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  1 Introduction
  2 ------------
  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
  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         +- ...
 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.
 24 Menu entries
 25 ------------
 27 Most entries define a config option; all other entries help to organize
 28 them. A single configuration option is defined like this:
 30 config MODVERSIONS
 31         bool "Set version information on all module symbols"
 32         depends on MODULES
 33         help
 34           Usually, modules have to be recompiled whenever you switch to a new
 35           kernel.  ...
 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.
 45 Menu attributes
 46 ---------------
 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
 55   are equivalent:
 57         bool "Networking support"
 58   and
 59         bool
 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
 65   with "if".
 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
 76   be overridden by him.
 77   Optionally, dependencies only for this default value can be added with
 78   "if".
 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:
 91         bool "foo" if BAR
 92         default y if BAR
 93   and
 94         depends on BAR
 95         bool "foo"
 96         default y
 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.
106   Note:
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
127   symbol.
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:
142   - "defconfig_list"
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.)
147   - "modules"
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.
152   - "env"=<value>
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
158     another symbol).
160   - "allnoconfig_y"
161     This declares the symbol as one that should have the value y when
162     using "allnoconfig". Used for symbols that hide other symbols.
164 Menu dependencies
165 -----------------
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)
175            '(' <expr> ')'                       (4)
176            '!' <expr>                           (5)
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',
186     otherwise 'n'.
187 (3) If the values of both symbols are equal, it returns 'n',
188     otherwise 'y'.
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 '\'.
206 Menu structure
207 --------------
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"
213         depends on NET
215 config NETDEVICES
216         ...
218 endmenu
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
229 must be true:
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
233 config MODULES
234         bool "Enable loadable module support"
236 config MODVERSIONS
237         bool "Set version information on all module symbols"
238         depends on MODULES
240 comment "module support disabled"
241         depends on !MODULES
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'.
248 Kconfig syntax
249 --------------
251 The configuration file describes a series of menu entries, where every
252 line starts with a keyword (except help texts). The following keywords
253 end a menu entry:
254 - config
255 - menuconfig
256 - choice/endchoice
257 - comment
258 - menu/endmenu
259 - if/endif
260 - source
261 The first five also start the definition of a menu entry.
263 config:
265         "config" <symbol>
266         <config options>
268 This defines a config symbol <symbol> and accepts any of above
269 attributes as options.
271 menuconfig:
272         "menuconfig" <symbol>
273         <config options>
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. To make sure all the suboptions will really
278 show up under the menuconfig entry and not outside of it, every item
279 from the <config options> list must depend on the menuconfig symbol.
280 In practice, this is achieved by using one of the next two constructs:
282 (1):
283 menuconfig M
284 if M
285     config C1
286     config C2
287 endif
289 (2):
290 menuconfig M
291 config C1
292     depends on M
293 config C2
294     depends on M
296 In the following examples (3) and (4), C1 and C2 still have the M
297 dependency, but will not appear under menuconfig M anymore, because
298 of C0, which doesn't depend on M:
300 (3):
301 menuconfig M
302     config C0
303 if M
304     config C1
305     config C2
306 endif
308 (4):
309 menuconfig M
310 config C0
311 config C1
312     depends on M
313 config C2
314     depends on M
316 choices:
318         "choice" [symbol]
319         <choice options>
320         <choice block>
321         "endchoice"
323 This defines a choice group and accepts any of the above attributes as
324 options. A choice can only be of type bool or tristate.  If no type is
325 specified for a choice, it's type will be determined by the type of
326 the first choice element in the group or remain unknown if none of the
327 choice elements have a type specified, as well.
329 While a boolean choice only allows a single config entry to be
330 selected, a tristate choice also allows any number of config entries
331 to be set to 'm'. This can be used if multiple drivers for a single
332 hardware exists and only a single driver can be compiled/loaded into
333 the kernel, but all drivers can be compiled as modules.
335 A choice accepts another option "optional", which allows to set the
336 choice to 'n' and no entry needs to be selected.
337 If no [symbol] is associated with a choice, then you can not have multiple
338 definitions of that choice. If a [symbol] is associated to the choice,
339 then you may define the same choice (ie. with the same entries) in another
340 place.
342 comment:
344         "comment" <prompt>
345         <comment options>
347 This defines a comment which is displayed to the user during the
348 configuration process and is also echoed to the output files. The only
349 possible options are dependencies.
