hostprogs-y := qconf | |
658 | qconf-cxxobjs := qconf.o | |
659 | ||
660 | In the example above the executable is composed of the C++ file | |
661 | qconf.cc - identified by $(qconf-cxxobjs). | |
39e6e9cf | 662 | |
39fed701 | 663 | If qconf is composed of a mixture of .c and .cc files, then an |
1da177e4 LT |
664 | additional line can be used to identify this. |
665 | ||
666 | Example: | |
667 | #scripts/kconfig/Makefile | |
668 | hostprogs-y := qconf | |
669 | qconf-cxxobjs := qconf.o | |
670 | qconf-objs := check.o | |
39e6e9cf | 671 | |
62e22107 | 672 | --- 4.4 Controlling compiler options for host programs |
1da177e4 LT |
673 | |
674 | When compiling host programs, it is possible to set specific flags. | |
675 | The programs will always be compiled utilising $(HOSTCC) passed | |
676 | the options specified in $(HOSTCFLAGS). | |
677 | To set flags that will take effect for all host programs created | |
a07f6033 | 678 | in that Makefile, use the variable HOST_EXTRACFLAGS. |
1da177e4 LT |
679 | |
680 | Example: | |
681 | #scripts/lxdialog/Makefile | |
682 | HOST_EXTRACFLAGS += -I/usr/include/ncurses | |
39e6e9cf | 683 | |
1da177e4 LT |
684 | To set specific flags for a single file the following construction |
685 | is used: | |
686 | ||
687 | Example: | |
688 | #arch/ppc64/boot/Makefile | |
689 | HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) | |
39e6e9cf | 690 | |
1da177e4 | 691 | It is also possible to specify additional options to the linker. |
39e6e9cf | 692 | |
1da177e4 LT |
693 | Example: |
694 | #scripts/kconfig/Makefile | |
695 | HOSTLOADLIBES_qconf := -L$(QTDIR)/lib | |
696 | ||
a07f6033 JE |
697 | When linking qconf, it will be passed the extra option |
698 | "-L$(QTDIR)/lib". | |
39e6e9cf | 699 | |
62e22107 | 700 | --- 4.5 When host programs are actually built |
1da177e4 LT |
701 | |
702 | Kbuild will only build host-programs when they are referenced | |
703 | as a prerequisite. | |
704 | This is possible in two ways: | |
705 | ||
706 | (1) List the prerequisite explicitly in a special rule. | |
707 | ||
708 | Example: | |
709 | #drivers/pci/Makefile | |
710 | hostprogs-y := gen-devlist | |
711 | $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist | |
712 | ( cd $(obj); ./gen-devlist ) < $< | |
713 | ||
39e6e9cf | 714 | The target $(obj)/devlist.h will not be built before |
1da177e4 LT |
715 | $(obj)/gen-devlist is updated. Note that references to |
716 | the host programs in special rules must be prefixed with $(obj). | |
717 | ||
718 | (2) Use $(always) | |
719 | When there is no suitable special rule, and the host program | |
720 | shall be built when a makefile is entered, the $(always) | |
721 | variable shall be used. | |
722 | ||
723 | Example: | |
724 | #scripts/lxdialog/Makefile | |
725 | hostprogs-y := lxdialog | |
726 | always := $(hostprogs-y) | |
727 | ||
728 | This will tell kbuild to build lxdialog even if not referenced in | |
729 | any rule. | |
730 | ||
62e22107 | 731 | --- 4.6 Using hostprogs-$(CONFIG_FOO) |
1da177e4 | 732 | |
39e6e9cf | 733 | A typical pattern in a Kbuild file looks like this: |
1da177e4 LT |
734 | |
735 | Example: | |
736 | #scripts/Makefile | |
737 | hostprogs-$(CONFIG_KALLSYMS) += kallsyms | |
738 | ||
739 | Kbuild knows about both 'y' for built-in and 'm' for module. | |
39fed701 | 740 | So if a config symbol evaluates to 'm', kbuild will still build |
a07f6033 JE |
741 | the binary. In other words, Kbuild handles hostprogs-m exactly |
742 | like hostprogs-y. But only hostprogs-y is recommended to be used | |
743 | when no CONFIG symbols are involved. | |
1da177e4 LT |
744 | |
745 | === 5 Kbuild clean infrastructure | |
746 | ||
a07f6033 | 747 | "make clean" deletes most generated files in the obj tree where the kernel |
1da177e4 LT |
748 | is compiled. This includes generated files such as host programs. |
749 | Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always), | |
750 | $(extra-y) and $(targets). They are all deleted during "make clean". | |
751 | Files matching the patterns "*.[oas]", "*.ko", plus some additional files | |
752 | generated by kbuild are deleted all over the kernel src tree when | |
753 | "make clean" is executed. | |
754 | ||
755 | Additional files can be specified in kbuild makefiles by use of $(clean-files). | |
756 | ||
757 | Example: | |
97659181 MM |
758 | #lib/Makefile |
759 | clean-files := crc32table.h | |
1da177e4 | 760 | |
bd55daf4 JR |
