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1================
2Kconfig Language
3================
4
5Introduction
6------------
7
8The configuration database is a collection of configuration options
9organized in a tree structure::
10
11 +- Code maturity level options
12 | +- Prompt for development and/or incomplete code/drivers
13 +- General setup
14 | +- Networking support
15 | +- System V IPC
16 | +- BSD Process Accounting
17 | +- Sysctl support
18 +- Loadable module support
19 | +- Enable loadable module support
20 | +- Set version information on all module symbols
21 | +- Kernel module loader
22 +- ...
23
24Every entry has its own dependencies. These dependencies are used
25to determine the visibility of an entry. Any child entry is only
26visible if its parent entry is also visible.
27
28Menu entries
29------------
30
31Most entries define a config option; all other entries help to organize
32them. A single configuration option is defined like this::
33
34 config MODVERSIONS
35 bool "Set version information on all module symbols"
36 depends on MODULES
37 help
38 Usually, modules have to be recompiled whenever you switch to a new
39 kernel. ...
40
41Every line starts with a key word and can be followed by multiple
42arguments. "config" starts a new config entry. The following lines
43define attributes for this config option. Attributes can be the type of
44the config option, input prompt, dependencies, help text and default
45values. A config option can be defined multiple times with the same
46name, but every definition can have only a single input prompt and the
47type must not conflict.
48
49Menu attributes
50---------------
51
52A menu entry can have a number of attributes. Not all of them are
53applicable everywhere (see syntax).
54
55- type definition: "bool"/"tristate"/"string"/"hex"/"int"
56
57 Every config option must have a type. There are only two basic types:
58 tristate and string; the other types are based on these two. The type
59 definition optionally accepts an input prompt, so these two examples
60 are equivalent::
61
62 bool "Networking support"
63
64 and::
65
66 bool
67 prompt "Networking support"
68
69- input prompt: "prompt" <prompt> ["if" <expr>]
70
71 Every menu entry can have at most one prompt, which is used to display
72 to the user. Optionally dependencies only for this prompt can be added
73 with "if". If a prompt is not present, the config option is a non-visible
74 symbol, meaning its value cannot be directly changed by the user (such as
75 altering the value in ``.config``) and the option will not appear in any
76 config menus. Its value can only be set via "default" and "select" (see
77 below).
78
79- default value: "default" <expr> ["if" <expr>]
80
81 A config option can have any number of default values. If multiple
82 default values are visible, only the first defined one is active.
83 Default values are not limited to the menu entry where they are
84 defined. This means the default can be defined somewhere else or be
85 overridden by an earlier definition.
86 The default value is only assigned to the config symbol if no other
87 value was set by the user (via the input prompt above). If an input
88 prompt is visible the default value is presented to the user and can
89 be overridden by him.
90 Optionally, dependencies only for this default value can be added with
91 "if".
92
93 The default value deliberately defaults to 'n' in order to avoid bloating the
94 build. With few exceptions, new config options should not change this. The
95 intent is for "make oldconfig" to add as little as possible to the config from
96 release to release.
97
98 Note:
99 Things that merit "default y/m" include:
100
101 a) A new Kconfig option for something that used to always be built
102 should be "default y".
103
104 b) A new gatekeeping Kconfig option that hides/shows other Kconfig
105 options (but does not generate any code of its own), should be
106 "default y" so people will see those other options.
107
108 c) Sub-driver behavior or similar options for a driver that is
109 "default n". This allows you to provide sane defaults.
110
111 d) Hardware or infrastructure that everybody expects, such as CONFIG_NET
112 or CONFIG_BLOCK. These are rare exceptions.
