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 m 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- module attribute: "modules" 227 This declares the symbol to be used as the MODULES symbol, which 228 enables the third modular state for all config symbols. 229 At most one symbol may have the "modules" option set. 230 231Menu dependencies 232----------------- 233 234Dependencies define the visibility of a menu entry and can also reduce 235the input range of tristate symbols. The tristate logic used in the 236expressions uses one more state than normal boolean logic to express the 237module state. Dependency expressions have the following syntax:: 238 239 <expr> ::= <symbol> (1) 240 <symbol> '=' <symbol> (2) 241 <symbol> '!=' <symbol> (3) 242 <symbol1> '<' <symbol2> (4) 243 <symbol1> '>' <symbol2> (4) 244 <symbol1> '<=' <symbol2> (4) 245 <symbol1> '>=' <symbol2> (4) 246 '(' <expr> ')' (5) 247 '!' <expr> (6) 248 <expr> '&&' <expr> (7) 249 <expr> '||' <expr> (8) 250 251Expressions are listed in decreasing order of precedence. 252 253(1) Convert the symbol into an expression. Boolean and tristate symbols 254 are simply converted into the respective expression values. All 255 other symbol types result in 'n'. 256(2) If the values of both symbols are equal, it returns 'y', 257 otherwise 'n'. 258(3) If the values of both symbols are equal, it returns 'n', 259 otherwise 'y'. 260(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, 261 or greater-or-equal than value of <symbol2>, it returns 'y', 262 otherwise 'n'. 263(5) Returns the value of the expression. Used to override precedence. 264(6) Returns the result of (2-/expr/). 265(7) Returns the result of min(/expr/, /expr/). 266(8) Returns the result of max(/expr/, /expr/). 267 268An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 269respectively for calculations). A menu entry becomes visible when its 270expression evaluates to 'm' or 'y'. 271 272There are two types of symbols: constant and non-constant symbols. 273Non-constant symbols are the most common ones and are defined with the 274'config' statement. Non-constant symbols consist entirely of alphanumeric 275characters or underscores. 276Constant symbols are only part of expressions. Constant symbols are 277always surrounded by single or double quotes. Within the quote, any 278other character is allowed and the quotes can be escaped using '\'. 279 280Menu structure 281-------------- 282 283The position of a menu entry in the tree is determined in two ways. First 284it can be specified explicitly:: 285 286 menu "Network device support" 287 depends on NET 288 289 config NETDEVICES 290 ... 291 292 endmenu 293 294All entries within the "menu" ... "endmenu" block become a submenu of 295"Network device support". All subentries inherit the dependencies from 296the menu entry, e.g. this means the dependency "NET" is added to the 297dependency list of the config option NETDEVICES. 298 299The other way to generate the menu structure is done by analyzing the 300dependencies. If a menu entry somehow depends on the previous entry, it 301can be made a submenu of it. First, the previous (parent) symbol must 302be part of the dependency list and then one of these two conditions 303must be true: 304 305- the child entry must become invisible, if the parent is set to 'n' 306- the child entry must only be visible, if the parent is visible:: 307 308 config MODULES 309 bool "Enable loadable module support" 310 311 config MODVERSIONS 312 bool "Set version information on all module symbols" 313 depends on MODULES 314 315 comment "module support disabled" 316 depends on !MODULES 317 318MODVERSIONS directly depends on MODULES, this means it's only visible if 319MODULES is different from 'n'. The comment on the other hand is only 320visible when MODULES is set to 'n'. 321 322 323Kconfig syntax 324-------------- 325 326The configuration file describes a series of menu entries, where every 327line starts with a keyword (except help texts). The following keywords 328end a menu entry: 329 330- config 331- menuconfig 332- choice/endchoice 333- comment 334- menu/endmenu 335- if/endif 336- source 337 338The first five also start the definition of a menu entry. 339 340config:: 341 342 "config" <symbol> 343 <config options> 344 345This defines a config symbol <symbol> and accepts any of above 346attributes as options. 347 348menuconfig:: 349 350 "menuconfig" <symbol> 351 <config options> 352 353This is similar to the simple config entry above, but it also gives a 354hint to front ends, that all suboptions should be displayed as a 355separate list of options. To make sure all the suboptions will really 356show up under the menuconfig entry and not outside of it, every item 357from the <config options> list must depend on the menuconfig symbol. 