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> 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- reverse dependencies: "select" <symbol> ["if" <expr>] 138 139 While normal dependencies reduce the upper limit of a symbol (see 140 below), reverse dependencies can be used to force a lower limit of 141 another symbol. The value of the current menu symbol is used as the 142 minimal value <symbol> can be set to. If <symbol> is selected multiple 143 times, the limit is set to the largest selection. 144 Reverse dependencies can only be used with boolean or tristate 145 symbols. 146 147 Note: 148 select should be used with care. select will force 149 a symbol to a value without visiting the dependencies. 150 By abusing select you are able to select a symbol FOO even 151 if FOO depends on BAR that is not set. 152 In general use select only for non-visible symbols 153 (no prompts anywhere) and for symbols with no dependencies. 154 That will limit the usefulness but on the other hand avoid 155 the illegal configurations all over. 156 157 If "select" <symbol> is followed by "if" <expr>, <symbol> will be 158 selected by the logical AND of the value of the current menu symbol 159 and <expr>. This means, the lower limit can be downgraded due to the 160 presence of "if" <expr>. This behavior may seem weird, but we rely on 161 it. (The future of this behavior is undecided.) 162 163- weak reverse dependencies: "imply" <symbol> ["if" <expr>] 164 165 This is similar to "select" as it enforces a lower limit on another 166 symbol except that the "implied" symbol's value may still be set to n 167 from a direct dependency or with a visible prompt. 168 169 Given the following example:: 170 171 config FOO 172 tristate "foo" 173 imply BAZ 174 175 config BAZ 176 tristate "baz" 177 depends on BAR 178 179 The following values are possible: 180 181 === === ============= ============== 182 FOO BAR BAZ's default choice for BAZ 183 === === ============= ============== 184 n y n N/m/y 185 m y m M/y/n 186 y y y Y/m/n 187 n m n N/m 188 m m m M/n 189 y m m M/n 190 y n * N 191 === === ============= ============== 192 193 This is useful e.g. with multiple drivers that want to indicate their 194 ability to hook into a secondary subsystem while allowing the user to 195 configure that subsystem out without also having to unset these drivers. 196 197 Note: If the combination of FOO=y and BAZ=m causes a link error, 198 you can guard the function call with IS_REACHABLE():: 199 200 foo_init() 201 { 202 if (IS_REACHABLE(CONFIG_BAZ)) 203 baz_register(&foo); 204 ... 205 } 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 to limit 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 245Menu dependencies 246----------------- 247 248Dependencies define the visibility of a menu entry and can also reduce 249the input range of tristate symbols. The tristate logic used in the 250expressions uses one more state than normal boolean logic to express the 251module state. Dependency expressions have the following syntax:: 252 253 <expr> ::= <symbol> (1) 254 <symbol> '=' <symbol> (2) 255 <symbol> '!=' <symbol> (3) 256 <symbol1> '<' <symbol2> (4) 257 <symbol1> '>' <symbol2> (4) 258 <symbol1> '<=' <symbol2> (4) 259 <symbol1> '>=' <symbol2> (4) 260 '(' <expr> ')' (5) 261 '!' <expr> (6) 262 <expr> '&&' <expr> (7) 263 <expr> '||' <expr> (8) 264 265Expressions are listed in decreasing order of precedence. 266 267(1) Convert the symbol into an expression. Boolean and tristate symbols 268 are simply converted into the respective expression values. All 269 other symbol types result in 'n'. 270(2) If the values of both symbols are equal, it returns 'y', 271 otherwise 'n'. 272(3) If the values of both symbols are equal, it returns 'n', 273 otherwise 'y'. 274(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, 275 or greater-or-equal than value of <symbol2>, it returns 'y', 276 otherwise 'n'. 277(5) Returns the value of the expression. Used to override precedence. 278(6) Returns the result of (2-/expr/). 279(7) Returns the result of min(/expr/, /expr/). 280(8) Returns the result of max(/expr/, /expr/). 281 282An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 283respectively for calculations). A menu entry becomes visible when its 284expression evaluates to 'm' or 'y'. 285 286There are two types of symbols: constant and non-constant symbols. 287Non-constant symbols are the most common ones and are defined with the 288'config' statement. Non-constant symbols consist entirely of alphanumeric 289characters or underscores. 