113
114- type definition + default value::
115
116 "def_bool"/"def_tristate" <expr> ["if" <expr>]
117
118 This is a shorthand notation for a type definition plus a value.
119 Optionally dependencies for this default value can be added with "if".
120
121- dependencies: "depends on" <expr> ["if" <expr>]
122
123 This defines a dependency for this menu entry. If multiple
124 dependencies are defined, they are connected with '&&'. Dependencies
125 are applied to all other options within this menu entry (which also
126 accept an "if" expression), so these two examples are equivalent::
127
128 bool "foo" if BAR
129 default y if BAR
130
131 and::
132
133 depends on BAR
134 bool "foo"
135 default y
136
137 The dependency definition itself may be conditional by appending "if"
138 followed by an expression. For example::
139
140 config FOO
141 tristate
142 depends on BAR if BAZ
143
144 meaning that FOO is constrained by the value of BAR only if BAZ is
145 also set.
146
147- reverse dependencies: "select" <symbol> ["if" <expr>]
148
149 While normal dependencies reduce the upper limit of a symbol (see
150 below), reverse dependencies can be used to force a lower limit of
151 another symbol. The value of the current menu symbol is used as the
152 minimal value <symbol> can be set to. If <symbol> is selected multiple
153 times, the limit is set to the largest selection.
154 Reverse dependencies can only be used with boolean or tristate
155 symbols.
156
157 Note:
158 select should be used with care. select will force
159 a symbol to a value without visiting the dependencies.
160 By abusing select you are able to select a symbol FOO even
161 if FOO depends on BAR that is not set.
162 In general use select only for non-visible symbols
163 (no prompts anywhere) and for symbols with no dependencies.
164 That will limit the usefulness but on the other hand avoid
165 the illegal configurations all over.
166
167 If "select" <symbol> is followed by "if" <expr>, <symbol> will be
168 selected by the logical AND of the value of the current menu symbol
169 and <expr>. This means, the lower limit can be downgraded due to the
170 presence of "if" <expr>. This behavior may seem weird, but we rely on
171 it. (The future of this behavior is undecided.)
172
173- weak reverse dependencies: "imply" <symbol> ["if" <expr>]
174
175 This is similar to "select" as it enforces a lower limit on another
176 symbol except that the "implied" symbol's value may still be set to n
177 from a direct dependency or with a visible prompt.
178
179 Given the following example::
180
181 config FOO
182 tristate "foo"
183 imply BAZ
184
185 config BAZ
186 tristate "baz"
187 depends on BAR
188
189 The following values are possible:
190
191 === === ============= ==============
192 FOO BAR BAZ's default choice for BAZ
193 === === ============= ==============
194 n y n N/m/y
195 m y m M/y/n
196 y y y Y/m/n
197 n m n N/m
198 m m m M/n
199 y m m M/n
200 y n * N
201 === === ============= ==============
202
203 This is useful e.g. with multiple drivers that want to indicate their
204 ability to hook into a secondary subsystem while allowing the user to
205 configure that subsystem out without also having to unset these drivers.
206
207 Note: If the feature provided by BAZ is highly desirable for FOO,
208 FOO should imply not only BAZ, but also its dependency BAR::
209
210 config FOO
211 tristate "foo"
212 imply BAR
213 imply BAZ
214
215 Note: If "imply" <symbol> is followed by "if" <expr>, the default of <symbol>
216 will be the logical AND of the value of the current menu symbol and <expr>.
217 (The future of this behavior is undecided.)
218
219- limiting menu display: "visible if" <expr>
220
221 This attribute is only applicable to menu blocks, if the condition is
222 false, the menu block is not displayed to the user (the symbols
223 contained there can still be selected by other symbols, though). It is
224 similar to a conditional "prompt" attribute for individual menu
225 entries. Default value of "visible" is true.
226
227- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
228
229 This allows limiting the range of possible input values for int
230 and hex symbols. The user can only input a value which is larger than
231 or equal to the first symbol and smaller than or equal to the second
232 symbol.
233
234- help text: "help"
235
236 This defines a help text. The end of the help text is determined by
237 the indentation level, this means it ends at the first line which has
238 a smaller indentation than the first line of the help text.
