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diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt index 350f733bf2c7..c52856da0cad 100644 --- a/Documentation/kbuild/kconfig-language.txt +++ b/Documentation/kbuild/kconfig-language.txt @@ -393,3 +393,164 @@ config FOO depends on BAR && m limits FOO to module (=m) or disabled (=n). + +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 three 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's 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 on 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. + +Having well defined semantics can be useful for tools for practical +evaluation of depenencies, for instance one such use known 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). + +Confirming this could prove useful as Kconfig stands as one of the 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] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf +[1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf +[2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf +[3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf + +Full SAT solver for Kconfig +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Although SAT solvers [0] 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 [1]. Another known related project +is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has +been introduced first with [5]. The basic concept of undertaker is to exract +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: + +http://kernelnewbies.org/KernelProjects/kconfig-sat + +[0] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf +[1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf +[2] https://cados.cs.fau.de +[3] https://vamos.cs.fau.de +[4] https://undertaker.cs.fau.de +[5] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf |