1 2 In this document you will find information about:
3 - how to build external modules
4 - how to make your module use the kbuild infrastructure
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
16 --- 2.5 Building separate files for a module
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
22 --- 5.3 External modules using several directories
23 === 6. Module installation
24 --- 6.1 INSTALL_MOD_PATH
25 --- 6.2 INSTALL_MOD_DIR
26 === 7. Module versioning & Module.symvers
27 --- 7.1 Symbols from the kernel (vmlinux + modules)
28 --- 7.2 Symbols and external modules
29 --- 7.3 Symbols from another external module
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.
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.
42 43 What is covered within this file is mainly information to authors
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
46 'make' to build the module. A complete example will be presented in
47 chapter 4, "Creating a kbuild file for an external module".
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
M=`pwd`
62
63 For the running kernel use:
64
65 make -C /lib/modules/`uname -r`/build M=`pwd`
66
67 For the above command to succeed, the kernel must have been
68 built with modules enabled.
69
70 To install the modules that were just built:
71
72 make -C M=`pwd` modules_install
73
74 More complex examples will be shown later, the above should
75 be enough to get you started.
76
77 --- 2.2 Available targets
78
79 $KDIR refers to the path to the kernel source top-level directory
80
81 make -C $KDIR M=`pwd`
82 Will build the module(s) located in current directory.
83 All output files will be located in the same directory
84 as the module source.
85 No attempts are made to update the kernel source, and it is
86 a precondition that a successful make has been executed
87 for the kernel.
88
89 make -C $KDIR M=`pwd` modules
90 The modules target is implied when no target is given.
91 Same functionality as if no target was specified.
92 See description above.
93
94 make -C $KDIR M=`pwd` modules_install
95 Install the external module(s).
96 Installation default is in /lib/modules//extra,
97 but may be prefixed with INSTALL_MOD_PATH - see separate
98 chapter.
99
100 make -C $KDIR M=`pwd` clean
101 Remove all generated files for the module - the kernel
102 source directory is not modified.
103
104 make -C $KDIR M=`pwd` help
105 help will list the available target when building external
106 modules.
107
108 --- 2.3 Available options:
109
110 $KDIR refers to the path to the kernel source top-level directory
111
112 make -C $KDIR
113 Used to specify where to find the kernel source.
114 '$KDIR' represent the directory where the kernel source is.
115 Make will actually change directory to the specified directory
116 when executed but change back when finished.
117
118 make -C $KDIR M=`pwd`
119 M= is used to tell kbuild that an external module is
120 being built.
121 The option given to M= is the directory where the external
122 module (kbuild file) is located.
123 When an external module is being built only a subset of the
124 usual targets are available.
125
126 make -C $KDIR SUBDIRS=`pwd`
127 Same as M=. The SUBDIRS= syntax is kept for backwards
128 compatibility.
129
130 --- 2.4 Preparing the kernel tree for module build
131
132 To make sure the kernel contains the information required to
133 build external modules the target 'modules_prepare' must be used.
134 'modules_prepare' exists solely as a simple way to prepare
135 a kernel source tree for building external modules.
136 Note: modules_prepare will not build Module.symvers even if
137 CONFIG_MODVERSIONS is set. Therefore a full kernel build
138 needs to be executed to make module versioning work.
139
140 --- 2.5 Building separate files for a module
141 It is possible to build single files which are part of a module.
142 This works equally well for the kernel, a module and even for
143 external modules.
144 Examples (module foo.ko, consist of bar.o, baz.o):
145 make -C $KDIR M=`pwd` bar.lst
146 make -C $KDIR M=`pwd` bar.o
147 make -C $KDIR M=`pwd` foo.ko
148 make -C $KDIR M=`pwd` /
149
150
151 === 3. Example commands
152
153 This example shows the actual commands to be executed when building
154 an external module for the currently running kernel.
155 In the example below, the distribution is supposed to use the
156 facility to locate output files for a kernel compile in a different
157 directory than the kernel source - but the examples will also work
158 when the source and the output files are mixed in the same directory.
