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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:
Matthew Wilcoxdfecbec2008-04-19 14:45:11 -0600107 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700115
Michal Marekdf835c22010-11-26 17:15:11 +0100116- 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
Masanari Iida40e47122012-03-04 23:16:11 +0900120 similar to a conditional "prompt" attribute for individual menu
Michal Marekdf835c22010-11-26 17:15:11 +0100121 entries. Default value of "visible" is true.
122
Linus Torvalds1da177e2005-04-16 15:20:36 -0700123- 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
Matt LaPlante53cb4722006-10-03 22:55:17 +0200134 used to help visually separate configuration logic from help within
Linus Torvalds1da177e2005-04-16 15:20:36 -0700135 the file as an aid to developers.
136
Roman Zippel93449082008-01-14 04:50:54 +0100137- 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).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700158
159Menu dependencies
160-----------------
161
162Dependencies define the visibility of a menu entry and can also reduce
163the input range of tristate symbols. The tristate logic used in the
164expressions uses one more state than normal boolean logic to express the
165module 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
175Expressions 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
189An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
Li Zefan4280eae2010-04-14 11:44:05 +0800190respectively for calculations). A menu entry becomes visible when its
Linus Torvalds1da177e2005-04-16 15:20:36 -0700191expression evaluates to 'm' or 'y'.
192
Randy Dunlap0486bc92007-11-12 16:17:55 -0800193There are two types of symbols: constant and non-constant symbols.
194Non-constant symbols are the most common ones and are defined with the
195'config' statement. Non-constant symbols consist entirely of alphanumeric
Linus Torvalds1da177e2005-04-16 15:20:36 -0700196characters or underscores.
197Constant symbols are only part of expressions. Constant symbols are
Jan Engelhardt83dcde42006-07-27 22:14:29 +0200198always surrounded by single or double quotes. Within the quote, any
Linus Torvalds1da177e2005-04-16 15:20:36 -0700199other character is allowed and the quotes can be escaped using '\'.
200
201Menu structure
202--------------
203
204The position of a menu entry in the tree is determined in two ways. First
205it can be specified explicitly:
206
207menu "Network device support"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100208 depends on NET
Linus Torvalds1da177e2005-04-16 15:20:36 -0700209
210config NETDEVICES
211 ...
212
213endmenu
214
215All entries within the "menu" ... "endmenu" block become a submenu of
216"Network device support". All subentries inherit the dependencies from
217the menu entry, e.g. this means the dependency "NET" is added to the
218dependency list of the config option NETDEVICES.
219
220The other way to generate the menu structure is done by analyzing the
221dependencies. If a menu entry somehow depends on the previous entry, it
222can be made a submenu of it. First, the previous (parent) symbol must
223be part of the dependency list and then one of these two conditions
224must 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
228config MODULES
229 bool "Enable loadable module support"
230
231config MODVERSIONS
232 bool "Set version information on all module symbols"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100233 depends on MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -0700234
235comment "module support disabled"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100236 depends on !MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -0700237
238MODVERSIONS directly depends on MODULES, this means it's only visible if
239MODULES is different from 'n'. The comment on the other hand is always
240visible when MODULES is visible (the (empty) dependency of MODULES is
241also part of the comment dependencies).
242
243
244Kconfig syntax
245--------------
246
247The configuration file describes a series of menu entries, where every
248line starts with a keyword (except help texts). The following keywords
249end a menu entry:
250- config
251- menuconfig
252- choice/endchoice
253- comment
254- menu/endmenu
255- if/endif
256- source
257The first five also start the definition of a menu entry.
258
259config:
260
261 "config" <symbol>
262 <config options>
263
264This defines a config symbol <symbol> and accepts any of above
265attributes as options.
266
267menuconfig:
268 "menuconfig" <symbol>
269 <config options>
270
Matt LaPlante53cb4722006-10-03 22:55:17 +0200271This is similar to the simple config entry above, but it also gives a
Linus Torvalds1da177e2005-04-16 15:20:36 -0700272hint to front ends, that all suboptions should be displayed as a
273separate list of options.
