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 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 Note: If "imply" <symbol> is followed by "if" <expr>, the default of <symbol> 206 will be the logical AND of the value of the current menu symbol and <expr>. 207 (The future of this behavior is undecided.) 208 209- limiting menu display: "visible if" <expr> 210 211 This attribute is only applicable to menu blocks, if the condition is 212 false, the menu block is not displayed to the user (the symbols 213 contained there can still be selected by other symbols, though). It is 214 similar to a conditional "prompt" attribute for individual menu 215 entries. Default value of "visible" is true. 216 217- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] 218 219 This allows to limit the range of possible input values for int 220 and hex symbols. The user can only input a value which is larger than 221 or equal to the first symbol and smaller than or equal to the second 222 symbol. 223 224- help text: "help" 225 226 This defines a help text. The end of the help text is determined by 227 the indentation level, this means it ends at the first line which has 228 a smaller indentation than the first line of the help text. 229 230- module attribute: "modules" 231 This declares the symbol to be used as the MODULES symbol, which 232 enables the third modular state for all config symbols. 233 At most one symbol may have the "modules" option set. 234 235- transitional attribute: "transitional" 236 This declares the symbol as transitional, meaning it should be processed 237 during configuration but omitted from newly written .config files. 238 Transitional symbols are useful for backward compatibility during config 239 option migrations - they allow olddefconfig to process existing .config 240 files while ensuring the old option doesn't appear in new configurations. 241 242 A transitional symbol: 243 - Has no prompt (is not visible to users in menus) 244 - Is processed normally during configuration (values are read and used) 245 - Can be referenced in default expressions of other symbols 246 - Is not written to new .config files 247 - Cannot have any other properties (it is a pass-through option) 248 249 Example migration from OLD_NAME to NEW_NAME:: 250 251 config NEW_NAME 252 bool "New option name" 253 default OLD_NAME 254 help 255 This replaces the old CONFIG_OLD_NAME option. 256 257 config OLD_NAME 258 bool 259 transitional 260 help 261 Transitional config for OLD_NAME to NEW_NAME migration. 262 263 With this setup, existing .config files with "CONFIG_OLD_NAME=y" will 264 result in "CONFIG_NEW_NAME=y" being set, while CONFIG_OLD_NAME will be 265 omitted from newly written .config files. 266 267Menu dependencies 268----------------- 269 270Dependencies define the visibility of a menu entry and can also reduce 271the input range of tristate symbols. The tristate logic used in the 272expressions uses one more state than normal boolean logic to express the 273module state. Dependency expressions have the following syntax:: 274 275 <expr> ::= <symbol> (1) 276 <symbol> '=' <symbol> (2) 277 <symbol> '!=' <symbol> (3) 278 <symbol1> '<' <symbol2> (4) 279 <symbol1> '>' <symbol2> (4) 280 <symbol1> '<=' <symbol2> (4) 281 <symbol1> '>=' <symbol2> (4) 282 '(' <expr> ')' (5) 283 '!' <expr> (6) 284 <expr> '&&' <expr> (7) 285 <expr> '||' <expr> (8) 286 287Expressions are listed in decreasing order of precedence. 288 289(1) Convert the symbol into an expression. Boolean and tristate symbols 290 are simply converted into the respective expression values. All 291 other symbol types result in 'n'. 292(2) If the values of both symbols are equal, it returns 'y', 293 otherwise 'n'. 294(3) If the values of both symbols are equal, it returns 'n', 295 otherwise 'y'. 296(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, 297 or greater-or-equal than value of <symbol2>, it returns 'y', 298 otherwise 'n'. 299(5) Returns the value of the expression. Used to override precedence. 300(6) Returns the result of (2-/expr/). 301(7) Returns the result of min(/expr/, /expr/). 302(8) Returns the result of max(/expr/, /expr/). 303 304An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 305respectively for calculations). A menu entry becomes visible when its 306expression evaluates to 'm' or 'y'. 307 308There are two types of symbols: constant and non-constant symbols. 309Non-constant symbols are the most common ones and are defined with the 310'config' statement. Non-constant symbols consist entirely of alphanumeric 311characters or underscores. 