name : Kconfig.recursion-issue-01 # 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 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