351 menu:
353         "menu" <prompt>
354         <menu options>
355         <menu block>
356         "endmenu"
358 This defines a menu block, see "Menu structure" above for more
359 information. The only possible options are dependencies and "visible"
360 attributes.
362 if:
364         "if" <expr>
365         <if block>
366         "endif"
368 This defines an if block. The dependency expression <expr> is appended
369 to all enclosed menu entries.
371 source:
373         "source" <prompt>
375 This reads the specified configuration file. This file is always parsed.
377 mainmenu:
379         "mainmenu" <prompt>
381 This sets the config program's title bar if the config program chooses
382 to use it. It should be placed at the top of the configuration, before any
383 other statement.
386 Kconfig hints
387 -------------
388 This is a collection of Kconfig tips, most of which aren't obvious at
389 first glance and most of which have become idioms in several Kconfig
390 files.
392 Adding common features and make the usage configurable
393 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
394 It is a common idiom to implement a feature/functionality that are
395 relevant for some architectures but not all.
396 The recommended way to do so is to use a config variable named HAVE_*
397 that is defined in a common Kconfig file and selected by the relevant
398 architectures.
399 An example is the generic IOMAP functionality.
401 We would in lib/Kconfig see:
403 # Generic IOMAP is used to ...
406 config GENERIC_IOMAP
407         depends on HAVE_GENERIC_IOMAP && FOO
409 And in lib/Makefile we would see:
410 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
412 For each architecture using the generic IOMAP functionality we would see:
414 config X86
415         select ...
416         select HAVE_GENERIC_IOMAP
417         select ...
419 Note: we use the existing config option and avoid creating a new
420 config variable to select HAVE_GENERIC_IOMAP.
422 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
423 introduced to overcome the limitation of select which will force a
424 config option to 'y' no matter the dependencies.
425 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
426 situation where select forces a symbol equals to 'y'.
428 Build as module only
429 ~~~~~~~~~~~~~~~~~~~~
430 To restrict a component build to module-only, qualify its config symbol
431 with "depends on m".  E.g.:
433 config FOO
434         depends on BAR && m
436 limits FOO to module (=m) or disabled (=n).
438 Kconfig recursive dependency limitations
439 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
441 If you've hit the Kconfig error: "recursive dependency detected" you've run
442 into a recursive dependency issue with Kconfig, a recursive dependency can be
443 summarized as a circular dependency. The kconfig tools need to ensure that
444 Kconfig files comply with specified configuration requirements. In order to do
445 that kconfig must determine the values that are possible for all Kconfig
446 symbols, this is currently not possible if there is a circular relation
447 between two or more Kconfig symbols. For more details refer to the "Simple
448 Kconfig recursive issue" subsection below. Kconfig does not do recursive
449 dependency resolution; this has a few implications for Kconfig file writers.
450 We'll first explain why this issues exists and then provide an example
451 technical limitation which this brings upon Kconfig developers. Eager
452 developers wishing to try to address this limitation should read the next
453 subsections.
455 Simple Kconfig recursive issue
456 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
458 Read: Documentation/kbuild/Kconfig.recursion-issue-01
460 Test with:
462 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
464 Cumulative Kconfig recursive issue
465 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
467 Read: Documentation/kbuild/Kconfig.recursion-issue-02
469 Test with:
471 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
473 Practical solutions to kconfig recursive issue
474 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
476 Developers who run into the recursive Kconfig issue have three options
477 at their disposal. We document them below and also provide a list of
478 historical issues resolved through these different solutions.
480   a) Remove any superfluous "select FOO" or "depends on FOO"
481   b) Match dependency semantics:
482         b1) Swap all "select FOO" to "depends on FOO" or,
483         b2) Swap all "depends on FOO" to "select FOO"
485 The resolution to a) can be tested with the sample Kconfig file
486 Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
487 of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
488 since CORE_BELL_A depends on CORE. At times it may not be possible to remove
489 some dependency criteria, for such cases you can work with solution b).
491 The two different resolutions for b) can be tested in the sample Kconfig file
492 Documentation/kbuild/Kconfig.recursion-issue-02.
494 Below is a list of examples of prior fixes for these types of recursive issues;
495 all errors appear to involve one or more select's and one or more "depends on".