290Constant symbols are only part of expressions. Constant symbols are 291always surrounded by single or double quotes. Within the quote, any 292other character is allowed and the quotes can be escaped using '\'. 293 294Menu structure 295-------------- 296 297The position of a menu entry in the tree is determined in two ways. First 298it can be specified explicitly:: 299 300 menu "Network device support" 301 depends on NET 302 303 config NETDEVICES 304 ... 305 306 endmenu 307 308All entries within the "menu" ... "endmenu" block become a submenu of 309"Network device support". All subentries inherit the dependencies from 310the menu entry, e.g. this means the dependency "NET" is added to the 311dependency list of the config option NETDEVICES. 312 313The other way to generate the menu structure is done by analyzing the 314dependencies. If a menu entry somehow depends on the previous entry, it 315can be made a submenu of it. First, the previous (parent) symbol must 316be part of the dependency list and then one of these two conditions 317must be true: 318 319- the child entry must become invisible, if the parent is set to 'n' 320- the child entry must only be visible, if the parent is visible:: 321 322 config MODULES 323 bool "Enable loadable module support" 324 325 config MODVERSIONS 326 bool "Set version information on all module symbols" 327 depends on MODULES 328 329 comment "module support disabled" 330 depends on !MODULES 331 332MODVERSIONS directly depends on MODULES, this means it's only visible if 333MODULES is different from 'n'. The comment on the other hand is only 334visible when MODULES is set to 'n'. 335 336 337Kconfig syntax 338-------------- 339 340The configuration file describes a series of menu entries, where every 341line starts with a keyword (except help texts). The following keywords 342end a menu entry: 343 344- config 345- menuconfig 346- choice/endchoice 347- comment 348- menu/endmenu 349- if/endif 350- source 351 352The first five also start the definition of a menu entry. 353 354config:: 355 356 "config" <symbol> 357 <config options> 358 359This defines a config symbol <symbol> and accepts any of above 360attributes as options. 361 362menuconfig:: 363 364 "menuconfig" <symbol> 365 <config options> 366 367This is similar to the simple config entry above, but it also gives a 368hint to front ends, that all suboptions should be displayed as a 369separate list of options. To make sure all the suboptions will really 370show up under the menuconfig entry and not outside of it, every item 371from the <config options> list must depend on the menuconfig symbol. 372In practice, this is achieved by using one of the next two constructs:: 373 374 (1): 375 menuconfig M 376 if M 377 config C1 378 config C2 379 endif 380 381 (2): 382 menuconfig M 383 config C1 384 depends on M 385 config C2 386 depends on M 387 388In the following examples (3) and (4), C1 and C2 still have the M 389dependency, but will not appear under menuconfig M anymore, because 390of C0, which doesn't depend on M:: 391 392 (3): 393 menuconfig M 394 config C0 395 if M 396 config C1 397 config C2 398 endif 399 400 (4): 401 menuconfig M 402 config C0 403 config C1 404 depends on M 405 config C2 406 depends on M 407 408choices:: 409 410 "choice" 411 <choice options> 412 <choice block> 413 "endchoice" 414 415This defines a choice group and accepts any of the above attributes as 416options. 417 418A choice only allows a single config entry to be selected. 419 420comment:: 421 422 "comment" <prompt> 423 <comment options> 424 425This defines a comment which is displayed to the user during the 426configuration process and is also echoed to the output files. The only 427possible options are dependencies. 428 429menu:: 430 431 "menu" <prompt> 432 <menu options> 433 <menu block> 434 "endmenu" 435 436This defines a menu block, see "Menu structure" above for more 437information. The only possible options are dependencies and "visible" 438attributes. 439 440if:: 441 442 "if" <expr> 443 <if block> 444 "endif" 445 446This defines an if block. The dependency expression <expr> is appended 447to all enclosed menu entries. 448 449source:: 450 451 "source" <prompt> 452 453This reads the specified configuration file. This file is always parsed. 454 455mainmenu:: 456 457 "mainmenu" <prompt> 458 459This sets the config program's title bar if the config program chooses 460to use it. It should be placed at the top of the configuration, before any 461other statement. 