761 | When executing "make clean", the file "crc32table.h" will be deleted. |
762 | Kbuild will assume files to be in the same relative directory as the | |
97659181 | 763 | Makefile, except if prefixed with $(objtree). |
1da177e4 | 764 | |
39e6e9cf BH |
765 | To delete a directory hierarchy use: |
766 | ||
1da177e4 LT |
767 | Example: |
768 | #scripts/package/Makefile | |
769 | clean-dirs := $(objtree)/debian/ | |
770 | ||
97659181 MM |
771 | This will delete the directory debian in the toplevel directory, including all |
772 | subdirectories. | |
1da177e4 | 773 | |
ef8ff89b MM |
774 | To exclude certain files from make clean, use the $(no-clean-files) variable. |
775 | This is only a special case used in the top level Kbuild file: | |
776 | ||
777 | Example: | |
778 | #Kbuild | |
779 | no-clean-files := $(bounds-file) $(offsets-file) | |
780 | ||
1da177e4 LT |
781 | Usually kbuild descends down in subdirectories due to "obj-* := dir/", |
782 | but in the architecture makefiles where the kbuild infrastructure | |
783 | is not sufficient this sometimes needs to be explicit. | |
784 | ||
785 | Example: | |
25eb650a | 786 | #arch/x86/boot/Makefile |
1da177e4 LT |
787 | subdir- := compressed/ |
788 | ||
789 | The above assignment instructs kbuild to descend down in the | |
790 | directory compressed/ when "make clean" is executed. | |
791 | ||
39fed701 | 792 | To support the clean infrastructure in the Makefiles that build the |
1da177e4 LT |
793 | final bootimage there is an optional target named archclean: |
794 | ||
795 | Example: | |
25eb650a | 796 | #arch/x86/Makefile |
1da177e4 | 797 | archclean: |
25eb650a | 798 | $(Q)$(MAKE) $(clean)=arch/x86/boot |
1da177e4 | 799 | |
25eb650a WG |
800 | When "make clean" is executed, make will descend down in arch/x86/boot, |
801 | and clean as usual. The Makefile located in arch/x86/boot/ may use | |
1da177e4 LT |
802 | the subdir- trick to descend further down. |
803 | ||
804 | Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is | |
805 | included in the top level makefile, and the kbuild infrastructure | |
806 | is not operational at that point. | |
807 | ||
808 | Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will | |
809 | be visited during "make clean". | |
810 | ||
811 | === 6 Architecture Makefiles | |
812 | ||
813 | The top level Makefile sets up the environment and does the preparation, | |
814 | before starting to descend down in the individual directories. | |
a07f6033 JE |
815 | The top level makefile contains the generic part, whereas |
816 | arch/$(ARCH)/Makefile contains what is required to set up kbuild | |
817 | for said architecture. | |
818 | To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines | |
1da177e4 LT |
819 | a few targets. |
820 | ||
a07f6033 JE |
821 | When kbuild executes, the following steps are followed (roughly): |
822 | 1) Configuration of the kernel => produce .config | |
1da177e4 | 823 | 2) Store kernel version in include/linux/version.h |
b22ae40e | 824 | 3) Updating all other prerequisites to the target prepare: |
1da177e4 | 825 | - Additional prerequisites are specified in arch/$(ARCH)/Makefile |
b22ae40e | 826 | 4) Recursively descend down in all directories listed in |
1da177e4 | 827 | init-* core* drivers-* net-* libs-* and build all targets. |
a07f6033 | 828 | - The values of the above variables are expanded in arch/$(ARCH)/Makefile. |
b22ae40e | 829 | 5) All object files are then linked and the resulting file vmlinux is |
a07f6033 | 830 | located at the root of the obj tree. |
1da177e4 LT |
831 | The very first objects linked are listed in head-y, assigned by |
832 | arch/$(ARCH)/Makefile. | |
b22ae40e | 833 | 6) Finally, the architecture-specific part does any required post processing |
1da177e4 LT |
834 | and builds the final bootimage. |
835 | - This includes building boot records | |
5c811e59 | 836 | - Preparing initrd images and the like |
1da177e4 LT |
837 | |
838 | ||
839 | --- 6.1 Set variables to tweak the build to the architecture | |
840 | ||
841 | LDFLAGS Generic $(LD) options | |
842 | ||
843 | Flags used for all invocations of the linker. | |
844 | Often specifying the emulation is sufficient. | |
845 | ||
846 | Example: | |
847 | #arch/s390/Makefile | |
848 | LDFLAGS := -m elf_s390 | |
f77bf014 | 849 | Note: ldflags-y can be used to further customise |
a9af3305 | 850 | the flags used. See chapter 3.7. |
39e6e9cf | 851 | |
1da177e4 LT |
852 | LDFLAGS_MODULE Options for $(LD) when linking modules |
