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1 | |
2 | In this document you will find information about: | |
3 | - how to build external modules | |
d9a7ff66 | 4 | - how to make your module use the kbuild infrastructure |
1da177e4 LT |
5 | - how kbuild will install a kernel |
6 | - how to install modules in a non-standard location | |
7 | ||
8 | === Table of Contents | |
9 | ||
10 | === 1 Introduction | |
11 | === 2 How to build external modules | |
12 | --- 2.1 Building external modules | |
13 | --- 2.2 Available targets | |
14 | --- 2.3 Available options | |
15 | --- 2.4 Preparing the kernel tree for module build | |
06300b21 | 16 | --- 2.5 Building separate files for a module |
1da177e4 LT |
17 | === 3. Example commands |
18 | === 4. Creating a kbuild file for an external module | |
19 | === 5. Include files | |
20 | --- 5.1 How to include files from the kernel include dir | |
21 | --- 5.2 External modules using an include/ dir | |
253dfa6e | 22 | --- 5.3 External modules using several directories |
1da177e4 LT |
23 | === 6. Module installation |
24 | --- 6.1 INSTALL_MOD_PATH | |
25 | --- 6.2 INSTALL_MOD_DIR | |
040fcc81 | 26 | === 7. Module versioning & Module.symvers |
2e99f319 | 27 | --- 7.1 Symbols from the kernel (vmlinux + modules) |
040fcc81 SR |
28 | --- 7.2 Symbols and external modules |
29 | --- 7.3 Symbols from another external module | |
1da177e4 LT |
30 | === 8. Tips & Tricks |
31 | --- 8.1 Testing for CONFIG_FOO_BAR | |
32 | ||
33 | ||
34 | ||
35 | === 1. Introduction | |
36 | ||
37 | kbuild includes functionality for building modules both | |
38 | within the kernel source tree and outside the kernel source tree. | |
d9a7ff66 JE |
39 | The latter is usually referred to as external or "out-of-tree" |
40 | modules and is used both during development and for modules that | |
41 | are not planned to be included in the kernel tree. | |
1da177e4 LT |
42 | |
43 | What is covered within this file is mainly information to authors | |
d9a7ff66 JE |
44 | of modules. The author of an external module should supply |
45 | a makefile that hides most of the complexity, so one only has to type | |
98a1e444 | 46 | 'make' to build the module. A complete example will be present in |
a7d7cb3c | 47 | chapter 4, "Creating a kbuild file for an external module". |
1da177e4 LT |
48 | |
49 | ||
50 | === 2. How to build external modules | |
51 | ||
52 | kbuild offers functionality to build external modules, with the | |
53 | prerequisite that there is a pre-built kernel available with full source. | |
54 | A subset of the targets available when building the kernel is available | |
55 | when building an external module. | |
56 | ||
57 | --- 2.1 Building external modules | |
58 | ||
59 | Use the following command to build an external module: | |
60 | ||
61 | make -C <path-to-kernel> M=`pwd` | |
62 | ||
63 | For the running kernel use: | |
64 | make -C /lib/modules/`uname -r`/build M=`pwd` | |
65 | ||
2e99f319 RD |
66 | For the above command to succeed, the kernel must have been |
67 | built with modules enabled. | |
1da177e4 LT |
68 | |
69 | To install the modules that were just built: | |
70 | ||
71 | make -C <path-to-kernel> M=`pwd` modules_install | |
72 | ||
2e99f319 RD |
73 | More complex examples will be shown later, the above should |
74 | be enough to get you started. | |
1da177e4 LT |
75 | |
76 | --- 2.2 Available targets | |
77 | ||
98a1e444 | 78 | $KDIR refers to the path to the kernel source top-level directory |
1da177e4 LT |
79 | |
80 | make -C $KDIR M=`pwd` | |
81 | Will build the module(s) located in current directory. | |
82 | All output files will be located in the same directory | |
83 | as the module source. | |
84 | No attempts are made to update the kernel source, and it is | |
85 | a precondition that a successful make has been executed | |
86 | for the kernel. | |
87 | ||
88 | make -C $KDIR M=`pwd` modules | |
89 | The modules target is implied when no target is given. | |
90 | Same functionality as if no target was specified. | |
91 | See description above. | |
92 | ||
2e99f319 | 93 | make -C $KDIR M=`pwd` modules_install |
