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diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst new file mode 100644 index 000000000..858ed5d80 --- /dev/null +++ b/Documentation/kbuild/kconfig-language.rst @@ -0,0 +1,746 @@ +================ +Kconfig Language +================ + +Introduction +------------ + +The configuration database is a collection of configuration options +organized in a tree structure:: + + +- Code maturity level options + | +- Prompt for development and/or incomplete code/drivers + +- General setup + | +- Networking support + | +- System V IPC + | +- BSD Process Accounting + | +- Sysctl support + +- Loadable module support + | +- Enable loadable module support + | +- Set version information on all module symbols + | +- Kernel module loader + +- ... + +Every entry has its own dependencies. These dependencies are used +to determine the visibility of an entry. Any child entry is only +visible if its parent entry is also visible. + +Menu entries +------------ + +Most entries define a config option; all other entries help to organize +them. A single configuration option is defined like this:: + + config MODVERSIONS + bool "Set version information on all module symbols" + depends on MODULES + help + Usually, modules have to be recompiled whenever you switch to a new + kernel. ... + +Every line starts with a key word and can be followed by multiple +arguments. "config" starts a new config entry. The following lines +define attributes for this config option. Attributes can be the type of +the config option, input prompt, dependencies, help text and default +values. A config option can be defined multiple times with the same +name, but every definition can have only a single input prompt and the +type must not conflict. + +Menu attributes +--------------- + +A menu entry can have a number of attributes. Not all of them are +applicable everywhere (see syntax). + +- type definition: "bool"/"tristate"/"string"/"hex"/"int" + + Every config option must have a type. There are only two basic types: + tristate and string; the other types are based on these two. The type + definition optionally accepts an input prompt, so these two examples + are equivalent:: + + bool "Networking support" + + and:: + + bool + prompt "Networking support" + +- input prompt: "prompt" <prompt> ["if" <expr>] + + Every menu entry can have at most one prompt, which is used to display + to the user. Optionally dependencies only for this prompt can be added + with "if". + +- default value: "default" <expr> ["if" <expr>] + + A config option can have any number of default values. If multiple + default values are visible, only the first defined one is active. + Default values are not limited to the menu entry where they are + defined. This means the default can be defined somewhere else or be + overridden by an earlier definition. + The default value is only assigned to the config symbol if no other + value was set by the user (via the input prompt above). If an input + prompt is visible the default value is presented to the user and can + be overridden by him. + Optionally, dependencies only for this default value can be added with + "if". + + The default value deliberately defaults to 'n' in order to avoid bloating the + build. With few exceptions, new config options should not change this. The + intent is for "make oldconfig" to add as little as possible to the config from + release to release. + + Note: + Things that merit "default y/m" include: + + a) A new Kconfig option for something that used to always be built + should be "default y". + + b) A new gatekeeping Kconfig option that hides/shows other Kconfig + options (but does not generate any code of its own), should be + "default y" so people will see those other options. + + c) Sub-driver behavior or similar options for a driver that is + "default n". This allows you to provide sane defaults. + + d) Hardware or infrastructure that everybody expects, such as CONFIG_NET + or CONFIG_BLOCK. These are rare exceptions. + +- type definition + default value:: + + "def_bool"/"def_tristate" <expr> ["if" <expr>] + + This is a shorthand notation for a type definition plus a value. + Optionally dependencies for this default value can be added with "if". + +- dependencies: "depends on" <expr> + + This defines a dependency for this menu entry. If multiple + dependencies are defined, they are connected with '&&'. Dependencies + are applied to all other options within this menu entry (which also + accept an "if" expression), so these two examples are equivalent:: + + bool "foo" if BAR + default y if BAR + + and:: + + depends on BAR + bool "foo" + default y + +- reverse dependencies: "select" <symbol> ["if" <expr>] + + While normal dependencies reduce the upper limit of a symbol (see + below), reverse dependencies can be used to force a lower limit of + another symbol. The value of the current menu symbol is used as the + minimal value <symbol> can be set to. If <symbol> is selected multiple + times, the limit is set to the largest selection. + Reverse dependencies can only be used with boolean or tristate + symbols. + + Note: + select should be used with care. select will force + a symbol to a value without visiting the dependencies. + By abusing select you are able to select a symbol FOO even + if FOO depends on BAR that is not set. + In general use select only for