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- weak reverse dependencies: "imply" <symbol> ["if" <expr>] 154 155 This is similar to "select" as it enforces a lower limit on another 156 symbol except that the "implied" symbol's value may still be set to n 157 from a direct dependency or with a visible prompt. 158 159 Given the following example:: 160 161 config FOO 162 tristate "foo" 163 imply BAZ 164 165 config BAZ 166 tristate "baz" 167 depends on BAR 168 169 The following values are possible: 170 171 === === ============= ============== 172 FOO BAR BAZ's default choice for BAZ 173 === === ============= ============== 174 n y n N/m/y 175 m y m M/y/n 176 y y y Y/m/n 177 n m n N/m 178 m m m M/n 179 y m n M/n 180 y n * N 181 === === ============= ============== 182 183 This is useful e.g. with multiple drivers that want to indicate their 184 ability to hook into a secondary subsystem while allowing the user to 185 configure that subsystem out without also having to unset these drivers. 186 187 Note: If the combination of FOO=y and BAR=m causes a link error, 188 you can guard the function call with IS_REACHABLE():: 189 190 foo_init() 191 { 192 if (IS_REACHABLE(CONFIG_BAZ)) 193 baz_register(&foo); 194 ... 195 } 196 197 Note: If the feature provided by BAZ is highly desirable for FOO, 198 FOO should imply not only BAZ, but also its dependency BAR:: 199 200 config FOO 201 tristate "foo" 202 imply BAR 203 imply BAZ 204 205- limiting menu display: "visible if" <expr> 206 207 This attribute is only applicable to menu blocks, if the condition is 208 false, the menu block is not displayed to the user (the symbols 209 contained there can still be selected by other symbols, though). It is 210 similar to a conditional "prompt" attribute for individual menu 211 entries. Default value of "visible" is true. 212 213- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] 214 215 This allows to limit the range of possible input values for int 216 and hex symbols. The user can only input a value which is larger than 217 or equal to the first symbol and smaller than or equal to the second 218 symbol. 219 220- help text: "help" 221 222 This defines a help text. The end of the help text is determined by 223 the indentation level, this means it ends at the first line which has 224 a smaller indentation than the first line of the help text. 225 226- misc options: "option" <symbol>[=<value>] 227 228 Various less common options can be defined via this option syntax, 229 which can modify the behaviour of the menu entry and its config 230 symbol. These options are currently possible: 231 232 - "modules" 233 This declares the symbol to be used as the MODULES symbol, which 234 enables the third modular state for all config symbols. 235 At most one symbol may have the "modules" option set. 236 237 - "allnoconfig_y" 238 This declares the symbol as one that should have the value y when 239 using "allnoconfig". Used for symbols that hide other symbols. 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" [symbol] 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 423A choice accepts another option "optional", which allows to set the 424choice to 'n' and no entry needs to be selected. 425If no [symbol] is associated with a choice, then you can not have multiple 426definitions of that choice. If a [symbol] is associated to the choice, 427then you may define the same choice (i.e. with the same entries) in another 428place. 429 430comment:: 431 432 "comment" <prompt> 433 <comment options> 434 435This defines a comment which is displayed to the user during the 436configuration process and is also echoed to the output files. The only 437possible options are dependencies. 438 439menu:: 440 441 "menu" <prompt> 442 <menu options> 443 <menu block> 444 "endmenu" 445 446This defines a menu block, see "Menu structure" above for more 447information. The only possible options are dependencies and "visible" 448attributes. 449 450if:: 451 452 "if" <expr> 453 <if block> 454 "endif" 455 456This defines an if block. The dependency expression <expr> is appended 457to all enclosed menu entries. 458 459source:: 460 461 "source" <prompt> 462 463This reads the specified configuration file. This file is always parsed. 464 465mainmenu:: 466 467 "mainmenu" <prompt> 468 469This sets the config program's title bar if the config program chooses 470to use it. It should be placed at the top of the configuration, before any 471other statement. 472 473'#' Kconfig source file comment: 474 475An unquoted '#' character anywhere in a source file line indicates 476the beginning of a source file comment. The remainder of that line 477is a comment. 478 479 480Kconfig hints 481------------- 482This is a collection of Kconfig tips, most of which aren't obvious at 483first glance and most of which have become idioms in several Kconfig 484files. 485 486Adding common features and make the usage configurable 487~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 488It is a common idiom to implement a feature/functionality that are 489relevant for some architectures but not all. 490The recommended way to do so is to use a config variable named HAVE_* 491that is defined in a common Kconfig file and selected by the relevant 492architectures. 493An example is the generic IOMAP functionality. 494 495We would in lib/Kconfig see:: 496 497 # Generic IOMAP is used to ... 498 config HAVE_GENERIC_IOMAP 499 500 config GENERIC_IOMAP 501 depends on HAVE_GENERIC_IOMAP && FOO 502 503And in lib/Makefile we would see:: 504 505 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 506 507For each architecture using the generic IOMAP functionality we would see:: 508 509 config X86 510 select ... 