239
240- module attribute: "modules"
241 This declares the symbol to be used as the MODULES symbol, which
242 enables the third modular state for all config symbols.
243 At most one symbol may have the "modules" option set.
244
245- transitional attribute: "transitional"
246 This declares the symbol as transitional, meaning it should be processed
247 during configuration but omitted from newly written .config files.
248 Transitional symbols are useful for backward compatibility during config
249 option migrations - they allow olddefconfig to process existing .config
250 files while ensuring the old option doesn't appear in new configurations.
251
252 A transitional symbol:
253 - Has no prompt (is not visible to users in menus)
254 - Is processed normally during configuration (values are read and used)
255 - Can be referenced in default expressions of other symbols
256 - Is not written to new .config files
257 - Cannot have any other properties (it is a pass-through option)
258
259 Example migration from OLD_NAME to NEW_NAME::
260
261 config NEW_NAME
262 bool "New option name"
263 default OLD_NAME
264 help
265 This replaces the old CONFIG_OLD_NAME option.
266
267 config OLD_NAME
268 bool
269 transitional
270 help
271 Transitional config for OLD_NAME to NEW_NAME migration.
272
273 With this setup, existing .config files with "CONFIG_OLD_NAME=y" will
274 result in "CONFIG_NEW_NAME=y" being set, while CONFIG_OLD_NAME will be
275 omitted from newly written .config files.
276
277Menu dependencies
278-----------------
279
280Dependencies define the visibility of a menu entry and can also reduce
281the input range of tristate symbols. The tristate logic used in the
282expressions uses one more state than normal boolean logic to express the
283module state. Dependency expressions have the following syntax::
284
285 <expr> ::= <symbol> (1)
286 <symbol> '=' <symbol> (2)
287 <symbol> '!=' <symbol> (3)
288 <symbol1> '<' <symbol2> (4)
289 <symbol1> '>' <symbol2> (4)
290 <symbol1> '<=' <symbol2> (4)
291 <symbol1> '>=' <symbol2> (4)
292 '(' <expr> ')' (5)
293 '!' <expr> (6)
294 <expr> '&&' <expr> (7)
295 <expr> '||' <expr> (8)
296
297Expressions are listed in decreasing order of precedence.
298
299(1) Convert the symbol into an expression. Boolean and tristate symbols
300 are simply converted into the respective expression values. All
301 other symbol types result in 'n'.
302(2) If the values of both symbols are equal, it returns 'y',
303 otherwise 'n'.
304(3) If the values of both symbols are equal, it returns 'n',
305 otherwise 'y'.
306(4) If value of <symbol1> is respectively lower, greater, lower-or-equal,
307 or greater-or-equal than value of <symbol2>, it returns 'y',
308 otherwise 'n'.
309(5) Returns the value of the expression. Used to override precedence.
310(6) Returns the result of (2-/expr/).
311(7) Returns the result of min(/expr/, /expr/).
312(8) Returns the result of max(/expr/, /expr/).
313
314An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
315respectively for calculations). A menu entry becomes visible when its
316expression evaluates to 'm' or 'y'.
317
318There are two types of symbols: constant and non-constant symbols.
319Non-constant symbols are the most common ones and are defined with the
320'config' statement. Non-constant symbols consist entirely of alphanumeric
321characters or underscores.
322Constant symbols are only part of expressions. Constant symbols are
323always surrounded by single or double quotes. Within the quote, any
324other character is allowed and the quotes can be escaped using '\'.
325
326Menu structure
327--------------
328
329The position of a menu entry in the tree is determined in two ways. First
330it can be specified explicitly::
331
332 menu "Network device support"
333 depends on NET
334
335 config NETDEVICES
336 ...
337
338 endmenu
339
340All entries within the "menu" ... "endmenu" block become a submenu of
341"Network device support". All subentries inherit the dependencies from
342the menu entry, e.g. this means the dependency "NET" is added to the
343dependency list of the config option NETDEVICES.
