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1 | Introduction |
2 | ------------ | |
3 | ||
e95be9a5 | 4 | The configuration database is a collection of configuration options |
1da177e4 LT |
5 | organized in a tree structure: |
6 | ||
7 | +- Code maturity level options | |
8 | | +- Prompt for development and/or incomplete code/drivers | |
9 | +- General setup | |
10 | | +- Networking support | |
11 | | +- System V IPC | |
12 | | +- BSD Process Accounting | |
13 | | +- Sysctl support | |
14 | +- Loadable module support | |
15 | | +- Enable loadable module support | |
16 | | +- Set version information on all module symbols | |
17 | | +- Kernel module loader | |
18 | +- ... | |
19 | ||
20 | Every entry has its own dependencies. These dependencies are used | |
21 | to determine the visibility of an entry. Any child entry is only | |
22 | visible if its parent entry is also visible. | |
23 | ||
24 | Menu entries | |
25 | ------------ | |
26 | ||
0486bc90 | 27 | Most entries define a config option; all other entries help to organize |
1da177e4 LT |
28 | them. A single configuration option is defined like this: |
29 | ||
30 | config MODVERSIONS | |
31 | bool "Set version information on all module symbols" | |
bef1f402 | 32 | depends on MODULES |
1da177e4 LT |
33 | help |
34 | Usually, modules have to be recompiled whenever you switch to a new | |
35 | kernel. ... | |
36 | ||
37 | Every line starts with a key word and can be followed by multiple | |
38 | arguments. "config" starts a new config entry. The following lines | |
39 | define attributes for this config option. Attributes can be the type of | |
40 | the config option, input prompt, dependencies, help text and default | |
41 | values. A config option can be defined multiple times with the same | |
42 | name, but every definition can have only a single input prompt and the | |
43 | type must not conflict. | |
44 | ||
45 | Menu attributes | |
46 | --------------- | |
47 | ||
48 | A menu entry can have a number of attributes. Not all of them are | |
49 | applicable everywhere (see syntax). | |
50 | ||
51 | - type definition: "bool"/"tristate"/"string"/"hex"/"int" | |
52 | Every config option must have a type. There are only two basic types: | |
0486bc90 | 53 | tristate and string; the other types are based on these two. The type |
1da177e4 LT |
54 | definition optionally accepts an input prompt, so these two examples |
55 | are equivalent: | |
56 | ||
57 | bool "Networking support" | |
58 | and | |
59 | bool | |
60 | prompt "Networking support" | |
61 | ||
62 | - input prompt: "prompt" <prompt> ["if" <expr>] | |
63 | Every menu entry can have at most one prompt, which is used to display | |
64 | to the user. Optionally dependencies only for this prompt can be added | |
65 | with "if". | |
66 | ||
67 | - default value: "default" <expr> ["if" <expr>] | |
68 | A config option can have any number of default values. If multiple | |
69 | default values are visible, only the first defined one is active. | |
83dcde4e JE |
70 | Default values are not limited to the menu entry where they are |
71 | defined. This means the default can be defined somewhere else or be | |
1da177e4 LT |
72 | overridden by an earlier definition. |
73 | The default value is only assigned to the config symbol if no other | |
74 | value was set by the user (via the input prompt above). If an input | |
75 | prompt is visible the default value is presented to the user and can | |
76 | be overridden by him. | |
83dcde4e | 77 | Optionally, dependencies only for this default value can be added with |
1da177e4 LT |
78 | "if". |
79 | ||
6e66b900 RD |
80 | - type definition + default value: |
81 | "def_bool"/"def_tristate" <expr> ["if" <expr>] | |
82 | This is a shorthand notation for a type definition plus a value. | |
83 | Optionally dependencies for this default value can be added with "if". | |
84 | ||
85 | - dependencies: "depends on" <expr> | |
1da177e4 | 86 | This defines a dependency for this menu entry. If multiple |
83dcde4e | 87 | dependencies are defined, they are connected with '&&'. Dependencies |
1da177e4 LT |
88 | are applied to all other options within this menu entry (which also |
89 | accept an "if" expression), so these two examples are equivalent: | |
90 | ||
91 | bool "foo" if BAR | |
92 | default y if BAR | |
93 | and | |
94 | depends on BAR | |
95 | bool "foo" | |
96 | default y | |
97 | ||
98 | - reverse dependencies: "select" <symbol> ["if" <expr>] | |
99 | While normal dependencies reduce the upper limit of a symbol (see | |
