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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Introduction
2------------
3
Daniel Walkere95be9a2006-10-04 02:15:21 -07004The configuration database is a collection of configuration options
Linus Torvalds1da177e2005-04-16 15:20:36 -07005organized 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
20Every entry has its own dependencies. These dependencies are used
21to determine the visibility of an entry. Any child entry is only
22visible if its parent entry is also visible.
23
24Menu entries
25------------
26
Randy Dunlap0486bc92007-11-12 16:17:55 -080027Most entries define a config option; all other entries help to organize
Linus Torvalds1da177e2005-04-16 15:20:36 -070028them. A single configuration option is defined like this:
29
30config MODVERSIONS
31 bool "Set version information on all module symbols"
Robert P. J. Daybef1f402006-12-12 20:04:19 +010032 depends on MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -070033 help
34 Usually, modules have to be recompiled whenever you switch to a new
35 kernel. ...
36
37Every line starts with a key word and can be followed by multiple
38arguments. "config" starts a new config entry. The following lines
39define attributes for this config option. Attributes can be the type of
40the config option, input prompt, dependencies, help text and default
41values. A config option can be defined multiple times with the same
42name, but every definition can have only a single input prompt and the
43type must not conflict.
44
45Menu attributes
46---------------
47
48A menu entry can have a number of attributes. Not all of them are
49applicable 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:
Randy Dunlap0486bc92007-11-12 16:17:55 -080053 tristate and string; the other types are based on these two. The type
Linus Torvalds1da177e2005-04-16 15:20:36 -070054 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.
Jan Engelhardt83dcde42006-07-27 22:14:29 +020070 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
Linus Torvalds1da177e2005-04-16 15:20:36 -070072 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.
Jan Engelhardt83dcde42006-07-27 22:14:29 +020077 Optionally, dependencies only for this default value can be added with
Linus Torvalds1da177e2005-04-16 15:20:36 -070078 "if".
79
Randy Dunlap6e66b902007-10-19 10:53:48 -070080- 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>
Linus Torvalds1da177e2005-04-16 15:20:36 -070086 This defines a dependency for this menu entry. If multiple
Jan Engelhardt83dcde42006-07-27 22:14:29 +020087 dependencies are defined, they are connected with '&&'. Dependencies
Linus Torvalds1da177e2005-04-16 15:20:36 -070088 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.
Jarek Poplawskif8a74592007-08-10 13:01:04 -0700106 Note:
107 select is evil.... select will by brute force set a symbol
108 equal to 'y' without visiting the dependencies. So abusing
109 select you are able to select a symbol FOO even if FOO depends
110 on BAR that is not set. In general use select only for
Randy Dunlap0486bc92007-11-12 16:17:55 -0800111 non-visible symbols (no prompts anywhere) and for symbols with
Jarek Poplawskif8a74592007-08-10 13:01:04 -0700112 no dependencies. That will limit the usefulness but on the
113 other hand avoid the illegal configurations all over. kconfig
114 should one day warn about such things.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700115
116- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
117 This allows to limit the range of possible input values for int
118 and hex symbols. The user can only input a value which is larger than
119 or equal to the first symbol and smaller than or equal to the second
120 symbol.
121
122- help text: "help" or "---help---"
123 This defines a help text. The end of the help text is determined by
124 the indentation level, this means it ends at the first line which has
125 a smaller indentation than the first line of the help text.
126 "---help---" and "help" do not differ in behaviour, "---help---" is
Matt LaPlante53cb4722006-10-03 22:55:17 +0200127 used to help visually separate configuration logic from help within
Linus Torvalds1da177e2005-04-16 15:20:36 -0700128 the file as an aid to developers.
129
Roman Zippel93449082008-01-14 04:50:54 +0100130- misc options: "option" <symbol>[=<value>]
131 Various less common options can be defined via this option syntax,
132 which can modify the behaviour of the menu entry and its config
133 symbol. These options are currently possible:
134
135 - "defconfig_list"
136 This declares a list of default entries which can be used when
137 looking for the default configuration (which is used when the main
138 .config doesn't exists yet.)
