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". 74 75- default value: "default" <expr> ["if" <expr>] 76 77 A config option can have any number of default values. If multiple 78 default values are visible, only the first defined one is active. 79 Default values are not limited to the menu entry where they are 80 defined. This means the default can be defined somewhere else or be 81 overridden by an earlier definition. 82 The default value is only assigned to the config symbol if no other 83 value was set by the user (via the input prompt above). If an input 84 prompt is visible the default value is presented to the user and can 85 be overridden by him. 86 Optionally, dependencies only for this default value can be added with 87 "if". 88 89 The default value deliberately defaults to 'n' in order to avoid bloating the 90 build. With few exceptions, new config options should not change this. The 91 intent is for "make oldconfig" to add as little as possible to the config from 92 release to release. 93 94 Note: 95 Things that merit "default y/m" include: 96 97 a) A new Kconfig option for something that used to always be built 98 should be "default y". 99 100 b) A new gatekeeping Kconfig option that hides/shows other Kconfig 101 options (but does not generate any code of its own), should be 102 "default y" so people will see those other options. 103 104 c) Sub-driver behavior or similar options for a driver that is 105 "default n". This allows you to provide sane defaults. 106 107 d) Hardware or infrastructure that everybody expects, such as CONFIG_NET 108 or CONFIG_BLOCK. These are rare exceptions. 109 110- type definition + default value:: 111 112 "def_bool"/"def_tristate" <expr> ["if" <expr>] 113 114 This is a shorthand notation for a type definition plus a value. 115 Optionally dependencies for this default value can be added with "if". 116 117- dependencies: "depends on" <expr> 118 119 This defines a dependency for this menu entry. If multiple 120 dependencies are defined, they are connected with '&&'. Dependencies 121 are applied to all other options within this menu entry (which also 122 accept an "if" expression), so these two examples are equivalent:: 123 124 bool "foo" if BAR 125 default y if BAR 126 127 and:: 128 129 depends on BAR 130 bool "foo" 131 default y 132 133- reverse dependencies: "select" <symbol> ["if" <expr>] 134 135 While normal dependencies reduce the upper limit of a symbol (see 136 below), reverse dependencies can be used to force a lower limit of 137 another symbol. The value of the current menu symbol is used as the 138 minimal value <symbol> can be set to. If <symbol> is selected multiple 139 times, the limit is set to the largest selection. 140 Reverse dependencies can only be used with boolean or tristate 141 symbols. 142 143 Note: 144 select should be used with care. select will force 145 a symbol to a value without visiting the dependencies. 146 By abusing select you are able to select a symbol FOO even 147 if FOO depends on BAR that is not set. 148 In general use select only for non-visible symbols 149 (no prompts anywhere) and for symbols with no dependencies. 150 That will limit the usefulness but on the other hand avoid 151 the illegal configurations all over. 152 153 If "select" <symbol> is followed by "if" <expr>, <symbol> will be 154 selected by the logical AND of the value of the current menu symbol 155 and <expr>. This means, the lower limit can be downgraded due to the 156 presence of "if" <expr>. This behavior may seem weird, but we rely on 157 it. (The future of this behavior is undecided.) 158 159- weak reverse dependencies: "imply" <symbol> ["if" <expr>] 160 161 This is similar to "select" as it enforces a lower limit on another 162 symbol except that the "implied" symbol's value may still be set to n 163 from a direct dependency or with a visible prompt. 164 165 Given the following example:: 166 167 config FOO 168 tristate "foo" 169 imply BAZ 170 171 config BAZ 172 tristate "baz" 173 depends on BAR 174 175 The following values are possible: 176 177 === === ============= ============== 178 FOO BAR BAZ's default choice for BAZ 179 === === ============= ============== 180 n y n N/m/y 181 m y m M/y/n 182 y y y Y/m/n 183 n m n N/m 184 m m m M/n 185 y m m M/n 186 y n * N 187 === === ============= ============== 188 189 This is useful e.g. with multiple drivers that want to indicate their 190 ability to hook into a secondary subsystem while allowing the user to 191 configure that subsystem out without also having to unset these drivers. 