159
160 # Kernel source
161 /lib/modules//source -> /usr/src/linux-
162
163 # Output from kernel compile
164 /lib/modules//build -> /usr/src/linux--up
165
166 Change to the directory where the kbuild file is located and execute
167 the following commands to build the module:
168
169 cd /home/user/src/module
170 make -C /usr/src/`uname -r`/source \
171 O=/lib/modules/`uname-r`/build \
172 M=`pwd`
173
174 Then, to install the module use the following command:
175
176 make -C /usr/src/`uname -r`/source \
177 O=/lib/modules/`uname-r`/build \
178 M=`pwd` \
179 modules_install
180
181 If you look closely you will see that this is the same command as
182 listed before - with the directories spelled out.
183
184 The above are rather long commands, and the following chapter
185 lists a few tricks to make it all easier.
186
187
188 === 4. Creating a kbuild file for an external module
189
190 kbuild is the build system for the kernel, and external modules
191 must use kbuild to stay compatible with changes in the build system
192 and to pick up the right flags to gcc etc.
193
194 The kbuild file used as input shall follow the syntax described
195 in Documentation/kbuild/makefiles.txt. This chapter will introduce a few
196 more tricks to be used when dealing with external modules.
197
198 In the following a Makefile will be created for a module with the
199 following files:
200 8123_if.c
201 8123_if.h
202 8123_pci.c
203 8123_bin.o_shipped <= Binary blob
204
205 --- 4.1 Shared Makefile for module and kernel
206
207 An external module always includes a wrapper Makefile supporting
208 building the module using 'make' with no arguments.
209 The Makefile provided will most likely include additional
210 functionality such as test targets etc. and this part shall
211 be filtered away from kbuild since it may impact kbuild if
212 name clashes occurs.
213
214 Example 1:
215 --> filename: Makefile
216 ifneq ($(KERNELRELEASE),)
217 # kbuild part of makefile
218 obj-m := 8123.o
219 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
220
221 else
222 # Normal Makefile
223
224 KERNELDIR := /lib/modules/`uname -r`/build
225 all::
226 $(MAKE) -C $(KERNELDIR) M=`pwd` $@
227
228 # Module specific targets
229 genbin:
230 echo "X" > 8123_bin.o_shipped
231
232 endif
233
234 In example 1, the check for KERNELRELEASE is used to separate
235 the two parts of the Makefile. kbuild will only see the two
236 assignments whereas make will see everything except the two
237 kbuild assignments.
238
239 In recent versions of the kernel, kbuild will look for a file named
240 Kbuild and as second option look for a file named Makefile.
241 Utilising the Kbuild file makes us split up the Makefile in example 1
242 into two files as shown in example 2:
243
244 Example 2:
245 --> filename: Kbuild
246 obj-m := 8123.o
247 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
248
249 --> filename: Makefile
250 KERNELDIR := /lib/modules/`uname -r`/build
251 all::
252 $(MAKE) -C $(KERNELDIR) M=`pwd` $@
253
254 # Module specific targets
255 genbin:
256 echo "X" > 8123_bin.o_shipped
257
258
259 In example 2, we are down to two fairly simple files and for simple
260 files as used in this example the split is questionable. But some
261 external modules use Makefiles of several hundred lines and here it
262 really pays off to separate the kbuild part from the rest.
263 Example 3 shows a backward compatible version.
264
265 Example 3:
266 --> filename: Kbuild
267 obj-m := 8123.o
268 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
269
270 --> filename: Makefile
271 ifneq ($(KERNELRELEASE),)
272 include Kbuild
273 else
274 # Normal Makefile
275
276 KERNELDIR := /lib/modules/`uname -r`/build
277 all::
278 $(MAKE) -C $(KERNELDIR) M=`pwd` $@
279
280 # Module specific targets
281 genbin:
282 echo "X" > 8123_bin.o_shipped
283
284 endif
285
286 The trick here is to include the Kbuild file from Makefile, so
287 if an older version of kbuild picks up the Makefile, the Kbuild
288 file will be included.
289
290 --- 4.2 Binary blobs included in a module
291
292 Some external modules needs to include a .o as a blob. kbuild
293 has support for this, but requires the blob file to be named
294 _shipped. In our example the blob is named
295 8123_bin.o_shipped and when the kbuild rules kick in the file
296 8123_bin.o is created as a simple copy off the 8213_bin.o_shipped file
297 with the _shipped part stripped of the filename.
298 This allows the 8123_bin.o filename to be used in the assignment to
299 the module.
300
301 Example 4:
302 obj-m := 8123.o
303 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
304
305 In example 4, there is no distinction between the ordinary .c/.h files
306 and the binary file. But kbuild will pick up different rules to create
307 the .o file.