511 select HAVE_GENERIC_IOMAP 512 select ... 513 514Note: we use the existing config option and avoid creating a new 515config variable to select HAVE_GENERIC_IOMAP. 516 517Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 518introduced to overcome the limitation of select which will force a 519config option to 'y' no matter the dependencies. 520The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 521situation where select forces a symbol equals to 'y'. 522 523Adding features that need compiler support 524~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 525 526There are several features that need compiler support. The recommended way 527to describe the dependency on the compiler feature is to use "depends on" 528followed by a test macro:: 529 530 config STACKPROTECTOR 531 bool "Stack Protector buffer overflow detection" 532 depends on $(cc-option,-fstack-protector) 533 ... 534 535If you need to expose a compiler capability to makefiles and/or C source files, 536`CC_HAS_` is the recommended prefix for the config option:: 537 538 config CC_HAS_ASM_GOTO 539 def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC)) 540 541Build as module only 542~~~~~~~~~~~~~~~~~~~~ 543To restrict a component build to module-only, qualify its config symbol 544with "depends on m". E.g.:: 545 546 config FOO 547 depends on BAR && m 548 549limits FOO to module (=m) or disabled (=n). 550 551Compile-testing 552~~~~~~~~~~~~~~~ 553If a config symbol has a dependency, but the code controlled by the config 554symbol can still be compiled if the dependency is not met, it is encouraged to 555increase build coverage by adding an "|| COMPILE_TEST" clause to the 556dependency. This is especially useful for drivers for more exotic hardware, as 557it allows continuous-integration systems to compile-test the code on a more 558common system, and detect bugs that way. 559Note that compile-tested code should avoid crashing when run on a system where 560the dependency is not met. 561 562Architecture and platform dependencies 563~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 564Due to the presence of stubs, most drivers can now be compiled on most 565architectures. However, this does not mean it makes sense to have all drivers 566available everywhere, as the actual hardware may only exist on specific 567architectures and platforms. This is especially true for on-SoC IP cores, 568which may be limited to a specific vendor or SoC family. 569 570To prevent asking the user about drivers that cannot be used on the system(s) 571the user is compiling a kernel for, and if it makes sense, config symbols 572controlling the compilation of a driver should contain proper dependencies, 573limiting the visibility of the symbol to (a superset of) the platform(s) the 574driver can be used on. The dependency can be an architecture (e.g. ARM) or 575platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for 576distro config owners, but also for every single developer or user who 577configures a kernel. 578 579Such a dependency can be relaxed by combining it with the compile-testing rule 580above, leading to: 581 582 config FOO 583 bool "Support for foo hardware" 584 depends on ARCH_FOO_VENDOR || COMPILE_TEST 585 586Kconfig recursive dependency limitations 587~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 588 589If you've hit the Kconfig error: "recursive dependency detected" you've run 590into a recursive dependency issue with Kconfig, a recursive dependency can be 591summarized as a circular dependency. The kconfig tools need to ensure that 592Kconfig files comply with specified configuration requirements. In order to do 593that kconfig must determine the values that are possible for all Kconfig 594symbols, this is currently not possible if there is a circular relation 595between two or more Kconfig symbols. For more details refer to the "Simple 596Kconfig recursive issue" subsection below. Kconfig does not do recursive 597dependency resolution; this has a few implications for Kconfig file writers. 598We'll first explain why this issues exists and then provide an example 599technical limitation which this brings upon Kconfig developers. Eager 600developers wishing to try to address this limitation should read the next 601subsections. 602 603Simple Kconfig recursive issue 604~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 605 606Read: Documentation/kbuild/Kconfig.recursion-issue-01 607 608Test with:: 609 610 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 611 612Cumulative Kconfig recursive issue 613~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 614 615Read: Documentation/kbuild/Kconfig.recursion-issue-02 616 617Test with:: 618 619 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 620 621Practical solutions to kconfig recursive issue 622~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 623 624Developers who run into the recursive Kconfig issue have two options 625at their disposal. We document them below and also provide a list of 626historical issues resolved through these different solutions. 627 628 a) Remove any superfluous "select FOO" or "depends on FOO" 629 b) Match dependency semantics: 630 631 b1) Swap all "select FOO" to "depends on FOO" or, 632 633 b2) Swap all "depends on FOO" to "select FOO" 634 635The resolution to a) can be tested with the sample Kconfig file 636Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 637of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 638since CORE_BELL_A depends on CORE. At times it may not be possible to remove 639some dependency criteria, for such cases you can work with solution b). 