1# Cumulative Kconfig recursive issue
2# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3#
4# Test with:
5#
6# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
7#
8# The recursive limitations with Kconfig has some non intuitive implications on
9# kconfig semantics which are documented here. One known practical implication
10# of the recursive limitation is that drivers cannot negate features from other
11# drivers if they share a common core requirement and use disjoint semantics to
12# annotate those requirements, ie, some drivers use "depends on" while others
13# use "select". For instance it means if a driver A and driver B share the same
14# core requirement, and one uses "select" while the other uses "depends on" to
15# annotate this, all features that driver A selects cannot now be negated by
16# driver B.
17#
18# A perhaps not so obvious implication of this is that, if semantics on these
19# core requirements are not carefully synced, as drivers evolve features
20# they select or depend on end up becoming shared requirements which cannot be
21# negated by other drivers.
22#
23# The example provided in Documentation/kbuild/Kconfig.recursion-issue-02
24# describes a simple driver core layout of example features a kernel might
25# have. Let's assume we have some CORE functionality, then the kernel has a
26# series of bells and whistles it desires to implement, its not so advanced so
27# it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If
28# CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects
29# CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which
30# other bells in the system cannot negate. The reason for this issue is
31# due to the disjoint use of semantics on expressing each bell's relationship
32# with CORE, one uses "depends on" while the other uses "select". Another
33# more important reason is that kconfig does not check for dependencies listed
34# under 'select' for a symbol, when such symbols are selected kconfig them
35# as mandatory required symbols. For more details on the heavy handed nature
36# of select refer to Documentation/kbuild/Kconfig.select-break
37#
38# To fix this the "depends on CORE" must be changed to "select CORE", or the
39# "select CORE" must be changed to "depends on CORE".
40#
41# For an example real world scenario issue refer to the attempt to remove
42# "select FW_LOADER" [0], in the end the simple alternative solution to this
43# problem consisted on matching semantics with newly introduced features.
44#
45# [0] https://lkml.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com
46
47mainmenu "Simple example to demo cumulative kconfig recursive dependency implication"
48
49config CORE
50	tristate
51
52config CORE_BELL_A
53	tristate
54	depends on CORE
55
56config CORE_BELL_A_ADVANCED
57	tristate
58	select CORE_BELL_A
59
60config CORE_BELL_B
61	tristate
62	depends on !CORE_BELL_A
63	select CORE
64