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> ["if" <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 The dependency definition itself may be conditional by appending "if" 138 followed by an expression. For example:: 139 140 config FOO 141 tristate 142 depends on BAR if BAZ 143 144 meaning that FOO is constrained by the value of BAR only if BAZ is 145 also set. 146 147- reverse dependencies: "select" <symbol> ["if" <expr>] 148 149 While normal dependencies reduce the upper limit of a symbol (see 150 below), reverse dependencies can be used to force a lower limit of 151 another symbol. The value of the current menu symbol is used as the 152 minimal value <symbol> can be set to. If <symbol> is selected multiple 153 times, the limit is set to the largest selection. 154 Reverse dependencies can only be used with boolean or tristate 155 symbols. 156 157 Note: 158 select should be used with care. select will force 159 a symbol to a value without visiting the dependencies. 160 By abusing select you are able to select a symbol FOO even 161 if FOO depends on BAR that is not set. 162 In general use select only for non-visible symbols 163 (no prompts anywhere) and for symbols with no dependencies. 164 That will limit the usefulness but on the other hand avoid 165 the illegal configurations all over. 166 167 If "select" <symbol> is followed by "if" <expr>, <symbol> will be 168 selected by the logical AND of the value of the current menu symbol 169 and <expr>. This means, the lower limit can be downgraded due to the 170 presence of "if" <expr>. This behavior may seem weird, but we rely on 171 it. (The future of this behavior is undecided.) 172 173- weak reverse dependencies: "imply" <symbol> ["if" <expr>] 174 175 This is similar to "select" as it enforces a lower limit on another 176 symbol except that the "implied" symbol's value may still be set to n 177 from a direct dependency or with a visible prompt. 178 179 Given the following example:: 180 181 config FOO 182 tristate "foo" 183 imply BAZ 184 185 config BAZ 186 tristate "baz" 187 depends on BAR 188 189 The following values are possible: 190 191 === === ============= ============== 192 FOO BAR BAZ's default choice for BAZ 193 === === ============= ============== 194 n y n N/m/y 195 m y m M/y/n 196 y y y Y/m/n 197 n m n N/m 198 m m m M/n 199 y m m M/n 200 y n * N 201 === === ============= ============== 202 203 This is useful e.g. with multiple drivers that want to indicate their 204 ability to hook into a secondary subsystem while allowing the user to 205 configure that subsystem out without also having to unset these drivers. 206 207 Note: If the feature provided by BAZ is highly desirable for FOO, 208 FOO should imply not only BAZ, but also its dependency BAR:: 209 210 config FOO 211 tristate "foo" 212 imply BAR 213 imply BAZ 214 215 Note: If "imply" <symbol> is followed by "if" <expr>, the default of <symbol> 216 will be the logical AND of the value of the current menu symbol and <expr>. 217 (The future of this behavior is undecided.) 218 219- limiting menu display: "visible if" <expr> 220 221 This attribute is only applicable to menu blocks, if the condition is 222 false, the menu block is not displayed to the user (the symbols 223 contained there can still be selected by other symbols, though). It is 224 similar to a conditional "prompt" attribute for individual menu 225 entries. Default value of "visible" is true. 226 227- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] 228 229 This allows limiting the range of possible input values for int 230 and hex symbols. The user can only input a value which is larger than 231 or equal to the first symbol and smaller than or equal to the second 232 symbol. 233 234- help text: "help" 235 236 This defines a help text. The end of the help text is determined by 237 the indentation level, this means it ends at the first line which has 238 a smaller indentation than the first line of the help text. 239 240- module attribute: "modules" 241 This declares the symbol to be used as the MODULES symbol, which 242 enables the third modular state for all config symbols. 243 At most one symbol may have the "modules" option set. 244 245- transitional attribute: "transitional" 246 This declares the symbol as transitional, meaning it should be processed 247 during configuration but omitted from newly written .config files. 