497 commit          fix
498 ======          ===
499 06b718c01208    select A -> depends on A
500 c22eacfe82f9    depends on A -> depends on B
501 6a91e854442c    select A -> depends on A
502 118c565a8f2e    select A -> select B
503 f004e5594705    select A -> depends on A
504 c7861f37b4c6    depends on A -> (null)
505 80c69915e5fb    select A -> (null)              (1)
506 c2218e26c0d0    select A -> depends on A        (1)
507 d6ae99d04e1c    select A -> depends on A
508 95ca19cf8cbf    select A -> depends on A
509 8f057d7bca54    depends on A -> (null)
510 8f057d7bca54    depends on A -> select A
511 a0701f04846e    select A -> depends on A
512 0c8b92f7f259    depends on A -> (null)
513 e4e9e0540928    select A -> depends on A        (2)
514 7453ea886e87    depends on A > (null)           (1)
515 7b1fff7e4fdf    select A -> depends on A
516 86c747d2a4f0    select A -> depends on A
517 d9f9ab51e55e    select A -> depends on A
518 0c51a4d8abd6    depends on A -> select A        (3)
519 e98062ed6dc4    select A -> depends on A        (3)
520 91e5d284a7f1    select A -> (null)
522 (1) Partial (or no) quote of error.
523 (2) That seems to be the gist of that fix.
524 (3) Same error.
526 Future kconfig work
527 ~~~~~~~~~~~~~~~~~~~
529 Work on kconfig is welcomed on both areas of clarifying semantics and on
530 evaluating the use of a full SAT solver for it. A full SAT solver can be
531 desirable to enable more complex dependency mappings and / or queries,
532 for instance on possible use case for a SAT solver could be that of handling
533 the current known recursive dependency issues. It is not known if this would
534 address such issues but such evaluation is desirable. If support for a full SAT
535 solver proves too complex or that it cannot address recursive dependency issues
536 Kconfig should have at least clear and well defined semantics which also
537 addresses and documents limitations or requirements such as the ones dealing
538 with recursive dependencies.
540 Further work on both of these areas is welcomed on Kconfig. We elaborate
541 on both of these in the next two subsections.
543 Semantics of Kconfig
544 ~~~~~~~~~~~~~~~~~~~~
546 The use of Kconfig is broad, Linux is now only one of Kconfig's users:
547 one study has completed a broad analysis of Kconfig use in 12 projects [0].
548 Despite its widespread use, and although this document does a reasonable job
549 in documenting basic Kconfig syntax a more precise definition of Kconfig
550 semantics is welcomed. One project deduced Kconfig semantics through
551 the use of the xconfig configurator [1]. Work should be done to confirm if
552 the deduced semantics matches our intended Kconfig design goals.
554 Having well defined semantics can be useful for tools for practical
555 evaluation of depenencies, for instance one such use known case was work to
556 express in boolean abstraction of the inferred semantics of Kconfig to
557 translate Kconfig logic into boolean formulas and run a SAT solver on this to
558 find dead code / features (always inactive), 114 dead features were found in
559 Linux using this methodology [1] (Section 8: Threats to validity).
561 Confirming this could prove useful as Kconfig stands as one of the the leading
562 industrial variability modeling languages [1] [2]. Its study would help
563 evaluate practical uses of such languages, their use was only theoretical
564 and real world requirements were not well understood. As it stands though
565 only reverse engineering techniques have been used to deduce semantics from
566 variability modeling languages such as Kconfig [3].
568 [0]
569 [1]
570 [2]
571 [3]
573 Full SAT solver for Kconfig
574 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
576 Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in
577 the previous subsection, work has been done however to express in boolean
578 abstraction the inferred semantics of Kconfig to translate Kconfig logic into
579 boolean formulas and run a SAT solver on it [1]. Another known related project
580 is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has
581 been introduced first with [5].  The basic concept of undertaker is to exract
582 variability models from Kconfig, and put them together with a propositional
583 formula extracted from CPP #ifdefs and build-rules into a SAT solver in order
584 to find dead code, dead files, and dead symbols. If using a SAT solver is
585 desirable on Kconfig one approach would be to evaluate repurposing such efforts
586 somehow on Kconfig. There is enough interest from mentors of existing projects
587 to not only help advise how to integrate this work upstream but also help
588 maintain it long term. Interested developers should visit:
592 [0]
593 [1]
594 [2]
595 [3]
596 [4]
597 [5]

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