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Linux/Documentation/kbuild/Kconfig.recursion-issue-02

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  1 # Cumulative Kconfig recursive issue
  2 # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  3 #
  4 # Test with:
  5 #
  6 # make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
  7 #
  8 # The recursive limitations with Kconfig has some non intuitive implications on
  9 # kconfig semantics which are documented here. One known practical implication
 10 # of the recursive limitation is that drivers cannot negate features from other
 11 # drivers if they share a common core requirement and use disjoint semantics to
 12 # annotate those requirements, ie, some drivers use "depends on" while others
 13 # use "select". For instance it means if a driver A and driver B share the same
 14 # core requirement, and one uses "select" while the other uses "depends on" to
 15 # annotate this, all features that driver A selects cannot now be negated by
 16 # driver B.
 17 #
 18 # A perhaps not so obvious implication of this is that, if semantics on these
 19 # core requirements are not carefully synced, as drivers evolve features
 20 # they select or depend on end up becoming shared requirements which cannot be
 21 # negated by other drivers.
 22 #
 23 # The example provided in Documentation/kbuild/Kconfig.recursion-issue-02
 24 # describes a simple driver core layout of example features a kernel might
 25 # have. Let's assume we have some CORE functionality, then the kernel has a
 26 # series of bells and whistles it desires to implement, its not so advanced so
 27 # it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If
 28 # CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects
 29 # CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which
 30 # other bells in the system cannot negate. The reason for this issue is
 31 # due to the disjoint use of semantics on expressing each bell's relationship
 32 # with CORE, one uses "depends on" while the other uses "select". Another
 33 # more important reason is that kconfig does not check for dependencies listed
 34 # under 'select' for a symbol, when such symbols are selected kconfig them
 35 # as mandatory required symbols. For more details on the heavy handed nature
 36 # of select refer to Documentation/kbuild/Kconfig.select-break
 37 #
 38 # To fix this the "depends on CORE" must be changed to "select CORE", or the
 39 # "select CORE" must be changed to "depends on CORE".
 40 #
 41 # For an example real world scenario issue refer to the attempt to remove
 42 # "select FW_LOADER" [0], in the end the simple alternative solution to this
 43 # problem consisted on matching semantics with newly introduced features.
 44 #
 45 # [0] https://lore.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com
 46 
 47 mainmenu "Simple example to demo cumulative kconfig recursive dependency implication"
 48 
 49 config CORE
 50         tristate
 51 
 52 config CORE_BELL_A
 53         tristate
 54         depends on CORE
 55 
 56 config CORE_BELL_A_ADVANCED
 57         tristate
 58         select CORE_BELL_A
 59 
 60 config CORE_BELL_B
 61         tristate
 62         depends on !CORE_BELL_A
 63         select CORE

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