248 Transitional symbols are useful for backward compatibility during config 249 option migrations - they allow olddefconfig to process existing .config 250 files while ensuring the old option doesn't appear in new configurations. 251 252 A transitional symbol: 253 - Has no prompt (is not visible to users in menus) 254 - Is processed normally during configuration (values are read and used) 255 - Can be referenced in default expressions of other symbols 256 - Is not written to new .config files 257 - Cannot have any other properties (it is a pass-through option) 258 259 Example migration from OLD_NAME to NEW_NAME:: 260 261 config NEW_NAME 262 bool "New option name" 263 default OLD_NAME 264 help 265 This replaces the old CONFIG_OLD_NAME option. 266 267 config OLD_NAME 268 bool 269 transitional 270 help 271 Transitional config for OLD_NAME to NEW_NAME migration. 272 273 With this setup, existing .config files with "CONFIG_OLD_NAME=y" will 274 result in "CONFIG_NEW_NAME=y" being set, while CONFIG_OLD_NAME will be 275 omitted from newly written .config files. 276 277Menu dependencies 278----------------- 279 280Dependencies define the visibility of a menu entry and can also reduce 281the input range of tristate symbols. The tristate logic used in the 282expressions uses one more state than normal boolean logic to express the 283module state. Dependency expressions have the following syntax:: 284 285 <expr> ::= <symbol> (1) 286 <symbol> '=' <symbol> (2) 287 <symbol> '!=' <symbol> (3) 288 <symbol1> '<' <symbol2> (4) 289 <symbol1> '>' <symbol2> (4) 290 <symbol1> '<=' <symbol2> (4) 291 <symbol1> '>=' <symbol2> (4) 292 '(' <expr> ')' (5) 293 '!' <expr> (6) 294 <expr> '&&' <expr> (7) 295 <expr> '||' <expr> (8) 296 297Expressions are listed in decreasing order of precedence. 298 299(1) Convert the symbol into an expression. Boolean and tristate symbols 300 are simply converted into the respective expression values. All 301 other symbol types result in 'n'. 302(2) If the values of both symbols are equal, it returns 'y', 303 otherwise 'n'. 304(3) If the values of both symbols are equal, it returns 'n', 305 otherwise 'y'. 306(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, 307 or greater-or-equal than value of <symbol2>, it returns 'y', 308 otherwise 'n'. 309(5) Returns the value of the expression. Used to override precedence. 310(6) Returns the result of (2-/expr/). 311(7) Returns the result of min(/expr/, /expr/). 312(8) Returns the result of max(/expr/, /expr/). 313 314An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 315respectively for calculations). A menu entry becomes visible when its 316expression evaluates to 'm' or 'y'. 317 318There are two types of symbols: constant and non-constant symbols. 319Non-constant symbols are the most common ones and are defined with the 320'config' statement. Non-constant symbols consist entirely of alphanumeric 321characters or underscores. 322Constant symbols are only part of expressions. Constant symbols are 323always surrounded by single or double quotes. Within the quote, any 324other character is allowed and the quotes can be escaped using '\'. 325 326Menu structure 327-------------- 328 329The position of a menu entry in the tree is determined in two ways. First 330it can be specified explicitly:: 331 332 menu "Network device support" 333 depends on NET 334 335 config NETDEVICES 336 ... 337 338 endmenu 339 340All entries within the "menu" ... "endmenu" block become a submenu of 341"Network device support". All subentries inherit the dependencies from 342the menu entry, e.g. this means the dependency "NET" is added to the 343dependency list of the config option NETDEVICES. 344 345The other way to generate the menu structure is done by analyzing the 346dependencies. If a menu entry somehow depends on the previous entry, it 347can be made a submenu of it. First, the previous (parent) symbol must 348be part of the dependency list and then one of these two conditions 349must be true: 350 351- the child entry must become invisible, if the parent is set to 'n' 352- the child entry must only be visible, if the parent is visible:: 353 354 config MODULES 355 bool "Enable loadable module support" 356 357 config MODVERSIONS 358 bool "Set version information on all module symbols" 359 depends on MODULES 360 361 comment "module support disabled" 362 depends on !MODULES 363 364MODVERSIONS directly depends on MODULES, this means it's only visible if 365MODULES is different from 'n'. The comment on the other hand is only 366visible when MODULES is set to 'n'. 