Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1 | |
| 2 | In this document you will find information about: |
| 3 | - how to build external modules |
| 4 | - how to make your module use 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 |
Sam Ravnborg | 06300b2 | 2006-01-25 07:13:18 +0100 | [diff] [blame] | 16 | --- 2.5 Building separate files for a module |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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 |
Sam Ravnborg | 253dfa6 | 2006-01-06 20:33:41 +0100 | [diff] [blame] | 22 | --- 5.3 External modules using several directories |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 23 | === 6. Module installation |
| 24 | --- 6.1 INSTALL_MOD_PATH |
| 25 | --- 6.2 INSTALL_MOD_DIR |
Sam Ravnborg | 040fcc8 | 2006-01-28 22:15:55 +0100 | [diff] [blame] | 26 | === 7. Module versioning & Module.symvers |
| 27 | --- 7.1 Symbols fron the kernel (vmlinux + modules) |
| 28 | --- 7.2 Symbols and external modules |
| 29 | --- 7.3 Symbols from another external module |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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 modules and is used |
| 40 | both during development and for modules that are not planned to be |
| 41 | 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 modules should supply |
| 45 | a makefile that hides most of the complexity so one only has to type |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 46 | 'make' to build the module. A complete example will be present in |
Brian Gerst | a7d7cb3 | 2006-03-25 14:48:37 -0500 | [diff] [blame^] | 47 | chapter 4, "Creating a kbuild file for an external module". |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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 | |
| 66 | For the above command to succeed the kernel must have been built with |
| 67 | modules enabled. |
| 68 | |
| 69 | To install the modules that were just built: |
| 70 | |
| 71 | make -C <path-to-kernel> M=`pwd` modules_install |
| 72 | |
| 73 | More complex examples later, the above should get you going. |
| 74 | |
| 75 | --- 2.2 Available targets |
| 76 | |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 77 | $KDIR refers to the path to the kernel source top-level directory |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 78 | |
| 79 | make -C $KDIR M=`pwd` |
| 80 | Will build the module(s) located in current directory. |
| 81 | All output files will be located in the same directory |
| 82 | as the module source. |
| 83 | No attempts are made to update the kernel source, and it is |
| 84 | a precondition that a successful make has been executed |
| 85 | for the kernel. |
| 86 | |
| 87 | make -C $KDIR M=`pwd` modules |
| 88 | The modules target is implied when no target is given. |
| 89 | Same functionality as if no target was specified. |
| 90 | See description above. |
| 91 | |
| 92 | make -C $KDIR M=$PWD modules_install |
| 93 | Install the external module(s). |
| 94 | Installation default is in /lib/modules/<kernel-version>/extra, |
Sam Ravnborg | 040fcc8 | 2006-01-28 22:15:55 +0100 | [diff] [blame] | 95 | but may be prefixed with INSTALL_MOD_PATH - see separate |
| 96 | chapter. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 97 | |
| 98 | make -C $KDIR M=$PWD clean |
| 99 | Remove all generated files for the module - the kernel |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 100 | source directory is not modified. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 101 | |
| 102 | make -C $KDIR M=`pwd` help |
| 103 | help will list the available target when building external |
| 104 | modules. |
| 105 | |
| 106 | --- 2.3 Available options: |
| 107 | |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 108 | $KDIR refers to the path to the kernel source top-level directory |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 109 | |
| 110 | make -C $KDIR |
| 111 | Used to specify where to find the kernel source. |
| 112 | '$KDIR' represent the directory where the kernel source is. |
| 113 | Make will actually change directory to the specified directory |
| 114 | when executed but change back when finished. |
| 115 | |
| 116 | make -C $KDIR M=`pwd` |
| 117 | M= is used to tell kbuild that an external module is |
| 118 | being built. |
| 119 | The option given to M= is the directory where the external |
| 120 | module (kbuild file) is located. |
| 121 | When an external module is being built only a subset of the |
| 122 | usual targets are available. |
| 123 | |
| 124 | make -C $KDIR SUBDIRS=`pwd` |
| 125 | Same as M=. The SUBDIRS= syntax is kept for backwards |
| 126 | compatibility. |
| 127 | |
| 128 | --- 2.4 Preparing the kernel tree for module build |
| 129 | |
| 130 | To make sure the kernel contains the information required to |
