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.
115 kconfig should one day warn about such things.
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
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
128 used to help visually separate configuration logic from help within
129 the file as an aid to developers.
131 - 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:
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.)
142 This declares the symbol to be used as the MODULES symbol, which
143 enables the third modular state for all config symbols.
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
156 Dependencies define the visibility of a menu entry and can also reduce
157 the input range of tristate symbols. The tristate logic used in the
158 expressions uses one more state than normal boolean logic to express the
159 module state. Dependency expressions have the following syntax:
161 <expr> ::= <symbol> (1)
162 <symbol> '=' <symbol> (2)
163 <symbol> '!=' <symbol> (3)
166 <expr> '&&' <expr> (6)
167 <expr> '||' <expr> (7)
169 Expressions are listed in decreasing order of precedence.
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',
176 (3) If the values of both symbols are equal, it returns 'n',
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/).
183 An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
184 respectively for calculations). A menu entry becomes visible when it's
185 expression evaluates to 'm' or 'y'.
187 There are two types of symbols: constant and non-constant symbols.
188 Non-constant symbols are the most common ones and are defined with the
189 'config' statement. Non-constant symbols consist entirely of alphanumeric
190 characters or underscores.
191 Constant symbols are only part of expressions. Constant symbols are
192 always surrounded by single or double quotes. Within the quote, any
193 other character is allowed and the quotes can be escaped using '\'.
198 The position of a menu entry in the tree is determined in two ways. First
199 it can be specified explicitly:
201 menu "Network device support"
209 All entries within the "menu" ... "endmenu" block become a submenu of
210 "Network device support". All subentries inherit the dependencies from
211 the menu entry, e.g. this means the dependency "NET" is added to the
212 dependency list of the config option NETDEVICES.
214 The other way to generate the menu structure is done by analyzing the
215 dependencies. If a menu entry somehow depends on the previous entry, it
216 can be made a submenu of it. First, the previous (parent) symbol must
217 be part of the dependency list and then one of these two conditions
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
223 bool "Enable loadable module support"
226 bool "Set version information on all module symbols"
229 comment "module support disabled"
232 MODVERSIONS directly depends on MODULES, this means it's only visible if
233 MODULES is different from 'n'. The comment on the other hand is always
234 visible when MODULES is visible (the (empty) dependency of MODULES is
235 also part of the comment dependencies).
241 The configuration file describes a series of menu entries, where every
242 line starts with a keyword (except help texts). The following keywords
251 The first five also start the definition of a menu entry.
258 This defines a config symbol <symbol> and accepts any of above
259 attributes as options.
262 "menuconfig" <symbol>
265 This is similar to the simple config entry above, but it also gives a
266 hint to front ends, that all suboptions should be displayed as a
267 separate list of options.
276 This defines a choice group and accepts any of the above attributes as
277 options. A choice can only be of type bool or tristate, while a boolean
278 choice only allows a single config entry to be selected, a tristate
279 choice also allows any number of config entries to be set to 'm'. This
280 can be used if multiple drivers for a single hardware exists and only a
281 single driver can be compiled/loaded into the kernel, but all drivers
282 can be compiled as modules.
283 A choice accepts another option "optional", which allows to set the
284 choice to 'n' and no entry needs to be selected.
291 This defines a comment which is displayed to the user during the
292 configuration process and is also echoed to the output files. The only
293 possible options are dependencies.
302 This defines a menu block, see "Menu structure" above for more
303 information. The only possible options are dependencies.
311 This defines an if block. The dependency expression <expr> is appended
312 to all enclosed menu entries.
318 This reads the specified configuration file. This file is always parsed.
324 This sets the config program's title bar if the config program chooses
330 This is a collection of Kconfig tips, most of which aren't obvious at
331 first glance and most of which have become idioms in several Kconfig
334 Adding common features and make the usage configurable
335 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
336 It is a common idiom to implement a feature/functionality that are
337 relevant for some architectures but not all.
338 The recommended way to do so is to use a config variable named HAVE_*
339 that is defined in a common Kconfig file and selected by the relevant
341 An example is the generic IOMAP functionality.
343 We would in lib/Kconfig see:
345 # Generic IOMAP is used to ...
346 config HAVE_GENERIC_IOMAP
349 depends on HAVE_GENERIC_IOMAP && FOO
351 And in lib/Makefile we would see:
352 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
354 For each architecture using the generic IOMAP functionality we would see:
358 select HAVE_GENERIC_IOMAP
361 Note: we use the existing config option and avoid creating a new
362 config variable to select HAVE_GENERIC_IOMAP.
364 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
365 introduced to overcome the limitation of select which will force a
366 config option to 'y' no matter the dependencies.
367 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
368 situation where select forces a symbol equals to 'y'.
372 To restrict a component build to module-only, qualify its config symbol
373 with "depends on m". E.g.:
378 limits FOO to module (=m) or disabled (=n).
381 Build limited by a third config symbol which may be =y or =m
382 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
383 A common idiom that we see (and sometimes have problems with) is this:
385 When option C in B (module or subsystem) uses interfaces from A (module
386 or subsystem), and both A and B are tristate (could be =y or =m if they
387 were independent of each other, but they aren't), then we need to limit
388 C such that it cannot be built statically if A is built as a loadable
389 module. (C already depends on B, so there is no dependency issue to
392 If A is linked statically into the kernel image, C can be built
393 statically or as loadable module(s). However, if A is built as loadable
394 module(s), then C must be restricted to loadable module(s) also. This
395 can be expressed in kconfig language as:
398 depends on A = y || A = B
400 or for real examples, use this command in a kernel tree:
402 $ find . -name Kconfig\* | xargs grep -ns "depends on.*=.*||.*=" | grep -v orig
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