367 368 369Kconfig syntax 370-------------- 371 372The configuration file describes a series of menu entries, where every 373line starts with a keyword (except help texts). The following keywords 374end a menu entry: 375 376- config 377- menuconfig 378- choice/endchoice 379- comment 380- menu/endmenu 381- if/endif 382- source 383 384The first five also start the definition of a menu entry. 385 386config:: 387 388 "config" <symbol> 389 <config options> 390 391This defines a config symbol <symbol> and accepts any of above 392attributes as options. 393 394menuconfig:: 395 396 "menuconfig" <symbol> 397 <config options> 398 399This is similar to the simple config entry above, but it also gives a 400hint to front ends, that all suboptions should be displayed as a 401separate list of options. To make sure all the suboptions will really 402show up under the menuconfig entry and not outside of it, every item 403from the <config options> list must depend on the menuconfig symbol. 404In practice, this is achieved by using one of the next two constructs:: 405 406 (1): 407 menuconfig M 408 if M 409 config C1 410 config C2 411 endif 412 413 (2): 414 menuconfig M 415 config C1 416 depends on M 417 config C2 418 depends on M 419 420In the following examples (3) and (4), C1 and C2 still have the M 421dependency, but will not appear under menuconfig M anymore, because 422of C0, which doesn't depend on M:: 423 424 (3): 425 menuconfig M 426 config C0 427 if M 428 config C1 429 config C2 430 endif 431 432 (4): 433 menuconfig M 434 config C0 435 config C1 436 depends on M 437 config C2 438 depends on M 439 440choices:: 441 442 "choice" 443 <choice options> 444 <choice block> 445 "endchoice" 446 447This defines a choice group and accepts "prompt", "default", "depends on", and 448"help" attributes as options. 449 450A choice only allows a single config entry to be selected. 451 452comment:: 453 454 "comment" <prompt> 455 <comment options> 456 457This defines a comment which is displayed to the user during the 458configuration process and is also echoed to the output files. The only 459possible options are dependencies. 460 461menu:: 462 463 "menu" <prompt> 464 <menu options> 465 <menu block> 466 "endmenu" 467 468This defines a menu block, see "Menu structure" above for more 469information. The only possible options are dependencies and "visible" 470attributes. 471 472if:: 473 474 "if" <expr> 475 <if block> 476 "endif" 477 478This defines an if block. The dependency expression <expr> is appended 479to all enclosed menu entries. 480 481source:: 482 483 "source" <prompt> 484 485This reads the specified configuration file. This file is always parsed. 486 487mainmenu:: 488 489 "mainmenu" <prompt> 490 491This sets the config program's title bar if the config program chooses 492to use it. It should be placed at the top of the configuration, before any 493other statement. 494 495'#' Kconfig source file comment: 496 497An unquoted '#' character anywhere in a source file line indicates 498the beginning of a source file comment. The remainder of that line 499is a comment. 500 501 502Kconfig hints 503------------- 504This is a collection of Kconfig tips, most of which aren't obvious at 505first glance and most of which have become idioms in several Kconfig 506files. 507 508Adding common features and make the usage configurable 509~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 510It is a common idiom to implement a feature/functionality that are 511relevant for some architectures but not all. 512The recommended way to do so is to use a config variable named HAVE_* 513that is defined in a common Kconfig file and selected by the relevant 514architectures. 515An example is the generic IOMAP functionality. 516 517We would in lib/Kconfig see:: 518 519 # Generic IOMAP is used to ... 520 config HAVE_GENERIC_IOMAP 521 522 config GENERIC_IOMAP 523 depends on HAVE_GENERIC_IOMAP && FOO 524 525And in lib/Makefile we would see:: 526 527 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 528 529For each architecture using the generic IOMAP functionality we would see:: 530 531 config X86 532 select ... 533 select HAVE_GENERIC_IOMAP 534 select ... 