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
+ imply BAZ
+
+ config BAZ
+ tristate
+ 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/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.
+
- 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
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 always
-visible when MODULES is visible (the (empty) dependency of MODULES is
-also part of the comment dependencies).
+MODULES is different from 'n'. The comment on the other hand is only
+visible when MODULES is set to 'n'.
Kconfig syntax
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.
+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:
"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, 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.
+options. A choice can only be of type bool or tristate. If no type is
+specified for a choice, it's 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
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"
+ c) Consider the use of "imply" instead of "select"
+
+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