100 | below), reverse dependencies can be used to force a lower limit of | |
101 | another symbol. The value of the current menu symbol is used as the | |
102 | minimal value <symbol> can be set to. If <symbol> is selected multiple | |
103 | times, the limit is set to the largest selection. | |
104 | Reverse dependencies can only be used with boolean or tristate | |
105 | symbols. | |
f8a74594 | 106 | Note: |
dfecbec8 MW |
107 | select should be used with care. select will force |
108 | a symbol to a value without visiting the dependencies. | |
109 | By abusing select you are able to select a symbol FOO even | |
110 | if FOO depends on BAR that is not set. | |
111 | In general use select only for non-visible symbols | |
112 | (no prompts anywhere) and for symbols with no dependencies. | |
113 | That will limit the usefulness but on the other hand avoid | |
114 | the illegal configurations all over. | |
1da177e4 | 115 | |
df835c2e MM |
116 | - limiting menu display: "visible if" <expr> |
117 | This attribute is only applicable to menu blocks, if the condition is | |
118 | false, the menu block is not displayed to the user (the symbols | |
119 | contained there can still be selected by other symbols, though). It is | |
40e47125 | 120 | similar to a conditional "prompt" attribute for individual menu |
df835c2e MM |
121 | entries. Default value of "visible" is true. |
122 | ||
1da177e4 LT |
123 | - numerical ranges: "range" <symbol> <symbol> ["if" <expr>] |
124 | This allows to limit the range of possible input values for int | |
125 | and hex symbols. The user can only input a value which is larger than | |
126 | or equal to the first symbol and smaller than or equal to the second | |
127 | symbol. | |
128 | ||
129 | - help text: "help" or "---help---" | |
130 | This defines a help text. The end of the help text is determined by | |
131 | the indentation level, this means it ends at the first line which has | |
132 | a smaller indentation than the first line of the help text. | |
133 | "---help---" and "help" do not differ in behaviour, "---help---" is | |
53cb4726 | 134 | used to help visually separate configuration logic from help within |
1da177e4 LT |
135 | the file as an aid to developers. |
136 | ||
93449082 RZ |
137 | - misc options: "option" <symbol>[=<value>] |
138 | Various less common options can be defined via this option syntax, | |
139 | which can modify the behaviour of the menu entry and its config | |
140 | symbol. These options are currently possible: | |
141 | ||
142 | - "defconfig_list" | |
143 | This declares a list of default entries which can be used when | |
144 | looking for the default configuration (which is used when the main | |
145 | .config doesn't exists yet.) | |
146 | ||
147 | - "modules" | |
148 | This declares the symbol to be used as the MODULES symbol, which | |
149 | enables the third modular state for all config symbols. | |
150 | ||
151 | - "env"=<value> | |
152 | This imports the environment variable into Kconfig. It behaves like | |
153 | a default, except that the value comes from the environment, this | |
154 | also means that the behaviour when mixing it with normal defaults is | |
155 | undefined at this point. The symbol is currently not exported back | |
156 | to the build environment (if this is desired, it can be done via | |
157 | another symbol). | |
1da177e4 LT |
158 | |
159 | Menu dependencies | |
160 | ----------------- | |
161 | ||
162 | Dependencies define the visibility of a menu entry and can also reduce | |
163 | the input range of tristate symbols. The tristate logic used in the | |
164 | expressions uses one more state than normal boolean logic to express the | |
165 | module state. Dependency expressions have the following syntax: | |
166 | ||
167 | <expr> ::= <symbol> (1) | |
168 | <symbol> '=' <symbol> (2) | |
169 | <symbol> '!=' <symbol> (3) | |
170 | '(' <expr> ')' (4) | |
171 | '!' <expr> (5) | |
172 | <expr> '&&' <expr> (6) | |
173 | <expr> '||' <expr> (7) | |
174 | ||
175 | Expressions are listed in decreasing order of precedence. | |
176 | ||
177 | (1) Convert the symbol into an expression. Boolean and tristate symbols | |
178 | are simply converted into the respective expression values. All | |
179 | other symbol types result in 'n'. | |
180 | (2) If the values of both symbols are equal, it returns 'y', | |
181 | otherwise 'n'. | |
182 | (3) If the values of both symbols are equal, it returns 'n', | |
183 | otherwise 'y'. | |
184 | (4) Returns the value of the expression. Used to override precedence. | |
185 | (5) Returns the result of (2-/expr/). | |
186 | (6) Returns the result of min(/expr/, /expr/). | |
187 | (7) Returns the result of max(/expr/, /expr/). | |
188 | ||
189 | An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 | |
4280eae0 | 190 | respectively for calculations). A menu entry becomes visible when its |
1da177e4 LT |
191 | expression evaluates to 'm' or 'y'. |
192 | ||
0486bc90 RD |
193 | There are two types of symbols: constant and non-constant symbols. |
194 | Non-constant symbols are the most common ones and are defined with the | |
195 | 'config' statement. Non-constant symbols consist entirely of alphanumeric | |
1da177e4 LT |
196 | characters or underscores. |
197 | Constant symbols are only part of expressions. Constant symbols are | |
83dcde4e | 198 | always surrounded by single or double quotes. Within the quote, any |
1da177e4 LT |
199 | other character is allowed and the quotes can be escaped using '\'. |
200 | ||
201 | Menu structure | |
202 | -------------- | |
203 | ||
204 | The position of a menu entry in the tree is determined in two ways. First | |
205 | it can be specified explicitly: | |
206 | ||
207 | menu "Network device support" | |
bef1f402 | 208 | depends on NET |
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209 | |
210 | config NETDEVICES | |
211 | ... | |
212 | ||
213 | endmenu | |
214 | ||
215 | All entries within the "menu" ... "endmenu" block become a submenu of | |
216 | "Network device support". All subentries inherit the dependencies from | |
217 | the menu entry, e.g. this means the dependency "NET" is added to the | |
218 | dependency list of the config option NETDEVICES. | |
219 | ||
220 | The other way to generate the menu structure is done by analyzing the | |
221 | dependencies. If a menu entry somehow depends on the previous entry, it | |
222 | can be made a submenu of it. First, the previous (parent) symbol must | |
223 | be part of the dependency list and then one of these two conditions | |
224 | must be true: | |
225 | - the child entry must become invisible, if the parent is set to 'n' | |
226 | - the child entry must only be visible, if the parent is visible | |
227 | ||
228 | config MODULES | |
229 | bool "Enable loadable module support" | |
230 | ||
231 | config MODVERSIONS | |
232 | bool "Set version information on all module symbols" | |
bef1f402 | 233 | depends on MODULES |
1da177e4 LT |
234 | |
235 | comment "module support disabled" | |
bef1f402 | 236 | depends on !MODULES |
1da177e4 LT |
237 | |
238 | MODVERSIONS directly depends on MODULES, this means it's only visible if | |
239 | MODULES is different from 'n'. The comment on the other hand is always | |
240 | visible when MODULES is visible (the (empty) dependency of MODULES is | |
241 | also part of the comment dependencies). | |
242 | ||
243 | ||
244 | Kconfig syntax | |
245 | -------------- | |
246 | ||
247 | The configuration file describes a series of menu entries, where every | |
248 | line starts with a keyword (except help texts). The following keywords | |
249 | end a menu entry: | |
250 | - config | |
251 | - menuconfig | |
252 | - choice/endchoice | |
253 | - comment | |
254 | - menu/endmenu | |
255 | - if/endif | |
256 | - source | |
257 | The first five also start the definition of a menu entry. | |
258 | ||
259 | config: | |
260 | ||
261 | "config" <symbol> | |
262 | <config options> | |
263 | ||
264 | This defines a config symbol <symbol> and accepts any of above | |
265 | attributes as options. | |
266 | ||
267 | menuconfig: | |
268 | "menuconfig" <symbol> | |
269 | <config options> | |
270 | ||
53cb4726 | 271 | This is similar to the simple config entry above, but it also gives a |
1da177e4 LT |
272 | hint to front ends, that all suboptions should be displayed as a |
273 | separate list of options. | |
274 | ||
275 | choices: | |
276 | ||
0719e1d2 | 277 | "choice" [symbol] |
1da177e4 LT |
278 | <choice options> |
279 | <choice block> | |
280 | "endchoice" | |
281 | ||
83dcde4e | 282 | This defines a choice group and accepts any of the above attributes as |
1da177e4 LT |
283 | options. A choice can only be of type bool or tristate, while a boolean |
284 | choice only allows a single config entry to be selected, a tristate | |
285 | choice also allows any number of config entries to be set to 'm'. This | |
286 | can be used if multiple drivers for a single hardware exists and only a | |
287 | single driver can be compiled/loaded into the kernel, but all drivers | |
288 | can be compiled as modules. | |