344
345The other way to generate the menu structure is done by analyzing the
346dependencies. If a menu entry somehow depends on the previous entry, it
347can be made a submenu of it. First, the previous (parent) symbol must
348be part of the dependency list and then one of these two conditions
349must be true:
350
351- the child entry must become invisible, if the parent is set to 'n'
352- the child entry must only be visible, if the parent is visible::
353
354 config MODULES
355 bool "Enable loadable module support"
356
357 config MODVERSIONS
358 bool "Set version information on all module symbols"
359 depends on MODULES
360
361 comment "module support disabled"
362 depends on !MODULES
363
364MODVERSIONS directly depends on MODULES, this means it's only visible if
365MODULES is different from 'n'. The comment on the other hand is only
366visible when MODULES is set to 'n'.
367
368
369Kconfig syntax
370--------------
371
372The configuration file describes a series of menu entries, where every
373line starts with a keyword (except help texts). The following keywords
374end a menu entry:
375
376- config
377- menuconfig
378- choice/endchoice
379- comment
380- menu/endmenu
381- if/endif
382- source
383
384The first five also start the definition of a menu entry.
385
386config::
387
388 "config" <symbol>
389 <config options>
390
391This defines a config symbol <symbol> and accepts any of above
392attributes as options.
393
394menuconfig::
395
396 "menuconfig" <symbol>
397 <config options>
398
399This is similar to the simple config entry above, but it also gives a
400hint to front ends, that all suboptions should be displayed as a
401separate list of options. To make sure all the suboptions will really
402show up under the menuconfig entry and not outside of it, every item
403from the <config options> list must depend on the menuconfig symbol.
404In practice, this is achieved by using one of the next two constructs::
405
406 (1):
407 menuconfig M
408 if M
409 config C1
410 config C2
411 endif
412
413 (2):
414 menuconfig M
415 config C1
416 depends on M
417 config C2
418 depends on M
419
420In the following examples (3) and (4), C1 and C2 still have the M
421dependency, but will not appear under menuconfig M anymore, because
422of C0, which doesn't depend on M::
423
424 (3):
425 menuconfig M
426 config C0
427 if M
428 config C1
429 config C2
430 endif
431
432 (4):
433 menuconfig M
434 config C0
435 config C1
436 depends on M
437 config C2
438 depends on M
439
440choices::
441
442 "choice"
443 <choice options>
444 <choice block>
445 "endchoice"
446
447This defines a choice group and accepts "prompt", "default", "depends on", and
448"help" attributes as options.
449
450A choice only allows a single config entry to be selected.
451
452comment::
453
454 "comment" <prompt>
455 <comment options>
456
457This defines a comment which is displayed to the user during the
458configuration process and is also echoed to the output files. The only
459possible options are dependencies.
460
461menu::
462
463 "menu" <prompt>
464 <menu options>
465 <menu block>
466 "endmenu"
467
468This defines a menu block, see "Menu structure" above for more
469information. The only possible options are dependencies and "visible"
470attributes.
471
472if::
473
474 "if" <expr>
475 <if block>
476 "endif"
477
478This defines an if block. The dependency expression <expr> is appended
479to all enclosed menu entries.
480
481source::
482
483 "source" <prompt>
484
485This reads the specified configuration file. This file is always parsed.
486
487mainmenu::
488
489 "mainmenu" <prompt>
490
491This sets the config program's title bar if the config program chooses
492to use it. It should be placed at the top of the configuration, before any
493other statement.
494
495'#' Kconfig source file comment:
496
497An unquoted '#' character anywhere in a source file line indicates
498the beginning of a source file comment. The remainder of that line
499is a comment.
500
501
502Kconfig hints
503-------------
504This is a collection of Kconfig tips, most of which aren't obvious at
505first glance and most of which have become idioms in several Kconfig
506files.
507
508Adding common features and make the usage configurable
509~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
510It is a common idiom to implement a feature/functionality that are
511relevant for some architectures but not all.