1da177e4 LT |
94 | Install the external module(s). |
95 | Installation default is in /lib/modules/<kernel-version>/extra, | |
040fcc81 SR |
96 | but may be prefixed with INSTALL_MOD_PATH - see separate |
97 | chapter. | |
1da177e4 | 98 | |
2e99f319 | 99 | make -C $KDIR M=`pwd` clean |
1da177e4 | 100 | Remove all generated files for the module - the kernel |
98a1e444 | 101 | source directory is not modified. |
1da177e4 LT |
102 | |
103 | make -C $KDIR M=`pwd` help | |
104 | help will list the available target when building external | |
105 | modules. | |
106 | ||
107 | --- 2.3 Available options: | |
108 | ||
98a1e444 | 109 | $KDIR refers to the path to the kernel source top-level directory |
1da177e4 LT |
110 | |
111 | make -C $KDIR | |
112 | Used to specify where to find the kernel source. | |
113 | '$KDIR' represent the directory where the kernel source is. | |
114 | Make will actually change directory to the specified directory | |
115 | when executed but change back when finished. | |
116 | ||
117 | make -C $KDIR M=`pwd` | |
118 | M= is used to tell kbuild that an external module is | |
119 | being built. | |
120 | The option given to M= is the directory where the external | |
121 | module (kbuild file) is located. | |
122 | When an external module is being built only a subset of the | |
123 | usual targets are available. | |
124 | ||
125 | make -C $KDIR SUBDIRS=`pwd` | |
126 | Same as M=. The SUBDIRS= syntax is kept for backwards | |
127 | compatibility. | |
128 | ||
129 | --- 2.4 Preparing the kernel tree for module build | |
130 | ||
131 | To make sure the kernel contains the information required to | |
132 | build external modules the target 'modules_prepare' must be used. | |
2e99f319 RD |
133 | 'module_prepare' exists solely as a simple way to prepare |
134 | a kernel source tree for building external modules. | |
1da177e4 | 135 | Note: modules_prepare will not build Module.symvers even if |
2e99f319 RD |
136 | CONFIG_MODULEVERSIONING is set. Therefore a full kernel build |
137 | needs to be executed to make module versioning work. | |
1da177e4 | 138 | |
06300b21 | 139 | --- 2.5 Building separate files for a module |
d9a7ff66 | 140 | It is possible to build single files which are part of a module. |
2e99f319 RD |
141 | This works equally well for the kernel, a module and even for |
142 | external modules. | |
06300b21 SR |
143 | Examples (module foo.ko, consist of bar.o, baz.o): |
144 | make -C $KDIR M=`pwd` bar.lst | |
145 | make -C $KDIR M=`pwd` bar.o | |
146 | make -C $KDIR M=`pwd` foo.ko | |
147 | make -C $KDIR M=`pwd` / | |
2e99f319 | 148 | |
1da177e4 LT |
149 | |
150 | === 3. Example commands | |
151 | ||
152 | This example shows the actual commands to be executed when building | |
153 | an external module for the currently running kernel. | |
d9a7ff66 | 154 | In the example below, the distribution is supposed to use the |
1da177e4 LT |
155 | facility to locate output files for a kernel compile in a different |
156 | directory than the kernel source - but the examples will also work | |
157 | when the source and the output files are mixed in the same directory. | |
158 | ||
159 | # Kernel source | |
160 | /lib/modules/<kernel-version>/source -> /usr/src/linux-<version> | |
161 | ||
162 | # Output from kernel compile | |
163 | /lib/modules/<kernel-version>/build -> /usr/src/linux-<version>-up | |
164 | ||
165 | Change to the directory where the kbuild file is located and execute | |
166 | the following commands to build the module: | |
167 | ||
168 | cd /home/user/src/module | |
169 | make -C /usr/src/`uname -r`/source \ | |
170 | O=/lib/modules/`uname-r`/build \ | |
171 | M=`pwd` | |
172 | ||
d9a7ff66 | 173 | Then, to install the module use the following command: |
1da177e4 LT |
174 | |
175 | make -C /usr/src/`uname -r`/source \ | |
176 | O=/lib/modules/`uname-r`/build \ | |
177 | M=`pwd` \ | |
178 | modules_install | |
179 | ||
2e99f319 | 180 | If you look closely you will see that this is the same command as |
1da177e4 LT |
181 | listed before - with the directories spelled out. |
182 | ||
183 | The above are rather long commands, and the following chapter | |
