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58 lines
2.2 KiB
Text
58 lines
2.2 KiB
Text
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# Simple 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-01 allnoconfig
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#
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# This Kconfig file has a simple recursive dependency issue. In order to
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# understand why this recursive dependency issue occurs lets consider what
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# Kconfig needs to address. We iterate over what Kconfig needs to address
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# by stepping through the questions it needs to address sequentially.
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#
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# * What values are possible for CORE?
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#
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# CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values
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# that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y',
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# CORE must be 'y' too.
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#
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# * What influences CORE_BELL_A_ADVANCED ?
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#
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# As the name implies CORE_BELL_A_ADVANCED is an advanced feature of
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# CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y'
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# we know CORE_BELL_A_ADVANCED can be 'y' too.
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#
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# * What influences CORE_BELL_A ?
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#
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# CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A.
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#
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# But that is a problem, because this means that in order to determine
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# what values are possible for CORE we ended up needing to address questions
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# regarding possible values of CORE itself again. Answering the original
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# question of what are the possible values of CORE would make the kconfig
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# tools run in a loop. When this happens Kconfig exits and complains about
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# the "recursive dependency detected" error.
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#
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# Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be
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# obvious that an easy to solution to this problem should just be the removal
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# of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
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# since CORE_BELL_A depends on CORE. Recursive dependency issues are not always
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# so trivial to resolve, we provide another example below of practical
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# implications of this recursive issue where the solution is perhaps not so
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# easy to understand. Note that matching semantics on the dependency on
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# CORE also consist of a solution to this recursive problem.
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mainmenu "Simple example to demo kconfig recursive dependency issue"
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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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depends on CORE_BELL_A
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select CORE
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