# 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 CONFIG_A and driver CONFIG_B share the same
# core requirement, and one uses "select" while the other uses "depends on" to
# annotate this, all features that driver CONFIG_A selects cannot now be negated by
# driver CONFIG_B.
#
# CONFIG_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 CONFIG_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: CONFIG_CORE_BELL_A and CONFIG_CORE_BELL_B. If
# CONFIG_CORE_BELL_A has some advanced feature CONFIG_CORE_BELL_A_ADVANCED which selects
# CONFIG_CORE_BELL_A then CONFIG_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 CONFIG_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 CONFIG_CORE
	tristate

config CONFIG_CORE_BELL_A
	tristate
	depends on CONFIG_CORE

config CONFIG_CORE_BELL_A_ADVANCED
	tristate
	select CONFIG_CORE_BELL_A

config CONFIG_CORE_BELL_B
	tristate
	depends on !CONFIG_CORE_BELL_A
	select CONFIG_CORE