192 193 Note: If the combination of FOO=y and BAZ=m causes a link error, 194 you can guard the function call with IS_REACHABLE():: 195 196 foo_init() 197 { 198 if (IS_REACHABLE(CONFIG_BAZ)) 199 baz_register(&foo); 200 ... 201 } 202 203 Note: If the feature provided by BAZ is highly desirable for FOO, 204 FOO should imply not only BAZ, but also its dependency BAR:: 205 206 config FOO 207 tristate "foo" 208 imply BAR 209 imply BAZ 210 211 Note: If "imply" <symbol> is followed by "if" <expr>, the default of <symbol> 212 will be the logical AND of the value of the current menu symbol and <expr>. 213 (The future of this behavior is undecided.) 214 215- limiting menu display: "visible if" <expr> 216 217 This attribute is only applicable to menu blocks, if the condition is 218 false, the menu block is not displayed to the user (the symbols 219 contained there can still be selected by other symbols, though). It is 220 similar to a conditional "prompt" attribute for individual menu 221 entries. Default value of "visible" is true. 222 223- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] 224 225 This allows to limit the range of possible input values for int 226 and hex symbols. The user can only input a value which is larger than 227 or equal to the first symbol and smaller than or equal to the second 228 symbol. 229 230- help text: "help" 231 232 This defines a help text. The end of the help text is determined by 233 the indentation level, this means it ends at the first line which has 234 a smaller indentation than the first line of the help text. 235 236- module attribute: "modules" 237 This declares the symbol to be used as the MODULES symbol, which 238 enables the third modular state for all config symbols. 239 At most one symbol may have the "modules" option set. 240 241Menu dependencies 242----------------- 243 244Dependencies define the visibility of a menu entry and can also reduce 245the input range of tristate symbols. The tristate logic used in the 246expressions uses one more state than normal boolean logic to express the 247module state. Dependency expressions have the following syntax:: 248 249 <expr> ::= <symbol> (1) 250 <symbol> '=' <symbol> (2) 251 <symbol> '!=' <symbol> (3) 252 <symbol1> '<' <symbol2> (4) 253 <symbol1> '>' <symbol2> (4) 254 <symbol1> '<=' <symbol2> (4) 255 <symbol1> '>=' <symbol2> (4) 256 '(' <expr> ')' (5) 257 '!' <expr> (6) 258 <expr> '&&' <expr> (7) 259 <expr> '||' <expr> (8) 260 261Expressions are listed in decreasing order of precedence. 262 263(1) Convert the symbol into an expression. Boolean and tristate symbols 264 are simply converted into the respective expression values. All 265 other symbol types result in 'n'. 266(2) If the values of both symbols are equal, it returns 'y', 267 otherwise 'n'. 268(3) If the values of both symbols are equal, it returns 'n', 269 otherwise 'y'. 270(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, 271 or greater-or-equal than value of <symbol2>, it returns 'y', 272 otherwise 'n'. 273(5) Returns the value of the expression. Used to override precedence. 274(6) Returns the result of (2-/expr/). 275(7) Returns the result of min(/expr/, /expr/). 276(8) Returns the result of max(/expr/, /expr/). 277 278An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 279respectively for calculations). A menu entry becomes visible when its 280expression evaluates to 'm' or 'y'. 281 282There are two types of symbols: constant and non-constant symbols. 283Non-constant symbols are the most common ones and are defined with the 284'config' statement. Non-constant symbols consist entirely of alphanumeric 285characters or underscores. 286Constant symbols are only part of expressions. Constant symbols are 287always surrounded by single or double quotes. Within the quote, any 288other character is allowed and the quotes can be escaped using '\'. 289 290Menu structure 291-------------- 292 293The position of a menu entry in the tree is determined in two ways. First 294it can be specified explicitly:: 295 296 menu "Network device support" 297 depends on NET 298 299 config NETDEVICES 300 ... 301 302 endmenu 303 304All entries within the "menu" ... "endmenu" block become a submenu of 305"Network device support". All subentries inherit the dependencies from 306the menu entry, e.g. this means the dependency "NET" is added to the 307dependency list of the config option NETDEVICES. 