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Linux/Documentation/kbuild/kconfig-language.txt

  1 Introduction
  2 ------------
  3 
  4 The configuration database is a collection of configuration options
  5 organized in a tree structure:
  6 
  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         +- ...
 19 
 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.
 23 
 24 Menu entries
 25 ------------
 26 
 27 Most entries define a config option; all other entries help to organize
 28 them. A single configuration option is defined like this:
 29 
 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.  ...
 36 
 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.
 44 
 45 Menu attributes
 46 ---------------
 47 
 48 A menu entry can have a number of attributes. Not all of them are
 49 applicable everywhere (see syntax).
 50 
 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:
 56 
 57         bool "Networking support"
 58   and
 59         bool
 60         prompt "Networking support"
 61 
 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".
 66 
 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".
 79 
 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".
 84 
 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:
 90 
 91         bool "foo" if BAR
 92         default y if BAR
 93   and
 94         depends on BAR
 95         bool "foo"
 96         default y
 97 
 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.
115 
116 - weak reverse dependencies: "imply" <symbol> ["if" <expr>]
117   This is similar to "select" as it enforces a lower limit on another
118   symbol except that the "implied" symbol's value may still be set to n
119   from a direct dependency or with a visible prompt.
120 
121   Given the following example:
122 
123   config FOO
124         tristate
125         imply BAZ
126 
127   config BAZ
128         tristate
129         depends on BAR
130 
131   The following values are possible:
132 
133         FOO             BAR             BAZ's default   choice for BAZ
134         ---             ---             -------------   --------------
135         n               y               n               N/m/y
136         m               y               m               M/y/n
137         y               y               y               Y/n
138         y               n               *               N
139 
140   This is useful e.g. with multiple drivers that want to indicate their
141   ability to hook into a secondary subsystem while allowing the user to
142   configure that subsystem out without also having to unset these drivers.
143 
144 - limiting menu display: "visible if" <expr>
145   This attribute is only applicable to menu blocks, if the condition is
146   false, the menu block is not displayed to the user (the symbols
147   contained there can still be selected by other symbols, though). It is
148   similar to a conditional "prompt" attribute for individual menu
149   entries. Default value of "visible" is true.
150 
151 - numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
152   This allows to limit the range of possible input values for int
153   and hex symbols. The user can only input a value which is larger than
154   or equal to the first symbol and smaller than or equal to the second
155   symbol.
156 
157 - help text: "help" or "---help---"
158   This defines a help text. The end of the help text is determined by
159   the indentation level, this means it ends at the first line which has
160   a smaller indentation than the first line of the help text.
161   "---help---" and "help" do not differ in behaviour, "---help---" is
162   used to help visually separate configuration logic from help within
163   the file as an aid to developers.
164 
165 - misc options: "option" <symbol>[=<value>]
166   Various less common options can be defined via this option syntax,
167   which can modify the behaviour of the menu entry and its config
168   symbol. These options are currently possible:
169 
170   - "defconfig_list"
171     This declares a list of default entries which can be used when
172     looking for the default configuration (which is used when the main
173     .config doesn't exists yet.)
174 
175   - "modules"
176     This declares the symbol to be used as the MODULES symbol, which
177     enables the third modular state for all config symbols.
178     At most one symbol may have the "modules" option set.
179 
180   - "env"=<value>
181     This imports the environment variable into Kconfig. It behaves like
182     a default, except that the value comes from the environment, this
183     also means that the behaviour when mixing it with normal defaults is
184     undefined at this point. The symbol is currently not exported back
185     to the build environment (if this is desired, it can be done via
186     another symbol).
187 
188   - "allnoconfig_y"
189     This declares the symbol as one that should have the value y when
190     using "allnoconfig". Used for symbols that hide other symbols.
191 
192 Menu dependencies
193 -----------------
194 
195 Dependencies define the visibility of a menu entry and can also reduce
196 the input range of tristate symbols. The tristate logic used in the
197 expressions uses one more state than normal boolean logic to express the
198 module state. Dependency expressions have the following syntax:
199 
200 <expr> ::= <symbol>                             (1)
201            <symbol> '=' <symbol>                (2)
202            <symbol> '!=' <symbol>               (3)
203            '(' <expr> ')'                       (4)
204            '!' <expr>                           (5)
205            <expr> '&&' <expr>                   (6)
206            <expr> '||' <expr>                   (7)
207 
208 Expressions are listed in decreasing order of precedence. 
209 
210 (1) Convert the symbol into an expression. Boolean and tristate symbols
211     are simply converted into the respective expression values. All
212     other symbol types result in 'n'.
213 (2) If the values of both symbols are equal, it returns 'y',
214     otherwise 'n'.
215 (3) If the values of both symbols are equal, it returns 'n',
216     otherwise 'y'.