462 463'#' Kconfig source file comment: 464 465An unquoted '#' character anywhere in a source file line indicates 466the beginning of a source file comment. The remainder of that line 467is a comment. 468 469 470Kconfig hints 471------------- 472This is a collection of Kconfig tips, most of which aren't obvious at 473first glance and most of which have become idioms in several Kconfig 474files. 475 476Adding common features and make the usage configurable 477~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 478It is a common idiom to implement a feature/functionality that are 479relevant for some architectures but not all. 480The recommended way to do so is to use a config variable named HAVE_* 481that is defined in a common Kconfig file and selected by the relevant 482architectures. 483An example is the generic IOMAP functionality. 484 485We would in lib/Kconfig see:: 486 487 # Generic IOMAP is used to ... 488 config HAVE_GENERIC_IOMAP 489 490 config GENERIC_IOMAP 491 depends on HAVE_GENERIC_IOMAP && FOO 492 493And in lib/Makefile we would see:: 494 495 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 496 497For each architecture using the generic IOMAP functionality we would see:: 498 499 config X86 500 select ... 501 select HAVE_GENERIC_IOMAP 502 select ... 503 504Note: we use the existing config option and avoid creating a new 505config variable to select HAVE_GENERIC_IOMAP. 506 507Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 508introduced to overcome the limitation of select which will force a 509config option to 'y' no matter the dependencies. 510The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 511situation where select forces a symbol equals to 'y'. 512 513Adding features that need compiler support 514~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 515 516There are several features that need compiler support. The recommended way 517to describe the dependency on the compiler feature is to use "depends on" 518followed by a test macro:: 519 520 config STACKPROTECTOR 521 bool "Stack Protector buffer overflow detection" 522 depends on $(cc-option,-fstack-protector) 523 ... 524 525If you need to expose a compiler capability to makefiles and/or C source files, 526`CC_HAS_` is the recommended prefix for the config option:: 527 528 config CC_HAS_FOO 529 def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC)) 530 531Build as module only 532~~~~~~~~~~~~~~~~~~~~ 533To restrict a component build to module-only, qualify its config symbol 534with "depends on m". E.g.:: 535 536 config FOO 537 depends on BAR && m 538 539limits FOO to module (=m) or disabled (=n). 540 541Compile-testing 542~~~~~~~~~~~~~~~ 543If a config symbol has a dependency, but the code controlled by the config 544symbol can still be compiled if the dependency is not met, it is encouraged to 545increase build coverage by adding an "|| COMPILE_TEST" clause to the 546dependency. This is especially useful for drivers for more exotic hardware, as 547it allows continuous-integration systems to compile-test the code on a more 548common system, and detect bugs that way. 549Note that compile-tested code should avoid crashing when run on a system where 550the dependency is not met. 551 552Architecture and platform dependencies 553~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 554Due to the presence of stubs, most drivers can now be compiled on most 555architectures. However, this does not mean it makes sense to have all drivers 556available everywhere, as the actual hardware may only exist on specific 557architectures and platforms. This is especially true for on-SoC IP cores, 558which may be limited to a specific vendor or SoC family. 559 560To prevent asking the user about drivers that cannot be used on the system(s) 561the user is compiling a kernel for, and if it makes sense, config symbols 562controlling the compilation of a driver should contain proper dependencies, 563limiting the visibility of the symbol to (a superset of) the platform(s) the 564driver can be used on. The dependency can be an architecture (e.g. ARM) or 565platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for 566distro config owners, but also for every single developer or user who 567configures a kernel. 568 569Such a dependency can be relaxed by combining it with the compile-testing rule 570above, leading to: 571 572 config FOO 573 bool "Support for foo hardware" 574 depends on ARCH_FOO_VENDOR || COMPILE_TEST 575 576Optional dependencies 577~~~~~~~~~~~~~~~~~~~~~ 578 579Some drivers are able to optionally use a feature from another module 580or build cleanly with that module disabled, but cause a link failure 581when trying to use that loadable module from a built-in driver. 