853 | ||
854 | LDFLAGS_MODULE is used to set specific flags for $(LD) when | |
855 | linking the .ko files used for modules. | |
856 | Default is "-r", for relocatable output. | |
857 | ||
858 | LDFLAGS_vmlinux Options for $(LD) when linking vmlinux | |
859 | ||
860 | LDFLAGS_vmlinux is used to specify additional flags to pass to | |
a07f6033 | 861 | the linker when linking the final vmlinux image. |
1da177e4 LT |
862 | LDFLAGS_vmlinux uses the LDFLAGS_$@ support. |
863 | ||
864 | Example: | |
25eb650a | 865 | #arch/x86/Makefile |
1da177e4 LT |
866 | LDFLAGS_vmlinux := -e stext |
867 | ||
868 | OBJCOPYFLAGS objcopy flags | |
869 | ||
870 | When $(call if_changed,objcopy) is used to translate a .o file, | |
a07f6033 | 871 | the flags specified in OBJCOPYFLAGS will be used. |
1da177e4 LT |
872 | $(call if_changed,objcopy) is often used to generate raw binaries on |
873 | vmlinux. | |
874 | ||
875 | Example: | |
876 | #arch/s390/Makefile | |
877 | OBJCOPYFLAGS := -O binary | |
878 | ||
879 | #arch/s390/boot/Makefile | |
880 | $(obj)/image: vmlinux FORCE | |
881 | $(call if_changed,objcopy) | |
882 | ||
a07f6033 | 883 | In this example, the binary $(obj)/image is a binary version of |
1da177e4 LT |
884 | vmlinux. The usage of $(call if_changed,xxx) will be described later. |
885 | ||
222d394d | 886 | KBUILD_AFLAGS $(AS) assembler flags |
1da177e4 LT |
887 | |
888 | Default value - see top level Makefile | |
889 | Append or modify as required per architecture. | |
890 | ||
891 | Example: | |
892 | #arch/sparc64/Makefile | |
222d394d | 893 | KBUILD_AFLAGS += -m64 -mcpu=ultrasparc |
1da177e4 | 894 | |
a0f97e06 | 895 | KBUILD_CFLAGS $(CC) compiler flags |
1da177e4 LT |
896 | |
897 | Default value - see top level Makefile | |
898 | Append or modify as required per architecture. | |
899 | ||
a0f97e06 | 900 | Often, the KBUILD_CFLAGS variable depends on the configuration. |
1da177e4 LT |
901 | |
902 | Example: | |
ff4eb04c PB |
903 | #arch/x86/boot/compressed/Makefile |
904 | cflags-$(CONFIG_X86_32) := -march=i386 | |
905 | cflags-$(CONFIG_X86_64) := -mcmodel=small | |
a0f97e06 | 906 | KBUILD_CFLAGS += $(cflags-y) |
1da177e4 LT |
907 | |
908 | Many arch Makefiles dynamically run the target C compiler to | |
909 | probe supported options: | |
910 | ||
25eb650a | 911 | #arch/x86/Makefile |
1da177e4 LT |
912 | |
913 | ... | |
914 | cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ | |
915 | -march=pentium2,-march=i686) | |
916 | ... | |
917 | # Disable unit-at-a-time mode ... | |
a0f97e06 | 918 | KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) |
1da177e4 LT |
919 | ... |
920 | ||
921 | ||
a07f6033 | 922 | The first example utilises the trick that a config option expands |
1da177e4 LT |
923 | to 'y' when selected. |
924 | ||
80c00ba9 | 925 | KBUILD_AFLAGS_KERNEL $(AS) options specific for built-in |
1da177e4 | 926 | |
80c00ba9 | 927 | $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile |
1da177e4 LT |
928 | resident kernel code. |
929 | ||
6588169d | 930 | KBUILD_AFLAGS_MODULE Options for $(AS) when building modules |
1da177e4 | 931 | |
39fed701 | 932 | $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that |
6588169d SR |
933 | are used for $(AS). |
934 | From commandline AFLAGS_MODULE shall be used (see kbuild.txt). | |
1da177e4 | 935 | |
80c00ba9 SR |
936 | KBUILD_CFLAGS_KERNEL $(CC) options specific for built-in |
937 | ||
938 | $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile | |
939 | resident kernel code. | |
940 | ||
6588169d SR |
941 | KBUILD_CFLAGS_MODULE Options for $(CC) when building modules |
942 | ||
39fed701 | 943 | $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that |
6588169d SR |
944 | are used for $(CC). |
945 | From commandline CFLAGS_MODULE shall be used (see kbuild.txt). | |
946 | ||
947 | KBUILD_LDFLAGS_MODULE Options for $(LD) when linking modules | |
948 | ||
39fed701 | 949 | $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options |
6588169d SR |
950 | used when linking modules. This is often a linker script. |
951 | From commandline LDFLAGS_MODULE shall be used (see kbuild.txt). | |
39e6e9cf | 952 | |
40df759e MM |
953 | KBUILD_ARFLAGS Options for $(AR) when creating archives |
954 | ||
955 | $(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic | |
956 | mode) if this option is supported by $(AR). | |
957 | ||
61754c18 MM |
958 | ARCH_CPPFLAGS, ARCH_AFLAGS, ARCH_CFLAGS Overrides the kbuild defaults |
