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
161 This declares the symbol as one that should have the value y when
162 using "allnoconfig". Used for symbols that hide other symbols.
167 Dependencies define the visibility of a menu entry and can also reduce
168 the input range of tristate symbols. The tristate logic used in the
169 expressions uses one more state than normal boolean logic to express the
170 module state. Dependency expressions have the following syntax:
172 <expr> ::= <symbol> (1)
173 <symbol> '=' <symbol> (2)
174 <symbol> '!=' <symbol> (3)
177 <expr> '&&' <expr> (6)
178 <expr> '||' <expr> (7)
180 Expressions are listed in decreasing order of precedence.
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',
187 (3) If the values of both symbols are equal, it returns 'n',
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/).
194 An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
195 respectively for calculations). A menu entry becomes visible when its
196 expression evaluates to 'm' or 'y'.
198 There are two types of symbols: constant and non-constant symbols.
199 Non-constant symbols are the most common ones and are defined with the
200 'config' statement. Non-constant symbols consist entirely of alphanumeric
201 characters or underscores.
202 Constant symbols are only part of expressions. Constant symbols are
203 always surrounded by single or double quotes. Within the quote, any
204 other character is allowed and the quotes can be escaped using '\'.
209 The position of a menu entry in the tree is determined in two ways. First
210 it can be specified explicitly:
212 menu "Network device support"
220 All entries within the "menu" ... "endmenu" block become a submenu of
221 "Network device support". All subentries inherit the dependencies from
222 the menu entry, e.g. this means the dependency "NET" is added to the
223 dependency list of the config option NETDEVICES.
225 The other way to generate the menu structure is done by analyzing the
226 dependencies. If a menu entry somehow depends on the previous entry, it
227 can be made a submenu of it. First, the previous (parent) symbol must
228 be part of the dependency list and then one of these two conditions
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
234 bool "Enable loadable module support"
237 bool "Set version information on all module symbols"
240 comment "module support disabled"
243 MODVERSIONS directly depends on MODULES, this means it's only visible if
244 MODULES is different from 'n'. The comment on the other hand is only
245 visible when MODULES is set to 'n'.
251 The configuration file describes a series of menu entries, where every
252 line starts with a keyword (except help texts). The following keywords
261 The first five also start the definition of a menu entry.
268 This defines a config symbol <symbol> and accepts any of above
269 attributes as options.
272 "menuconfig" <symbol>
275 This is similar to the simple config entry above, but it also gives a
276 hint to front ends, that all suboptions should be displayed as a
277 separate list of options.
286 This defines a choice group and accepts any of the above attributes as
287 options. A choice can only be of type bool or tristate, while a boolean
288 choice only allows a single config entry to be selected, a tristate
289 choice also allows any number of config entries to be set to 'm'. This
290 can be used if multiple drivers for a single hardware exists and only a
291 single driver can be compiled/loaded into the kernel, but all drivers
292 can be compiled as modules.
293 A choice accepts another option "optional", which allows to set the
294 choice to 'n' and no entry needs to be selected.
295 If no [symbol] is associated with a choice, then you can not have multiple
296 definitions of that choice. If a [symbol] is associated to the choice,
297 then you may define the same choice (ie. with the same entries) in another
305 This defines a comment which is displayed to the user during the
306 configuration process and is also echoed to the output files. The only
307 possible options are dependencies.
316 This defines a menu block, see "Menu structure" above for more
317 information. The only possible options are dependencies and "visible"
326 This defines an if block. The dependency expression <expr> is appended
327 to all enclosed menu entries.
333 This reads the specified configuration file. This file is always parsed.
339 This sets the config program's title bar if the config program chooses
340 to use it. It should be placed at the top of the configuration, before any
346 This is a collection of Kconfig tips, most of which aren't obvious at
347 first glance and most of which have become idioms in several Kconfig
350 Adding common features and make the usage configurable
351 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
352 It is a common idiom to implement a feature/functionality that are
353 relevant for some architectures but not all.
354 The recommended way to do so is to use a config variable named HAVE_*
355 that is defined in a common Kconfig file and selected by the relevant
357 An example is the generic IOMAP functionality.
359 We would in lib/Kconfig see:
361 # Generic IOMAP is used to ...
362 config HAVE_GENERIC_IOMAP
365 depends on HAVE_GENERIC_IOMAP && FOO
367 And in lib/Makefile we would see:
368 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
370 For each architecture using the generic IOMAP functionality we would see:
374 select HAVE_GENERIC_IOMAP
377 Note: we use the existing config option and avoid creating a new
378 config variable to select HAVE_GENERIC_IOMAP.
380 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
381 introduced to overcome the limitation of select which will force a
382 config option to 'y' no matter the dependencies.
383 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
384 situation where select forces a symbol equals to 'y'.
388 To restrict a component build to module-only, qualify its config symbol
389 with "depends on m". E.g.:
394 limits FOO to module (=m) or disabled (=n).
396 Kconfig recursive dependency limitations
397 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
399 If you've hit the Kconfig error: "recursive dependency detected" you've run
400 into a recursive dependency issue with Kconfig, a recursive dependency can be
401 summarized as a circular dependency. The kconfig tools need to ensure that
402 Kconfig files comply with specified configuration requirements. In order to do
403 that kconfig must determine the values that are possible for all Kconfig
404 symbols, this is currently not possible if there is a circular relation
405 between two or more Kconfig symbols. For more details refer to the "Simple
406 Kconfig recursive issue" subsection below. Kconfig does not do recursive
407 dependency resolution; this has a few implications for Kconfig file writers.