308
309
310 === 5. Include files
311
312 Include files are a necessity when a .c file uses something from other .c
313 files (not strictly in the sense of C, but if good programming practice is
314 used). Any module that consists of more than one .c file will have a .h file
315 for one of the .c files.
316
317 - If the .h file only describes a module internal interface, then the .h file
318 shall be placed in the same directory as the .c files.
319 - If the .h files describe an interface used by other parts of the kernel
320 located in different directories, the .h files shall be located in
321 include/linux/ or other include/ directories as appropriate.
322
323 One exception for this rule is larger subsystems that have their own directory
324 under include/ such as include/scsi. Another exception is arch-specific
325 .h files which are located under include/asm-$(ARCH)/*.
326
327 External modules have a tendency to locate include files in a separate include/
328 directory and therefore need to deal with this in their kbuild file.
329
330 --- 5.1 How to include files from the kernel include dir
331
332 When a module needs to include a file from include/linux/, then one
333 just uses:
334
335 #include
336
337 kbuild will make sure to add options to gcc so the relevant
338 directories are searched.
339 Likewise for .h files placed in the same directory as the .c file.
340
341 #include "8123_if.h"
342
343 will do the job.
344
345 --- 5.2 External modules using an include/ dir
346
347 External modules often locate their .h files in a separate include/
348 directory although this is not usual kernel style. When an external
349 module uses an include/ dir then kbuild needs to be told so.
350 The trick here is to use either EXTRA_CFLAGS (take effect for all .c
351 files) or CFLAGS_$F.o (take effect only for a single file).
352
353 In our example, if we move 8123_if.h to a subdirectory named include/
354 the resulting Kbuild file would look like:
355
356 --> filename: Kbuild
357 obj-m := 8123.o
358
359 EXTRA_CFLAGS := -Iinclude
360 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
361
362 Note that in the assignment there is no space between -I and the path.
363 This is a kbuild limitation: there must be no space present.
364
365 --- 5.3 External modules using several directories
366
367 If an external module does not follow the usual kernel style, but
368 decides to spread files over several directories, then kbuild can
369 handle this too.
370
371 Consider the following example:
372
373 |
374 +- src/complex_main.c
375 | +- hal/hardwareif.c
376 | +- hal/include/hardwareif.h
377 +- include/complex.h
378
379 To build a single module named complex.ko, we then need the following
380 kbuild file:
381
382 Kbuild:
383 obj-m := complex.o
384 complex-y := src/complex_main.o
385 complex-y += src/hal/hardwareif.o
386
387 EXTRA_CFLAGS := -I$(src)/include
388 EXTRA_CFLAGS += -I$(src)src/hal/include
389
390
391 kbuild knows how to handle .o files located in another directory -
392 although this is NOT recommended practice. The syntax is to specify
393 the directory relative to the directory where the Kbuild file is
394 located.
395
396 To find the .h files, we have to explicitly tell kbuild where to look
397 for the .h files. When kbuild executes, the current directory is always
398 the root of the kernel tree (argument to -C) and therefore we have to
399 tell kbuild how to find the .h files using absolute paths.
400 $(src) will specify the absolute path to the directory where the
401 Kbuild file are located when being build as an external module.
402 Therefore -I$(src)/ is used to point out the directory of the Kbuild
403 file and any additional path are just appended.
404
405 === 6. Module installation
406
407 Modules which are included in the kernel are installed in the directory:
408
409 /lib/modules/$(KERNELRELEASE)/kernel
410
411 External modules are installed in the directory:
412
413 /lib/modules/$(KERNELRELEASE)/extra
414
415 --- 6.1 INSTALL_MOD_PATH
416
417 Above are the default directories, but as always, some level of
418 customization is possible. One can prefix the path using the variable
419 INSTALL_MOD_PATH:
420
421 $ make INSTALL_MOD_PATH=/frodo modules_install
422 => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel
423
424 INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the
425 example above, can be specified on the command line when calling make.
426 INSTALL_MOD_PATH has effect both when installing modules included in
427 the kernel as well as when installing external modules.
428
429 --- 6.2 INSTALL_MOD_DIR
430
431 When installing external modules they are by default installed to a
432 directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish
433 to locate modules for a specific functionality in a separate
434 directory. For this purpose, one can use INSTALL_MOD_DIR to specify an
435 alternative name to 'extra'.