535 536Note: we use the existing config option and avoid creating a new 537config variable to select HAVE_GENERIC_IOMAP. 538 539Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 540introduced to overcome the limitation of select which will force a 541config option to 'y' no matter the dependencies. 542The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 543situation where select forces a symbol equals to 'y'. 544 545Adding features that need compiler support 546~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 547 548There are several features that need compiler support. The recommended way 549to describe the dependency on the compiler feature is to use "depends on" 550followed by a test macro:: 551 552 config STACKPROTECTOR 553 bool "Stack Protector buffer overflow detection" 554 depends on $(cc-option,-fstack-protector) 555 ... 556 557If you need to expose a compiler capability to makefiles and/or C source files, 558`CC_HAS_` is the recommended prefix for the config option:: 559 560 config CC_HAS_FOO 561 def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC)) 562 563Build as module only 564~~~~~~~~~~~~~~~~~~~~ 565To restrict a component build to module-only, qualify its config symbol 566with "depends on m". E.g.:: 567 568 config FOO 569 depends on BAR && m 570 571limits FOO to module (=m) or disabled (=n). 572 573Compile-testing 574~~~~~~~~~~~~~~~ 575If a config symbol has a dependency, but the code controlled by the config 576symbol can still be compiled if the dependency is not met, it is encouraged to 577increase build coverage by adding an "|| COMPILE_TEST" clause to the 578dependency. This is especially useful for drivers for more exotic hardware, as 579it allows continuous-integration systems to compile-test the code on a more 580common system, and detect bugs that way. 581Note that compile-tested code should avoid crashing when run on a system where 582the dependency is not met. 583 584Architecture and platform dependencies 585~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 586Due to the presence of stubs, most drivers can now be compiled on most 587architectures. However, this does not mean it makes sense to have all drivers 588available everywhere, as the actual hardware may only exist on specific 589architectures and platforms. This is especially true for on-SoC IP cores, 590which may be limited to a specific vendor or SoC family. 591 592To prevent asking the user about drivers that cannot be used on the system(s) 593the user is compiling a kernel for, and if it makes sense, config symbols 594controlling the compilation of a driver should contain proper dependencies, 595limiting the visibility of the symbol to (a superset of) the platform(s) the 596driver can be used on. The dependency can be an architecture (e.g. ARM) or 597platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for 598distro config owners, but also for every single developer or user who 599configures a kernel. 600 601Such a dependency can be relaxed by combining it with the compile-testing rule 602above, leading to: 603 604 config FOO 605 bool "Support for foo hardware" 606 depends on ARCH_FOO_VENDOR || COMPILE_TEST 607 608Optional dependencies 609~~~~~~~~~~~~~~~~~~~~~ 610 611Some drivers are able to optionally use a feature from another module 612or build cleanly with that module disabled, but cause a link failure 613when trying to use that loadable module from a built-in driver. 614 615The recommended way to express this optional dependency in Kconfig logic 616uses the conditional form:: 617 618 config FOO 619 tristate "Support for foo hardware" 620 depends on BAR if BAR 621 622This slightly counterintuitive style is also widely used:: 623 624 config FOO 625 tristate "Support for foo hardware" 626 depends on BAR || !BAR 627 628This means that there is either a dependency on BAR that disallows 629the combination of FOO=y with BAR=m, or BAR is completely disabled. The BAR 630module must provide all the stubs for !BAR case. 631 632For a more formalized approach if there are multiple drivers that have 633the same dependency, a helper symbol can be used, like:: 634 635 config FOO 636 tristate "Support for foo hardware" 637 depends on BAR_OPTIONAL 638 639 config BAR_OPTIONAL 640 def_tristate BAR || !BAR 641 642Much less favorable way to express optional dependency is IS_REACHABLE() within 643the module code, useful for example when the module BAR does not provide 644!BAR stubs:: 645 646 foo_init() 647 { 648 if (IS_REACHABLE(CONFIG_BAR)) 649 bar_register(&foo); 650 ... 