| 131 | build external modules the target 'modules_prepare' must be used. |
| 132 | 'module_prepare' solely exists as a simple way to prepare |
| 133 | a kernel for building external modules. |
| 134 | Note: modules_prepare will not build Module.symvers even if |
| 135 | CONFIG_MODULEVERSIONING is set. |
| 136 | Therefore a full kernel build needs to be executed to make |
| 137 | module versioning work. |
| 138 | |
Sam Ravnborg | 06300b2 | 2006-01-25 07:13:18 +0100 | [diff] [blame] | 139 | --- 2.5 Building separate files for a module |
| 140 | It is possible to build single files which is part of a module. |
| 141 | This works equal for the kernel, a module and even for external |
| 142 | modules. |
| 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` / |
| 148 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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. |
| 154 | In the example below the distribution is supposed to use the |
| 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 | |
| 173 | Then to install the module use the following command: |
| 174 | |
| 175 | make -C /usr/src/`uname -r`/source \ |
| 176 | O=/lib/modules/`uname-r`/build \ |
| 177 | M=`pwd` \ |
| 178 | modules_install |
| 179 | |
| 180 | If one looks closely you will see that this is the same commands as |
| 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:: |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 225 | $(MAKE) -C $(KERNELDIR) M=`pwd` $@ |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 226 | |
| 227 | # Module specific targets |
| 228 | genbin: |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 229 | echo "X" > 8123_bin.o_shipped |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 230 | |
| 231 | endif |
| 232 | |
| 233 | In example 1 the check for KERNELRELEASE is used to separate |
| 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` $@ |
| 252 | |
| 253 | # Module specific targets |
| 254 | genbin: |
| 255 | echo "X" > 8123_bin_shipped |
| 256 | |
| 257 | |
| 258 | In example 2 we are down to two fairly simple files and for simple |
| 259 | files as used in this example the split is questionable. But some |
| 260 | external modules use Makefiles of several hundred lines and here it |
| 261 | really pays off to separate the kbuild part from the rest. |
| 262 | Example 3 shows a backward compatible version. |
| 263 | |
| 264 | Example 3: |
| 265 | --> filename: Kbuild |
| 266 | obj-m := 8123.o |
| 267 | 8123-y := 8123_if.o 8123_pci.o 8123_bin.o |
| 268 | |
| 269 | --> filename: Makefile |
| 270 | ifneq ($(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 | |
| 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. |
| 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 | |
| 304 | In example 4 there is no distinction between the ordinary .c/.h files |
| 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 | |
| 311 | Include files are a necessity when a .c file uses something from another .c |
| 312 | files (not strictly in the sense of .c but if good programming practice is |
| 313 | used). Any module that consist of more than one .c file will have a .h file |
| 314 | for one of the .c files. |
| 315 | - If the .h file only describes a module internal interface then the .h file |
| 316 | shall be placed in the same directory as the .c files. |
| 317 | - If the .h files describe an interface used by other parts of the kernel |
| 318 | located in different directories, the .h files shall be located in |
| 319 | include/linux/ or other include/ directories as appropriate. |
| 320 | |
| 321 | One exception for this rule is larger subsystems that have their own directory |
| 322 | under include/ such as include/scsi. Another exception is arch-specific |
| 323 | .h files which are located under include/asm-$(ARCH)/*. |
| 324 | |
| 325 | External modules have a tendency to locate include files in a separate include/ |
| 326 | directory and therefore needs to deal with this in their kbuild file. |
| 327 | |
| 328 | --- 5.1 How to include files from the kernel include dir |
| 329 | |
| 330 | When a module needs to include a file from include/linux/ then one |
| 331 | just uses: |
| 332 | |
| 333 | #include <linux/modules.h> |
| 334 | |
| 335 | kbuild will make sure to add options to gcc so the relevant |
| 336 | directories are searched. |
| 337 | Likewise for .h files placed in the same directory as the .c file. |
| 338 | |
| 339 | #include "8123_if.h" |
| 340 | |
| 341 | will do the job. |
| 342 | |
| 343 | --- 5.2 External modules using an include/ dir |
| 344 | |
| 345 | External modules often locate their .h files in a separate include/ |
| 346 | directory although this is not usual kernel style. When an external |
| 347 | module uses an include/ dir then kbuild needs to be told so. |
| 348 | The trick here is to use either EXTRA_CFLAGS (take effect for all .c |