non-visible symbols + (no prompts anywhere) and for symbols with no dependencies. + That will limit the usefulness but on the other hand avoid + the illegal configurations all over. + +- weak reverse dependencies: "imply" <symbol> ["if" <expr>] + + This is similar to "select" as it enforces a lower limit on another + symbol except that the "implied" symbol's value may still be set to n + from a direct dependency or with a visible prompt. + + Given the following example:: + + config FOO + tristate "foo" + imply BAZ + + config BAZ + tristate "baz" + depends on BAR + + The following values are possible: + + === === ============= ============== + FOO BAR BAZ's default choice for BAZ + === === ============= ============== + n y n N/m/y + m y m M/y/n + y y y Y/m/n + n m n N/m + m m m M/n + y m m M/n + y n * N + === === ============= ============== + + This is useful e.g. with multiple drivers that want to indicate their + ability to hook into a secondary subsystem while allowing the user to + configure that subsystem out without also having to unset these drivers. + + Note: If the combination of FOO=y and BAR=m causes a link error, + you can guard the function call with IS_REACHABLE():: + + foo_init() + { + if (IS_REACHABLE(CONFIG_BAZ)) + baz_register(&foo); + ... + } + + Note: If the feature provided by BAZ is highly desirable for FOO, + FOO should imply not only BAZ, but also its dependency BAR:: + + config FOO + tristate "foo" + imply BAR + imply BAZ + +- limiting menu display: "visible if" <expr> + + This attribute is only applicable to menu blocks, if the condition is + false, the menu block is not displayed to the user (the symbols + contained there can still be selected by other symbols, though). It is + similar to a conditional "prompt" attribute for individual menu + entries. Default value of "visible" is true. + +- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] + + This allows to limit the range of possible input values for int + and hex symbols. The user can only input a value which is larger than + or equal to the first symbol and smaller than or equal to the second + symbol. + +- help text: "help" + + This defines a help text. The end of the help text is determined by + the indentation level, this means it ends at the first line which has + a smaller indentation than the first line of the help text. + +- module attribute: "modules" + This declares the symbol to be used as the MODULES symbol, which + enables the third modular state for all config symbols. + At most one symbol may have the "modules" option set. + +Menu dependencies +----------------- + +Dependencies define the visibility of a menu entry and can also reduce +the input range of tristate symbols. The tristate logic used in the +expressions uses one more state than normal boolean logic to express the +module state. Dependency expressions have the following syntax:: + + <expr> ::= <symbol> (1) + <symbol> '=' <symbol> (2) + <symbol> '!=' <symbol> (3) + <symbol1> '<' <symbol2> (4) + <symbol1> '>' <symbol2> (4) + <symbol1> '<=' <symbol2> (4) + <symbol1> '>=' <symbol2> (4) + '(' <expr> ')' (5) + '!' <expr> (6) + <expr> '&&' <expr> (7) + <expr> '||' <expr> (8) + +Expressions are listed in decreasing order of precedence. + +(1) Convert the symbol into an expression. Boolean and tristate symbols + are simply converted into the respective expression values. All + other symbol types result in 'n'. +(2) If the values of both symbols are equal, it returns 'y', + otherwise 'n'. +(3) If the values of both symbols are equal, it returns 'n', + otherwise 'y'. +(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, + or greater-or-equal than value of <symbol2>, it returns 'y', + otherwise 'n'. +(5) Returns the value of the expression. Used to override precedence. +(6) Returns the result of (2-/expr/). +(7) Returns the result of min(/expr/, /expr/). +(8) Returns the result of max(/expr/, /expr/). + +An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 +respectively for calculations). A menu entry becomes visible when its +expression evaluates to 'm' or 'y'. + +There are two types of symbols: constant and non-constant symbols. +Non-constant symbols are the most common ones and are defined with the +'config' statement. Non-constant symbols consist entirely of alphanumeric +characters or underscores. +Constant symbols are only part of expressions. Constant symbols are +always surrounded by single or double quotes. Within the quote, any +other character is allowed and the quotes can be escaped using '\'. + +Menu structure +-------------- + +The position of a menu entry in the tree is determined in two ways. First +it can be specified explicitly:: + + menu "Network device support" + depends on NET + + config NETDEVICES + ... + + endmenu + +All entries within the "menu" ... "endmenu" block become a submenu of +"Network device support". All subentries inherit the dependencies from +the menu entry, e.g. this means the dependency "NET" is added to the +dependency list of the config option NETDEVICES. + +The other way to generate the menu structure is done by analyzing the +dependencies. If a menu entry somehow depends on the previous entry, it +can be made a submenu of it. First, the previous (parent) symbol must +be part of the dependency list and then one of these two conditions +must be true: + +- the child entry must become invisible, if the