139
140 - "modules"
141 This declares the symbol to be used as the MODULES symbol, which
142 enables the third modular state for all config symbols.
143
144 - "env"=<value>
145 This imports the environment variable into Kconfig. It behaves like
146 a default, except that the value comes from the environment, this
147 also means that the behaviour when mixing it with normal defaults is
148 undefined at this point. The symbol is currently not exported back
149 to the build environment (if this is desired, it can be done via
150 another symbol).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700151
152Menu dependencies
153-----------------
154
155Dependencies define the visibility of a menu entry and can also reduce
156the input range of tristate symbols. The tristate logic used in the
157expressions uses one more state than normal boolean logic to express the
158module state. Dependency expressions have the following syntax:
159
160<expr> ::= <symbol> (1)
161 <symbol> '=' <symbol> (2)
162 <symbol> '!=' <symbol> (3)
163 '(' <expr> ')' (4)
164 '!' <expr> (5)
165 <expr> '&&' <expr> (6)
166 <expr> '||' <expr> (7)
167
168Expressions are listed in decreasing order of precedence.
169
170(1) Convert the symbol into an expression. Boolean and tristate symbols
171 are simply converted into the respective expression values. All
172 other symbol types result in 'n'.
173(2) If the values of both symbols are equal, it returns 'y',
174 otherwise 'n'.
175(3) If the values of both symbols are equal, it returns 'n',
176 otherwise 'y'.
177(4) Returns the value of the expression. Used to override precedence.
178(5) Returns the result of (2-/expr/).
179(6) Returns the result of min(/expr/, /expr/).
180(7) Returns the result of max(/expr/, /expr/).
181
182An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
183respectively for calculations). A menu entry becomes visible when it's
184expression evaluates to 'm' or 'y'.
185
Randy Dunlap0486bc92007-11-12 16:17:55 -0800186There are two types of symbols: constant and non-constant symbols.
187Non-constant symbols are the most common ones and are defined with the
188'config' statement. Non-constant symbols consist entirely of alphanumeric
Linus Torvalds1da177e2005-04-16 15:20:36 -0700189characters or underscores.
190Constant symbols are only part of expressions. Constant symbols are
Jan Engelhardt83dcde42006-07-27 22:14:29 +0200191always surrounded by single or double quotes. Within the quote, any
Linus Torvalds1da177e2005-04-16 15:20:36 -0700192other character is allowed and the quotes can be escaped using '\'.
193
194Menu structure
195--------------
196
197The position of a menu entry in the tree is determined in two ways. First
198it can be specified explicitly:
199
200menu "Network device support"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100201 depends on NET
Linus Torvalds1da177e2005-04-16 15:20:36 -0700202
203config NETDEVICES
204 ...
205
206endmenu
207
208All entries within the "menu" ... "endmenu" block become a submenu of
209"Network device support". All subentries inherit the dependencies from
210the menu entry, e.g. this means the dependency "NET" is added to the
211dependency list of the config option NETDEVICES.
212
213The other way to generate the menu structure is done by analyzing the
214dependencies. If a menu entry somehow depends on the previous entry, it
215can be made a submenu of it. First, the previous (parent) symbol must
216be part of the dependency list and then one of these two conditions
217must be true:
218- the child entry must become invisible, if the parent is set to 'n'
219- the child entry must only be visible, if the parent is visible
220
221config MODULES
222 bool "Enable loadable module support"
223
224config MODVERSIONS
225 bool "Set version information on all module symbols"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100226 depends on MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -0700227
228comment "module support disabled"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100229 depends on !MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -0700230
231MODVERSIONS directly depends on MODULES, this means it's only visible if
232MODULES is different from 'n'. The comment on the other hand is always
233visible when MODULES is visible (the (empty) dependency of MODULES is
234also part of the comment dependencies).