512The recommended way to do so is to use a config variable named HAVE_*
513that is defined in a common Kconfig file and selected by the relevant
514architectures.
515An example is the generic IOMAP functionality.
516
517We would in lib/Kconfig see::
518
519 # Generic IOMAP is used to ...
520 config HAVE_GENERIC_IOMAP
521
522 config GENERIC_IOMAP
523 depends on HAVE_GENERIC_IOMAP && FOO
524
525And in lib/Makefile we would see::
526
527 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
528
529For each architecture using the generic IOMAP functionality we would see::
530
531 config X86
532 select ...
533 select HAVE_GENERIC_IOMAP
534 select ...
535
536Note: we use the existing config option and avoid creating a new
537config variable to select HAVE_GENERIC_IOMAP.
538
539Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
540introduced to overcome the limitation of select which will force a
541config option to 'y' no matter the dependencies.
542The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
543situation where select forces a symbol equals to 'y'.
544
545Adding features that need compiler support
546~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
547
548There are several features that need compiler support. The recommended way
549to describe the dependency on the compiler feature is to use "depends on"
550followed by a test macro::
551
552 config STACKPROTECTOR
553 bool "Stack Protector buffer overflow detection"
554 depends on $(cc-option,-fstack-protector)
555 ...
556
557If you need to expose a compiler capability to makefiles and/or C source files,
558`CC_HAS_` is the recommended prefix for the config option::
559
560 config CC_HAS_FOO
561 def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC))
562
563Build as module only
564~~~~~~~~~~~~~~~~~~~~
565To restrict a component build to module-only, qualify its config symbol
566with "depends on m". E.g.::
567
568 config FOO
569 depends on BAR && m
570
571limits FOO to module (=m) or disabled (=n).
572
573Compile-testing
574~~~~~~~~~~~~~~~
575If a config symbol has a dependency, but the code controlled by the config
576symbol can still be compiled if the dependency is not met, it is encouraged to
577increase build coverage by adding an "|| COMPILE_TEST" clause to the
578dependency. This is especially useful for drivers for more exotic hardware, as
579it allows continuous-integration systems to compile-test the code on a more
580common system, and detect bugs that way.
581Note that compile-tested code should avoid crashing when run on a system where
582the dependency is not met.
583
584Architecture and platform dependencies
585~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
586Due to the presence of stubs, most drivers can now be compiled on most
587architectures. However, this does not mean it makes sense to have all drivers
588available everywhere, as the actual hardware may only exist on specific
589architectures and platforms. This is especially true for on-SoC IP cores,
590which may be limited to a specific vendor or SoC family.
591
592To prevent asking the user about drivers that cannot be used on the system(s)
593the user is compiling a kernel for, and if it makes sense, config symbols
594controlling the compilation of a driver should contain proper dependencies,
595limiting the visibility of the symbol to (a superset of) the platform(s) the
596driver can be used on. The dependency can be an architecture (e.g. ARM) or
597platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for
598distro config owners, but also for every single developer or user who
599configures a kernel.
600
601Such a dependency can be relaxed by combining it with the compile-testing rule
602above, leading to:
603
604 config FOO
605 bool "Support for foo hardware"
606 depends on ARCH_FOO_VENDOR || COMPILE_TEST
607
608Optional dependencies
609~~~~~~~~~~~~~~~~~~~~~
610
611Some drivers are able to optionally use a feature from another module
612or build cleanly with that module disabled, but cause a link failure
613when trying to use that loadable module from a built-in driver.
614
615The recommended way to express this optional dependency in Kconfig logic
616uses the conditional form::
617
618 config FOO
619 tristate "Support for foo hardware"
620 depends on BAR if BAR
621
622This slightly counterintuitive style is also widely used::
623
624 config FOO
625 tristate "Support for foo hardware"
626 depends on BAR || !BAR
627
628This means that there is either a dependency on BAR that disallows
629the combination of FOO=y with BAR=m, or BAR is completely disabled. The BAR
630module must provide all the stubs for !BAR case.