parent is set to 'n' +- the child entry must only be visible, if the parent is visible:: + + config MODULES + bool "Enable loadable module support" + + config MODVERSIONS + bool "Set version information on all module symbols" + depends on MODULES + + comment "module support disabled" + depends on !MODULES + +MODVERSIONS directly depends on MODULES, this means it's only visible if +MODULES is different from 'n'. The comment on the other hand is only +visible when MODULES is set to 'n'. + + +Kconfig syntax +-------------- + +The configuration file describes a series of menu entries, where every +line starts with a keyword (except help texts). The following keywords +end a menu entry: + +- config +- menuconfig +- choice/endchoice +- comment +- menu/endmenu +- if/endif +- source + +The first five also start the definition of a menu entry. + +config:: + + "config" <symbol> + <config options> + +This defines a config symbol <symbol> and accepts any of above +attributes as options. + +menuconfig:: + + "menuconfig" <symbol> + <config options> + +This is similar to the simple config entry above, but it also gives a +hint to front ends, that all suboptions should be displayed as a +separate list of options. To make sure all the suboptions will really +show up under the menuconfig entry and not outside of it, every item +from the <config options> list must depend on the menuconfig symbol. +In practice, this is achieved by using one of the next two constructs:: + + (1): + menuconfig M + if M + config C1 + config C2 + endif + + (2): + menuconfig M + config C1 + depends on M + config C2 + depends on M + +In the following examples (3) and (4), C1 and C2 still have the M +dependency, but will not appear under menuconfig M anymore, because +of C0, which doesn't depend on M:: + + (3): + menuconfig M + config C0 + if M + config C1 + config C2 + endif + + (4): + menuconfig M + config C0 + config C1 + depends on M + config C2 + depends on M + +choices:: + + "choice" [symbol] + <choice options> + <choice block> + "endchoice" + +This defines a choice group and accepts any of the above attributes as +options. A choice can only be of type bool or tristate. If no type is +specified for a choice, its type will be determined by the type of +the first choice element in the group or remain unknown if none of the +choice elements have a type specified, as well. + +While a boolean choice only allows a single config entry to be +selected, a tristate choice also allows any number of config entries +to be set to 'm'. This can be used if multiple drivers for a single +hardware exists and only a single driver can be compiled/loaded into +the kernel, but all drivers can be compiled as modules. + +A choice accepts another option "optional", which allows to set the +choice to 'n' and no entry needs to be selected. +If no [symbol] is associated with a choice, then you can not have multiple +definitions of that choice. If a [symbol] is associated to the choice, +then you may define the same choice (i.e. with the same entries) in another +place. + +comment:: + + "comment" <prompt> + <comment options> + +This defines a comment which is displayed to the user during the +configuration process and is also echoed to the output files. The only +possible options are dependencies. + +menu:: + + "menu" <prompt> + <menu options> + <menu block> + "endmenu" + +This defines a menu block, see "Menu structure" above for more +information. The only possible options are dependencies and "visible" +attributes. + +if:: + + "if" <expr> + <if block> + "endif" + +This defines an if block. The dependency expression <expr> is appended +to all enclosed menu entries. + +source:: + + "source" <prompt> + +This reads the specified configuration file. This file is always parsed. + +mainmenu:: + + "mainmenu" <prompt> + +This sets the config program's title bar if the config program chooses +to use it. It should be placed at the top of the configuration, before any +other statement. + +'#' Kconfig source file comment: + +An unquoted '#' character anywhere in a source file line indicates +the beginning of a source file comment. The remainder of that line +is a comment. + + +Kconfig hints +------------- +This is a collection of Kconfig tips, most of which aren't obvious at +first glance and most of which have become idioms in several Kconfig +files. + +Adding common features and make the usage configurable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It is a common idiom to implement a feature/functionality that are +relevant for some architectures but not all. +The recommended way to do so is to use a config variable named HAVE_* +that is defined in a common Kconfig file and selected by the relevant +architectures. +An example is the generic IOMAP functionality. + +We would in lib/Kconfig see:: + + # Generic IOMAP is used to ... + config HAVE_GENERIC_IOMAP + + config GENERIC_IOMAP + depends on HAVE_GENERIC_IOMAP && FOO + +And in lib/Makefile we would see:: + + obj-$(CONFIG_GENERIC_IOMAP) += iomap.o + +For each architecture using the generic IOMAP functionality we would see:: + + config X86 + select ... + select HAVE_GENERIC_IOMAP + select ... + +Note: we use the existing config option and avoid creating a new +config variable to select HAVE_GENERIC_IOMAP. + +Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is +introduced to overcome the limitation of select which