217 (4) Returns the value of the expression. Used to override precedence.
218 (5) Returns the result of (2-/expr/).
219 (6) Returns the result of min(/expr/, /expr/).
220 (7) Returns the result of max(/expr/, /expr/).
221 
222 An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
223 respectively for calculations). A menu entry becomes visible when its
224 expression evaluates to 'm' or 'y'.
225 
226 There are two types of symbols: constant and non-constant symbols.
227 Non-constant symbols are the most common ones and are defined with the
228 'config' statement. Non-constant symbols consist entirely of alphanumeric
229 characters or underscores.
230 Constant symbols are only part of expressions. Constant symbols are
231 always surrounded by single or double quotes. Within the quote, any
232 other character is allowed and the quotes can be escaped using '\'.
233 
234 Menu structure
235 --------------
236 
237 The position of a menu entry in the tree is determined in two ways. First
238 it can be specified explicitly:
239 
240 menu "Network device support"
241         depends on NET
242 
243 config NETDEVICES
244         ...
245 
246 endmenu
247 
248 All entries within the "menu" ... "endmenu" block become a submenu of
249 "Network device support". All subentries inherit the dependencies from
250 the menu entry, e.g. this means the dependency "NET" is added to the
251 dependency list of the config option NETDEVICES.
252 
253 The other way to generate the menu structure is done by analyzing the
254 dependencies. If a menu entry somehow depends on the previous entry, it
255 can be made a submenu of it. First, the previous (parent) symbol must
256 be part of the dependency list and then one of these two conditions
257 must be true:
258 - the child entry must become invisible, if the parent is set to 'n'
259 - the child entry must only be visible, if the parent is visible
260 
261 config MODULES
262         bool "Enable loadable module support"
263 
264 config MODVERSIONS
265         bool "Set version information on all module symbols"
266         depends on MODULES
267 
268 comment "module support disabled"
269         depends on !MODULES
270 
271 MODVERSIONS directly depends on MODULES, this means it's only visible if
272 MODULES is different from 'n'. The comment on the other hand is only
273 visible when MODULES is set to 'n'.
274 
275 
276 Kconfig syntax
277 --------------
278 
279 The configuration file describes a series of menu entries, where every
280 line starts with a keyword (except help texts). The following keywords
281 end a menu entry:
282 - config
283 - menuconfig
284 - choice/endchoice
285 - comment
286 - menu/endmenu
287 - if/endif
288 - source
289 The first five also start the definition of a menu entry.
290 
291 config:
292 
293         "config" <symbol>
294         <config options>
295 
296 This defines a config symbol <symbol> and accepts any of above
297 attributes as options.
298 
299 menuconfig:
300         "menuconfig" <symbol>
301         <config options>
302 
303 This is similar to the simple config entry above, but it also gives a
304 hint to front ends, that all suboptions should be displayed as a
305 separate list of options. To make sure all the suboptions will really
306 show up under the menuconfig entry and not outside of it, every item
307 from the <config options> list must depend on the menuconfig symbol.
308 In practice, this is achieved by using one of the next two constructs:
309 
310 (1):
311 menuconfig M
312 if M
313     config C1
314     config C2
315 endif
316 
317 (2):
318 menuconfig M
319 config C1
320     depends on M
321 config C2
322     depends on M
323 
324 In the following examples (3) and (4), C1 and C2 still have the M
325 dependency, but will not appear under menuconfig M anymore, because
326 of C0, which doesn't depend on M:
327 
328 (3):
329 menuconfig M
330     config C0
331 if M
332     config C1
333     config C2
334 endif
335 
336 (4):
337 menuconfig M
338 config C0
339 config C1
340     depends on M
341 config C2
342     depends on M
343 
344 choices:
345 
346         "choice" [symbol]
347         <choice options>
348         <choice block>
349         "endchoice"
350 
351 This defines a choice group and accepts any of the above attributes as
352 options. A choice can only be of type bool or tristate.  If no type is
353 specified for a choice, it's type will be determined by the type of
354 the first choice element in the group or remain unknown if none of the
355 choice elements have a type specified, as well.
356 
357 While a boolean choice only allows a single config entry to be
358 selected, a tristate choice also allows any number of config entries
359 to be set to 'm'. This can be used if multiple drivers for a single
360 hardware exists and only a single driver can be compiled/loaded into
361 the kernel, but all drivers can be compiled as modules.
362 
363 A choice accepts another option "optional", which allows to set the
364 choice to 'n' and no entry needs to be selected.
365 If no [symbol] is associated with a choice, then you can not have multiple
366 definitions of that choice. If a [symbol] is associated to the choice,
367 then you may define the same choice (ie. with the same entries) in another
368 place.