631
632For a more formalized approach if there are multiple drivers that have
633the same dependency, a helper symbol can be used, like::
634
635 config FOO
636 tristate "Support for foo hardware"
637 depends on BAR_OPTIONAL
638
639 config BAR_OPTIONAL
640 def_tristate BAR || !BAR
641
642Much less favorable way to express optional dependency is IS_REACHABLE() within
643the module code, useful for example when the module BAR does not provide
644!BAR stubs::
645
646 foo_init()
647 {
648 if (IS_REACHABLE(CONFIG_BAR))
649 bar_register(&foo);
650 ...
651 }
652
653IS_REACHABLE() is generally discouraged, because the code will be silently
654discarded, when CONFIG_BAR=m and this code is built-in. This is not what users
655usually expect when enabling BAR as module.
656
657Kconfig recursive dependency limitations
658~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
659
660If you've hit the Kconfig error: "recursive dependency detected" you've run
661into a recursive dependency issue with Kconfig, a recursive dependency can be
662summarized as a circular dependency. The kconfig tools need to ensure that
663Kconfig files comply with specified configuration requirements. In order to do
664that kconfig must determine the values that are possible for all Kconfig
665symbols, this is currently not possible if there is a circular relation
666between two or more Kconfig symbols. For more details refer to the "Simple
667Kconfig recursive issue" subsection below. Kconfig does not do recursive
668dependency resolution; this has a few implications for Kconfig file writers.
669We'll first explain why this issues exists and then provide an example
670technical limitation which this brings upon Kconfig developers. Eager
671developers wishing to try to address this limitation should read the next
672subsections.
673
674Simple Kconfig recursive issue
675~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
676
677Read: Documentation/kbuild/Kconfig.recursion-issue-01
678
679Test with::
680
681 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
682
683Cumulative Kconfig recursive issue
684~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
685
686Read: Documentation/kbuild/Kconfig.recursion-issue-02
687
688Test with::
689
690 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
691
692Practical solutions to kconfig recursive issue
693~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
694
695Developers who run into the recursive Kconfig issue have two options
696at their disposal. We document them below and also provide a list of
697historical issues resolved through these different solutions.
698
699 a) Remove any superfluous "select FOO" or "depends on FOO"
700 b) Match dependency semantics:
701
702 b1) Swap all "select FOO" to "depends on FOO" or,
703
704 b2) Swap all "depends on FOO" to "select FOO"
705
706The resolution to a) can be tested with the sample Kconfig file
707Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
708of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
709since CORE_BELL_A depends on CORE. At times it may not be possible to remove
710some dependency criteria, for such cases you can work with solution b).
711
712The two different resolutions for b) can be tested in the sample Kconfig file
713Documentation/kbuild/Kconfig.recursion-issue-02.
714
715Below is a list of examples of prior fixes for these types of recursive issues;
716all errors appear to involve one or more "select" statements and one or more
717"depends on".
718
719============ ===================================
720commit fix
721============ ===================================
72206b718c01208 select A -> depends on A
723c22eacfe82f9 depends on A -> depends on B
7246a91e854442c select A -> depends on A
725118c565a8f2e select A -> select B
726f004e5594705 select A -> depends on A
727c7861f37b4c6 depends on A -> (null)
72880c69915e5fb select A -> (null) (1)
729c2218e26c0d0 select A -> depends on A (1)
730d6ae99d04e1c select A -> depends on A
73195ca19cf8cbf select A -> depends on A
7328f057d7bca54 depends on A -> (null)
7338f057d7bca54 depends on A -> select A
734a0701f04846e select A -> depends on A
7350c8b92f7f259 depends on A -> (null)
736e4e9e0540928 select A -> depends on A (2)
7377453ea886e87 depends on A > (null) (1)
7387b1fff7e4fdf select A -> depends on A
73986c747d2a4f0 select A -> depends on A
740d9f9ab51e55e select A -> depends on A
7410c51a4d8abd6 depends on A -> select A (3)
742e98062ed6dc4 select A -> depends on A (3)
74391e5d284a7f1 select A -> (null)
744============ ===================================
745
746(1) Partial (or no) quote of error.