640 641The two different resolutions for b) can be tested in the sample Kconfig file 642Documentation/kbuild/Kconfig.recursion-issue-02. 643 644Below is a list of examples of prior fixes for these types of recursive issues; 645all errors appear to involve one or more "select" statements and one or more 646"depends on". 647 648============ =================================== 649commit fix 650============ =================================== 65106b718c01208 select A -> depends on A 652c22eacfe82f9 depends on A -> depends on B 6536a91e854442c select A -> depends on A 654118c565a8f2e select A -> select B 655f004e5594705 select A -> depends on A 656c7861f37b4c6 depends on A -> (null) 65780c69915e5fb select A -> (null) (1) 658c2218e26c0d0 select A -> depends on A (1) 659d6ae99d04e1c select A -> depends on A 66095ca19cf8cbf select A -> depends on A 6618f057d7bca54 depends on A -> (null) 6628f057d7bca54 depends on A -> select A 663a0701f04846e select A -> depends on A 6640c8b92f7f259 depends on A -> (null) 665e4e9e0540928 select A -> depends on A (2) 6667453ea886e87 depends on A > (null) (1) 6677b1fff7e4fdf select A -> depends on A 66886c747d2a4f0 select A -> depends on A 669d9f9ab51e55e select A -> depends on A 6700c51a4d8abd6 depends on A -> select A (3) 671e98062ed6dc4 select A -> depends on A (3) 67291e5d284a7f1 select A -> (null) 673============ =================================== 674 675(1) Partial (or no) quote of error. 676(2) That seems to be the gist of that fix. 677(3) Same error. 678 679Future kconfig work 680~~~~~~~~~~~~~~~~~~~ 681 682Work on kconfig is welcomed on both areas of clarifying semantics and on 683evaluating the use of a full SAT solver for it. A full SAT solver can be 684desirable to enable more complex dependency mappings and / or queries, 685for instance on possible use case for a SAT solver could be that of handling 686the current known recursive dependency issues. It is not known if this would 687address such issues but such evaluation is desirable. If support for a full SAT 688solver proves too complex or that it cannot address recursive dependency issues 689Kconfig should have at least clear and well defined semantics which also 690addresses and documents limitations or requirements such as the ones dealing 691with recursive dependencies. 692 693Further work on both of these areas is welcomed on Kconfig. We elaborate 694on both of these in the next two subsections. 695 696Semantics of Kconfig 697~~~~~~~~~~~~~~~~~~~~ 698 699The use of Kconfig is broad, Linux is now only one of Kconfig's users: 700one study has completed a broad analysis of Kconfig use in 12 projects [0]_. 701Despite its widespread use, and although this document does a reasonable job 702in documenting basic Kconfig syntax a more precise definition of Kconfig 703semantics is welcomed. One project deduced Kconfig semantics through 704the use of the xconfig configurator [1]_. Work should be done to confirm if 705the deduced semantics matches our intended Kconfig design goals. 706 707Having well defined semantics can be useful for tools for practical 708evaluation of dependencies, for instance one such case was work to 709express in boolean abstraction of the inferred semantics of Kconfig to 710translate Kconfig logic into boolean formulas and run a SAT solver on this to 711find dead code / features (always inactive), 114 dead features were found in 712Linux using this methodology [1]_ (Section 8: Threats to validity). 713 714Confirming this could prove useful as Kconfig stands as one of the leading 715industrial variability modeling languages [1]_ [2]_. Its study would help 716evaluate practical uses of such languages, their use was only theoretical 717and real world requirements were not well understood. As it stands though 718only reverse engineering techniques have been used to deduce semantics from 719variability modeling languages such as Kconfig [3]_. 720 721.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 722.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 723.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 724.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 725 726Full SAT solver for Kconfig 727~~~~~~~~~~~~~~~~~~~~~~~~~~~ 728 729Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted 730in the previous subsection, work has been done however to express in boolean 731abstraction the inferred semantics of Kconfig to translate Kconfig logic into 732boolean formulas and run a SAT solver on it [5]_. Another known related project 733is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which 734has been introduced first with [9]_. The basic concept of undertaker is to 735extract variability models from Kconfig and put them together with a 736propositional formula extracted from CPP #ifdefs and build-rules into a SAT 737solver in order to find dead code, dead files, and dead symbols. If using a SAT 738solver is desirable on Kconfig one approach would be to evaluate repurposing 739such efforts somehow on Kconfig. There is enough interest from mentors of 740existing projects to not only help advise how to integrate this work upstream 741but also help maintain it long term. Interested developers should visit: 742 743https://kernelnewbies.org/KernelProjects/kconfig-sat 744 745.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 746.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 747.. [6] https://cados.cs.fau.de 748.. [7] https://vamos.cs.fau.de 749.. [8] https://undertaker.cs.fau.de 750.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf 751