| 349 | files) or CFLAGS_$F.o (take effect only for a single file). |
| 350 | |
| 351 | In our example if we move 8123_if.h to a subdirectory named include/ |
| 352 | the resulting Kbuild file would look like: |
| 353 | |
| 354 | --> filename: Kbuild |
| 355 | obj-m := 8123.o |
| 356 | |
| 357 | EXTRA_CFLAGS := -Iinclude |
| 358 | 8123-y := 8123_if.o 8123_pci.o 8123_bin.o |
| 359 | |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 360 | Note that in the assignment there is no space between -I and the path. |
| 361 | This is a kbuild limitation: there must be no space present. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 362 | |
Sam Ravnborg | 253dfa6 | 2006-01-06 20:33:41 +0100 | [diff] [blame] | 363 | --- 5.3 External modules using several directories |
| 364 | |
| 365 | If an external module does not follow the usual kernel style but |
| 366 | decide to spread files over several directories then kbuild can |
| 367 | support this too. |
| 368 | |
| 369 | Consider the following example: |
| 370 | |
| 371 | | |
| 372 | +- src/complex_main.c |
| 373 | | +- hal/hardwareif.c |
| 374 | | +- hal/include/hardwareif.h |
| 375 | +- include/complex.h |
| 376 | |
| 377 | To build a single module named complex.ko we then need the following |
| 378 | kbuild file: |
| 379 | |
| 380 | Kbuild: |
| 381 | obj-m := complex.o |
| 382 | complex-y := src/complex_main.o |
| 383 | complex-y += src/hal/hardwareif.o |
| 384 | |
| 385 | EXTRA_CFLAGS := -I$(src)/include |
| 386 | EXTRA_CFLAGS += -I$(src)src/hal/include |
| 387 | |
| 388 | |
| 389 | kbuild knows how to handle .o files located in another directory - |
| 390 | although this is NOT reccommended practice. The syntax is to specify |
| 391 | the directory relative to the directory where the Kbuild file is |
| 392 | located. |
| 393 | |
| 394 | To find the .h files we have to explicitly tell kbuild where to look |
| 395 | for the .h files. When kbuild executes current directory is always |
| 396 | the root of the kernel tree (argument to -C) and therefore we have to |
| 397 | tell kbuild how to find the .h files using absolute paths. |
| 398 | $(src) will specify the absolute path to the directory where the |
| 399 | Kbuild file are located when being build as an external module. |
| 400 | Therefore -I$(src)/ is used to point out the directory of the Kbuild |
| 401 | file and any additional path are just appended. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 402 | |
| 403 | === 6. Module installation |
| 404 | |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 405 | Modules which are included in the kernel are installed in the directory: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 406 | |
| 407 | /lib/modules/$(KERNELRELEASE)/kernel |
| 408 | |
| 409 | External modules are installed in the directory: |
| 410 | |
| 411 | /lib/modules/$(KERNELRELEASE)/extra |
| 412 | |
| 413 | --- 6.1 INSTALL_MOD_PATH |
| 414 | |
| 415 | Above are the default directories, but as always some level of |
| 416 | customization is possible. One can prefix the path using the variable |
| 417 | INSTALL_MOD_PATH: |
| 418 | |
| 419 | $ make INSTALL_MOD_PATH=/frodo modules_install |
| 420 | => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel |
| 421 | |
| 422 | INSTALL_MOD_PATH may be set as an ordinary shell variable or as in the |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 423 | example above be specified on the command line when calling make. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 424 | INSTALL_MOD_PATH has effect both when installing modules included in |
| 425 | the kernel as well as when installing external modules. |
| 426 | |
| 427 | --- 6.2 INSTALL_MOD_DIR |
| 428 | |
| 429 | When installing external modules they are default installed in a |
| 430 | directory under /lib/modules/$(KERNELRELEASE)/extra, but one may wish |
| 431 | to locate modules for a specific functionality in a separate |
| 432 | directory. For this purpose one can use INSTALL_MOD_DIR to specify an |
| 433 | alternative name than 'extra'. |
| 434 | |
| 435 | $ make INSTALL_MOD_DIR=gandalf -C KERNELDIR \ |
| 436 | M=`pwd` modules_install |
| 437 | => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf |
| 438 | |
| 439 | |
Sam Ravnborg | 040fcc8 | 2006-01-28 22:15:55 +0100 | [diff] [blame] | 440 | === 7. Module versioning & Module.symvers |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 441 | |
Brian Strand | 98a1e44 | 2005-11-22 01:23:08 +0000 | [diff] [blame] | 442 | Module versioning is enabled by the CONFIG_MODVERSIONS tag. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 443 | |
| 444 | Module versioning is used as a simple ABI consistency check. The Module |
| 445 | versioning creates a CRC value of the full prototype for an exported symbol and |
