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 always
245 visible when MODULES is visible (the (empty) dependency of MODULES is
246 also part of the comment dependencies).
252 The configuration file describes a series of menu entries, where every
253 line starts with a keyword (except help texts). The following keywords
262 The first five also start the definition of a menu entry.
269 This defines a config symbol <symbol> and accepts any of above
270 attributes as options.
273 "menuconfig" <symbol>
276 This is similar to the simple config entry above, but it also gives a
277 hint to front ends, that all suboptions should be displayed as a
278 separate list of options.
287 This defines a choice group and accepts any of the above attributes as
288 options. A choice can only be of type bool or tristate, while a boolean
289 choice only allows a single config entry to be selected, a tristate
290 choice also allows any number of config entries to be set to 'm'. This
291 can be used if multiple drivers for a single hardware exists and only a
292 single driver can be compiled/loaded into the kernel, but all drivers
293 can be compiled as modules.
294 A choice accepts another option "optional", which allows to set the
295 choice to 'n' and no entry needs to be selected.
296 If no [symbol] is associated with a choice, then you can not have multiple
297 definitions of that choice. If a [symbol] is associated to the choice,
298 then you may define the same choice (ie. with the same entries) in another
306 This defines a comment which is displayed to the user during the
307 configuration process and is also echoed to the output files. The only
308 possible options are dependencies.
317 This defines a menu block, see "Menu structure" above for more
318 information. The only possible options are dependencies and "visible"
327 This defines an if block. The dependency expression <expr> is appended
328 to all enclosed menu entries.
334 This reads the specified configuration file. This file is always parsed.
340 This sets the config program's title bar if the config program chooses
341 to use it. It should be placed at the top of the configuration, before any
347 This is a collection of Kconfig tips, most of which aren't obvious at
348 first glance and most of which have become idioms in several Kconfig
351 Adding common features and make the usage configurable
352 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
353 It is a common idiom to implement a feature/functionality that are
354 relevant for some architectures but not all.
355 The recommended way to do so is to use a config variable named HAVE_*
356 that is defined in a common Kconfig file and selected by the relevant
358 An example is the generic IOMAP functionality.
360 We would in lib/Kconfig see:
362 # Generic IOMAP is used to ...
363 config HAVE_GENERIC_IOMAP
366 depends on HAVE_GENERIC_IOMAP && FOO
368 And in lib/Makefile we would see:
369 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
371 For each architecture using the generic IOMAP functionality we would see:
375 select HAVE_GENERIC_IOMAP
378 Note: we use the existing config option and avoid creating a new
379 config variable to select HAVE_GENERIC_IOMAP.
381 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
382 introduced to overcome the limitation of select which will force a
383 config option to 'y' no matter the dependencies.
384 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
385 situation where select forces a symbol equals to 'y'.
389 To restrict a component build to module-only, qualify its config symbol
390 with "depends on m". E.g.:
395 limits FOO to module (=m) or disabled (=n).
397 Kconfig recursive dependency limitations
398 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
400 If you've hit the Kconfig error: "recursive dependency detected" you've run
401 into a recursive dependency issue with Kconfig, a recursive dependency can be
402 summarized as a circular dependency. The kconfig tools need to ensure that
403 Kconfig files comply with specified configuration requirements. In order to do
404 that kconfig must determine the values that are possible for all Kconfig
405 symbols, this is currently not possible if there is a circular relation
406 between two or more Kconfig symbols. For more details refer to the "Simple
407 Kconfig recursive issue" subsection below. Kconfig does not do recursive
408 dependency resolution; this has a few implications for Kconfig file writers.
409 We'll first explain why this issues exists and then provide an example
410 technical limitation which this brings upon Kconfig developers. Eager
411 developers wishing to try to address this limitation should read the next
414 Simple Kconfig recursive issue
415 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
417 Read: Documentation/kbuild/Kconfig.recursion-issue-01
421 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
423 Cumulative Kconfig recursive issue
424 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
426 Read: Documentation/kbuild/Kconfig.recursion-issue-02
430 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
432 Practical solutions to kconfig recursive issue
433 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
435 Developers who run into the recursive Kconfig issue have three options
436 at their disposal. We document them below and also provide a list of
437 historical issues resolved through these different solutions.
439 a) Remove any superfluous "select FOO" or "depends on FOO"
440 b) Match dependency semantics:
441 b1) Swap all "select FOO" to "depends on FOO" or,
442 b2) Swap all "depends on FOO" to "select FOO"
444 The resolution to a) can be tested with the sample Kconfig file
445 Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
446 of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
447 since CORE_BELL_A depends on CORE. At times it may not be possible to remove
448 some dependency criteria, for such cases you can work with solution b).
