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