747(2) That seems to be the gist of that fix.
748(3) Same error.
749
750Future kconfig work
751~~~~~~~~~~~~~~~~~~~
752
753Work on kconfig is welcomed on both areas of clarifying semantics and on
754evaluating the use of a full SAT solver for it. A full SAT solver can be
755desirable to enable more complex dependency mappings and / or queries,
756for instance one possible use case for a SAT solver could be that of handling
757the current known recursive dependency issues. It is not known if this would
758address such issues but such evaluation is desirable. If support for a full SAT
759solver proves too complex or that it cannot address recursive dependency issues
760Kconfig should have at least clear and well defined semantics which also
761addresses and documents limitations or requirements such as the ones dealing
762with recursive dependencies.
763
764Further work on both of these areas is welcomed on Kconfig. We elaborate
765on both of these in the next two subsections.
766
767Semantics of Kconfig
768~~~~~~~~~~~~~~~~~~~~
769
770The use of Kconfig is broad, Linux is now only one of Kconfig's users:
771one study has completed a broad analysis of Kconfig use in 12 projects [0]_.
772Despite its widespread use, and although this document does a reasonable job
773in documenting basic Kconfig syntax a more precise definition of Kconfig
774semantics is welcomed. One project deduced Kconfig semantics through
775the use of the xconfig configurator [1]_. Work should be done to confirm if
776the deduced semantics matches our intended Kconfig design goals.
777Another project formalized a denotational semantics of a core subset of
778the Kconfig language [10]_.
779
780Having well defined semantics can be useful for tools for practical
781evaluation of dependencies, for instance one such case was work to
782express in boolean abstraction of the inferred semantics of Kconfig to
783translate Kconfig logic into boolean formulas and run a SAT solver on this to
784find dead code / features (always inactive), 114 dead features were found in
785Linux using this methodology [1]_ (Section 8: Threats to validity).
786The kismet tool, based on the semantics in [10]_, finds abuses of reverse
787dependencies and has led to dozens of committed fixes to Linux Kconfig files [11]_.
788
789Confirming this could prove useful as Kconfig stands as one of the leading
790industrial variability modeling languages [1]_ [2]_. Its study would help
791evaluate practical uses of such languages, their use was only theoretical
792and real world requirements were not well understood. As it stands though
793only reverse engineering techniques have been used to deduce semantics from
794variability modeling languages such as Kconfig [3]_.
795
796.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
797.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
798.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
799.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf
800
801Full SAT solver for Kconfig
802~~~~~~~~~~~~~~~~~~~~~~~~~~~
803
804Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted
805in the previous subsection, work has been done however to express in boolean
806abstraction the inferred semantics of Kconfig to translate Kconfig logic into
807boolean formulas and run a SAT solver on it [5]_. Another known related project
808is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which
809has been introduced first with [9]_. The basic concept of undertaker is to
810extract variability models from Kconfig and put them together with a
811propositional formula extracted from CPP #ifdefs and build-rules into a SAT
812solver in order to find dead code, dead files, and dead symbols. If using a SAT
813solver is desirable on Kconfig one approach would be to evaluate repurposing
814such efforts somehow on Kconfig. There is enough interest from mentors of
815existing projects to not only help advise how to integrate this work upstream
816but also help maintain it long term. Interested developers should visit:
817
818https://kernelnewbies.org/KernelProjects/kconfig-sat
819
820.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
821.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
822.. [6] https://cados.cs.fau.de
823.. [7] https://vamos.cs.fau.de
824.. [8] https://undertaker.cs.fau.de
825.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf
826.. [10] https://paulgazzillo.com/papers/esecfse21.pdf
827.. [11] https://github.com/paulgazz/kmax