will force a +config option to 'y' no matter the dependencies. +The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the +situation where select forces a symbol equals to 'y'. + +Adding features that need compiler support +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +There are several features that need compiler support. The recommended way +to describe the dependency on the compiler feature is to use "depends on" +followed by a test macro:: + + config STACKPROTECTOR + bool "Stack Protector buffer overflow detection" + depends on $(cc-option,-fstack-protector) + ... + +If you need to expose a compiler capability to makefiles and/or C source files, +`CC_HAS_` is the recommended prefix for the config option:: + + config CC_HAS_FOO + def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC)) + +Build as module only +~~~~~~~~~~~~~~~~~~~~ +To restrict a component build to module-only, qualify its config symbol +with "depends on m". E.g.:: + + config FOO + depends on BAR && m + +limits FOO to module (=m) or disabled (=n). + +Compile-testing +~~~~~~~~~~~~~~~ +If a config symbol has a dependency, but the code controlled by the config +symbol can still be compiled if the dependency is not met, it is encouraged to +increase build coverage by adding an "|| COMPILE_TEST" clause to the +dependency. This is especially useful for drivers for more exotic hardware, as +it allows continuous-integration systems to compile-test the code on a more +common system, and detect bugs that way. +Note that compile-tested code should avoid crashing when run on a system where +the dependency is not met. + +Architecture and platform dependencies +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Due to the presence of stubs, most drivers can now be compiled on most +architectures. However, this does not mean it makes sense to have all drivers +available everywhere, as the actual hardware may only exist on specific +architectures and platforms. This is especially true for on-SoC IP cores, +which may be limited to a specific vendor or SoC family. + +To prevent asking the user about drivers that cannot be used on the system(s) +the user is compiling a kernel for, and if it makes sense, config symbols +controlling the compilation of a driver should contain proper dependencies, +limiting the visibility of the symbol to (a superset of) the platform(s) the +driver can be used on. The dependency can be an architecture (e.g. ARM) or +platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for +distro config owners, but also for every single developer or user who +configures a kernel. + +Such a dependency can be relaxed by combining it with the compile-testing rule +above, leading to: + + config FOO + bool "Support for foo hardware" + depends on ARCH_FOO_VENDOR || COMPILE_TEST + +Kconfig recursive dependency limitations +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If you've hit the Kconfig error: "recursive dependency detected" you've run +into a recursive dependency issue with Kconfig, a recursive dependency can be +summarized as a circular dependency. The kconfig tools need to ensure that +Kconfig files comply with specified configuration requirements. In order to do +that kconfig must determine the values that are possible for all Kconfig +symbols, this is currently not possible if there is a circular relation +between two or more Kconfig symbols. For more details refer to the "Simple +Kconfig recursive issue" subsection below. Kconfig does not do recursive +dependency resolution; this has a few implications for Kconfig file writers. +We'll first explain why this issues exists and then provide an example +technical limitation which this brings upon Kconfig developers. Eager +developers wishing to try to address this limitation should read the next +subsections. + +Simple Kconfig recursive issue +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Read: Documentation/kbuild/Kconfig.recursion-issue-01 + +Test with:: + + make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig + +Cumulative Kconfig recursive issue +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Read: Documentation/kbuild/Kconfig.recursion-issue-02 + +Test with:: + + make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig + +Practical solutions to kconfig recursive issue +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Developers who run into the recursive Kconfig issue have two options +at their disposal. We document them below and also provide a list of +historical issues resolved through these different solutions. + + a) Remove any superfluous "select FOO" or "depends on FOO" + b) Match dependency semantics: + + b1) Swap all "select FOO" to "depends on FOO" or, + + b2) Swap all "depends on FOO" to "select FOO" + +The resolution to a) can be tested with the sample Kconfig file +Documentation/kbuild/Kconfig.recursion-issue-01 through the removal +of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already +since CORE_BELL_A depends on CORE. At times it may not be possible to remove +some dependency criteria, for such cases you can work with solution b). + +The two different resolutions for b) can be tested in the sample Kconfig file +Documentation/kbuild/Kconfig.recursion-issue-02. + +Below is a list of examples of prior fixes for these types of recursive issues; +all errors appear to involve one or more "select" statements