959 | ||
960 | These variables are appended to the KBUILD_CPPFLAGS, | |
961 | KBUILD_AFLAGS, and KBUILD_CFLAGS, respectively, after the | |
962 | top-level Makefile has set any other flags. This provides a | |
963 | means for an architecture to override the defaults. | |
964 | ||
965 | ||
052ad274 PA |
966 | --- 6.2 Add prerequisites to archheaders: |
967 | ||
968 | The archheaders: rule is used to generate header files that | |
969 | may be installed into user space by "make header_install" or | |
970 | "make headers_install_all". In order to support | |
971 | "make headers_install_all", this target has to be able to run | |
972 | on an unconfigured tree, or a tree configured for another | |
973 | architecture. | |
974 | ||
975 | It is run before "make archprepare" when run on the | |
976 | architecture itself. | |
977 | ||
978 | ||
979 | --- 6.3 Add prerequisites to archprepare: | |
1da177e4 | 980 | |
a07f6033 | 981 | The archprepare: rule is used to list prerequisites that need to be |
1da177e4 | 982 | built before starting to descend down in the subdirectories. |
a07f6033 | 983 | This is usually used for header files containing assembler constants. |
1da177e4 LT |
984 | |
985 | Example: | |
5bb78269 SR |
986 | #arch/arm/Makefile |
987 | archprepare: maketools | |
1da177e4 | 988 | |
a07f6033 | 989 | In this example, the file target maketools will be processed |
5bb78269 | 990 | before descending down in the subdirectories. |
1da177e4 LT |
991 | See also chapter XXX-TODO that describe how kbuild supports |
992 | generating offset header files. | |
993 | ||
994 | ||
052ad274 | 995 | --- 6.4 List directories to visit when descending |
1da177e4 LT |
996 | |
997 | An arch Makefile cooperates with the top Makefile to define variables | |
998 | which specify how to build the vmlinux file. Note that there is no | |
999 | corresponding arch-specific section for modules; the module-building | |
1000 | machinery is all architecture-independent. | |
1001 | ||
39e6e9cf | 1002 | |
1da177e4 LT |
1003 | head-y, init-y, core-y, libs-y, drivers-y, net-y |
1004 | ||
a07f6033 JE |
1005 | $(head-y) lists objects to be linked first in vmlinux. |
1006 | $(libs-y) lists directories where a lib.a archive can be located. | |
5c811e59 | 1007 | The rest list directories where a built-in.o object file can be |
a07f6033 | 1008 | located. |
1da177e4 LT |
1009 | |
1010 | $(init-y) objects will be located after $(head-y). | |
1011 | Then the rest follows in this order: | |
1012 | $(core-y), $(libs-y), $(drivers-y) and $(net-y). | |
1013 | ||
a07f6033 | 1014 | The top level Makefile defines values for all generic directories, |
5c811e59 | 1015 | and arch/$(ARCH)/Makefile only adds architecture-specific directories. |
1da177e4 LT |
1016 | |
1017 | Example: | |
1018 | #arch/sparc64/Makefile | |
1019 | core-y += arch/sparc64/kernel/ | |
1020 | libs-y += arch/sparc64/prom/ arch/sparc64/lib/ | |
1021 | drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ | |
1022 | ||
1023 | ||
052ad274 | 1024 | --- 6.5 Architecture-specific boot images |
1da177e4 LT |
1025 | |
1026 | An arch Makefile specifies goals that take the vmlinux file, compress | |
1027 | it, wrap it in bootstrapping code, and copy the resulting files | |
1028 | somewhere. This includes various kinds of installation commands. | |
1029 | The actual goals are not standardized across architectures. | |
1030 | ||
1031 | It is common to locate any additional processing in a boot/ | |
1032 | directory below arch/$(ARCH)/. | |
1033 | ||
1034 | Kbuild does not provide any smart way to support building a | |
1035 | target specified in boot/. Therefore arch/$(ARCH)/Makefile shall | |
1036 | call make manually to build a target in boot/. | |
1037 | ||
1038 | The recommended approach is to include shortcuts in | |
1039 | arch/$(ARCH)/Makefile, and use the full path when calling down | |
1040 | into the arch/$(ARCH)/boot/Makefile. | |
1041 | ||
1042 | Example: | |
25eb650a WG |
1043 | #arch/x86/Makefile |
1044 | boot := arch/x86/boot | |
1da177e4 LT |
1045 | bzImage: vmlinux |
1046 | $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ | |
1047 | ||
1048 | "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke | |
1049 | make in a subdirectory. | |
1050 | ||
5c811e59 | 1051 | There are no rules for naming architecture-specific targets, |
1da177e4 | 1052 | but executing "make help" will list all relevant targets. |