312Constant symbols are only part of expressions. Constant symbols are 313always surrounded by single or double quotes. Within the quote, any 314other character is allowed and the quotes can be escaped using '\'. 315 316Menu structure 317-------------- 318 319The position of a menu entry in the tree is determined in two ways. First 320it can be specified explicitly:: 321 322 menu "Network device support" 323 depends on NET 324 325 config NETDEVICES 326 ... 327 328 endmenu 329 330All entries within the "menu" ... "endmenu" block become a submenu of 331"Network device support". All subentries inherit the dependencies from 332the menu entry, e.g. this means the dependency "NET" is added to the 333dependency list of the config option NETDEVICES. 334 335The other way to generate the menu structure is done by analyzing the 336dependencies. If a menu entry somehow depends on the previous entry, it 337can be made a submenu of it. First, the previous (parent) symbol must 338be part of the dependency list and then one of these two conditions 339must be true: 340 341- the child entry must become invisible, if the parent is set to 'n' 342- the child entry must only be visible, if the parent is visible:: 343 344 config MODULES 345 bool "Enable loadable module support" 346 347 config MODVERSIONS 348 bool "Set version information on all module symbols" 349 depends on MODULES 350 351 comment "module support disabled" 352 depends on !MODULES 353 354MODVERSIONS directly depends on MODULES, this means it's only visible if 355MODULES is different from 'n'. The comment on the other hand is only 356visible when MODULES is set to 'n'. 357 358 359Kconfig syntax 360-------------- 361 362The configuration file describes a series of menu entries, where every 363line starts with a keyword (except help texts). The following keywords 364end a menu entry: 365 366- config 367- menuconfig 368- choice/endchoice 369- comment 370- menu/endmenu 371- if/endif 372- source 373 374The first five also start the definition of a menu entry. 375 376config:: 377 378 "config" <symbol> 379 <config options> 380 381This defines a config symbol <symbol> and accepts any of above 382attributes as options. 383 384menuconfig:: 385 386 "menuconfig" <symbol> 387 <config options> 388 389This is similar to the simple config entry above, but it also gives a 390hint to front ends, that all suboptions should be displayed as a 391separate list of options. To make sure all the suboptions will really 392show up under the menuconfig entry and not outside of it, every item 393from the <config options> list must depend on the menuconfig symbol. 394In practice, this is achieved by using one of the next two constructs:: 395 396 (1): 397 menuconfig M 398 if M 399 config C1 400 config C2 401 endif 402 403 (2): 404 menuconfig M 405 config C1 406 depends on M 407 config C2 408 depends on M 409 410In the following examples (3) and (4), C1 and C2 still have the M 411dependency, but will not appear under menuconfig M anymore, because 412of C0, which doesn't depend on M:: 413 414 (3): 415 menuconfig M 416 config C0 417 if M 418 config C1 419 config C2 420 endif 421 422 (4): 423 menuconfig M 424 config C0 425 config C1 426 depends on M 427 config C2 428 depends on M 429 430choices:: 431 432 "choice" 433 <choice options> 434 <choice block> 435 "endchoice" 436 437This defines a choice group and accepts "prompt", "default", "depends on", and 438"help" attributes as options. 439 440A choice only allows a single config entry to be selected. 441 442comment:: 443 444 "comment" <prompt> 445 <comment options> 446 447This defines a comment which is displayed to the user during the 448configuration process and is also echoed to the output files. The only 449possible options are dependencies. 450 451menu:: 452 453 "menu" <prompt> 454 <menu options> 455 <menu block> 456 "endmenu" 457 458This defines a menu block, see "Menu structure" above for more 459information. The only possible options are dependencies and "visible" 460attributes. 461 462if:: 463 464 "if" <expr> 465 <if block> 466 "endif" 467 468This defines an if block. The dependency expression <expr> is appended 469to all enclosed menu entries. 470 471source:: 472 473 "source" <prompt> 474 475This reads the specified configuration file. This file is always parsed. 476 477mainmenu:: 478 479 "mainmenu" <prompt> 480 481This sets the config program's title bar if the config program chooses 482to use it. It should be placed at the top of the configuration, before any 483other statement. 484 485'#' Kconfig source file comment: 486 487An unquoted '#' character anywhere in a source file line indicates 488the beginning of a source file comment. The remainder of that line 489is a comment. 