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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.
115 kconfig should one day warn about such things.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700116
117- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
118 This allows to limit the range of possible input values for int
119 and hex symbols. The user can only input a value which is larger than
120 or equal to the first symbol and smaller than or equal to the second
121 symbol.
122
123- help text: "help" or "---help---"
124 This defines a help text. The end of the help text is determined by
125 the indentation level, this means it ends at the first line which has
126 a smaller indentation than the first line of the help text.
127 "---help---" and "help" do not differ in behaviour, "---help---" is
Matt LaPlante53cb4722006-10-03 22:55:17 +0200128 used to help visually separate configuration logic from help within
Linus Torvalds1da177e2005-04-16 15:20:36 -0700129 the file as an aid to developers.
130
Roman Zippel93449082008-01-14 04:50:54 +0100131- misc options: "option" <symbol>[=<value>]
132 Various less common options can be defined via this option syntax,
133 which can modify the behaviour of the menu entry and its config
134 symbol. These options are currently possible:
135
136 - "defconfig_list"
137 This declares a list of default entries which can be used when
138 looking for the default configuration (which is used when the main
139 .config doesn't exists yet.)
140
141 - "modules"
142 This declares the symbol to be used as the MODULES symbol, which
143 enables the third modular state for all config symbols.
144
145 - "env"=<value>
146 This imports the environment variable into Kconfig. It behaves like
147 a default, except that the value comes from the environment, this
148 also means that the behaviour when mixing it with normal defaults is
149 undefined at this point. The symbol is currently not exported back
150 to the build environment (if this is desired, it can be done via
151 another symbol).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700152
153Menu dependencies
154-----------------
155
156Dependencies define the visibility of a menu entry and can also reduce
157the input range of tristate symbols. The tristate logic used in the
158expressions uses one more state than normal boolean logic to express the
159module state. Dependency expressions have the following syntax:
160
161<expr> ::= <symbol> (1)
162 <symbol> '=' <symbol> (2)
163 <symbol> '!=' <symbol> (3)
164 '(' <expr> ')' (4)
165 '!' <expr> (5)
166 <expr> '&&' <expr> (6)
167 <expr> '||' <expr> (7)
168
169Expressions are listed in decreasing order of precedence.
170
171(1) Convert the symbol into an expression. Boolean and tristate symbols
172 are simply converted into the respective expression values. All
173 other symbol types result in 'n'.
174(2) If the values of both symbols are equal, it returns 'y',
175 otherwise 'n'.
176(3) If the values of both symbols are equal, it returns 'n',
177 otherwise 'y'.
178(4) Returns the value of the expression. Used to override precedence.
179(5) Returns the result of (2-/expr/).
180(6) Returns the result of min(/expr/, /expr/).
181(7) Returns the result of max(/expr/, /expr/).
182
183An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
184respectively for calculations). A menu entry becomes visible when it's
185expression evaluates to 'm' or 'y'.
186
Randy Dunlap0486bc92007-11-12 16:17:55 -0800187There are two types of symbols: constant and non-constant symbols.
188Non-constant symbols are the most common ones and are defined with the
189'config' statement. Non-constant symbols consist entirely of alphanumeric
Linus Torvalds1da177e2005-04-16 15:20:36 -0700190characters or underscores.
191Constant symbols are only part of expressions. Constant symbols are
Jan Engelhardt83dcde42006-07-27 22:14:29 +0200192always surrounded by single or double quotes. Within the quote, any
Linus Torvalds1da177e2005-04-16 15:20:36 -0700193other character is allowed and the quotes can be escaped using '\'.
194
195Menu structure
196--------------
197
198The position of a menu entry in the tree is determined in two ways. First
199it can be specified explicitly:
200
201menu "Network device support"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100202 depends on NET
Linus Torvalds1da177e2005-04-16 15:20:36 -0700203
204config NETDEVICES
205 ...
206
207endmenu
208
209All entries within the "menu" ... "endmenu" block become a submenu of
210"Network device support". All subentries inherit the dependencies from
211the menu entry, e.g. this means the dependency "NET" is added to the
212dependency list of the config option NETDEVICES.
213
214The other way to generate the menu structure is done by analyzing the
215dependencies. If a menu entry somehow depends on the previous entry, it
216can be made a submenu of it. First, the previous (parent) symbol must
217be part of the dependency list and then one of these two conditions
218must be true:
219- the child entry must become invisible, if the parent is set to 'n'
220- the child entry must only be visible, if the parent is visible
221
222config MODULES
223 bool "Enable loadable module support"
224
225config MODVERSIONS
226 bool "Set version information on all module symbols"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100227 depends on MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -0700228
229comment "module support disabled"
Robert P. J. Daybef1f402006-12-12 20:04:19 +0100230 depends on !MODULES
Linus Torvalds1da177e2005-04-16 15:20:36 -0700231
232MODVERSIONS directly depends on MODULES, this means it's only visible if
233MODULES is different from 'n'. The comment on the other hand is always
234visible when MODULES is visible (the (empty) dependency of MODULES is
235also part of the comment dependencies).