184 | lists a few tricks to make it all easier. | |
185 | ||
186 | ||
187 | === 4. Creating a kbuild file for an external module | |
188 | ||
189 | kbuild is the build system for the kernel, and external modules | |
190 | must use kbuild to stay compatible with changes in the build system | |
191 | and to pick up the right flags to gcc etc. | |
192 | ||
193 | The kbuild file used as input shall follow the syntax described | |
194 | in Documentation/kbuild/makefiles.txt. This chapter will introduce a few | |
195 | more tricks to be used when dealing with external modules. | |
196 | ||
197 | In the following a Makefile will be created for a module with the | |
198 | following files: | |
199 | 8123_if.c | |
200 | 8123_if.h | |
201 | 8123_pci.c | |
202 | 8123_bin.o_shipped <= Binary blob | |
203 | ||
204 | --- 4.1 Shared Makefile for module and kernel | |
205 | ||
206 | An external module always includes a wrapper Makefile supporting | |
207 | building the module using 'make' with no arguments. | |
208 | The Makefile provided will most likely include additional | |
209 | functionality such as test targets etc. and this part shall | |
210 | be filtered away from kbuild since it may impact kbuild if | |
211 | name clashes occurs. | |
212 | ||
213 | Example 1: | |
214 | --> filename: Makefile | |
215 | ifneq ($(KERNELRELEASE),) | |
216 | # kbuild part of makefile | |
217 | obj-m := 8123.o | |
218 | 8123-y := 8123_if.o 8123_pci.o 8123_bin.o | |
219 | ||
220 | else | |
221 | # Normal Makefile | |
222 | ||
223 | KERNELDIR := /lib/modules/`uname -r`/build | |
224 | all:: | |
98a1e444 | 225 | $(MAKE) -C $(KERNELDIR) M=`pwd` $@ |
1da177e4 LT |
226 | |
227 | # Module specific targets | |
228 | genbin: | |
98a1e444 | 229 | echo "X" > 8123_bin.o_shipped |
1da177e4 LT |
230 | |
231 | endif | |
232 | ||
d9a7ff66 | 233 | In example 1, the check for KERNELRELEASE is used to separate |
1da177e4 LT |
234 | the two parts of the Makefile. kbuild will only see the two |
235 | assignments whereas make will see everything except the two | |
236 | kbuild assignments. | |
237 | ||
238 | In recent versions of the kernel, kbuild will look for a file named | |
239 | Kbuild and as second option look for a file named Makefile. | |
240 | Utilising the Kbuild file makes us split up the Makefile in example 1 | |
241 | into two files as shown in example 2: | |
242 | ||
243 | Example 2: | |
244 | --> filename: Kbuild | |
245 | obj-m := 8123.o | |
246 | 8123-y := 8123_if.o 8123_pci.o 8123_bin.o | |
247 | ||
248 | --> filename: Makefile | |
249 | KERNELDIR := /lib/modules/`uname -r`/build | |
250 | all:: | |
251 | $(MAKE) -C $KERNELDIR M=`pwd` $@ | |
252 | ||
253 | # Module specific targets | |
254 | genbin: | |
255 | echo "X" > 8123_bin_shipped | |
256 | ||
257 | ||
d9a7ff66 | 258 | In example 2, we are down to two fairly simple files and for simple |
1da177e4 LT |
259 | files as used in this example the split is questionable. But some |
260 | external modules use Makefiles of several hundred lines and here it | |
261 | really pays off to separate the kbuild part from the rest. | |
262 | Example 3 shows a backward compatible version. | |
263 | ||
264 | Example 3: | |
265 | --> filename: Kbuild | |
266 | obj-m := 8123.o | |
267 | 8123-y := 8123_if.o 8123_pci.o 8123_bin.o | |
268 | ||
269 | --> filename: Makefile | |
270 | ifneq ($(KERNELRELEASE),) | |
271 | include Kbuild | |
272 | else | |
273 | # Normal Makefile | |
274 | ||
275 | KERNELDIR := /lib/modules/`uname -r`/build | |
276 | all:: | |
277 | $(MAKE) -C $KERNELDIR M=`pwd` $@ | |
278 | ||
279 | # Module specific targets | |
280 | genbin: | |
281 | echo "X" > 8123_bin_shipped | |
282 | ||
283 | endif | |
284 | ||
d9a7ff66 JE |
285 | The trick here is to include the Kbuild file from Makefile, so |
286 | if an older version of kbuild picks up the Makefile, the Kbuild | |
287 | file will be included. | |
1da177e4 LT |
288 | |
289 | --- 4.2 Binary blobs included in a module | |
290 | ||
291 | Some external modules needs to include a .o as a blob. kbuild | |
292 | has support for this, but requires the blob file to be named | |