358In practice, this is achieved by using one of the next two constructs:: 359 360 (1): 361 menuconfig M 362 if M 363 config C1 364 config C2 365 endif 366 367 (2): 368 menuconfig M 369 config C1 370 depends on M 371 config C2 372 depends on M 373 374In the following examples (3) and (4), C1 and C2 still have the M 375dependency, but will not appear under menuconfig M anymore, because 376of C0, which doesn't depend on M:: 377 378 (3): 379 menuconfig M 380 config C0 381 if M 382 config C1 383 config C2 384 endif 385 386 (4): 387 menuconfig M 388 config C0 389 config C1 390 depends on M 391 config C2 392 depends on M 393 394choices:: 395 396 "choice" [symbol] 397 <choice options> 398 <choice block> 399 "endchoice" 400 401This defines a choice group and accepts any of the above attributes as 402options. A choice can only be of type bool or tristate. If no type is 403specified for a choice, its type will be determined by the type of 404the first choice element in the group or remain unknown if none of the 405choice elements have a type specified, as well. 406 407While a boolean choice only allows a single config entry to be 408selected, a tristate choice also allows any number of config entries 409to be set to 'm'. This can be used if multiple drivers for a single 410hardware exists and only a single driver can be compiled/loaded into 411the kernel, but all drivers can be compiled as modules. 412 413A choice accepts another option "optional", which allows to set the 414choice to 'n' and no entry needs to be selected. 415If no [symbol] is associated with a choice, then you can not have multiple 416definitions of that choice. If a [symbol] is associated to the choice, 417then you may define the same choice (i.e. with the same entries) in another 418place. 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_ASM_GOTO 529 def_bool $(success,$(srctree)/scripts/gcc-goto.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 576Kconfig recursive dependency limitations 577~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 578 579If you've hit the Kconfig error: "recursive dependency detected" you've run 580into a recursive dependency issue with Kconfig, a recursive dependency can be 581summarized as a circular dependency. The kconfig tools need to ensure that 582Kconfig files comply with specified configuration requirements. In order to do 583that kconfig must determine the values that are possible for all Kconfig 584symbols, this is currently not possible if there is a circular relation 585between two or more Kconfig symbols. For more details refer to the "Simple 586Kconfig recursive issue" subsection below. Kconfig does not do recursive 587dependency resolution; this has a few implications for Kconfig file writers. 588We'll first explain why this issues exists and then provide an example 589technical limitation which this brings upon Kconfig developers. Eager 590developers wishing to try to address this limitation should read the next 591subsections. 592 593Simple Kconfig recursive issue 594~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 595 596Read: Documentation/kbuild/Kconfig.recursion-issue-01 597 598Test with:: 599 600 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 601 602Cumulative Kconfig recursive issue 603~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 604 605Read: Documentation/kbuild/Kconfig.recursion-issue-02 606 607Test with:: 608 609 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 610 611Practical solutions to kconfig recursive issue 612~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 613 614Developers who run into the recursive Kconfig issue have two options 615at their disposal. We document them below and also provide a list of 616historical issues resolved through these different solutions. 617 618 a) Remove any superfluous "select FOO" or "depends on FOO" 619 b) Match dependency semantics: 620 621 b1) Swap all "select FOO" to "depends on FOO" or, 622 623 b2) Swap all "depends on FOO" to "select FOO" 624 625The resolution to a) can be tested with the sample Kconfig file 626Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 627of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 628since CORE_BELL_A depends on CORE. At times it may not be possible to remove 629some dependency criteria, for such cases you can work with solution b). 630 631The two different resolutions for b) can be tested in the sample Kconfig file 632Documentation/kbuild/Kconfig.recursion-issue-02. 633 634Below is a list of examples of prior fixes for these types of recursive issues; 635all errors appear to involve one or more "select" statements and one or more 636"depends on". 