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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.
Yann E. MORINe0627812013-09-03 22:22:26 +0200150 At most one symbol may have the "modules" option set.
Roman Zippel93449082008-01-14 04:50:54 +0100151
152 - "env"=<value>
153 This imports the environment variable into Kconfig. It behaves like
154 a default, except that the value comes from the environment, this
155 also means that the behaviour when mixing it with normal defaults is
156 undefined at this point. The symbol is currently not exported back
157 to the build environment (if this is desired, it can be done via
158 another symbol).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700159
Josh Triplett5d2acfc2014-04-07 15:39:09 -0700160 - "allnoconfig_y"
161 This declares the symbol as one that should have the value y when
162 using "allnoconfig". Used for symbols that hide other symbols.
163
Linus Torvalds1da177e2005-04-16 15:20:36 -0700164Menu dependencies
165-----------------
166
167Dependencies define the visibility of a menu entry and can also reduce
168the input range of tristate symbols. The tristate logic used in the
169expressions uses one more state than normal boolean logic to express the
170module state. Dependency expressions have the following syntax:
171
172<expr> ::= <symbol> (1)
173 <symbol> '=' <symbol> (2)
174 <symbol> '!=' <symbol> (3)
175 '(' <expr> ')' (4)
176 '!' <expr> (5)
177 <expr> '&&' <expr> (6)
178 <expr> '||' <expr> (7)
179
180Expressions are listed in decreasing order of precedence.
181
182(1) Convert the symbol into an expression. Boolean and tristate symbols
183 are simply converted into the respective expression values. All
184 other symbol types result in 'n'.
185(2) If the values of both symbols are equal, it returns 'y',
186 otherwise 'n'.
187(3) If the values of both symbols are equal, it returns 'n',
188 otherwise 'y'.
189(4) Returns the value of the expression. Used to override precedence.
190(5) Returns the result of (2-/expr/).
191(6) Returns the result of min(/expr/, /expr/).
192(7) Returns the result of max(/expr/, /expr/).
193
194An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
Li Zefan4280eae2010-04-14 11:44:05 +0800195respectively for calculations). A menu entry becomes visible when its
Linus Torvalds1da177e2005-04-16 15:20:36 -0700196expression evaluates to 'm' or 'y'.
197
Randy Dunlap0486bc92007-11-12 16:17:55 -0800198There are two types of symbols: constant and non-constant symbols.
199Non-constant symbols are the most common ones and are defined with the
200'config' statement. Non-constant symbols consist entirely of alphanumeric
Linus Torvalds1da177e2005-04-16 15:20:36 -0700201characters or underscores.
202Constant symbols are only part of expressions. Constant symbols are
Jan Engelhardt83dcde42006-07-27 22:14:29 +0200203always surrounded by single or double quotes. Within the quote, any
Linus Torvalds1da177e2005-04-16 15:20:36 -0700204other character is allowed and the quotes can be escaped using '\'.
205
206Menu structure
207--------------
208
209The position of a menu entry in the tree is determined in two ways. First
210it can be specified explicitly:
211
212menu "Network device support"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100213 depends on NET
Linus Torvalds1da177e2005-04-16 15:20:36 -0700214
215config NETDEVICES
216 ...
217
218endmenu
219
220All entries within the "menu" ... "endmenu" block become a submenu of
221"Network device support". All subentries inherit the dependencies from
222the menu entry, e.g. this means the dependency "NET" is added to the
223dependency list of the config option NETDEVICES.
224
225The other way to generate the menu structure is done by analyzing the
226dependencies. If a menu entry somehow depends on the previous entry, it
227can be made a submenu of it. First, the previous (parent) symbol must
228be part of the dependency list and then one of these two conditions
229must be true:
230- the child entry must become invisible, if the parent is set to 'n'
231- the child entry must only be visible, if the parent is visible
232
233config MODULES
234 bool "Enable loadable module support"
235
236config MODVERSIONS
237 bool "Set version information on all module symbols"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100238 depends on MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -0700239
240comment "module support disabled"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100241 depends on !MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -0700242
243MODVERSIONS directly depends on MODULES, this means it's only visible if
244MODULES is different from 'n'. The comment on the other hand is always
245visible when MODULES is visible (the (empty) dependency of MODULES is
246also part of the comment dependencies).
247
248
249Kconfig syntax
250--------------
251
252The configuration file describes a series of menu entries, where every
253line starts with a keyword (except help texts). The following keywords
254end a menu entry:
255- config
256- menuconfig
257- choice/endchoice
258- comment
259- menu/endmenu
260- if/endif
261- source
262The first five also start the definition of a menu entry.