490 491 492Kconfig hints 493------------- 494This is a collection of Kconfig tips, most of which aren't obvious at 495first glance and most of which have become idioms in several Kconfig 496files. 497 498Adding common features and make the usage configurable 499~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 500It is a common idiom to implement a feature/functionality that are 501relevant for some architectures but not all. 502The recommended way to do so is to use a config variable named HAVE_* 503that is defined in a common Kconfig file and selected by the relevant 504architectures. 505An example is the generic IOMAP functionality. 506 507We would in lib/Kconfig see:: 508 509 # Generic IOMAP is used to ... 510 config HAVE_GENERIC_IOMAP 511 512 config GENERIC_IOMAP 513 depends on HAVE_GENERIC_IOMAP && FOO 514 515And in lib/Makefile we would see:: 516 517 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 518 519For each architecture using the generic IOMAP functionality we would see:: 520 521 config X86 522 select ... 523 select HAVE_GENERIC_IOMAP 524 select ... 525 526Note: we use the existing config option and avoid creating a new 527config variable to select HAVE_GENERIC_IOMAP. 528 529Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 530introduced to overcome the limitation of select which will force a 531config option to 'y' no matter the dependencies. 532The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 533situation where select forces a symbol equals to 'y'. 534 535Adding features that need compiler support 536~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 537 538There are several features that need compiler support. The recommended way 539to describe the dependency on the compiler feature is to use "depends on" 540followed by a test macro:: 541 542 config STACKPROTECTOR 543 bool "Stack Protector buffer overflow detection" 544 depends on $(cc-option,-fstack-protector) 545 ... 546 547If you need to expose a compiler capability to makefiles and/or C source files, 548`CC_HAS_` is the recommended prefix for the config option:: 549 550 config CC_HAS_FOO 551 def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC)) 552 553Build as module only 554~~~~~~~~~~~~~~~~~~~~ 555To restrict a component build to module-only, qualify its config symbol 556with "depends on m". E.g.:: 557 558 config FOO 559 depends on BAR && m 560 561limits FOO to module (=m) or disabled (=n). 562 563Compile-testing 564~~~~~~~~~~~~~~~ 565If a config symbol has a dependency, but the code controlled by the config 566symbol can still be compiled if the dependency is not met, it is encouraged to 567increase build coverage by adding an "|| COMPILE_TEST" clause to the 568dependency. This is especially useful for drivers for more exotic hardware, as 569it allows continuous-integration systems to compile-test the code on a more 570common system, and detect bugs that way. 571Note that compile-tested code should avoid crashing when run on a system where 572the dependency is not met. 573 574Architecture and platform dependencies 575~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 576Due to the presence of stubs, most drivers can now be compiled on most 577architectures. However, this does not mean it makes sense to have all drivers 578available everywhere, as the actual hardware may only exist on specific 579architectures and platforms. This is especially true for on-SoC IP cores, 580which may be limited to a specific vendor or SoC family. 581 582To prevent asking the user about drivers that cannot be used on the system(s) 583the user is compiling a kernel for, and if it makes sense, config symbols 584controlling the compilation of a driver should contain proper dependencies, 585limiting the visibility of the symbol to (a superset of) the platform(s) the 586driver can be used on. The dependency can be an architecture (e.g. ARM) or 587platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for 588distro config owners, but also for every single developer or user who 589configures a kernel. 590 591Such a dependency can be relaxed by combining it with the compile-testing rule 592above, leading to: 593 594 config FOO 595 bool "Support for foo hardware" 596 depends on ARCH_FOO_VENDOR || COMPILE_TEST 597 598Optional dependencies 599~~~~~~~~~~~~~~~~~~~~~ 600 601Some drivers are able to optionally use a feature from another module 602or build cleanly with that module disabled, but cause a link failure 603when trying to use that loadable module from a built-in driver. 