274
275choices:
276
Yann E. MORIN0719e1d2010-12-16 00:19:00 +0100277 "choice" [symbol]
Linus Torvalds1da177e2005-04-16 15:20:36 -0700278 <choice options>
279 <choice block>
280 "endchoice"
281
Jan Engelhardt83dcde42006-07-27 22:14:29 +0200282This defines a choice group and accepts any of the above attributes as
Linus Torvalds1da177e2005-04-16 15:20:36 -0700283options. A choice can only be of type bool or tristate, while a boolean
284choice only allows a single config entry to be selected, a tristate
285choice also allows any number of config entries to be set to 'm'. This
286can be used if multiple drivers for a single hardware exists and only a
287single driver can be compiled/loaded into the kernel, but all drivers
288can be compiled as modules.
289A choice accepts another option "optional", which allows to set the
290choice to 'n' and no entry needs to be selected.
Yann E. MORIN0719e1d2010-12-16 00:19:00 +0100291If no [symbol] is associated with a choice, then you can not have multiple
292definitions of that choice. If a [symbol] is associated to the choice,
293then you may define the same choice (ie. with the same entries) in another
294place.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700295
296comment:
297
298 "comment" <prompt>
299 <comment options>
300
301This defines a comment which is displayed to the user during the
302configuration process and is also echoed to the output files. The only
303possible options are dependencies.
304
305menu:
306
307 "menu" <prompt>
308 <menu options>
309 <menu block>
310 "endmenu"
311
312This defines a menu block, see "Menu structure" above for more
Michal Marekdf835c22010-11-26 17:15:11 +0100313information. The only possible options are dependencies and "visible"
314attributes.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700315
316if:
317
318 "if" <expr>
319 <if block>
320 "endif"
321
322This defines an if block. The dependency expression <expr> is appended
323to all enclosed menu entries.
324
325source:
326
327 "source" <prompt>
328
329This reads the specified configuration file. This file is always parsed.
Randy Dunlap6e66b902007-10-19 10:53:48 -0700330
331mainmenu:
332
333 "mainmenu" <prompt>
334
335This sets the config program's title bar if the config program chooses
Arnaud Lacombe8ea13e22010-08-16 22:55:31 -0400336to use it. It should be placed at the top of the configuration, before any
337other statement.
Randy Dunlap0486bc92007-11-12 16:17:55 -0800338
339
340Kconfig hints
341-------------
342This is a collection of Kconfig tips, most of which aren't obvious at
343first glance and most of which have become idioms in several Kconfig
344files.
345
Sam Ravnborg9b3e4da2008-01-28 21:49:46 +0100346Adding common features and make the usage configurable
347~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
348It is a common idiom to implement a feature/functionality that are
349relevant for some architectures but not all.
350The recommended way to do so is to use a config variable named HAVE_*
351that is defined in a common Kconfig file and selected by the relevant
352architectures.
353An example is the generic IOMAP functionality.
354
355We would in lib/Kconfig see:
356
357# Generic IOMAP is used to ...
358config HAVE_GENERIC_IOMAP
359
360config GENERIC_IOMAP
361 depends on HAVE_GENERIC_IOMAP && FOO
362
363And in lib/Makefile we would see:
364obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
365
366For each architecture using the generic IOMAP functionality we would see:
367
368config X86
369 select ...
370 select HAVE_GENERIC_IOMAP
371 select ...
372
373Note: we use the existing config option and avoid creating a new
374config variable to select HAVE_GENERIC_IOMAP.
375
376Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
377introduced to overcome the limitation of select which will force a
378config option to 'y' no matter the dependencies.
379The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
380situation where select forces a symbol equals to 'y'.
381
Randy Dunlap0486bc92007-11-12 16:17:55 -0800382Build as module only
383~~~~~~~~~~~~~~~~~~~~
384To restrict a component build to module-only, qualify its config symbol
385with "depends on m". E.g.:
386
387config FOO
388 depends on BAR && m
389
390limits FOO to module (=m) or disabled (=n).