308 309The other way to generate the menu structure is done by analyzing the 310dependencies. If a menu entry somehow depends on the previous entry, it 311can be made a submenu of it. First, the previous (parent) symbol must 312be part of the dependency list and then one of these two conditions 313must be true: 314 315- the child entry must become invisible, if the parent is set to 'n' 316- the child entry must only be visible, if the parent is visible:: 317 318 config MODULES 319 bool "Enable loadable module support" 320 321 config MODVERSIONS 322 bool "Set version information on all module symbols" 323 depends on MODULES 324 325 comment "module support disabled" 326 depends on !MODULES 327 328MODVERSIONS directly depends on MODULES, this means it's only visible if 329MODULES is different from 'n'. The comment on the other hand is only 330visible when MODULES is set to 'n'. 331 332 333Kconfig syntax 334-------------- 335 336The configuration file describes a series of menu entries, where every 337line starts with a keyword (except help texts). The following keywords 338end a menu entry: 339 340- config 341- menuconfig 342- choice/endchoice 343- comment 344- menu/endmenu 345- if/endif 346- source 347 348The first five also start the definition of a menu entry. 349 350config:: 351 352 "config" <symbol> 353 <config options> 354 355This defines a config symbol <symbol> and accepts any of above 356attributes as options. 357 358menuconfig:: 359 360 "menuconfig" <symbol> 361 <config options> 362 363This is similar to the simple config entry above, but it also gives a 364hint to front ends, that all suboptions should be displayed as a 365separate list of options. To make sure all the suboptions will really 366show up under the menuconfig entry and not outside of it, every item 367from the <config options> list must depend on the menuconfig symbol. 368In practice, this is achieved by using one of the next two constructs:: 369 370 (1): 371 menuconfig M 372 if M 373 config C1 374 config C2 375 endif 376 377 (2): 378 menuconfig M 379 config C1 380 depends on M 381 config C2 382 depends on M 383 384In the following examples (3) and (4), C1 and C2 still have the M 385dependency, but will not appear under menuconfig M anymore, because 386of C0, which doesn't depend on M:: 387 388 (3): 389 menuconfig M 390 config C0 391 if M 392 config C1 393 config C2 394 endif 395 396 (4): 397 menuconfig M 398 config C0 399 config C1 400 depends on M 401 config C2 402 depends on M 403 404choices:: 405 406 "choice" 407 <choice options> 408 <choice block> 409 "endchoice" 410 411This defines a choice group and accepts any of the above attributes as 412options. A choice can only be of type bool or tristate. If no type is 413specified for a choice, its type will be determined by the type of 414the first choice element in the group or remain unknown if none of the 415choice elements have a type specified, as well. 416 417While a boolean choice only allows a single config entry to be 418selected, a tristate choice also allows any number of config entries 419to be set to 'm'. This can be used if multiple drivers for a single 420hardware exists and only a single driver can be compiled/loaded into 421the kernel, but all drivers can be compiled as modules. 422 423comment:: 424 425 "comment" <prompt> 426 <comment options> 427 428This defines a comment which is displayed to the user during the 429configuration process and is also echoed to the output files. The only 430possible options are dependencies. 431 432menu:: 433 434 "menu" <prompt> 435 <menu options> 436 <menu block> 437 "endmenu" 438 439This defines a menu block, see "Menu structure" above for more 440information. The only possible options are dependencies and "visible" 441attributes. 442 443if:: 444 445 "if" <expr> 446 <if block> 447 "endif" 448 449This defines an if block. The dependency expression <expr> is appended 450to all enclosed menu entries. 451 452source:: 453 454 "source" <prompt> 455 456This reads the specified configuration file. This file is always parsed. 457 458mainmenu:: 459 460 "mainmenu" <prompt> 461 462This sets the config program's title bar if the config program chooses 463to use it. It should be placed at the top of the configuration, before any 464other statement. 465 466'#' Kconfig source file comment: 467 468An unquoted '#' character anywhere in a source file line indicates 469the beginning of a source file comment. The remainder of that line 470is a comment. 