293 | <filename>_shipped. In our example the blob is named | |
294 | 8123_bin.o_shipped and when the kbuild rules kick in the file | |
295 | 8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file | |
296 | with the _shipped part stripped of the filename. | |
297 | This allows the 8123_bin.o filename to be used in the assignment to | |
298 | the module. | |
299 | ||
300 | Example 4: | |
301 | obj-m := 8123.o | |
302 | 8123-y := 8123_if.o 8123_pci.o 8123_bin.o | |
303 | ||
d9a7ff66 | 304 | In example 4, there is no distinction between the ordinary .c/.h files |
1da177e4 LT |
305 | and the binary file. But kbuild will pick up different rules to create |
306 | the .o file. | |
307 | ||
308 | ||
309 | === 5. Include files | |
310 | ||
d9a7ff66 JE |
311 | Include files are a necessity when a .c file uses something from other .c |
312 | files (not strictly in the sense of C, but if good programming practice is | |
313 | used). Any module that consists of more than one .c file will have a .h file | |
2e99f319 | 314 | for one of the .c files. |
d9a7ff66 JE |
315 | |
316 | - If the .h file only describes a module internal interface, then the .h file | |
1da177e4 LT |
317 | shall be placed in the same directory as the .c files. |
318 | - If the .h files describe an interface used by other parts of the kernel | |
319 | located in different directories, the .h files shall be located in | |
320 | include/linux/ or other include/ directories as appropriate. | |
321 | ||
322 | One exception for this rule is larger subsystems that have their own directory | |
323 | under include/ such as include/scsi. Another exception is arch-specific | |
324 | .h files which are located under include/asm-$(ARCH)/*. | |
325 | ||
326 | External modules have a tendency to locate include files in a separate include/ | |
d9a7ff66 | 327 | directory and therefore need to deal with this in their kbuild file. |
1da177e4 LT |
328 | |
329 | --- 5.1 How to include files from the kernel include dir | |
330 | ||
d9a7ff66 | 331 | When a module needs to include a file from include/linux/, then one |
1da177e4 LT |
332 | just uses: |
333 | ||
334 | #include <linux/modules.h> | |
335 | ||
336 | kbuild will make sure to add options to gcc so the relevant | |
337 | directories are searched. | |
338 | Likewise for .h files placed in the same directory as the .c file. | |
339 | ||
340 | #include "8123_if.h" | |
341 | ||
342 | will do the job. | |
343 | ||
344 | --- 5.2 External modules using an include/ dir | |
345 | ||
346 | External modules often locate their .h files in a separate include/ | |
347 | directory although this is not usual kernel style. When an external | |
348 | module uses an include/ dir then kbuild needs to be told so. | |
349 | The trick here is to use either EXTRA_CFLAGS (take effect for all .c | |
350 | files) or CFLAGS_$F.o (take effect only for a single file). | |
351 | ||
d9a7ff66 | 352 | In our example, if we move 8123_if.h to a subdirectory named include/ |
1da177e4 LT |
353 | the resulting Kbuild file would look like: |
354 | ||
355 | --> filename: Kbuild | |
356 | obj-m := 8123.o | |
357 | ||
358 | EXTRA_CFLAGS := -Iinclude | |
359 | 8123-y := 8123_if.o 8123_pci.o 8123_bin.o | |
360 | ||
98a1e444 BS |
361 | Note that in the assignment there is no space between -I and the path. |
362 | This is a kbuild limitation: there must be no space present. | |
1da177e4 | 363 | |
253dfa6e SR |
364 | --- 5.3 External modules using several directories |
365 | ||
d9a7ff66 JE |
366 | If an external module does not follow the usual kernel style, but |
367 | decides to spread files over several directories, then kbuild can | |
368 | handle this too. | |
253dfa6e SR |
369 | |
370 | Consider the following example: | |
2e99f319 | 371 | |
253dfa6e SR |
372 | | |
373 | +- src/complex_main.c | |
374 | | +- hal/hardwareif.c | |
375 | | +- hal/include/hardwareif.h | |
376 | +- include/complex.h | |
2e99f319 | 377 | |
d9a7ff66 | 378 | To build a single module named complex.ko, we then need the following |
253dfa6e SR |
379 | kbuild file: |
380 | ||
381 | Kbuild: | |
382 | obj-m := complex.o | |