604 605The most common way to express this optional dependency in Kconfig logic 606uses the slightly counterintuitive:: 607 608 config FOO 609 tristate "Support for foo hardware" 610 depends on BAR || !BAR 611 612This means that there is either a dependency on BAR that disallows 613the combination of FOO=y with BAR=m, or BAR is completely disabled. The BAR 614module must provide all the stubs for !BAR case. 615 616For a more formalized approach if there are multiple drivers that have 617the same dependency, a helper symbol can be used, like:: 618 619 config FOO 620 tristate "Support for foo hardware" 621 depends on BAR_OPTIONAL 622 623 config BAR_OPTIONAL 624 def_tristate BAR || !BAR 625 626Much less favorable way to express optional dependency is IS_REACHABLE() within 627the module code, useful for example when the module BAR does not provide 628!BAR stubs:: 629 630 foo_init() 631 { 632 if (IS_REACHABLE(CONFIG_BAR)) 633 bar_register(&foo); 634 ... 635 } 636 637IS_REACHABLE() is generally discouraged, because the code will be silently 638discarded, when CONFIG_BAR=m and this code is built-in. This is not what users 639usually expect when enabling BAR as module. 640 641Kconfig recursive dependency limitations 642~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 643 644If you've hit the Kconfig error: "recursive dependency detected" you've run 645into a recursive dependency issue with Kconfig, a recursive dependency can be 646summarized as a circular dependency. The kconfig tools need to ensure that 647Kconfig files comply with specified configuration requirements. In order to do 648that kconfig must determine the values that are possible for all Kconfig 649symbols, this is currently not possible if there is a circular relation 650between two or more Kconfig symbols. For more details refer to the "Simple 651Kconfig recursive issue" subsection below. Kconfig does not do recursive 652dependency resolution; this has a few implications for Kconfig file writers. 653We'll first explain why this issues exists and then provide an example 654technical limitation which this brings upon Kconfig developers. Eager 655developers wishing to try to address this limitation should read the next 656subsections. 657 658Simple Kconfig recursive issue 659~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 660 661Read: Documentation/kbuild/Kconfig.recursion-issue-01 662 663Test with:: 664 665 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 666 667Cumulative Kconfig recursive issue 668~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 669 670Read: Documentation/kbuild/Kconfig.recursion-issue-02 671 672Test with:: 673 674 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 675 676Practical solutions to kconfig recursive issue 677~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 678 679Developers who run into the recursive Kconfig issue have two options 680at their disposal. We document them below and also provide a list of 681historical issues resolved through these different solutions. 682 683 a) Remove any superfluous "select FOO" or "depends on FOO" 684 b) Match dependency semantics: 685 686 b1) Swap all "select FOO" to "depends on FOO" or, 687 688 b2) Swap all "depends on FOO" to "select FOO" 689 690The resolution to a) can be tested with the sample Kconfig file 691Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 692of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 693since CORE_BELL_A depends on CORE. At times it may not be possible to remove 694some dependency criteria, for such cases you can work with solution b). 695 696The two different resolutions for b) can be tested in the sample Kconfig file 697Documentation/kbuild/Kconfig.recursion-issue-02. 698 699Below is a list of examples of prior fixes for these types of recursive issues; 700all errors appear to involve one or more "select" statements and one or more 701"depends on". 702 703============ =================================== 704commit fix 705============ =================================== 70606b718c01208 select A -> depends on A 707c22eacfe82f9 depends on A -> depends on B 7086a91e854442c select A -> depends on A 709118c565a8f2e select A -> select B 710f004e5594705 select A -> depends on A 711c7861f37b4c6 depends on A -> (null) 71280c69915e5fb select A -> (null) (1) 713c2218e26c0d0 select A -> depends on A (1) 714d6ae99d04e1c select A -> depends on A 71595ca19cf8cbf select A -> depends on A 7168f057d7bca54 depends on A -> (null) 7178f057d7bca54 depends on A -> select A 718a0701f04846e select A -> depends on A 7190c8b92f7f259 depends on A -> (null) 720e4e9e0540928 select A -> depends on A (2) 7217453ea886e87 depends on A > (null) (1) 7227b1fff7e4fdf select A -> depends on A 72386c747d2a4f0 select A -> depends on A 724d9f9ab51e55e select A -> depends on A 7250c51a4d8abd6 depends on A -> select A (3) 726e98062ed6dc4 select A -> depends on A (3) 72791e5d284a7f1 select A -> (null) 728============ =================================== 729 730(1) Partial (or no) quote of error. 