a07f6033 | 1053 | To support this, $(archhelp) must be defined. |
1da177e4 LT |
1054 | |
1055 | Example: | |
25eb650a | 1056 | #arch/x86/Makefile |
1da177e4 LT |
1057 | define archhelp |
1058 | echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' | |
39e6e9cf | 1059 | endif |
1da177e4 LT |
1060 | |
1061 | When make is executed without arguments, the first goal encountered | |
1062 | will be built. In the top level Makefile the first goal present | |
1063 | is all:. | |
a07f6033 JE |
1064 | An architecture shall always, per default, build a bootable image. |
1065 | In "make help", the default goal is highlighted with a '*'. | |
1da177e4 LT |
1066 | Add a new prerequisite to all: to select a default goal different |
1067 | from vmlinux. | |
1068 | ||
1069 | Example: | |
25eb650a | 1070 | #arch/x86/Makefile |
39e6e9cf | 1071 | all: bzImage |
1da177e4 LT |
1072 | |
1073 | When "make" is executed without arguments, bzImage will be built. | |
1074 | ||
052ad274 | 1075 | --- 6.6 Building non-kbuild targets |
1da177e4 LT |
1076 | |
1077 | extra-y | |
1078 | ||
39fed701 | 1079 | extra-y specifies additional targets created in the current |
1da177e4 LT |
1080 | directory, in addition to any targets specified by obj-*. |
1081 | ||
1082 | Listing all targets in extra-y is required for two purposes: | |
1083 | 1) Enable kbuild to check changes in command lines | |
1084 | - When $(call if_changed,xxx) is used | |
1085 | 2) kbuild knows what files to delete during "make clean" | |
1086 | ||
1087 | Example: | |
25eb650a | 1088 | #arch/x86/kernel/Makefile |
1da177e4 LT |
1089 | extra-y := head.o init_task.o |
1090 | ||
a07f6033 | 1091 | In this example, extra-y is used to list object files that |
1da177e4 LT |
1092 | shall be built, but shall not be linked as part of built-in.o. |
1093 | ||
39e6e9cf | 1094 | |
052ad274 | 1095 | --- 6.7 Commands useful for building a boot image |
1da177e4 LT |
1096 | |
1097 | Kbuild provides a few macros that are useful when building a | |
1098 | boot image. | |
1099 | ||
1100 | if_changed | |
1101 | ||
1102 | if_changed is the infrastructure used for the following commands. | |
1103 | ||
1104 | Usage: | |
1105 | target: source(s) FORCE | |
ef80f0a1 | 1106 | $(call if_changed,ld/objcopy/gzip/...) |
1da177e4 | 1107 | |
a07f6033 | 1108 | When the rule is evaluated, it is checked to see if any files |
5c811e59 | 1109 | need an update, or the command line has changed since the last |
1da177e4 LT |
1110 | invocation. The latter will force a rebuild if any options |
1111 | to the executable have changed. | |
1112 | Any target that utilises if_changed must be listed in $(targets), | |
1113 | otherwise the command line check will fail, and the target will | |
1114 | always be built. | |
1115 | Assignments to $(targets) are without $(obj)/ prefix. | |
1116 | if_changed may be used in conjunction with custom commands as | |
052ad274 | 1117 | defined in 6.8 "Custom kbuild commands". |
49490571 | 1118 | |
1da177e4 | 1119 | Note: It is a typical mistake to forget the FORCE prerequisite. |
49490571 PBG |
1120 | Another common pitfall is that whitespace is sometimes |
1121 | significant; for instance, the below will fail (note the extra space | |
1122 | after the comma): | |
1123 | target: source(s) FORCE | |
ef80f0a1 | 1124 | #WRONG!# $(call if_changed, ld/objcopy/gzip/...) |
1da177e4 LT |
1125 | |
1126 | ld | |
a07f6033 | 1127 | Link target. Often, LDFLAGS_$@ is used to set specific options to ld. |
39e6e9cf | 1128 | |
1da177e4 LT |
1129 | objcopy |
1130 | Copy binary. Uses OBJCOPYFLAGS usually specified in | |
1131 | arch/$(ARCH)/Makefile. | |
1132 | OBJCOPYFLAGS_$@ may be used to set additional options. | |
1133 | ||
1134 | gzip | |
1135 | Compress target. Use maximum compression to compress target. | |
1136 | ||
1137 | Example: | |
25eb650a | 1138 | #arch/x86/boot/Makefile |
1da177e4 LT |
1139 | LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary |
1140 | LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext | |
1141 | ||
1142 | targets += setup setup.o bootsect bootsect.o | |
1143 | $(obj)/setup $(obj)/bootsect: %: %.o FORCE | |
1144 | $(call if_changed,ld) | |
1145 | ||
a07f6033 JE |
1146 | In this example, there are two possible targets, requiring different |
1147 | options to the linker. The linker options are specified using the | |
1da177e4 | 1148 | LDFLAGS_$@ syntax - one for each potential target. |
5d3f083d | 1149 | $(targets) are assigned all potential targets, by which kbuild knows |
