[mirror_ubuntu-artful-kernel.git] / Documentation / kbuild / 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 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
Commit	Line	Data
1c199f28 LR	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
	45	mainmenu "Simple example to demo kconfig recursive dependency issue"
	46
	47	config CORE
	48	tristate
	49
	50	config CORE_BELL_A
	51	tristate
	52	depends on CORE
	53
	54	config CORE_BELL_A_ADVANCED
	55	tristate
	56	depends on CORE_BELL_A
	57	select CORE