383 | complex-y := src/complex_main.o | |
384 | complex-y += src/hal/hardwareif.o | |
385 | ||
386 | EXTRA_CFLAGS := -I$(src)/include | |
387 | EXTRA_CFLAGS += -I$(src)src/hal/include | |
388 | ||
389 | ||
390 | kbuild knows how to handle .o files located in another directory - | |
d9a7ff66 | 391 | although this is NOT recommended practice. The syntax is to specify |
253dfa6e SR |
392 | the directory relative to the directory where the Kbuild file is |
393 | located. | |
394 | ||
d9a7ff66 JE |
395 | To find the .h files, we have to explicitly tell kbuild where to look |
396 | for the .h files. When kbuild executes, the current directory is always | |
253dfa6e SR |
397 | the root of the kernel tree (argument to -C) and therefore we have to |
398 | tell kbuild how to find the .h files using absolute paths. | |
399 | $(src) will specify the absolute path to the directory where the | |
400 | Kbuild file are located when being build as an external module. | |
401 | Therefore -I$(src)/ is used to point out the directory of the Kbuild | |
402 | file and any additional path are just appended. | |
1da177e4 LT |
403 | |
404 | === 6. Module installation | |
405 | ||
98a1e444 | 406 | Modules which are included in the kernel are installed in the directory: |
1da177e4 LT |
407 | |
408 | /lib/modules/$(KERNELRELEASE)/kernel | |
409 | ||
410 | External modules are installed in the directory: | |
411 | ||
412 | /lib/modules/$(KERNELRELEASE)/extra | |
413 | ||
414 | --- 6.1 INSTALL_MOD_PATH | |
415 | ||
d9a7ff66 | 416 | Above are the default directories, but as always, some level of |
1da177e4 LT |
417 | customization is possible. One can prefix the path using the variable |
418 | INSTALL_MOD_PATH: | |
419 | ||
420 | $ make INSTALL_MOD_PATH=/frodo modules_install | |
421 | => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel | |
422 | ||
423 | INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the | |
d9a7ff66 | 424 | example above, can be specified on the command line when calling make. |
1da177e4 LT |
425 | INSTALL_MOD_PATH has effect both when installing modules included in |
426 | the kernel as well as when installing external modules. | |
427 | ||
428 | --- 6.2 INSTALL_MOD_DIR | |
429 | ||
d9a7ff66 | 430 | When installing external modules they are by default installed to a |
1da177e4 LT |
431 | directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish |
432 | to locate modules for a specific functionality in a separate | |
d9a7ff66 JE |
433 | directory. For this purpose, one can use INSTALL_MOD_DIR to specify an |
434 | alternative name to 'extra'. | |
1da177e4 LT |
435 | |
436 | $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \ | |
437 | M=`pwd` modules_install | |
438 | => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf | |
439 | ||
440 | ||
040fcc81 | 441 | === 7. Module versioning & Module.symvers |
1da177e4 | 442 | |
98a1e444 | 443 | Module versioning is enabled by the CONFIG_MODVERSIONS tag. |
1da177e4 LT |
444 | |
445 | Module versioning is used as a simple ABI consistency check. The Module | |
446 | versioning creates a CRC value of the full prototype for an exported symbol and | |
447 | when a module is loaded/used then the CRC values contained in the kernel are | |
d9a7ff66 | 448 | compared with similar values in the module. If they are not equal, then the |
1da177e4 LT |
449 | kernel refuses to load the module. |
450 | ||
040fcc81 | 451 | Module.symvers contains a list of all exported symbols from a kernel build. |
1da177e4 | 452 | |
040fcc81 SR |
453 | --- 7.1 Symbols fron the kernel (vmlinux + modules) |
454 | ||
d9a7ff66 | 455 | During a kernel build, a file named Module.symvers will be generated. |
040fcc81 | 456 | Module.symvers contains all exported symbols from the kernel and |
d9a7ff66 | 457 | compiled modules. For each symbols, the corresponding CRC value |
040fcc81 SR |
458 | is stored too. |
459 | ||
460 | The syntax of the Module.symvers file is: | |
461 | <CRC> <Symbol> <module> | |
462 | Sample: | |
463 | 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod | |
464 | ||