289 | A choice accepts another option "optional", which allows to set the | |
290 | choice to 'n' and no entry needs to be selected. | |
0719e1d2 YM |
291 | If no [symbol] is associated with a choice, then you can not have multiple |
292 | definitions of that choice. If a [symbol] is associated to the choice, | |
293 | then you may define the same choice (ie. with the same entries) in another | |
294 | place. | |
1da177e4 LT |
295 | |
296 | comment: | |
297 | ||
298 | "comment" <prompt> | |
299 | <comment options> | |
300 | ||
301 | This defines a comment which is displayed to the user during the | |
302 | configuration process and is also echoed to the output files. The only | |
303 | possible options are dependencies. | |
304 | ||
305 | menu: | |
306 | ||
307 | "menu" <prompt> | |
308 | <menu options> | |
309 | <menu block> | |
310 | "endmenu" | |
311 | ||
312 | This defines a menu block, see "Menu structure" above for more | |
df835c2e MM |
313 | information. The only possible options are dependencies and "visible" |
314 | attributes. | |
1da177e4 LT |
315 | |
316 | if: | |
317 | ||
318 | "if" <expr> | |
319 | <if block> | |
320 | "endif" | |
321 | ||
322 | This defines an if block. The dependency expression <expr> is appended | |
323 | to all enclosed menu entries. | |
324 | ||
325 | source: | |
326 | ||
327 | "source" <prompt> | |
328 | ||
329 | This reads the specified configuration file. This file is always parsed. | |
6e66b900 RD |
330 | |
331 | mainmenu: | |
332 | ||
333 | "mainmenu" <prompt> | |
334 | ||
335 | This sets the config program's title bar if the config program chooses | |
8ea13e2c AL |
336 | to use it. It should be placed at the top of the configuration, before any |
337 | other statement. | |
0486bc90 RD |
338 | |
339 | ||
340 | Kconfig hints | |
341 | ------------- | |
342 | This is a collection of Kconfig tips, most of which aren't obvious at | |
343 | first glance and most of which have become idioms in several Kconfig | |
344 | files. | |
345 | ||
9b3e4dad SR |
346 | Adding common features and make the usage configurable |
347 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
348 | It is a common idiom to implement a feature/functionality that are | |
349 | relevant for some architectures but not all. | |
350 | The recommended way to do so is to use a config variable named HAVE_* | |
351 | that is defined in a common Kconfig file and selected by the relevant | |
352 | architectures. | |
353 | An example is the generic IOMAP functionality. | |
354 | ||
355 | We would in lib/Kconfig see: | |
356 | ||
357 | # Generic IOMAP is used to ... | |
358 | config HAVE_GENERIC_IOMAP | |
359 | ||
360 | config GENERIC_IOMAP | |
361 | depends on HAVE_GENERIC_IOMAP && FOO | |
362 | ||
363 | And in lib/Makefile we would see: | |
364 | obj-$(CONFIG_GENERIC_IOMAP) += iomap.o | |
365 | ||
366 | For each architecture using the generic IOMAP functionality we would see: | |
367 | ||
368 | config X86 | |
369 | select ... | |
370 | select HAVE_GENERIC_IOMAP | |
371 | select ... | |
372 | ||
373 | Note: we use the existing config option and avoid creating a new | |
374 | config variable to select HAVE_GENERIC_IOMAP. | |
375 | ||
376 | Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is | |
377 | introduced to overcome the limitation of select which will force a | |
378 | config option to 'y' no matter the dependencies. | |
379 | The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the | |
380 | situation where select forces a symbol equals to 'y'. | |
381 | ||
0486bc90 RD |
382 | Build as module only |
383 | ~~~~~~~~~~~~~~~~~~~~ | |
384 | To restrict a component build to module-only, qualify its config symbol | |
385 | with "depends on m". E.g.: | |
386 | ||
387 | config FOO | |
388 | depends on BAR && m | |
389 | ||
390 | limits FOO to module (=m) or disabled (=n). | |
391 | ||
64b81ed7 RD |
392 | Kconfig symbol existence |
393 | ~~~~~~~~~~~~~~~~~~~~~~~~ | |
394 | The following two methods produce the same kconfig symbol dependencies | |
395 | but differ greatly in kconfig symbol existence (production) in the | |
396 | generated config file. | |
397 | ||
398 | case 1: | |
399 | ||
400 | config FOO | |
401 | tristate "about foo" | |
402 | depends on BAR | |
403 | ||
404 | vs. case 2: | |
405 | ||
406 | if BAR | |
407 | config FOO | |
408 | tristate "about foo" | |
409 | endif | |
410 | ||
411 | In case 1, the symbol FOO will always exist in the config file (given | |
412 | no other dependencies). In case 2, the symbol FOO will only exist in | |
413 | the config file if BAR is enabled. |