4 The configuration database is a collection of configuration options
5 organized in a tree structure:
7 +- Code maturity level options
8 | +- Prompt for development and/or incomplete code/drivers
10 | +- Networking support
12 | +- BSD Process Accounting
14 +- Loadable module support
15 | +- Enable loadable module support
16 | +- Set version information on all module symbols
17 | +- Kernel module loader
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.
27 Most entries define a config option; all other entries help to organize
28 them. A single configuration option is defined like this:
31 bool "Set version information on all module symbols"
34 Usually, modules have to be recompiled whenever you switch to a new
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.
48 A menu entry can have a number of attributes. Not all of them are
49 applicable everywhere (see syntax).
51 - type definition: "bool"/"tristate"/"string"/"hex"/"int"
52 Every config option must have a type. There are only two basic types:
53 tristate and string; the other types are based on these two. The type
54 definition optionally accepts an input prompt, so these two examples
57 bool "Networking support"
60 prompt "Networking support"
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
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.
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
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
77 Optionally, dependencies only for this default value can be added with
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".
85 - dependencies: "depends on" <expr>
86 This defines a dependency for this menu entry. If multiple
87 dependencies are defined, they are connected with '&&'. Dependencies
88 are applied to all other options within this menu entry (which also
89 accept an "if" expression), so these two examples are equivalent:
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
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.
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
120 similar to a conditional "prompt" attribute for individual menu
121 entries. Default value of "visible" is true.
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
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
134 used to help visually separate configuration logic from help within
135 the file as an aid to developers.
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:
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.)
148 This declares the symbol to be used as the MODULES symbol, which
149 enables the third modular state for all config symbols.
150 At most one symbol may have the "modules" option set.
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
163 Dependencies define the visibility of a menu entry and can also reduce
164 the input range of tristate symbols. The tristate logic used in the
165 expressions uses one more state than normal boolean logic to express the
166 module state. Dependency expressions have the following syntax:
168 <expr> ::= <symbol> (1)
169 <symbol> '=' <symbol> (2)
170 <symbol> '!=' <symbol> (3)
173 <expr> '&&' <expr> (6)
174 <expr> '||' <expr> (7)
176 Expressions are listed in decreasing order of precedence.
178 (1) Convert the symbol into an expression. Boolean and tristate symbols
179 are simply converted into the respective expression values. All
180 other symbol types result in 'n'.
181 (2) If the values of both symbols are equal, it returns 'y',
183 (3) If the values of both symbols are equal, it returns 'n',
185 (4) Returns the value of the expression. Used to override precedence.
186 (5) Returns the result of (2-/expr/).
187 (6) Returns the result of min(/expr/, /expr/).
188 (7) Returns the result of max(/expr/, /expr/).
190 An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
191 respectively for calculations). A menu entry becomes visible when its
192 expression evaluates to 'm' or 'y'.
194 There are two types of symbols: constant and non-constant symbols.
195 Non-constant symbols are the most common ones and are defined with the
196 'config' statement. Non-constant symbols consist entirely of alphanumeric
197 characters or underscores.
198 Constant symbols are only part of expressions. Constant symbols are
199 always surrounded by single or double quotes. Within the quote, any
200 other character is allowed and the quotes can be escaped using '\'.
205 The position of a menu entry in the tree is determined in two ways. First
206 it can be specified explicitly:
208 menu "Network device support"
216 All entries within the "menu" ... "endmenu" block become a submenu of
217 "Network device support". All subentries inherit the dependencies from
218 the menu entry, e.g. this means the dependency "NET" is added to the
219 dependency list of the config option NETDEVICES.
221 The other way to generate the menu structure is done by analyzing the
222 dependencies. If a menu entry somehow depends on the previous entry, it
223 can be made a submenu of it. First, the previous (parent) symbol must
224 be part of the dependency list and then one of these two conditions
226 - the child entry must become invisible, if the parent is set to 'n'
227 - the child entry must only be visible, if the parent is visible
230 bool "Enable loadable module support"
233 bool "Set version information on all module symbols"
236 comment "module support disabled"
239 MODVERSIONS directly depends on MODULES, this means it's only visible if
240 MODULES is different from 'n'. The comment on the other hand is always
241 visible when MODULES is visible (the (empty) dependency of MODULES is
242 also part of the comment dependencies).
248 The configuration file describes a series of menu entries, where every
249 line starts with a keyword (except help texts). The following keywords
258 The first five also start the definition of a menu entry.
265 This defines a config symbol <symbol> and accepts any of above
266 attributes as options.
269 "menuconfig" <symbol>
272 This is similar to the simple config entry above, but it also gives a
273 hint to front ends, that all suboptions should be displayed as a
274 separate list of options.
283 This defines a choice group and accepts any of the above attributes as
284 options. A choice can only be of type bool or tristate, while a boolean
285 choice only allows a single config entry to be selected, a tristate
286 choice also allows any number of config entries to be set to 'm'. This
287 can be used if multiple drivers for a single hardware exists and only a
288 single driver can be compiled/loaded into the kernel, but all drivers
289 can be compiled as modules.
290 A choice accepts another option "optional", which allows to set the
291 choice to 'n' and no entry needs to be selected.
292 If no [symbol] is associated with a choice, then you can not have multiple
293 definitions of that choice. If a [symbol] is associated to the choice,
294 then you may define the same choice (ie. with the same entries) in another
302 This defines a comment which is displayed to the user during the
303 configuration process and is also echoed to the output files. The only
304 possible options are dependencies.
313 This defines a menu block, see "Menu structure" above for more
314 information. The only possible options are dependencies and "visible"
323 This defines an if block. The dependency expression <expr> is appended
324 to all enclosed menu entries.
330 This reads the specified configuration file. This file is always parsed.
336 This sets the config program's title bar if the config program chooses
337 to use it. It should be placed at the top of the configuration, before any
343 This is a collection of Kconfig tips, most of which aren't obvious at
344 first glance and most of which have become idioms in several Kconfig
347 Adding common features and make the usage configurable
348 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
349 It is a common idiom to implement a feature/functionality that are
350 relevant for some architectures but not all.
351 The recommended way to do so is to use a config variable named HAVE_*
352 that is defined in a common Kconfig file and selected by the relevant
354 An example is the generic IOMAP functionality.
356 We would in lib/Kconfig see:
358 # Generic IOMAP is used to ...
359 config HAVE_GENERIC_IOMAP
362 depends on HAVE_GENERIC_IOMAP && FOO
364 And in lib/Makefile we would see:
365 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
367 For each architecture using the generic IOMAP functionality we would see:
371 select HAVE_GENERIC_IOMAP
374 Note: we use the existing config option and avoid creating a new
375 config variable to select HAVE_GENERIC_IOMAP.
377 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
378 introduced to overcome the limitation of select which will force a
379 config option to 'y' no matter the dependencies.
380 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
381 situation where select forces a symbol equals to 'y'.
385 To restrict a component build to module-only, qualify its config symbol
386 with "depends on m". E.g.:
391 limits FOO to module (=m) or disabled (=n).
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