235
236
237Kconfig syntax
238--------------
239
240The configuration file describes a series of menu entries, where every
241line starts with a keyword (except help texts). The following keywords
242end a menu entry:
243- config
244- menuconfig
245- choice/endchoice
246- comment
247- menu/endmenu
248- if/endif
249- source
250The first five also start the definition of a menu entry.
251
252config:
253
254 "config" <symbol>
255 <config options>
256
257This defines a config symbol <symbol> and accepts any of above
258attributes as options.
259
260menuconfig:
261 "menuconfig" <symbol>
262 <config options>
263
Matt LaPlante53cb4722006-10-03 22:55:17 +0200264This is similar to the simple config entry above, but it also gives a
Linus Torvalds1da177e2005-04-16 15:20:36 -0700265hint to front ends, that all suboptions should be displayed as a
266separate list of options.
267
268choices:
269
270 "choice"
271 <choice options>
272 <choice block>
273 "endchoice"
274
Jan Engelhardt83dcde42006-07-27 22:14:29 +0200275This defines a choice group and accepts any of the above attributes as
Linus Torvalds1da177e2005-04-16 15:20:36 -0700276options. A choice can only be of type bool or tristate, while a boolean
277choice only allows a single config entry to be selected, a tristate
278choice also allows any number of config entries to be set to 'm'. This
279can be used if multiple drivers for a single hardware exists and only a
280single driver can be compiled/loaded into the kernel, but all drivers
281can be compiled as modules.
282A choice accepts another option "optional", which allows to set the
283choice to 'n' and no entry needs to be selected.
284
285comment:
286
287 "comment" <prompt>
288 <comment options>
289
290This defines a comment which is displayed to the user during the
291configuration process and is also echoed to the output files. The only
292possible options are dependencies.
293
294menu:
295
296 "menu" <prompt>
297 <menu options>
298 <menu block>
299 "endmenu"
300
301This defines a menu block, see "Menu structure" above for more
302information. The only possible options are dependencies.
303
304if:
305
306 "if" <expr>
307 <if block>
308 "endif"
309
310This defines an if block. The dependency expression <expr> is appended
311to all enclosed menu entries.
312
313source:
314
315 "source" <prompt>
316
317This reads the specified configuration file. This file is always parsed.
Randy Dunlap6e66b902007-10-19 10:53:48 -0700318
319mainmenu:
320
321 "mainmenu" <prompt>
322
323This sets the config program's title bar if the config program chooses
324to use it.
Randy Dunlap0486bc92007-11-12 16:17:55 -0800325
326
327Kconfig hints
328-------------
329This is a collection of Kconfig tips, most of which aren't obvious at
330first glance and most of which have become idioms in several Kconfig
331files.
332
333Build as module only
334~~~~~~~~~~~~~~~~~~~~
335To restrict a component build to module-only, qualify its config symbol
336with "depends on m". E.g.:
337
338config FOO
339 depends on BAR && m
340
341limits FOO to module (=m) or disabled (=n).
342
343
344Build limited by a third config symbol which may be =y or =m
345~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
346A common idiom that we see (and sometimes have problems with) is this:
347
348When option C in B (module or subsystem) uses interfaces from A (module
349or subsystem), and both A and B are tristate (could be =y or =m if they
350were independent of each other, but they aren't), then we need to limit
351C such that it cannot be built statically if A is built as a loadable
352module. (C already depends on B, so there is no dependency issue to
353take care of here.)
354
355If A is linked statically into the kernel image, C can be built
356statically or as loadable module(s). However, if A is built as loadable
357module(s), then C must be restricted to loadable module(s) also. This
358can be expressed in kconfig language as:
359
360config C
361 depends on A = y || A = B
362
363or for real examples, use this command in a kernel tree:
364
365$ find . -name Kconfig\* | xargs grep -ns "depends on.*=.*||.*=" | grep -v orig
366