471 472 473Kconfig hints 474------------- 475This is a collection of Kconfig tips, most of which aren't obvious at 476first glance and most of which have become idioms in several Kconfig 477files. 478 479Adding common features and make the usage configurable 480~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 481It is a common idiom to implement a feature/functionality that are 482relevant for some architectures but not all. 483The recommended way to do so is to use a config variable named HAVE_* 484that is defined in a common Kconfig file and selected by the relevant 485architectures. 486An example is the generic IOMAP functionality. 487 488We would in lib/Kconfig see:: 489 490 # Generic IOMAP is used to ... 491 config HAVE_GENERIC_IOMAP 492 493 config GENERIC_IOMAP 494 depends on HAVE_GENERIC_IOMAP && FOO 495 496And in lib/Makefile we would see:: 497 498 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 499 500For each architecture using the generic IOMAP functionality we would see:: 501 502 config X86 503 select ... 504 select HAVE_GENERIC_IOMAP 505 select ... 506 507Note: we use the existing config option and avoid creating a new 508config variable to select HAVE_GENERIC_IOMAP. 509 510Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 511introduced to overcome the limitation of select which will force a 512config option to 'y' no matter the dependencies. 513The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 514situation where select forces a symbol equals to 'y'. 515 516Adding features that need compiler support 517~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 518 519There are several features that need compiler support. The recommended way 520to describe the dependency on the compiler feature is to use "depends on" 521followed by a test macro:: 522 523 config STACKPROTECTOR 524 bool "Stack Protector buffer overflow detection" 525 depends on $(cc-option,-fstack-protector) 526 ... 527 528If you need to expose a compiler capability to makefiles and/or C source files, 529`CC_HAS_` is the recommended prefix for the config option:: 530 531 config CC_HAS_FOO 532 def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC)) 533 534Build as module only 535~~~~~~~~~~~~~~~~~~~~ 536To restrict a component build to module-only, qualify its config symbol 537with "depends on m". E.g.:: 538 539 config FOO 540 depends on BAR && m 541 542limits FOO to module (=m) or disabled (=n). 543 544Compile-testing 545~~~~~~~~~~~~~~~ 546If a config symbol has a dependency, but the code controlled by the config 547symbol can still be compiled if the dependency is not met, it is encouraged to 548increase build coverage by adding an "|| COMPILE_TEST" clause to the 549dependency. This is especially useful for drivers for more exotic hardware, as 550it allows continuous-integration systems to compile-test the code on a more 551common system, and detect bugs that way. 552Note that compile-tested code should avoid crashing when run on a system where 553the dependency is not met. 554 555Architecture and platform dependencies 556~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 557Due to the presence of stubs, most drivers can now be compiled on most 558architectures. However, this does not mean it makes sense to have all drivers 559available everywhere, as the actual hardware may only exist on specific 560architectures and platforms. This is especially true for on-SoC IP cores, 561which may be limited to a specific vendor or SoC family. 562 563To prevent asking the user about drivers that cannot be used on the system(s) 564the user is compiling a kernel for, and if it makes sense, config symbols 565controlling the compilation of a driver should contain proper dependencies, 566limiting the visibility of the symbol to (a superset of) the platform(s) the 567driver can be used on. The dependency can be an architecture (e.g. ARM) or 568platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for 569distro config owners, but also for every single developer or user who 570configures a kernel. 571 572Such a dependency can be relaxed by combining it with the compile-testing rule 573above, leading to: 574 575 config FOO 576 bool "Support for foo hardware" 577 depends on ARCH_FOO_VENDOR || COMPILE_TEST 578 579Optional dependencies 580~~~~~~~~~~~~~~~~~~~~~ 581 582Some drivers are able to optionally use a feature from another module 583or build cleanly with that module disabled, but cause a link failure 584when trying to use that loadable module from a built-in driver. 