1da177e4 LT |
1150 | the targets and will: |
1151 | 1) check for commandline changes | |
1152 | 2) delete target during make clean | |
1153 | ||
1154 | The ": %: %.o" part of the prerequisite is a shorthand that | |
39fed701 | 1155 | frees us from listing the setup.o and bootsect.o files. |
ef80f0a1 | 1156 | Note: It is a common mistake to forget the "targets :=" assignment, |
1da177e4 LT |
1157 | resulting in the target file being recompiled for no |
1158 | obvious reason. | |
1159 | ||
aab94339 | 1160 | dtc |
c1410562 | 1161 | Create flattened device tree blob object suitable for linking |
aab94339 DB |
1162 | into vmlinux. Device tree blobs linked into vmlinux are placed |
1163 | in an init section in the image. Platform code *must* copy the | |
1164 | blob to non-init memory prior to calling unflatten_device_tree(). | |
1165 | ||
90b335fb SW |
1166 | To use this command, simply add *.dtb into obj-y or targets, or make |
1167 | some other target depend on %.dtb | |
aab94339 | 1168 | |
90b335fb SW |
1169 | A central rule exists to create $(obj)/%.dtb from $(src)/%.dts; |
1170 | architecture Makefiles do no need to explicitly write out that rule. | |
aab94339 | 1171 | |
90b335fb SW |
1172 | Example: |
1173 | targets += $(dtb-y) | |
1174 | clean-files += *.dtb | |
1175 | DTC_FLAGS ?= -p 1024 | |
1da177e4 | 1176 | |
052ad274 | 1177 | --- 6.8 Custom kbuild commands |
1da177e4 | 1178 | |
a07f6033 | 1179 | When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand |
1da177e4 LT |
1180 | of a command is normally displayed. |
1181 | To enable this behaviour for custom commands kbuild requires | |
1182 | two variables to be set: | |
1183 | quiet_cmd_<command> - what shall be echoed | |
1184 | cmd_<command> - the command to execute | |
1185 | ||
1186 | Example: | |
1187 | # | |
1188 | quiet_cmd_image = BUILD $@ | |
1189 | cmd_image = $(obj)/tools/build $(BUILDFLAGS) \ | |
1190 | $(obj)/vmlinux.bin > $@ | |
1191 | ||
1192 | targets += bzImage | |
1193 | $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE | |
1194 | $(call if_changed,image) | |
1195 | @echo 'Kernel: $@ is ready' | |
1196 | ||
a07f6033 | 1197 | When updating the $(obj)/bzImage target, the line |
1da177e4 | 1198 | |
25eb650a | 1199 | BUILD arch/x86/boot/bzImage |
1da177e4 LT |
1200 | |
1201 | will be displayed with "make KBUILD_VERBOSE=0". | |
39e6e9cf | 1202 | |
1da177e4 | 1203 | |
052ad274 | 1204 | --- 6.9 Preprocessing linker scripts |
1da177e4 | 1205 | |
a07f6033 | 1206 | When the vmlinux image is built, the linker script |
1da177e4 LT |
1207 | arch/$(ARCH)/kernel/vmlinux.lds is used. |
1208 | The script is a preprocessed variant of the file vmlinux.lds.S | |
1209 | located in the same directory. | |
a07f6033 | 1210 | kbuild knows .lds files and includes a rule *lds.S -> *lds. |
39e6e9cf | 1211 | |
1da177e4 | 1212 | Example: |
25eb650a | 1213 | #arch/x86/kernel/Makefile |
1da177e4 | 1214 | always := vmlinux.lds |
39e6e9cf | 1215 | |
1da177e4 LT |
1216 | #Makefile |
1217 | export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) | |
39e6e9cf BH |
1218 | |
1219 | The assignment to $(always) is used to tell kbuild to build the | |
a07f6033 JE |
1220 | target vmlinux.lds. |
1221 | The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the | |
1da177e4 | 1222 | specified options when building the target vmlinux.lds. |
39e6e9cf | 1223 | |
a07f6033 | 1224 | When building the *.lds target, kbuild uses the variables: |
06c5040c | 1225 | KBUILD_CPPFLAGS : Set in top-level Makefile |
f77bf014 | 1226 | cppflags-y : May be set in the kbuild makefile |
39fed701 | 1227 | CPPFLAGS_$(@F) : Target-specific flags. |
1da177e4 LT |
1228 | Note that the full filename is used in this |
1229 | assignment. | |
1230 | ||
39fed701 | 1231 | The kbuild infrastructure for *lds files is used in several |
5c811e59 | 1232 | architecture-specific files. |
1da177e4 | 1233 | |
052ad274 | 1234 | --- 6.10 Generic header files |
d8ecc5cd SR |
1235 | |
1236 | The directory include/asm-generic contains the header files | |
1237 | that may be shared between individual architectures. | |
1238 | The recommended approach how to use a generic header file is | |
1239 | to list the file in the Kbuild file. | |
fcc8487d | 1240 | See "7.3 generic-y" for further info on syntax etc. |
d8ecc5cd | 1241 | |
fbe6e37d NP |
1242 | --- 6.11 Post-link pass |
1243 | ||
1244 | If the file arch/xxx/Makefile.postlink exists, this makefile | |