2e99f319 | 465 | For a kernel build without CONFIG_MODVERSIONS enabled, the crc |
040fcc81 SR |
466 | would read: 0x00000000 |
467 | ||
d9a7ff66 JE |
468 | Module.symvers serves two purposes: |
469 | 1) It lists all exported symbols both from vmlinux and all modules | |
2e99f319 | 470 | 2) It lists the CRC if CONFIG_MODVERSIONS is enabled |
040fcc81 SR |
471 | |
472 | --- 7.2 Symbols and external modules | |
473 | ||
d9a7ff66 | 474 | When building an external module, the build system needs access to |
040fcc81 SR |
475 | the symbols from the kernel to check if all external symbols are |
476 | defined. This is done in the MODPOST step and to obtain all | |
d9a7ff66 | 477 | symbols, modpost reads Module.symvers from the kernel. |
040fcc81 | 478 | If a Module.symvers file is present in the directory where |
d9a7ff66 JE |
479 | the external module is being built, this file will be read too. |
480 | During the MODPOST step, a new Module.symvers file will be written | |
481 | containing all exported symbols that were not defined in the kernel. | |
2e99f319 | 482 | |
040fcc81 SR |
483 | --- 7.3 Symbols from another external module |
484 | ||
d9a7ff66 | 485 | Sometimes, an external module uses exported symbols from another |
040fcc81 SR |
486 | external module. Kbuild needs to have full knowledge on all symbols |
487 | to avoid spitting out warnings about undefined symbols. | |
488 | Two solutions exist to let kbuild know all symbols of more than | |
489 | one external module. | |
490 | The method with a top-level kbuild file is recommended but may be | |
491 | impractical in certain situations. | |
492 | ||
493 | Use a top-level Kbuild file | |
d9a7ff66 JE |
494 | If you have two modules: 'foo' and 'bar', and 'foo' needs |
495 | symbols from 'bar', then one can use a common top-level kbuild | |
496 | file so both modules are compiled in same build. | |
040fcc81 SR |
497 | |
498 | Consider following directory layout: | |
499 | ./foo/ <= contains the foo module | |
500 | ./bar/ <= contains the bar module | |
501 | The top-level Kbuild file would then look like: | |
2e99f319 | 502 | |
040fcc81 SR |
503 | #./Kbuild: (this file may also be named Makefile) |
504 | obj-y := foo/ bar/ | |
505 | ||
506 | Executing: | |
507 | make -C $KDIR M=`pwd` | |
508 | ||
509 | will then do the expected and compile both modules with full | |
510 | knowledge on symbols from both modules. | |
511 | ||
512 | Use an extra Module.symvers file | |
d9a7ff66 | 513 | When an external module is built, a Module.symvers file is |
040fcc81 SR |
514 | generated containing all exported symbols which are not |
515 | defined in the kernel. | |
d9a7ff66 | 516 | To get access to symbols from module 'bar', one can copy the |
040fcc81 | 517 | Module.symvers file from the compilation of the 'bar' module |
d9a7ff66 JE |
518 | to the directory where the 'foo' module is built. |
519 | During the module build, kbuild will read the Module.symvers | |
040fcc81 | 520 | file in the directory of the external module and when the |
d9a7ff66 | 521 | build is finished, a new Module.symvers file is created |
040fcc81 SR |
522 | containing the sum of all symbols defined and not part of the |
523 | kernel. | |
2e99f319 | 524 | |
1da177e4 LT |
525 | === 8. Tips & Tricks |
526 | ||
527 | --- 8.1 Testing for CONFIG_FOO_BAR | |
528 | ||
d9a7ff66 | 529 | Modules often need to check for certain CONFIG_ options to decide if |
1da177e4 LT |
530 | a specific feature shall be included in the module. When kbuild is used |
531 | this is done by referencing the CONFIG_ variable directly. | |
532 | ||
533 | #fs/ext2/Makefile | |
534 | obj-$(CONFIG_EXT2_FS) += ext2.o | |
535 | ||
536 | ext2-y := balloc.o bitmap.o dir.o | |
537 | ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o | |
538 | ||
539 | External modules have traditionally used grep to check for specific | |
540 | CONFIG_ settings directly in .config. This usage is broken. | |
d9a7ff66 JE |
541 | As introduced before, external modules shall use kbuild when building |
542 | and therefore can use the same methods as in-kernel modules when | |
543 | testing for CONFIG_ definitions. | |
1da177e4 | 544 |