| 446 | when a module is loaded/used then the CRC values contained in the kernel are |
| 447 | compared with similar values in the module. If they are not equal then the |
| 448 | kernel refuses to load the module. |
| 449 | |
Sam Ravnborg | 040fcc8 | 2006-01-28 22:15:55 +0100 | [diff] [blame] | 450 | Module.symvers contains a list of all exported symbols from a kernel build. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 451 | |
Sam Ravnborg | 040fcc8 | 2006-01-28 22:15:55 +0100 | [diff] [blame] | 452 | --- 7.1 Symbols fron the kernel (vmlinux + modules) |
| 453 | |
| 454 | During a kernel build a file named Module.symvers will be generated. |
| 455 | Module.symvers contains all exported symbols from the kernel and |
| 456 | compiled modules. For each symbols the corresponding CRC value |
| 457 | is stored too. |
| 458 | |
| 459 | The syntax of the Module.symvers file is: |
| 460 | <CRC> <Symbol> <module> |
| 461 | Sample: |
| 462 | 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod |
| 463 | |
| 464 | For a kernel build without CONFIG_MODVERSIONING enabled the crc |
| 465 | would read: 0x00000000 |
| 466 | |
| 467 | Module.symvers serve two purposes. |
| 468 | 1) It list all exported symbols both from vmlinux and all modules |
| 469 | 2) It list CRC if CONFIG_MODVERSION is enabled |
| 470 | |
| 471 | --- 7.2 Symbols and external modules |
| 472 | |
| 473 | When building an external module the build system needs access to |
| 474 | the symbols from the kernel to check if all external symbols are |
| 475 | defined. This is done in the MODPOST step and to obtain all |
| 476 | symbols modpost reads Module.symvers from the kernel. |
| 477 | If a Module.symvers file is present in the directory where |
| 478 | the external module is being build this file will be read too. |
| 479 | During the MODPOST step a new Module.symvers file will be written |
| 480 | containing all exported symbols that was not defined in the kernel. |
| 481 | |
| 482 | --- 7.3 Symbols from another external module |
| 483 | |
| 484 | Sometimes one external module uses exported symbols from another |
| 485 | external module. Kbuild needs to have full knowledge on all symbols |
| 486 | to avoid spitting out warnings about undefined symbols. |
| 487 | Two solutions exist to let kbuild know all symbols of more than |
| 488 | one external module. |
| 489 | The method with a top-level kbuild file is recommended but may be |
| 490 | impractical in certain situations. |
| 491 | |
| 492 | Use a top-level Kbuild file |
| 493 | If you have two modules: 'foo', 'bar' and 'foo' needs symbols |
| 494 | from 'bar' then one can use a common top-level kbuild file so |
| 495 | both modules are compiled in same build. |
| 496 | |
| 497 | Consider following directory layout: |
| 498 | ./foo/ <= contains the foo module |
| 499 | ./bar/ <= contains the bar module |
| 500 | The top-level Kbuild file would then look like: |
| 501 | |
| 502 | #./Kbuild: (this file may also be named Makefile) |
| 503 | obj-y := foo/ bar/ |
| 504 | |
| 505 | Executing: |
| 506 | make -C $KDIR M=`pwd` |
| 507 | |
| 508 | will then do the expected and compile both modules with full |
| 509 | knowledge on symbols from both modules. |
| 510 | |
| 511 | Use an extra Module.symvers file |
| 512 | When an external module is build a Module.symvers file is |
| 513 | generated containing all exported symbols which are not |
| 514 | defined in the kernel. |
| 515 | To get access to symbols from module 'bar' one can copy the |
| 516 | Module.symvers file from the compilation of the 'bar' module |
| 517 | to the directory where the 'foo' module is build. |
| 518 | During the module build kbuild will read the Module.symvers |
| 519 | file in the directory of the external module and when the |
| 520 | build is finished a new Module.symvers file is created |
| 521 | containing the sum of all symbols defined and not part of the |
| 522 | kernel. |
| 523 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 524 | === 8. Tips & Tricks |
| 525 | |
| 526 | --- 8.1 Testing for CONFIG_FOO_BAR |
| 527 | |
| 528 | Modules often needs to check for certain CONFIG_ options to decide if |
| 529 | a specific feature shall be included in the module. When kbuild is used |
| 530 | this is done by referencing the CONFIG_ variable directly. |
| 531 | |
| 532 | #fs/ext2/Makefile |
| 533 | obj-$(CONFIG_EXT2_FS) += ext2.o |
| 534 | |
| 535 | ext2-y := balloc.o bitmap.o dir.o |
| 536 | ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o |
| 537 | |
| 538 | External modules have traditionally used grep to check for specific |
| 539 | CONFIG_ settings directly in .config. This usage is broken. |
| 540 | As introduced before external modules shall use kbuild when building |
| 541 | and therefore can use the same methods as in-kernel modules when testing |
| 542 | for CONFIG_ definitions. |
| 543 | |