651 } 652 653IS_REACHABLE() is generally discouraged, because the code will be silently 654discarded, when CONFIG_BAR=m and this code is built-in. This is not what users 655usually expect when enabling BAR as module. 656 657Kconfig recursive dependency limitations 658~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 659 660If you've hit the Kconfig error: "recursive dependency detected" you've run 661into a recursive dependency issue with Kconfig, a recursive dependency can be 662summarized as a circular dependency. The kconfig tools need to ensure that 663Kconfig files comply with specified configuration requirements. In order to do 664that kconfig must determine the values that are possible for all Kconfig 665symbols, this is currently not possible if there is a circular relation 666between two or more Kconfig symbols. For more details refer to the "Simple 667Kconfig recursive issue" subsection below. Kconfig does not do recursive 668dependency resolution; this has a few implications for Kconfig file writers. 669We'll first explain why this issues exists and then provide an example 670technical limitation which this brings upon Kconfig developers. Eager 671developers wishing to try to address this limitation should read the next 672subsections. 673 674Simple Kconfig recursive issue 675~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 676 677Read: Documentation/kbuild/Kconfig.recursion-issue-01 678 679Test with:: 680 681 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 682 683Cumulative Kconfig recursive issue 684~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 685 686Read: Documentation/kbuild/Kconfig.recursion-issue-02 687 688Test with:: 689 690 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 691 692Practical solutions to kconfig recursive issue 693~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 694 695Developers who run into the recursive Kconfig issue have two options 696at their disposal. We document them below and also provide a list of 697historical issues resolved through these different solutions. 698 699 a) Remove any superfluous "select FOO" or "depends on FOO" 700 b) Match dependency semantics: 701 702 b1) Swap all "select FOO" to "depends on FOO" or, 703 704 b2) Swap all "depends on FOO" to "select FOO" 705 706The resolution to a) can be tested with the sample Kconfig file 707Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 708of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 709since CORE_BELL_A depends on CORE. At times it may not be possible to remove 710some dependency criteria, for such cases you can work with solution b). 711 712The two different resolutions for b) can be tested in the sample Kconfig file 713Documentation/kbuild/Kconfig.recursion-issue-02. 714 715Below is a list of examples of prior fixes for these types of recursive issues; 716all errors appear to involve one or more "select" statements and one or more 717"depends on". 718 719============ =================================== 720commit fix 721============ =================================== 72206b718c01208 select A -> depends on A 723c22eacfe82f9 depends on A -> depends on B 7246a91e854442c select A -> depends on A 725118c565a8f2e select A -> select B 726f004e5594705 select A -> depends on A 727c7861f37b4c6 depends on A -> (null) 72880c69915e5fb select A -> (null) (1) 729c2218e26c0d0 select A -> depends on A (1) 730d6ae99d04e1c select A -> depends on A 73195ca19cf8cbf select A -> depends on A 7328f057d7bca54 depends on A -> (null) 7338f057d7bca54 depends on A -> select A 734a0701f04846e select A -> depends on A 7350c8b92f7f259 depends on A -> (null) 736e4e9e0540928 select A -> depends on A (2) 7377453ea886e87 depends on A > (null) (1) 7387b1fff7e4fdf select A -> depends on A 73986c747d2a4f0 select A -> depends on A 740d9f9ab51e55e select A -> depends on A 7410c51a4d8abd6 depends on A -> select A (3) 742e98062ed6dc4 select A -> depends on A (3) 74391e5d284a7f1 select A -> (null) 744============ =================================== 745 746(1) Partial (or no) quote of error. 747(2) That seems to be the gist of that fix. 748(3) Same error. 