236
237
238Kconfig syntax
239--------------
240
241The configuration file describes a series of menu entries, where every
242line starts with a keyword (except help texts). The following keywords
243end a menu entry:
244- config
245- menuconfig
246- choice/endchoice
247- comment
248- menu/endmenu
249- if/endif
250- source
251The first five also start the definition of a menu entry.
252
253config:
254
255 "config" <symbol>
256 <config options>
257
258This defines a config symbol <symbol> and accepts any of above
259attributes as options.
260
261menuconfig:
262 "menuconfig" <symbol>
263 <config options>
264
Matt LaPlante53cb4722006-10-03 22:55:17 +0200265This is similar to the simple config entry above, but it also gives a
Linus Torvalds1da177e2005-04-16 15:20:36 -0700266hint to front ends, that all suboptions should be displayed as a
267separate list of options.
268
269choices:
270
271 "choice"
272 <choice options>
273 <choice block>
274 "endchoice"
275
Jan Engelhardt83dcde42006-07-27 22:14:29 +0200276This defines a choice group and accepts any of the above attributes as
Linus Torvalds1da177e2005-04-16 15:20:36 -0700277options. A choice can only be of type bool or tristate, while a boolean
278choice only allows a single config entry to be selected, a tristate
279choice also allows any number of config entries to be set to 'm'. This
280can be used if multiple drivers for a single hardware exists and only a
281single driver can be compiled/loaded into the kernel, but all drivers
282can be compiled as modules.
283A choice accepts another option "optional", which allows to set the
284choice to 'n' and no entry needs to be selected.
285
286comment:
287
288 "comment" <prompt>
289 <comment options>
290
291This defines a comment which is displayed to the user during the
292configuration process and is also echoed to the output files. The only
293possible options are dependencies.
294
295menu:
296
297 "menu" <prompt>
298 <menu options>
299 <menu block>
300 "endmenu"
301
302This defines a menu block, see "Menu structure" above for more
303information. The only possible options are dependencies.
304
305if:
306
307 "if" <expr>
308 <if block>
309 "endif"
310
311This defines an if block. The dependency expression <expr> is appended
312to all enclosed menu entries.
313
314source:
315
316 "source" <prompt>
317
318This reads the specified configuration file. This file is always parsed.
Randy Dunlap6e66b902007-10-19 10:53:48 -0700319
320mainmenu:
321
322 "mainmenu" <prompt>
323
324This sets the config program's title bar if the config program chooses
325to use it.
Randy Dunlap0486bc92007-11-12 16:17:55 -0800326
327
328Kconfig hints
329-------------
330This is a collection of Kconfig tips, most of which aren't obvious at
331first glance and most of which have become idioms in several Kconfig
332files.
333
Sam Ravnborg9b3e4da2008-01-28 21:49:46 +0100334Adding common features and make the usage configurable
335~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
336It is a common idiom to implement a feature/functionality that are
337relevant for some architectures but not all.
338The recommended way to do so is to use a config variable named HAVE_*
339that is defined in a common Kconfig file and selected by the relevant
340architectures.
341An example is the generic IOMAP functionality.
342
343We would in lib/Kconfig see:
344
345# Generic IOMAP is used to ...
346config HAVE_GENERIC_IOMAP
347
348config GENERIC_IOMAP
349 depends on HAVE_GENERIC_IOMAP && FOO
350
351And in lib/Makefile we would see:
352obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
353
354For each architecture using the generic IOMAP functionality we would see:
355
356config X86
357 select ...
358 select HAVE_GENERIC_IOMAP
359 select ...
360
361Note: we use the existing config option and avoid creating a new
362config variable to select HAVE_GENERIC_IOMAP.
363
364Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
365introduced to overcome the limitation of select which will force a
366config option to 'y' no matter the dependencies.
367The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
368situation where select forces a symbol equals to 'y'.
369
Randy Dunlap0486bc92007-11-12 16:17:55 -0800370Build as module only
371~~~~~~~~~~~~~~~~~~~~
372To restrict a component build to module-only, qualify its config symbol
373with "depends on m". E.g.:
374
375config FOO
376 depends on BAR && m
377
378limits FOO to module (=m) or disabled (=n).
379