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Kconfig.select-break 0000644 00000002057 14722072306 0010416 0 ustar 00 # Select broken dependency issue # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ # # Test with: # # make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.select-break menuconfig # # kconfig will not complain and enable this layout for configuration. This is # currently a feature of kconfig, given select was designed to be heavy handed. # Kconfig currently does not check the list of symbols listed on a symbol's # "select" list, this is done on purpose to help load a set of known required # symbols. Because of this use of select should be used with caution. An # example of this issue is below. # # The option B and C are clearly contradicting with respect to A. # However, when A is set, C can be set as well because Kconfig does not # visit the dependencies of the select target (in this case B). And since # Kconfig does not visit the dependencies, it breaks the dependencies of B # (!A). mainmenu "Simple example to demo kconfig select broken dependency issue" config A bool "CONFIG A" config B bool "CONFIG B" depends on !A config C bool "CONFIG C" depends on A select B Kconfig.recursion-issue-01 0000644 00000004320 14722072306 0011425 0 ustar 00 # Simple Kconfig recursive issue # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ # # Test with: # # make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig # # This Kconfig file has a simple recursive dependency issue. In order to # understand why this recursive dependency issue occurs lets consider what # Kconfig needs to address. We iterate over what Kconfig needs to address # by stepping through the questions it needs to address sequentially. # # * What values are possible for CORE? # # CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values # that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y', # CORE must be 'y' too. # # * What influences CORE_BELL_A_ADVANCED ? # # As the name implies CORE_BELL_A_ADVANCED is an advanced feature of # CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y' # we know CORE_BELL_A_ADVANCED can be 'y' too. # # * What influences CORE_BELL_A ? # # CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A. # # But that is a problem, because this means that in order to determine # what values are possible for CORE we ended up needing to address questions # regarding possible values of CORE itself again. Answering the original # question of what are the possible values of CORE would make the kconfig # tools run in a loop. When this happens Kconfig exits and complains about # the "recursive dependency detected" error. # # Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be # obvious that an easy to solution to this problem should just be the removal # of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already # since CORE_BELL_A depends on CORE. Recursive dependency issues are not always # so trivial to resolve, we provide another example below of practical # implications of this recursive issue where the solution is perhaps not so # easy to understand. Note that matching semantics on the dependency on # CORE also consist of a solution to this recursive problem. mainmenu "Simple example to demo kconfig recursive dependency issue" config CORE tristate config CORE_BELL_A tristate depends on CORE config CORE_BELL_A_ADVANCED tristate depends on CORE_BELL_A select CORE Kconfig.recursion-issue-02 0000644 00000005453 14722072306 0011436 0 ustar 00 # 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 sematics 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] http://lkml.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
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