450 The two different resolutions for b) can be tested in the sample Kconfig file
451 Documentation/kbuild/Kconfig.recursion-issue-02.
453 Below is a list of examples of prior fixes for these types of recursive issues;
454 all errors appear to involve one or more select's and one or more "depends on".
458 06b718c01208 select A -> depends on A
459 c22eacfe82f9 depends on A -> depends on B
460 6a91e854442c select A -> depends on A
461 118c565a8f2e select A -> select B
462 f004e5594705 select A -> depends on A
463 c7861f37b4c6 depends on A -> (null)
464 80c69915e5fb select A -> (null) (1)
465 c2218e26c0d0 select A -> depends on A (1)
466 d6ae99d04e1c select A -> depends on A
467 95ca19cf8cbf select A -> depends on A
468 8f057d7bca54 depends on A -> (null)
469 8f057d7bca54 depends on A -> select A
470 a0701f04846e select A -> depends on A
471 0c8b92f7f259 depends on A -> (null)
472 e4e9e0540928 select A -> depends on A (2)
473 7453ea886e87 depends on A > (null) (1)
474 7b1fff7e4fdf select A -> depends on A
475 86c747d2a4f0 select A -> depends on A
476 d9f9ab51e55e select A -> depends on A
477 0c51a4d8abd6 depends on A -> select A (3)
478 e98062ed6dc4 select A -> depends on A (3)
479 91e5d284a7f1 select A -> (null)
481 (1) Partial (or no) quote of error.
482 (2) That seems to be the gist of that fix.
488 Work on kconfig is welcomed on both areas of clarifying semantics and on
489 evaluating the use of a full SAT solver for it. A full SAT solver can be
490 desirable to enable more complex dependency mappings and / or queries,
491 for instance on possible use case for a SAT solver could be that of handling
492 the current known recursive dependency issues. It is not known if this would
493 address such issues but such evaluation is desirable. If support for a full SAT
494 solver proves too complex or that it cannot address recursive dependency issues
495 Kconfig should have at least clear and well defined semantics which also
496 addresses and documents limitations or requirements such as the ones dealing
497 with recursive dependencies.
499 Further work on both of these areas is welcomed on Kconfig. We elaborate
500 on both of these in the next two subsections.
505 The use of Kconfig is broad, Linux is now only one of Kconfig's users:
506 one study has completed a broad analysis of Kconfig use in 12 projects [0].
507 Despite its widespread use, and although this document does a reasonable job
508 in documenting basic Kconfig syntax a more precise definition of Kconfig
509 semantics is welcomed. One project deduced Kconfig semantics through
510 the use of the xconfig configurator [1]. Work should be done to confirm if
511 the deduced semantics matches our intended Kconfig design goals.
513 Having well defined semantics can be useful for tools for practical
514 evaluation of depenencies, for instance one such use known case was work to
515 express in boolean abstraction of the inferred semantics of Kconfig to
516 translate Kconfig logic into boolean formulas and run a SAT solver on this to
517 find dead code / features (always inactive), 114 dead features were found in
518 Linux using this methodology [1] (Section 8: Threats to validity).
520 Confirming this could prove useful as Kconfig stands as one of the the leading
521 industrial variability modeling languages [1] [2]. Its study would help
522 evaluate practical uses of such languages, their use was only theoretical
523 and real world requirements were not well understood. As it stands though
524 only reverse engineering techniques have been used to deduce semantics from
525 variability modeling languages such as Kconfig [3].
527 [0] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
528 [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
529 [2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
530 [3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf
532 Full SAT solver for Kconfig
533 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
535 Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in
536 the previous subsection, work has been done however to express in boolean
537 abstraction the inferred semantics of Kconfig to translate Kconfig logic into
538 boolean formulas and run a SAT solver on it [1]. Another known related project
539 is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has
540 been introduced first with [5]. The basic concept of undertaker is to exract
541 variability models from Kconfig, and put them together with a propositional
542 formula extracted from CPP #ifdefs and build-rules into a SAT solver in order
543 to find dead code, dead files, and dead symbols. If using a SAT solver is
544 desirable on Kconfig one approach would be to evaluate repurposing such efforts
545 somehow on Kconfig. There is enough interest from mentors of existing projects
546 to not only help advise how to integrate this work upstream but also help
547 maintain it long term. Interested developers should visit:
549 http://kernelnewbies.org/KernelProjects/kconfig-sat
551 [0] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
552 [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
553 [2] https://cados.cs.fau.de
554 [3] https://vamos.cs.fau.de
555 [4] https://undertaker.cs.fau.de
556 [5] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf
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