585 586The most common way to express this optional dependency in Kconfig logic 587uses the slightly counterintuitive:: 588 589 config FOO 590 tristate "Support for foo hardware" 591 depends on BAR || !BAR 592 593This means that there is either a dependency on BAR that disallows 594the combination of FOO=y with BAR=m, or BAR is completely disabled. 595For a more formalized approach if there are multiple drivers that have 596the same dependency, a helper symbol can be used, like:: 597 598 config FOO 599 tristate "Support for foo hardware" 600 depends on BAR_OPTIONAL 601 602 config BAR_OPTIONAL 603 def_tristate BAR || !BAR 604 605Kconfig recursive dependency limitations 606~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 607 608If you've hit the Kconfig error: "recursive dependency detected" you've run 609into a recursive dependency issue with Kconfig, a recursive dependency can be 610summarized as a circular dependency. The kconfig tools need to ensure that 611Kconfig files comply with specified configuration requirements. In order to do 612that kconfig must determine the values that are possible for all Kconfig 613symbols, this is currently not possible if there is a circular relation 614between two or more Kconfig symbols. For more details refer to the "Simple 615Kconfig recursive issue" subsection below. Kconfig does not do recursive 616dependency resolution; this has a few implications for Kconfig file writers. 617We'll first explain why this issues exists and then provide an example 618technical limitation which this brings upon Kconfig developers. Eager 619developers wishing to try to address this limitation should read the next 620subsections. 621 622Simple Kconfig recursive issue 623~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 624 625Read: Documentation/kbuild/Kconfig.recursion-issue-01 626 627Test with:: 628 629 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 630 631Cumulative Kconfig recursive issue 632~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 633 634Read: Documentation/kbuild/Kconfig.recursion-issue-02 635 636Test with:: 637 638 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 639 640Practical solutions to kconfig recursive issue 641~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 642 643Developers who run into the recursive Kconfig issue have two options 644at their disposal. We document them below and also provide a list of 645historical issues resolved through these different solutions. 646 647 a) Remove any superfluous "select FOO" or "depends on FOO" 648 b) Match dependency semantics: 649 650 b1) Swap all "select FOO" to "depends on FOO" or, 651 652 b2) Swap all "depends on FOO" to "select FOO" 653 654The resolution to a) can be tested with the sample Kconfig file 655Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 656of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 657since CORE_BELL_A depends on CORE. At times it may not be possible to remove 658some dependency criteria, for such cases you can work with solution b). 659 660The two different resolutions for b) can be tested in the sample Kconfig file 661Documentation/kbuild/Kconfig.recursion-issue-02. 662 663Below is a list of examples of prior fixes for these types of recursive issues; 664all errors appear to involve one or more "select" statements and one or more 665"depends on". 666 667============ =================================== 668commit fix 669============ =================================== 67006b718c01208 select A -> depends on A 671c22eacfe82f9 depends on A -> depends on B 6726a91e854442c select A -> depends on A 673118c565a8f2e select A -> select B 674f004e5594705 select A -> depends on A 675c7861f37b4c6 depends on A -> (null) 67680c69915e5fb select A -> (null) (1) 677c2218e26c0d0 select A -> depends on A (1) 678d6ae99d04e1c select A -> depends on A 67995ca19cf8cbf select A -> depends on A 6808f057d7bca54 depends on A -> (null) 6818f057d7bca54 depends on A -> select A 682a0701f04846e select A -> depends on A 6830c8b92f7f259 depends on A -> (null) 684e4e9e0540928 select A -> depends on A (2) 6857453ea886e87 depends on A > (null) (1) 6867b1fff7e4fdf select A -> depends on A 68786c747d2a4f0 select A -> depends on A 688d9f9ab51e55e select A -> depends on A 6890c51a4d8abd6 depends on A -> select A (3) 690e98062ed6dc4 select A -> depends on A (3) 69191e5d284a7f1 select A -> (null) 692============ =================================== 693 694(1) Partial (or no) quote of error. 