582 583The most common way to express this optional dependency in Kconfig logic 584uses the slightly counterintuitive:: 585 586 config FOO 587 tristate "Support for foo hardware" 588 depends on BAR || !BAR 589 590This means that there is either a dependency on BAR that disallows 591the combination of FOO=y with BAR=m, or BAR is completely disabled. 592For a more formalized approach if there are multiple drivers that have 593the same dependency, a helper symbol can be used, like:: 594 595 config FOO 596 tristate "Support for foo hardware" 597 depends on BAR_OPTIONAL 598 599 config BAR_OPTIONAL 600 def_tristate BAR || !BAR 601 602Kconfig recursive dependency limitations 603~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 604 605If you've hit the Kconfig error: "recursive dependency detected" you've run 606into a recursive dependency issue with Kconfig, a recursive dependency can be 607summarized as a circular dependency. The kconfig tools need to ensure that 608Kconfig files comply with specified configuration requirements. In order to do 609that kconfig must determine the values that are possible for all Kconfig 610symbols, this is currently not possible if there is a circular relation 611between two or more Kconfig symbols. For more details refer to the "Simple 612Kconfig recursive issue" subsection below. Kconfig does not do recursive 613dependency resolution; this has a few implications for Kconfig file writers. 614We'll first explain why this issues exists and then provide an example 615technical limitation which this brings upon Kconfig developers. Eager 616developers wishing to try to address this limitation should read the next 617subsections. 618 619Simple Kconfig recursive issue 620~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 621 622Read: Documentation/kbuild/Kconfig.recursion-issue-01 623 624Test with:: 625 626 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 627 628Cumulative Kconfig recursive issue 629~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 630 631Read: Documentation/kbuild/Kconfig.recursion-issue-02 632 633Test with:: 634 635 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 636 637Practical solutions to kconfig recursive issue 638~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 639 640Developers who run into the recursive Kconfig issue have two options 641at their disposal. We document them below and also provide a list of 642historical issues resolved through these different solutions. 643 644 a) Remove any superfluous "select FOO" or "depends on FOO" 645 b) Match dependency semantics: 646 647 b1) Swap all "select FOO" to "depends on FOO" or, 648 649 b2) Swap all "depends on FOO" to "select FOO" 650 651The resolution to a) can be tested with the sample Kconfig file 652Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 653of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 654since CORE_BELL_A depends on CORE. At times it may not be possible to remove 655some dependency criteria, for such cases you can work with solution b). 656 657The two different resolutions for b) can be tested in the sample Kconfig file 658Documentation/kbuild/Kconfig.recursion-issue-02. 659 660Below is a list of examples of prior fixes for these types of recursive issues; 661all errors appear to involve one or more "select" statements and one or more 662"depends on". 663 664============ =================================== 665commit fix 666============ =================================== 66706b718c01208 select A -> depends on A 668c22eacfe82f9 depends on A -> depends on B 6696a91e854442c select A -> depends on A 670118c565a8f2e select A -> select B 671f004e5594705 select A -> depends on A 672c7861f37b4c6 depends on A -> (null) 67380c69915e5fb select A -> (null) (1) 674c2218e26c0d0 select A -> depends on A (1) 675d6ae99d04e1c select A -> depends on A 67695ca19cf8cbf select A -> depends on A 6778f057d7bca54 depends on A -> (null) 6788f057d7bca54 depends on A -> select A 679a0701f04846e select A -> depends on A 6800c8b92f7f259 depends on A -> (null) 681e4e9e0540928 select A -> depends on A (2) 6827453ea886e87 depends on A > (null) (1) 6837b1fff7e4fdf select A -> depends on A 68486c747d2a4f0 select A -> depends on A 685d9f9ab51e55e select A -> depends on A 6860c51a4d8abd6 depends on A -> select A (3) 687e98062ed6dc4 select A -> depends on A (3) 68891e5d284a7f1 select A -> (null) 689============ =================================== 690 691(1) Partial (or no) quote of error. 