731(2) That seems to be the gist of that fix. 732(3) Same error. 733 734Future kconfig work 735~~~~~~~~~~~~~~~~~~~ 736 737Work on kconfig is welcomed on both areas of clarifying semantics and on 738evaluating the use of a full SAT solver for it. A full SAT solver can be 739desirable to enable more complex dependency mappings and / or queries, 740for instance one possible use case for a SAT solver could be that of handling 741the current known recursive dependency issues. It is not known if this would 742address such issues but such evaluation is desirable. If support for a full SAT 743solver proves too complex or that it cannot address recursive dependency issues 744Kconfig should have at least clear and well defined semantics which also 745addresses and documents limitations or requirements such as the ones dealing 746with recursive dependencies. 747 748Further work on both of these areas is welcomed on Kconfig. We elaborate 749on both of these in the next two subsections. 750 751Semantics of Kconfig 752~~~~~~~~~~~~~~~~~~~~ 753 754The use of Kconfig is broad, Linux is now only one of Kconfig's users: 755one study has completed a broad analysis of Kconfig use in 12 projects [0]_. 756Despite its widespread use, and although this document does a reasonable job 757in documenting basic Kconfig syntax a more precise definition of Kconfig 758semantics is welcomed. One project deduced Kconfig semantics through 759the use of the xconfig configurator [1]_. Work should be done to confirm if 760the deduced semantics matches our intended Kconfig design goals. 761Another project formalized a denotational semantics of a core subset of 762the Kconfig language [10]_. 763 764Having well defined semantics can be useful for tools for practical 765evaluation of dependencies, for instance one such case was work to 766express in boolean abstraction of the inferred semantics of Kconfig to 767translate Kconfig logic into boolean formulas and run a SAT solver on this to 768find dead code / features (always inactive), 114 dead features were found in 769Linux using this methodology [1]_ (Section 8: Threats to validity). 770The kismet tool, based on the semantics in [10]_, finds abuses of reverse 771dependencies and has led to dozens of committed fixes to Linux Kconfig files [11]_. 772 773Confirming this could prove useful as Kconfig stands as one of the leading 774industrial variability modeling languages [1]_ [2]_. Its study would help 775evaluate practical uses of such languages, their use was only theoretical 776and real world requirements were not well understood. As it stands though 777only reverse engineering techniques have been used to deduce semantics from 778variability modeling languages such as Kconfig [3]_. 779 780.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 781.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 782.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 783.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 784 785Full SAT solver for Kconfig 786~~~~~~~~~~~~~~~~~~~~~~~~~~~ 787 788Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted 789in the previous subsection, work has been done however to express in boolean 790abstraction the inferred semantics of Kconfig to translate Kconfig logic into 791boolean formulas and run a SAT solver on it [5]_. Another known related project 792is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which 793has been introduced first with [9]_. The basic concept of undertaker is to 794extract variability models from Kconfig and put them together with a 795propositional formula extracted from CPP #ifdefs and build-rules into a SAT 796solver in order to find dead code, dead files, and dead symbols. If using a SAT 797solver is desirable on Kconfig one approach would be to evaluate repurposing 798such efforts somehow on Kconfig. There is enough interest from mentors of 799existing projects to not only help advise how to integrate this work upstream 800but also help maintain it long term. Interested developers should visit: 801 802https://kernelnewbies.org/KernelProjects/kconfig-sat 803 804.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 805.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 806.. [6] https://cados.cs.fau.de 807.. [7] https://vamos.cs.fau.de 808.. [8] https://undertaker.cs.fau.de 809.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf 810.. [10] https://paulgazzillo.com/papers/esecfse21.pdf 811.. [11] https://github.com/paulgazz/kmax 812