1# Simple Kconfig recursive issue 2# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 3# 4# Test with: 5# 6# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 7# 8# This Kconfig file has a simple recursive dependency issue. In order to 9# understand why this recursive dependency issue occurs lets consider what 10# Kconfig needs to address. We iterate over what Kconfig needs to address 11# by stepping through the questions it needs to address sequentially. 12# 13# * What values are possible for CORE? 14# 15# CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values 16# that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y', 17# CORE must be 'y' too. 18# 19# * What influences CORE_BELL_A_ADVANCED ? 20# 21# As the name implies CORE_BELL_A_ADVANCED is an advanced feature of 22# CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y' 23# we know CORE_BELL_A_ADVANCED can be 'y' too. 24# 25# * What influences CORE_BELL_A ? 26# 27# CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A. 28# 29# But that is a problem, because this means that in order to determine 30# what values are possible for CORE we ended up needing to address questions 31# regarding possible values of CORE itself again. Answering the original 32# question of what are the possible values of CORE would make the kconfig 33# tools run in a loop. When this happens Kconfig exits and complains about 34# the "recursive dependency detected" error. 35# 36# Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be 37# obvious that an easy to solution to this problem should just be the removal 38# of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 39# since CORE_BELL_A depends on CORE. Recursive dependency issues are not always 40# so trivial to resolve, we provide another example below of practical 41# implications of this recursive issue where the solution is perhaps not so 42# easy to understand. Note that matching semantics on the dependency on 43# CORE also consist of a solution to this recursive problem. 44 45mainmenu "Simple example to demo kconfig recursive dependency issue" 46 47config CORE 48 tristate 49 50config CORE_BELL_A 51 tristate 52 depends on CORE 53 54config CORE_BELL_A_ADVANCED 55 tristate 56 depends on CORE_BELL_A 57 select CORE 58