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Luis R. Rodriguez1c199f22015-10-07 16:16:33 -07001# 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