and one or more +"depends on". + +============ =================================== +commit fix +============ =================================== +06b718c01208 select A -> depends on A +c22eacfe82f9 depends on A -> depends on B +6a91e854442c select A -> depends on A +118c565a8f2e select A -> select B +f004e5594705 select A -> depends on A +c7861f37b4c6 depends on A -> (null) +80c69915e5fb select A -> (null) (1) +c2218e26c0d0 select A -> depends on A (1) +d6ae99d04e1c select A -> depends on A +95ca19cf8cbf select A -> depends on A +8f057d7bca54 depends on A -> (null) +8f057d7bca54 depends on A -> select A +a0701f04846e select A -> depends on A +0c8b92f7f259 depends on A -> (null) +e4e9e0540928 select A -> depends on A (2) +7453ea886e87 depends on A > (null) (1) +7b1fff7e4fdf select A -> depends on A +86c747d2a4f0 select A -> depends on A +d9f9ab51e55e select A -> depends on A +0c51a4d8abd6 depends on A -> select A (3) +e98062ed6dc4 select A -> depends on A (3) +91e5d284a7f1 select A -> (null) +============ =================================== + +(1) Partial (or no) quote of error. +(2) That seems to be the gist of that fix. +(3) Same error. + +Future kconfig work +~~~~~~~~~~~~~~~~~~~ + +Work on kconfig is welcomed on both areas of clarifying semantics and on +evaluating the use of a full SAT solver for it. A full SAT solver can be +desirable to enable more complex dependency mappings and / or queries, +for instance one possible use case for a SAT solver could be that of handling +the current known recursive dependency issues. It is not known if this would +address such issues but such evaluation is desirable. If support for a full SAT +solver proves too complex or that it cannot address recursive dependency issues +Kconfig should have at least clear and well defined semantics which also +addresses and documents limitations or requirements such as the ones dealing +with recursive dependencies. + +Further work on both of these areas is welcomed on Kconfig. We elaborate +on both of these in the next two subsections. + +Semantics of Kconfig +~~~~~~~~~~~~~~~~~~~~ + +The use of Kconfig is broad, Linux is now only one of Kconfig's users: +one study has completed a broad analysis of Kconfig use in 12 projects [0]_. +Despite its widespread use, and although this document does a reasonable job +in documenting basic Kconfig syntax a more precise definition of Kconfig +semantics is welcomed. One project deduced Kconfig semantics through +the use of the xconfig configurator [1]_. Work should be done to confirm if +the deduced semantics matches our intended Kconfig design goals. +Another project formalized a denotational semantics of a core subset of +the Kconfig language [10]_. + +Having well defined semantics can be useful for tools for practical +evaluation of dependencies, for instance one such case was work to +express in boolean abstraction of the inferred semantics of Kconfig to +translate Kconfig logic into boolean formulas and run a SAT solver on this to +find dead code / features (always inactive), 114 dead features were found in +Linux using this methodology [1]_ (Section 8: Threats to validity). +The kismet tool, based on the semantics in [10]_, finds abuses of reverse +dependencies and has led to dozens of committed fixes to Linux Kconfig files [11]_. + +Confirming this could prove useful as Kconfig stands as one of the leading +industrial variability modeling languages [1]_ [2]_. Its study would help +evaluate practical uses of such languages, their use was only theoretical +and real world requirements were not well understood. As it stands though +only reverse engineering techniques have been used to deduce semantics from +variability modeling languages such as Kconfig [3]_. + +.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf +.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf +.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf +.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf + +Full SAT solver for Kconfig +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted +in the previous subsection, work has been done however to express in boolean +abstraction the inferred semantics of Kconfig to translate Kconfig logic into +boolean formulas and run a SAT solver on it [5]_. Another known related project +is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which +has been introduced first with [9]_. The basic concept of undertaker is to +extract variability models from Kconfig and put them together with a +propositional formula extracted from CPP #ifdefs and build-rules into a SAT +solver in order to find dead code, dead files, and dead symbols. If using a SAT +solver is desirable on Kconfig one approach would be to evaluate repurposing +such efforts somehow on Kconfig. There is enough interest from mentors of +existing projects to not only help advise how to integrate this work upstream +but also help maintain it long term. Interested developers should visit: + +https://kernelnewbies.org/KernelProjects/kconfig-sat + +.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf +.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf +.. [6] https://cados.cs.fau.de +.. [7] https://vamos.cs.fau.de +.. [8] https://undertaker.cs.fau.de +.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf +.. [10] https://paulgazzillo.com/papers/esecfse21.pdf +.. [11] https://github.com/paulgazz/kmax |