1*4882a593Smuzhiyun# Cumulative Kconfig recursive issue 2*4882a593Smuzhiyun# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 3*4882a593Smuzhiyun# 4*4882a593Smuzhiyun# Test with: 5*4882a593Smuzhiyun# 6*4882a593Smuzhiyun# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 7*4882a593Smuzhiyun# 8*4882a593Smuzhiyun# The recursive limitations with Kconfig has some non intuitive implications on 9*4882a593Smuzhiyun# kconfig sematics which are documented here. One known practical implication 10*4882a593Smuzhiyun# of the recursive limitation is that drivers cannot negate features from other 11*4882a593Smuzhiyun# drivers if they share a common core requirement and use disjoint semantics to 12*4882a593Smuzhiyun# annotate those requirements, ie, some drivers use "depends on" while others 13*4882a593Smuzhiyun# use "select". For instance it means if a driver A and driver B share the same 14*4882a593Smuzhiyun# core requirement, and one uses "select" while the other uses "depends on" to 15*4882a593Smuzhiyun# annotate this, all features that driver A selects cannot now be negated by 16*4882a593Smuzhiyun# driver B. 17*4882a593Smuzhiyun# 18*4882a593Smuzhiyun# A perhaps not so obvious implication of this is that, if semantics on these 19*4882a593Smuzhiyun# core requirements are not carefully synced, as drivers evolve features 20*4882a593Smuzhiyun# they select or depend on end up becoming shared requirements which cannot be 21*4882a593Smuzhiyun# negated by other drivers. 22*4882a593Smuzhiyun# 23*4882a593Smuzhiyun# The example provided in Documentation/kbuild/Kconfig.recursion-issue-02 24*4882a593Smuzhiyun# describes a simple driver core layout of example features a kernel might 25*4882a593Smuzhiyun# have. Let's assume we have some CORE functionality, then the kernel has a 26*4882a593Smuzhiyun# series of bells and whistles it desires to implement, its not so advanced so 27*4882a593Smuzhiyun# it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If 28*4882a593Smuzhiyun# CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects 29*4882a593Smuzhiyun# CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which 30*4882a593Smuzhiyun# other bells in the system cannot negate. The reason for this issue is 31*4882a593Smuzhiyun# due to the disjoint use of semantics on expressing each bell's relationship 32*4882a593Smuzhiyun# with CORE, one uses "depends on" while the other uses "select". Another 33*4882a593Smuzhiyun# more important reason is that kconfig does not check for dependencies listed 34*4882a593Smuzhiyun# under 'select' for a symbol, when such symbols are selected kconfig them 35*4882a593Smuzhiyun# as mandatory required symbols. For more details on the heavy handed nature 36*4882a593Smuzhiyun# of select refer to Documentation/kbuild/Kconfig.select-break 37*4882a593Smuzhiyun# 38*4882a593Smuzhiyun# To fix this the "depends on CORE" must be changed to "select CORE", or the 39*4882a593Smuzhiyun# "select CORE" must be changed to "depends on CORE". 40*4882a593Smuzhiyun# 41*4882a593Smuzhiyun# For an example real world scenario issue refer to the attempt to remove 42*4882a593Smuzhiyun# "select FW_LOADER" [0], in the end the simple alternative solution to this 43*4882a593Smuzhiyun# problem consisted on matching semantics with newly introduced features. 44*4882a593Smuzhiyun# 45*4882a593Smuzhiyun# [0] https://lkml.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com 46*4882a593Smuzhiyun 47*4882a593Smuzhiyunmainmenu "Simple example to demo cumulative kconfig recursive dependency implication" 48*4882a593Smuzhiyun 49*4882a593Smuzhiyunconfig CORE 50*4882a593Smuzhiyun tristate 51*4882a593Smuzhiyun 52*4882a593Smuzhiyunconfig CORE_BELL_A 53*4882a593Smuzhiyun tristate 54*4882a593Smuzhiyun depends on CORE 55*4882a593Smuzhiyun 56*4882a593Smuzhiyunconfig CORE_BELL_A_ADVANCED 57*4882a593Smuzhiyun tristate 58*4882a593Smuzhiyun select CORE_BELL_A 59*4882a593Smuzhiyun 60*4882a593Smuzhiyunconfig CORE_BELL_B 61*4882a593Smuzhiyun tristate 62*4882a593Smuzhiyun depends on !CORE_BELL_A 63*4882a593Smuzhiyun select CORE 64