692(2) That seems to be the gist of that fix. 693(3) Same error. 694 695Future kconfig work 696~~~~~~~~~~~~~~~~~~~ 697 698Work on kconfig is welcomed on both areas of clarifying semantics and on 699evaluating the use of a full SAT solver for it. A full SAT solver can be 700desirable to enable more complex dependency mappings and / or queries, 701for instance one possible use case for a SAT solver could be that of handling 702the current known recursive dependency issues. It is not known if this would 703address such issues but such evaluation is desirable. If support for a full SAT 704solver proves too complex or that it cannot address recursive dependency issues 705Kconfig should have at least clear and well defined semantics which also 706addresses and documents limitations or requirements such as the ones dealing 707with recursive dependencies. 708 709Further work on both of these areas is welcomed on Kconfig. We elaborate 710on both of these in the next two subsections. 711 712Semantics of Kconfig 713~~~~~~~~~~~~~~~~~~~~ 714 715The use of Kconfig is broad, Linux is now only one of Kconfig's users: 716one study has completed a broad analysis of Kconfig use in 12 projects [0]_. 717Despite its widespread use, and although this document does a reasonable job 718in documenting basic Kconfig syntax a more precise definition of Kconfig 719semantics is welcomed. One project deduced Kconfig semantics through 720the use of the xconfig configurator [1]_. Work should be done to confirm if 721the deduced semantics matches our intended Kconfig design goals. 722Another project formalized a denotational semantics of a core subset of 723the Kconfig language [10]_. 724 725Having well defined semantics can be useful for tools for practical 726evaluation of dependencies, for instance one such case was work to 727express in boolean abstraction of the inferred semantics of Kconfig to 728translate Kconfig logic into boolean formulas and run a SAT solver on this to 729find dead code / features (always inactive), 114 dead features were found in 730Linux using this methodology [1]_ (Section 8: Threats to validity). 731The kismet tool, based on the semantics in [10]_, finds abuses of reverse 732dependencies and has led to dozens of committed fixes to Linux Kconfig files [11]_. 733 734Confirming this could prove useful as Kconfig stands as one of the leading 735industrial variability modeling languages [1]_ [2]_. Its study would help 736evaluate practical uses of such languages, their use was only theoretical 737and real world requirements were not well understood. As it stands though 738only reverse engineering techniques have been used to deduce semantics from 739variability modeling languages such as Kconfig [3]_. 740 741.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 742.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 743.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 744.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 745 746Full SAT solver for Kconfig 747~~~~~~~~~~~~~~~~~~~~~~~~~~~ 748 749Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted 750in the previous subsection, work has been done however to express in boolean 751abstraction the inferred semantics of Kconfig to translate Kconfig logic into 752boolean formulas and run a SAT solver on it [5]_. Another known related project 753is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which 754has been introduced first with [9]_. The basic concept of undertaker is to 755extract variability models from Kconfig and put them together with a 756propositional formula extracted from CPP #ifdefs and build-rules into a SAT 757solver in order to find dead code, dead files, and dead symbols. If using a SAT 758solver is desirable on Kconfig one approach would be to evaluate repurposing 759such efforts somehow on Kconfig. There is enough interest from mentors of 760existing projects to not only help advise how to integrate this work upstream 761but also help maintain it long term. Interested developers should visit: 762 763https://kernelnewbies.org/KernelProjects/kconfig-sat 764 765.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 766.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 767.. [6] https://cados.cs.fau.de 768.. [7] https://vamos.cs.fau.de 769.. [8] https://undertaker.cs.fau.de 770.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf 771.. [10] https://paulgazzillo.com/papers/esecfse21.pdf 772.. [11] https://github.com/paulgazz/kmax 773