408 We'll first explain why this issues exists and then provide an example
409 technical limitation which this brings upon Kconfig developers. Eager
410 developers wishing to try to address this limitation should read the next
413 Simple Kconfig recursive issue
414 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
416 Read: Documentation/kbuild/Kconfig.recursion-issue-01
420 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
422 Cumulative Kconfig recursive issue
423 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
425 Read: Documentation/kbuild/Kconfig.recursion-issue-02
429 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
431 Practical solutions to kconfig recursive issue
432 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
434 Developers who run into the recursive Kconfig issue have three options
435 at their disposal. We document them below and also provide a list of
436 historical issues resolved through these different solutions.
438 a) Remove any superfluous "select FOO" or "depends on FOO"
439 b) Match dependency semantics:
440 b1) Swap all "select FOO" to "depends on FOO" or,
441 b2) Swap all "depends on FOO" to "select FOO"
443 The resolution to a) can be tested with the sample Kconfig file
444 Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
445 of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
446 since CORE_BELL_A depends on CORE. At times it may not be possible to remove
447 some dependency criteria, for such cases you can work with solution b).
449 The two different resolutions for b) can be tested in the sample Kconfig file
450 Documentation/kbuild/Kconfig.recursion-issue-02.
452 Below is a list of examples of prior fixes for these types of recursive issues;
453 all errors appear to involve one or more select's and one or more "depends on".
457 06b718c01208 select A -> depends on A
458 c22eacfe82f9 depends on A -> depends on B
459 6a91e854442c select A -> depends on A
460 118c565a8f2e select A -> select B
461 f004e5594705 select A -> depends on A
462 c7861f37b4c6 depends on A -> (null)
463 80c69915e5fb select A -> (null) (1)
464 c2218e26c0d0 select A -> depends on A (1)
465 d6ae99d04e1c select A -> depends on A
466 95ca19cf8cbf select A -> depends on A
467 8f057d7bca54 depends on A -> (null)
468 8f057d7bca54 depends on A -> select A
469 a0701f04846e select A -> depends on A
470 0c8b92f7f259 depends on A -> (null)
471 e4e9e0540928 select A -> depends on A (2)
472 7453ea886e87 depends on A > (null) (1)
473 7b1fff7e4fdf select A -> depends on A
474 86c747d2a4f0 select A -> depends on A
475 d9f9ab51e55e select A -> depends on A
476 0c51a4d8abd6 depends on A -> select A (3)
477 e98062ed6dc4 select A -> depends on A (3)
478 91e5d284a7f1 select A -> (null)
480 (1) Partial (or no) quote of error.
481 (2) That seems to be the gist of that fix.
487 Work on kconfig is welcomed on both areas of clarifying semantics and on
488 evaluating the use of a full SAT solver for it. A full SAT solver can be
489 desirable to enable more complex dependency mappings and / or queries,
490 for instance on possible use case for a SAT solver could be that of handling
491 the current known recursive dependency issues. It is not known if this would
492 address such issues but such evaluation is desirable. If support for a full SAT
493 solver proves too complex or that it cannot address recursive dependency issues
494 Kconfig should have at least clear and well defined semantics which also
495 addresses and documents limitations or requirements such as the ones dealing
496 with recursive dependencies.
498 Further work on both of these areas is welcomed on Kconfig. We elaborate
499 on both of these in the next two subsections.
504 The use of Kconfig is broad, Linux is now only one of Kconfig's users:
505 one study has completed a broad analysis of Kconfig use in 12 projects [0].
506 Despite its widespread use, and although this document does a reasonable job
507 in documenting basic Kconfig syntax a more precise definition of Kconfig
508 semantics is welcomed. One project deduced Kconfig semantics through
509 the use of the xconfig configurator [1]. Work should be done to confirm if
510 the deduced semantics matches our intended Kconfig design goals.
512 Having well defined semantics can be useful for tools for practical
513 evaluation of depenencies, for instance one such use known case was work to
514 express in boolean abstraction of the inferred semantics of Kconfig to
515 translate Kconfig logic into boolean formulas and run a SAT solver on this to
516 find dead code / features (always inactive), 114 dead features were found in
517 Linux using this methodology [1] (Section 8: Threats to validity).
519 Confirming this could prove useful as Kconfig stands as one of the the leading
520 industrial variability modeling languages [1] [2]. Its study would help
521 evaluate practical uses of such languages, their use was only theoretical
522 and real world requirements were not well understood. As it stands though
523 only reverse engineering techniques have been used to deduce semantics from
524 variability modeling languages such as Kconfig [3].
526 [0] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
527 [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
528 [2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
529 [3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf
531 Full SAT solver for Kconfig
532 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
534 Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in
535 the previous subsection, work has been done however to express in boolean
536 abstraction the inferred semantics of Kconfig to translate Kconfig logic into
537 boolean formulas and run a SAT solver on it [1]. Another known related project
538 is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has
539 been introduced first with [5]. The basic concept of undertaker is to exract
540 variability models from Kconfig, and put them together with a propositional
541 formula extracted from CPP #ifdefs and build-rules into a SAT solver in order
542 to find dead code, dead files, and dead symbols. If using a SAT solver is
543 desirable on Kconfig one approach would be to evaluate repurposing such efforts
544 somehow on Kconfig. There is enough interest from mentors of existing projects
545 to not only help advise how to integrate this work upstream but also help
546 maintain it long term. Interested developers should visit:
548 http://kernelnewbies.org/KernelProjects/kconfig-sat
550 [0] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
551 [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
552 [2] https://cados.cs.fau.de
553 [3] https://vamos.cs.fau.de
554 [4] https://undertaker.cs.fau.de
555 [5] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf
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