369 
370 comment:
371 
372         "comment" <prompt>
373         <comment options>
374 
375 This defines a comment which is displayed to the user during the
376 configuration process and is also echoed to the output files. The only
377 possible options are dependencies.
378 
379 menu:
380 
381         "menu" <prompt>
382         <menu options>
383         <menu block>
384         "endmenu"
385 
386 This defines a menu block, see "Menu structure" above for more
387 information. The only possible options are dependencies and "visible"
388 attributes.
389 
390 if:
391 
392         "if" <expr>
393         <if block>
394         "endif"
395 
396 This defines an if block. The dependency expression <expr> is appended
397 to all enclosed menu entries.
398 
399 source:
400 
401         "source" <prompt>
402 
403 This reads the specified configuration file. This file is always parsed.
404 
405 mainmenu:
406 
407         "mainmenu" <prompt>
408 
409 This sets the config program's title bar if the config program chooses
410 to use it. It should be placed at the top of the configuration, before any
411 other statement.
412 
413 
414 Kconfig hints
415 -------------
416 This is a collection of Kconfig tips, most of which aren't obvious at
417 first glance and most of which have become idioms in several Kconfig
418 files.
419 
420 Adding common features and make the usage configurable
421 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
422 It is a common idiom to implement a feature/functionality that are
423 relevant for some architectures but not all.
424 The recommended way to do so is to use a config variable named HAVE_*
425 that is defined in a common Kconfig file and selected by the relevant
426 architectures.
427 An example is the generic IOMAP functionality.
428 
429 We would in lib/Kconfig see:
430 
431 # Generic IOMAP is used to ...
432 config HAVE_GENERIC_IOMAP
433 
434 config GENERIC_IOMAP
435         depends on HAVE_GENERIC_IOMAP && FOO
436 
437 And in lib/Makefile we would see:
438 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
439 
440 For each architecture using the generic IOMAP functionality we would see:
441 
442 config X86
443         select ...
444         select HAVE_GENERIC_IOMAP
445         select ...
446 
447 Note: we use the existing config option and avoid creating a new
448 config variable to select HAVE_GENERIC_IOMAP.
449 
450 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
451 introduced to overcome the limitation of select which will force a
452 config option to 'y' no matter the dependencies.
453 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
454 situation where select forces a symbol equals to 'y'.
455 
456 Build as module only
457 ~~~~~~~~~~~~~~~~~~~~
458 To restrict a component build to module-only, qualify its config symbol
459 with "depends on m".  E.g.:
460 
461 config FOO
462         depends on BAR && m
463 
464 limits FOO to module (=m) or disabled (=n).
465 
466 Kconfig recursive dependency limitations
467 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
468 
469 If you've hit the Kconfig error: "recursive dependency detected" you've run
470 into a recursive dependency issue with Kconfig, a recursive dependency can be
471 summarized as a circular dependency. The kconfig tools need to ensure that
472 Kconfig files comply with specified configuration requirements. In order to do
473 that kconfig must determine the values that are possible for all Kconfig
474 symbols, this is currently not possible if there is a circular relation
475 between two or more Kconfig symbols. For more details refer to the "Simple
476 Kconfig recursive issue" subsection below. Kconfig does not do recursive
477 dependency resolution; this has a few implications for Kconfig file writers.
478 We'll first explain why this issues exists and then provide an example
479 technical limitation which this brings upon Kconfig developers. Eager
480 developers wishing to try to address this limitation should read the next
481 subsections.
482 
483 Simple Kconfig recursive issue
484 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
485 
486 Read: Documentation/kbuild/Kconfig.recursion-issue-01
487 
488 Test with:
489 
490 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
491 
492 Cumulative Kconfig recursive issue
493 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
494 
495 Read: Documentation/kbuild/Kconfig.recursion-issue-02
496 
497 Test with:
498 
499 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
500 
501 Practical solutions to kconfig recursive issue
502 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
503 
504 Developers who run into the recursive Kconfig issue have three options
505 at their disposal. We document them below and also provide a list of
506 historical issues resolved through these different solutions.
507 
508   a) Remove any superfluous "select FOO" or "depends on FOO"
509   b) Match dependency semantics:
510         b1) Swap all "select FOO" to "depends on FOO" or,
511         b2) Swap all "depends on FOO" to "select FOO"
512   c) Consider the use of "imply" instead of "select"
513 
514 The resolution to a) can be tested with the sample Kconfig file
515 Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
516 of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
517 since CORE_BELL_A depends on CORE. At times it may not be possible to remove
518 some dependency criteria, for such cases you can work with solution b).
519 
520 The two different resolutions for b) can be tested in the sample Kconfig file
521 Documentation/kbuild/Kconfig.recursion-issue-02.
522 
523 Below is a list of examples of prior fixes for these types of recursive issues;
524 all errors appear to involve one or more select's and one or more "depends on".
