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 sematics 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] http://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