436
437 $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \
438 M=`pwd` modules_install
439 => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
440
441
442 === 7. Module versioning & Module.symvers
443
444 Module versioning is enabled by the CONFIG_MODVERSIONS tag.
445
446 Module versioning is used as a simple ABI consistency check. The Module
447 versioning creates a CRC value of the full prototype for an exported symbol and
448 when a module is loaded/used then the CRC values contained in the kernel are
449 compared with similar values in the module. If they are not equal, then the
450 kernel refuses to load the module.
451
452 Module.symvers contains a list of all exported symbols from a kernel build.
453
454 --- 7.1 Symbols from the kernel (vmlinux + modules)
455
456 During a kernel build, a file named Module.symvers will be generated.
457 Module.symvers contains all exported symbols from the kernel and
458 compiled modules. For each symbols, the corresponding CRC value
459 is stored too.
460
461 The syntax of the Module.symvers file is:
462
463 Sample:
464 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod
465
466 For a kernel build without CONFIG_MODVERSIONS enabled, the crc
467 would read: 0x00000000
468
469 Module.symvers serves two purposes:
470 1) It lists all exported symbols both from vmlinux and all modules
471 2) It lists the CRC if CONFIG_MODVERSIONS is enabled
472
473 --- 7.2 Symbols and external modules
474
475 When building an external module, the build system needs access to
476 the symbols from the kernel to check if all external symbols are
477 defined. This is done in the MODPOST step and to obtain all
478 symbols, modpost reads Module.symvers from the kernel.
479 If a Module.symvers file is present in the directory where
480 the external module is being built, this file will be read too.
481 During the MODPOST step, a new Module.symvers file will be written
482 containing all exported symbols that were not defined in the kernel.
483
484 --- 7.3 Symbols from another external module
485
486 Sometimes, an external module uses exported symbols from another
487 external module. Kbuild needs to have full knowledge on all symbols
488 to avoid spitting out warnings about undefined symbols.
489 Three solutions exist to let kbuild know all symbols of more than
490 one external module.
491 The method with a top-level kbuild file is recommended but may be
492 impractical in certain situations.
493
494 Use a top-level Kbuild file
495 If you have two modules: 'foo' and 'bar', and 'foo' needs
496 symbols from 'bar', then one can use a common top-level kbuild
497 file so both modules are compiled in same build.
498
499 Consider following directory layout:
500 ./foo/ <= contains the foo module
501 ./bar/ <= contains the bar module
502 The top-level Kbuild file would then look like:
503
504 #./Kbuild: (this file may also be named Makefile)
505 obj-y := foo/ bar/
506
507 Executing:
508 make -C $KDIR M=`pwd`
509
510 will then do the expected and compile both modules with full
511 knowledge on symbols from both modules.
512
513 Use an extra Module.symvers file
514 When an external module is built, a Module.symvers file is
515 generated containing all exported symbols which are not
516 defined in the kernel.
517 To get access to symbols from module 'bar', one can copy the
518 Module.symvers file from the compilation of the 'bar' module
519 to the directory where the 'foo' module is built.
520 During the module build, kbuild will read the Module.symvers
521 file in the directory of the external module and when the
522 build is finished, a new Module.symvers file is created
523 containing the sum of all symbols defined and not part of the
524 kernel.
525
526 Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile
527 If it is impractical to copy Module.symvers from another
528 module, you can assign a space separated list of files to
529 KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be
530 loaded by modpost during the initialisation of its symbol
531 tables.
532
533 === 8. Tips & Tricks
534
535 --- 8.1 Testing for CONFIG_FOO_BAR
536
537 Modules often need to check for certain CONFIG_ options to decide if
538 a specific feature shall be included in the module. When kbuild is used
539 this is done by referencing the CONFIG_ variable directly.
540
541 #fs/ext2/Makefile
542 obj-$(CONFIG_EXT2_FS) += ext2.o
543
544 ext2-y := balloc.o bitmap.o dir.o
545 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
546
547 External modules have traditionally used grep to check for specific
548 CONFIG_ settings directly in .config. This usage is broken.
549 As introduced before, external modules shall use kbuild when building
550 and therefore can use the same methods as in-kernel modules when
551 testing for CONFIG_ definitions.
