name : Kconfig.recursion-issue-02 # Cumulative Kconfig recursive issue # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ # # Test with: # # make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig # # The recursive limitations with Kconfig has some non intuitive implications on # kconfig semantics which are documented here. One known practical implication # of the recursive limitation is that drivers cannot negate features from other # drivers if they share a common core requirement and use disjoint semantics to # annotate those requirements, ie, some drivers use "depends on" while others # use "select". For instance it means if a driver A and driver B share the same # core requirement, and one uses "select" while the other uses "depends on" to # annotate this, all features that driver A selects cannot now be negated by # driver B. # # A perhaps not so obvious implication of this is that, if semantics on these # core requirements are not carefully synced, as drivers evolve features # they select or depend on end up becoming shared requirements which cannot be # negated by other drivers. # # The example provided in Documentation/kbuild/Kconfig.recursion-issue-02 # describes a simple driver core layout of example features a kernel might # have. Let's assume we have some CORE functionality, then the kernel has a # series of bells and whistles it desires to implement, its not so advanced so # it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If # CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects # CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which # other bells in the system cannot negate. The reason for this issue is # due to the disjoint use of semantics on expressing each bell's relationship # with CORE, one uses "depends on" while the other uses "select". Another # more important reason is that kconfig does not check for dependencies listed # under 'select' for a symbol, when such symbols are selected kconfig them # as mandatory required symbols. For more details on the heavy handed nature # of select refer to Documentation/kbuild/Kconfig.select-break # # To fix this the "depends on CORE" must be changed to "select CORE", or the # "select CORE" must be changed to "depends on CORE". # # For an example real world scenario issue refer to the attempt to remove # "select FW_LOADER" [0], in the end the simple alternative solution to this # problem consisted on matching semantics with newly introduced features. # # [0] https://lore.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com mainmenu "Simple example to demo cumulative kconfig recursive dependency implication" config CORE tristate config CORE_BELL_A tristate depends on CORE config CORE_BELL_A_ADVANCED tristate select CORE_BELL_A config CORE_BELL_B tristate depends on !CORE_BELL_A select CORE