263
264config:
265
266 "config" <symbol>
267 <config options>
268
269This defines a config symbol <symbol> and accepts any of above
270attributes as options.
271
272menuconfig:
273 "menuconfig" <symbol>
274 <config options>
275
Matt LaPlante53cb4722006-10-03 22:55:17 +0200276This is similar to the simple config entry above, but it also gives a
Linus Torvalds1da177e2005-04-16 15:20:36 -0700277hint to front ends, that all suboptions should be displayed as a
278separate list of options.
279
280choices:
281
Yann E. MORIN0719e1d2010-12-16 00:19:00 +0100282 "choice" [symbol]
Linus Torvalds1da177e2005-04-16 15:20:36 -0700283 <choice options>
284 <choice block>
285 "endchoice"
286
Jan Engelhardt83dcde42006-07-27 22:14:29 +0200287This defines a choice group and accepts any of the above attributes as
Linus Torvalds1da177e2005-04-16 15:20:36 -0700288options. A choice can only be of type bool or tristate, while a boolean
289choice only allows a single config entry to be selected, a tristate
290choice also allows any number of config entries to be set to 'm'. This
291can be used if multiple drivers for a single hardware exists and only a
292single driver can be compiled/loaded into the kernel, but all drivers
293can be compiled as modules.
294A choice accepts another option "optional", which allows to set the
295choice to 'n' and no entry needs to be selected.
Yann E. MORIN0719e1d2010-12-16 00:19:00 +0100296If no [symbol] is associated with a choice, then you can not have multiple
297definitions of that choice. If a [symbol] is associated to the choice,
298then you may define the same choice (ie. with the same entries) in another
299place.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700300
301comment:
302
303 "comment" <prompt>
304 <comment options>
305
306This defines a comment which is displayed to the user during the
307configuration process and is also echoed to the output files. The only
308possible options are dependencies.
309
310menu:
311
312 "menu" <prompt>
313 <menu options>
314 <menu block>
315 "endmenu"
316
317This defines a menu block, see "Menu structure" above for more
Michal Marekdf835c22010-11-26 17:15:11 +0100318information. The only possible options are dependencies and "visible"
319attributes.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700320
321if:
322
323 "if" <expr>
324 <if block>
325 "endif"
326
327This defines an if block. The dependency expression <expr> is appended
328to all enclosed menu entries.
329
330source:
331
332 "source" <prompt>
333
334This reads the specified configuration file. This file is always parsed.
Randy Dunlap6e66b902007-10-19 10:53:48 -0700335
336mainmenu:
337
338 "mainmenu" <prompt>
339
340This sets the config program's title bar if the config program chooses
Arnaud Lacombe8ea13e22010-08-16 22:55:31 -0400341to use it. It should be placed at the top of the configuration, before any
342other statement.
Randy Dunlap0486bc92007-11-12 16:17:55 -0800343
344
345Kconfig hints
346-------------
347This is a collection of Kconfig tips, most of which aren't obvious at
348first glance and most of which have become idioms in several Kconfig
349files.
350
Sam Ravnborg9b3e4da2008-01-28 21:49:46 +0100351Adding common features and make the usage configurable
352~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
353It is a common idiom to implement a feature/functionality that are
354relevant for some architectures but not all.
355The recommended way to do so is to use a config variable named HAVE_*
356that is defined in a common Kconfig file and selected by the relevant
357architectures.
358An example is the generic IOMAP functionality.
359
360We would in lib/Kconfig see:
361
362# Generic IOMAP is used to ...
363config HAVE_GENERIC_IOMAP
364
365config GENERIC_IOMAP
366 depends on HAVE_GENERIC_IOMAP && FOO
367
368And in lib/Makefile we would see:
369obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
370
371For each architecture using the generic IOMAP functionality we would see:
372
373config X86
374 select ...
375 select HAVE_GENERIC_IOMAP
376 select ...
377
378Note: we use the existing config option and avoid creating a new
379config variable to select HAVE_GENERIC_IOMAP.
380
381Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
382introduced to overcome the limitation of select which will force a
383config option to 'y' no matter the dependencies.
384The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
385situation where select forces a symbol equals to 'y'.
386
Randy Dunlap0486bc92007-11-12 16:17:55 -0800387Build as module only
388~~~~~~~~~~~~~~~~~~~~
389To restrict a component build to module-only, qualify its config symbol
390with "depends on m". E.g.:
391
392config FOO
393 depends on BAR && m
394
395limits FOO to module (=m) or disabled (=n).