1245 | will be invoked for post-link objects (vmlinux and modules.ko) | |
1246 | for architectures to run post-link passes on. Must also handle | |
1247 | the clean target. | |
1248 | ||
1249 | This pass runs after kallsyms generation. If the architecture | |
1250 | needs to modify symbol locations, rather than manipulate the | |
1251 | kallsyms, it may be easier to add another postlink target for | |
1252 | .tmp_vmlinux? targets to be called from link-vmlinux.sh. | |
1253 | ||
1254 | For example, powerpc uses this to check relocation sanity of | |
1255 | the linked vmlinux file. | |
1256 | ||
c7bb349e SR |
1257 | === 7 Kbuild syntax for exported headers |
1258 | ||
39fed701 | 1259 | The kernel includes a set of headers that is exported to userspace. |
c95940f2 | 1260 | Many headers can be exported as-is but other headers require a |
c7bb349e SR |
1261 | minimal pre-processing before they are ready for user-space. |
1262 | The pre-processing does: | |
39fed701 | 1263 | - drop kernel-specific annotations |
c7bb349e | 1264 | - drop include of compiler.h |
c95940f2 | 1265 | - drop all sections that are kernel internal (guarded by ifdef __KERNEL__) |
c7bb349e | 1266 | |
fcc8487d ND |
1267 | All headers under include/uapi/, include/generated/uapi/, |
1268 | arch/<arch>/include/uapi/asm/ and arch/<arch>/include/generated/uapi/asm/ | |
1269 | are exported. | |
c7bb349e | 1270 | |
fcc8487d ND |
1271 | A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and |
1272 | arch/<arch>/include/asm/ to list asm files coming from asm-generic. | |
1273 | See subsequent chapter for the syntax of the Kbuild file. | |
c7bb349e | 1274 | |
fcc8487d | 1275 | --- 7.1 no-export-headers |
c7bb349e | 1276 | |
fcc8487d ND |
1277 | no-export-headers is essentially used by include/uapi/linux/Kbuild to |
1278 | avoid exporting specific headers (e.g. kvm.h) on architectures that do | |
1279 | not support it. It should be avoided as much as possible. | |
c7bb349e | 1280 | |
40f1d4c2 | 1281 | --- 7.2 genhdr-y |
c7bb349e | 1282 | |
fcc8487d | 1283 | genhdr-y specifies asm files to be generated. |
c7bb349e SR |
1284 | |
1285 | Example: | |
fcc8487d ND |
1286 | #arch/x86/include/uapi/asm/Kbuild |
1287 | genhdr-y += unistd_32.h | |
1288 | genhdr-y += unistd_64.h | |
1289 | genhdr-y += unistd_x32.h | |
1290 | ||
c7bb349e | 1291 | |
bd73a328 | 1292 | --- 7.3 generic-y |
d8ecc5cd SR |
1293 | |
1294 | If an architecture uses a verbatim copy of a header from | |
1295 | include/asm-generic then this is listed in the file | |
1296 | arch/$(ARCH)/include/asm/Kbuild like this: | |
1297 | ||
1298 | Example: | |
1299 | #arch/x86/include/asm/Kbuild | |
1300 | generic-y += termios.h | |
1301 | generic-y += rtc.h | |
1302 | ||
1303 | During the prepare phase of the build a wrapper include | |
1304 | file is generated in the directory: | |
1305 | ||
1306 | arch/$(ARCH)/include/generated/asm | |
1307 | ||
1308 | When a header is exported where the architecture uses | |
1309 | the generic header a similar wrapper is generated as part | |
1310 | of the set of exported headers in the directory: | |
1311 | ||
1312 | usr/include/asm | |
1313 | ||
1314 | The generated wrapper will in both cases look like the following: | |
1315 | ||
1316 | Example: termios.h | |
1317 | #include <asm-generic/termios.h> | |
c7bb349e | 1318 | |
bd73a328 | 1319 | --- 7.4 generated-y |
54b880ca JH |
1320 | |
1321 | If an architecture generates other header files alongside generic-y | |
1322 | wrappers, and not included in genhdr-y, then generated-y specifies | |
1323 | them. | |
1324 | ||
1325 | This prevents them being treated as stale asm-generic wrappers and | |
1326 | removed. | |
1327 | ||
1328 | Example: | |
1329 | #arch/x86/include/asm/Kbuild | |
1330 | generated-y += syscalls_32.h | |
1331 | ||
fcc8487d ND |
1332 | --- 7.5 mandatory-y |
1333 | ||
1334 | mandatory-y is essentially used by include/uapi/asm-generic/Kbuild.asm | |
1335 | to define the minimun set of headers that must be exported in | |
1336 | include/asm. | |
1337 | ||
1338 | The convention is to list one subdir per line and | |
1339 | preferably in alphabetic order. | |
1340 | ||
1341 | --- 7.6 subdir-y | |
1342 | ||
1343 | subdir-y may be used to specify a subdirectory to be exported. | |
1344 | ||
1345 | Example: | |
1346 | #arch/cris/include/uapi/asm/Kbuild | |
1347 | subdir-y += ../arch-v10/arch/ | |
1348 | subdir-y += ../arch-v32/arch/ | |
1349 | ||