525 
526 commit          fix
527 ======          ===
528 06b718c01208    select A -> depends on A
529 c22eacfe82f9    depends on A -> depends on B
530 6a91e854442c    select A -> depends on A
531 118c565a8f2e    select A -> select B
532 f004e5594705    select A -> depends on A
533 c7861f37b4c6    depends on A -> (null)
534 80c69915e5fb    select A -> (null)              (1)
535 c2218e26c0d0    select A -> depends on A        (1)
536 d6ae99d04e1c    select A -> depends on A
537 95ca19cf8cbf    select A -> depends on A
538 8f057d7bca54    depends on A -> (null)
539 8f057d7bca54    depends on A -> select A
540 a0701f04846e    select A -> depends on A
541 0c8b92f7f259    depends on A -> (null)
542 e4e9e0540928    select A -> depends on A        (2)
543 7453ea886e87    depends on A > (null)           (1)
544 7b1fff7e4fdf    select A -> depends on A
545 86c747d2a4f0    select A -> depends on A
546 d9f9ab51e55e    select A -> depends on A
547 0c51a4d8abd6    depends on A -> select A        (3)
548 e98062ed6dc4    select A -> depends on A        (3)
549 91e5d284a7f1    select A -> (null)
550 
551 (1) Partial (or no) quote of error.
552 (2) That seems to be the gist of that fix.
553 (3) Same error.
554 
555 Future kconfig work
556 ~~~~~~~~~~~~~~~~~~~
557 
558 Work on kconfig is welcomed on both areas of clarifying semantics and on
559 evaluating the use of a full SAT solver for it. A full SAT solver can be
560 desirable to enable more complex dependency mappings and / or queries,
561 for instance on possible use case for a SAT solver could be that of handling
562 the current known recursive dependency issues. It is not known if this would
563 address such issues but such evaluation is desirable. If support for a full SAT
564 solver proves too complex or that it cannot address recursive dependency issues
565 Kconfig should have at least clear and well defined semantics which also
566 addresses and documents limitations or requirements such as the ones dealing
567 with recursive dependencies.
568 
569 Further work on both of these areas is welcomed on Kconfig. We elaborate
570 on both of these in the next two subsections.
571 
572 Semantics of Kconfig
573 ~~~~~~~~~~~~~~~~~~~~
574 
575 The use of Kconfig is broad, Linux is now only one of Kconfig's users:
576 one study has completed a broad analysis of Kconfig use in 12 projects [0].
577 Despite its widespread use, and although this document does a reasonable job
578 in documenting basic Kconfig syntax a more precise definition of Kconfig
579 semantics is welcomed. One project deduced Kconfig semantics through
580 the use of the xconfig configurator [1]. Work should be done to confirm if
581 the deduced semantics matches our intended Kconfig design goals.
582 
583 Having well defined semantics can be useful for tools for practical
584 evaluation of depenencies, for instance one such use known case was work to
585 express in boolean abstraction of the inferred semantics of Kconfig to
586 translate Kconfig logic into boolean formulas and run a SAT solver on this to
587 find dead code / features (always inactive), 114 dead features were found in
588 Linux using this methodology [1] (Section 8: Threats to validity).
589 
590 Confirming this could prove useful as Kconfig stands as one of the the leading
591 industrial variability modeling languages [1] [2]. Its study would help
592 evaluate practical uses of such languages, their use was only theoretical
593 and real world requirements were not well understood. As it stands though
594 only reverse engineering techniques have been used to deduce semantics from
595 variability modeling languages such as Kconfig [3].
596 
597 [0] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
598 [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
599 [2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
600 [3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf
601 
602 Full SAT solver for Kconfig
603 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
604 
605 Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in
606 the previous subsection, work has been done however to express in boolean
607 abstraction the inferred semantics of Kconfig to translate Kconfig logic into
608 boolean formulas and run a SAT solver on it [1]. Another known related project
609 is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has
610 been introduced first with [5].  The basic concept of undertaker is to exract
611 variability models from Kconfig, and put them together with a propositional
612 formula extracted from CPP #ifdefs and build-rules into a SAT solver in order
613 to find dead code, dead files, and dead symbols. If using a SAT solver is
614 desirable on Kconfig one approach would be to evaluate repurposing such efforts
615 somehow on Kconfig. There is enough interest from mentors of existing projects
616 to not only help advise how to integrate this work upstream but also help
617 maintain it long term. Interested developers should visit:
618 
619 http://kernelnewbies.org/KernelProjects/kconfig-sat
620 
621 [0] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
622 [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
623 [2] https://cados.cs.fau.de
624 [3] https://vamos.cs.fau.de
625 [4] https://undertaker.cs.fau.de
626 [5] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf

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