749 750Future kconfig work 751~~~~~~~~~~~~~~~~~~~ 752 753Work on kconfig is welcomed on both areas of clarifying semantics and on 754evaluating the use of a full SAT solver for it. A full SAT solver can be 755desirable to enable more complex dependency mappings and / or queries, 756for instance one possible use case for a SAT solver could be that of handling 757the current known recursive dependency issues. It is not known if this would 758address such issues but such evaluation is desirable. If support for a full SAT 759solver proves too complex or that it cannot address recursive dependency issues 760Kconfig should have at least clear and well defined semantics which also 761addresses and documents limitations or requirements such as the ones dealing 762with recursive dependencies. 763 764Further work on both of these areas is welcomed on Kconfig. We elaborate 765on both of these in the next two subsections. 766 767Semantics of Kconfig 768~~~~~~~~~~~~~~~~~~~~ 769 770The use of Kconfig is broad, Linux is now only one of Kconfig's users: 771one study has completed a broad analysis of Kconfig use in 12 projects [0]_. 772Despite its widespread use, and although this document does a reasonable job 773in documenting basic Kconfig syntax a more precise definition of Kconfig 774semantics is welcomed. One project deduced Kconfig semantics through 775the use of the xconfig configurator [1]_. Work should be done to confirm if 776the deduced semantics matches our intended Kconfig design goals. 777Another project formalized a denotational semantics of a core subset of 778the Kconfig language [10]_. 779 780Having well defined semantics can be useful for tools for practical 781evaluation of dependencies, for instance one such case was work to 782express in boolean abstraction of the inferred semantics of Kconfig to 783translate Kconfig logic into boolean formulas and run a SAT solver on this to 784find dead code / features (always inactive), 114 dead features were found in 785Linux using this methodology [1]_ (Section 8: Threats to validity). 786The kismet tool, based on the semantics in [10]_, finds abuses of reverse 787dependencies and has led to dozens of committed fixes to Linux Kconfig files [11]_. 788 789Confirming this could prove useful as Kconfig stands as one of the leading 790industrial variability modeling languages [1]_ [2]_. Its study would help 791evaluate practical uses of such languages, their use was only theoretical 792and real world requirements were not well understood. As it stands though 793only reverse engineering techniques have been used to deduce semantics from 794variability modeling languages such as Kconfig [3]_. 795 796.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 797.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 798.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 799.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 800 801Full SAT solver for Kconfig 802~~~~~~~~~~~~~~~~~~~~~~~~~~~ 803 804Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted 805in the previous subsection, work has been done however to express in boolean 806abstraction the inferred semantics of Kconfig to translate Kconfig logic into 807boolean formulas and run a SAT solver on it [5]_. Another known related project 808is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which 809has been introduced first with [9]_. The basic concept of undertaker is to 810extract variability models from Kconfig and put them together with a 811propositional formula extracted from CPP #ifdefs and build-rules into a SAT 812solver in order to find dead code, dead files, and dead symbols. If using a SAT 813solver is desirable on Kconfig one approach would be to evaluate repurposing 814such efforts somehow on Kconfig. There is enough interest from mentors of 815existing projects to not only help advise how to integrate this work upstream 816but also help maintain it long term. Interested developers should visit: 817 818https://kernelnewbies.org/KernelProjects/kconfig-sat 819 820.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 821.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 822.. [6] https://cados.cs.fau.de 823.. [7] https://vamos.cs.fau.de 824.. [8] https://undertaker.cs.fau.de 825.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf 826.. [10] https://paulgazzillo.com/papers/esecfse21.pdf 827.. [11] https://github.com/paulgazz/kmax 828