1350 | The convention is to list one subdir per line and | |
1351 | preferably in alphabetic order. | |
1352 | ||
c7bb349e | 1353 | === 8 Kbuild Variables |
1da177e4 LT |
1354 | |
1355 | The top Makefile exports the following variables: | |
1356 | ||
1357 | VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION | |
1358 | ||
1359 | These variables define the current kernel version. A few arch | |
1360 | Makefiles actually use these values directly; they should use | |
1361 | $(KERNELRELEASE) instead. | |
1362 | ||
1363 | $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic | |
1364 | three-part version number, such as "2", "4", and "0". These three | |
1365 | values are always numeric. | |
1366 | ||
1367 | $(EXTRAVERSION) defines an even tinier sublevel for pre-patches | |
1368 | or additional patches. It is usually some non-numeric string | |
1369 | such as "-pre4", and is often blank. | |
1370 | ||
1371 | KERNELRELEASE | |
1372 | ||
1373 | $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable | |
1374 | for constructing installation directory names or showing in | |
1375 | version strings. Some arch Makefiles use it for this purpose. | |
1376 | ||
1377 | ARCH | |
1378 | ||
1379 | This variable defines the target architecture, such as "i386", | |
1380 | "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to | |
1381 | determine which files to compile. | |
1382 | ||
1383 | By default, the top Makefile sets $(ARCH) to be the same as the | |
1384 | host system architecture. For a cross build, a user may | |
1385 | override the value of $(ARCH) on the command line: | |
1386 | ||
1387 | make ARCH=m68k ... | |
1388 | ||
1389 | ||
1390 | INSTALL_PATH | |
1391 | ||
1392 | This variable defines a place for the arch Makefiles to install | |
1393 | the resident kernel image and System.map file. | |
5c811e59 | 1394 | Use this for architecture-specific install targets. |
1da177e4 LT |
1395 | |
1396 | INSTALL_MOD_PATH, MODLIB | |
1397 | ||
1398 | $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module | |
1399 | installation. This variable is not defined in the Makefile but | |
1400 | may be passed in by the user if desired. | |
1401 | ||
1402 | $(MODLIB) specifies the directory for module installation. | |
1403 | The top Makefile defines $(MODLIB) to | |
1404 | $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may | |
1405 | override this value on the command line if desired. | |
1406 | ||
ac031f26 TT |
1407 | INSTALL_MOD_STRIP |
1408 | ||
39fed701 | 1409 | If this variable is specified, it will cause modules to be stripped |
ac031f26 | 1410 | after they are installed. If INSTALL_MOD_STRIP is '1', then the |
39fed701 | 1411 | default option --strip-debug will be used. Otherwise, the |
177b241d GE |
1412 | INSTALL_MOD_STRIP value will be used as the option(s) to the strip |
1413 | command. | |
ac031f26 TT |
1414 | |
1415 | ||
c7bb349e | 1416 | === 9 Makefile language |
1da177e4 | 1417 | |
a07f6033 | 1418 | The kernel Makefiles are designed to be run with GNU Make. The Makefiles |
1da177e4 LT |
1419 | use only the documented features of GNU Make, but they do use many |
1420 | GNU extensions. | |
1421 | ||
1422 | GNU Make supports elementary list-processing functions. The kernel | |
1423 | Makefiles use a novel style of list building and manipulation with few | |
1424 | "if" statements. | |
1425 | ||
1426 | GNU Make has two assignment operators, ":=" and "=". ":=" performs | |
1427 | immediate evaluation of the right-hand side and stores an actual string | |
1428 | into the left-hand side. "=" is like a formula definition; it stores the | |
1429 | right-hand side in an unevaluated form and then evaluates this form each | |
1430 | time the left-hand side is used. | |
1431 | ||
1432 | There are some cases where "=" is appropriate. Usually, though, ":=" | |
1433 | is the right choice. | |
1434 | ||
c7bb349e | 1435 | === 10 Credits |
1da177e4 LT |
1436 | |
1437 | Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> | |
1438 | Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> | |
1439 | Updates by Sam Ravnborg <sam@ravnborg.org> | |
a07f6033 | 1440 | Language QA by Jan Engelhardt <jengelh@gmx.de> |
1da177e4 | 1441 | |
c7bb349e | 1442 | === 11 TODO |
1da177e4 | 1443 | |
a07f6033 | 1444 | - Describe how kbuild supports shipped files with _shipped. |
1da177e4 LT |
1445 | - Generating offset header files. |
1446 | - Add more variables to section 7? | |
1447 | ||
39e6e9cf BH |
1448 | |
1449 |