695(2) That seems to be the gist of that fix. 696(3) Same error. 697 698Future kconfig work 699~~~~~~~~~~~~~~~~~~~ 700 701Work on kconfig is welcomed on both areas of clarifying semantics and on 702evaluating the use of a full SAT solver for it. A full SAT solver can be 703desirable to enable more complex dependency mappings and / or queries, 704for instance one possible use case for a SAT solver could be that of handling 705the current known recursive dependency issues. It is not known if this would 706address such issues but such evaluation is desirable. If support for a full SAT 707solver proves too complex or that it cannot address recursive dependency issues 708Kconfig should have at least clear and well defined semantics which also 709addresses and documents limitations or requirements such as the ones dealing 710with recursive dependencies. 711 712Further work on both of these areas is welcomed on Kconfig. We elaborate 713on both of these in the next two subsections. 714 715Semantics of Kconfig 716~~~~~~~~~~~~~~~~~~~~ 717 718The use of Kconfig is broad, Linux is now only one of Kconfig's users: 719one study has completed a broad analysis of Kconfig use in 12 projects [0]_. 720Despite its widespread use, and although this document does a reasonable job 721in documenting basic Kconfig syntax a more precise definition of Kconfig 722semantics is welcomed. One project deduced Kconfig semantics through 723the use of the xconfig configurator [1]_. Work should be done to confirm if 724the deduced semantics matches our intended Kconfig design goals. 725Another project formalized a denotational semantics of a core subset of 726the Kconfig language [10]_. 727 728Having well defined semantics can be useful for tools for practical 729evaluation of dependencies, for instance one such case was work to 730express in boolean abstraction of the inferred semantics of Kconfig to 731translate Kconfig logic into boolean formulas and run a SAT solver on this to 732find dead code / features (always inactive), 114 dead features were found in 733Linux using this methodology [1]_ (Section 8: Threats to validity). 734The kismet tool, based on the semantics in [10]_, finds abuses of reverse 735dependencies and has led to dozens of committed fixes to Linux Kconfig files [11]_. 736 737Confirming this could prove useful as Kconfig stands as one of the leading 738industrial variability modeling languages [1]_ [2]_. Its study would help 739evaluate practical uses of such languages, their use was only theoretical 740and real world requirements were not well understood. As it stands though 741only reverse engineering techniques have been used to deduce semantics from 742variability modeling languages such as Kconfig [3]_. 743 744.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 745.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 746.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 747.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 748 749Full SAT solver for Kconfig 750~~~~~~~~~~~~~~~~~~~~~~~~~~~ 751 752Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted 753in the previous subsection, work has been done however to express in boolean 754abstraction the inferred semantics of Kconfig to translate Kconfig logic into 755boolean formulas and run a SAT solver on it [5]_. Another known related project 756is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which 757has been introduced first with [9]_. The basic concept of undertaker is to 758extract variability models from Kconfig and put them together with a 759propositional formula extracted from CPP #ifdefs and build-rules into a SAT 760solver in order to find dead code, dead files, and dead symbols. If using a SAT 761solver is desirable on Kconfig one approach would be to evaluate repurposing 762such efforts somehow on Kconfig. There is enough interest from mentors of 763existing projects to not only help advise how to integrate this work upstream 764but also help maintain it long term. Interested developers should visit: 765 766https://kernelnewbies.org/KernelProjects/kconfig-sat 767 768.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 769.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 770.. [6] https://cados.cs.fau.de 771.. [7] https://vamos.cs.fau.de 772.. [8] https://undertaker.cs.fau.de 773.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf 774.. [10] https://paulgazzillo.com/papers/esecfse21.pdf 775.. [11] https://github.com/paulgazz/kmax 776