637 638============ =================================== 639commit fix 640============ =================================== 64106b718c01208 select A -> depends on A 642c22eacfe82f9 depends on A -> depends on B 6436a91e854442c select A -> depends on A 644118c565a8f2e select A -> select B 645f004e5594705 select A -> depends on A 646c7861f37b4c6 depends on A -> (null) 64780c69915e5fb select A -> (null) (1) 648c2218e26c0d0 select A -> depends on A (1) 649d6ae99d04e1c select A -> depends on A 65095ca19cf8cbf select A -> depends on A 6518f057d7bca54 depends on A -> (null) 6528f057d7bca54 depends on A -> select A 653a0701f04846e select A -> depends on A 6540c8b92f7f259 depends on A -> (null) 655e4e9e0540928 select A -> depends on A (2) 6567453ea886e87 depends on A > (null) (1) 6577b1fff7e4fdf select A -> depends on A 65886c747d2a4f0 select A -> depends on A 659d9f9ab51e55e select A -> depends on A 6600c51a4d8abd6 depends on A -> select A (3) 661e98062ed6dc4 select A -> depends on A (3) 66291e5d284a7f1 select A -> (null) 663============ =================================== 664 665(1) Partial (or no) quote of error. 666(2) That seems to be the gist of that fix. 667(3) Same error. 668 669Future kconfig work 670~~~~~~~~~~~~~~~~~~~ 671 672Work on kconfig is welcomed on both areas of clarifying semantics and on 673evaluating the use of a full SAT solver for it. A full SAT solver can be 674desirable to enable more complex dependency mappings and / or queries, 675for instance on possible use case for a SAT solver could be that of handling 676the current known recursive dependency issues. It is not known if this would 677address such issues but such evaluation is desirable. If support for a full SAT 678solver proves too complex or that it cannot address recursive dependency issues 679Kconfig should have at least clear and well defined semantics which also 680addresses and documents limitations or requirements such as the ones dealing 681with recursive dependencies. 682 683Further work on both of these areas is welcomed on Kconfig. We elaborate 684on both of these in the next two subsections. 685 686Semantics of Kconfig 687~~~~~~~~~~~~~~~~~~~~ 688 689The use of Kconfig is broad, Linux is now only one of Kconfig's users: 690one study has completed a broad analysis of Kconfig use in 12 projects [0]_. 691Despite its widespread use, and although this document does a reasonable job 692in documenting basic Kconfig syntax a more precise definition of Kconfig 693semantics is welcomed. One project deduced Kconfig semantics through 694the use of the xconfig configurator [1]_. Work should be done to confirm if 695the deduced semantics matches our intended Kconfig design goals. 696 697Having well defined semantics can be useful for tools for practical 698evaluation of dependencies, for instance one such case was work to 699express in boolean abstraction of the inferred semantics of Kconfig to 700translate Kconfig logic into boolean formulas and run a SAT solver on this to 701find dead code / features (always inactive), 114 dead features were found in 702Linux using this methodology [1]_ (Section 8: Threats to validity). 703 704Confirming this could prove useful as Kconfig stands as one of the leading 705industrial variability modeling languages [1]_ [2]_. Its study would help 706evaluate practical uses of such languages, their use was only theoretical 707and real world requirements were not well understood. As it stands though 708only reverse engineering techniques have been used to deduce semantics from 709variability modeling languages such as Kconfig [3]_. 710 711.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 712.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 713.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 714.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 715 716Full SAT solver for Kconfig 717~~~~~~~~~~~~~~~~~~~~~~~~~~~ 718 719Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted 720in the previous subsection, work has been done however to express in boolean 721abstraction the inferred semantics of Kconfig to translate Kconfig logic into 722boolean formulas and run a SAT solver on it [5]_. Another known related project 723is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which 724has been introduced first with [9]_. The basic concept of undertaker is to 725extract variability models from Kconfig and put them together with a 726propositional formula extracted from CPP #ifdefs and build-rules into a SAT 727solver in order to find dead code, dead files, and dead symbols. If using a SAT 728solver is desirable on Kconfig one approach would be to evaluate repurposing 729such efforts somehow on Kconfig. There is enough interest from mentors of 730existing projects to not only help advise how to integrate this work upstream 731but also help maintain it long term. Interested developers should visit: 732 733https://kernelnewbies.org/KernelProjects/kconfig-sat 734 735.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 736.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 737.. [6] https://cados.cs.fau.de 738.. [7] https://vamos.cs.fau.de 739.. [8] https://undertaker.cs.fau.de 740.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf 741