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| -rw-r--r-- | Documentation/kbuild/Kconfig.recursion-issue-02 | 63 |
1 files changed, 63 insertions, 0 deletions
diff --git a/Documentation/kbuild/Kconfig.recursion-issue-02 b/Documentation/kbuild/Kconfig.recursion-issue-02 new file mode 100644 index 000000000..df245fd76 --- /dev/null +++ b/Documentation/kbuild/Kconfig.recursion-issue-02 @@ -0,0 +1,63 @@ +# Cumulative Kconfig recursive issue +# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +# +# Test with: +# +# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig +# +# The recursive limitations with Kconfig has some non intuitive implications on +# kconfig sematics which are documented here. One known practical implication +# of the recursive limitation is that drivers cannot negate features from other +# drivers if they share a common core requirement and use disjoint semantics to +# annotate those requirements, ie, some drivers use "depends on" while others +# use "select". For instance it means if a driver A and driver B share the same +# core requirement, and one uses "select" while the other uses "depends on" to +# annotate this, all features that driver A selects cannot now be negated by +# driver B. +# +# A perhaps not so obvious implication of this is that, if semantics on these +# core requirements are not carefully synced, as drivers evolve features +# they select or depend on end up becoming shared requirements which cannot be +# negated by other drivers. +# +# The example provided in Documentation/kbuild/Kconfig.recursion-issue-02 +# describes a simple driver core layout of example features a kernel might +# have. Let's assume we have some CORE functionality, then the kernel has a +# series of bells and whistles it desires to implement, its not so advanced so +# it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If +# CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects +# CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which +# other bells in the system cannot negate. The reason for this issue is +# due to the disjoint use of semantics on expressing each bell's relationship +# with CORE, one uses "depends on" while the other uses "select". Another +# more important reason is that kconfig does not check for dependencies listed +# under 'select' for a symbol, when such symbols are selected kconfig them +# as mandatory required symbols. For more details on the heavy handed nature +# of select refer to Documentation/kbuild/Kconfig.select-break +# +# To fix this the "depends on CORE" must be changed to "select CORE", or the +# "select CORE" must be changed to "depends on CORE". +# +# For an example real world scenario issue refer to the attempt to remove +# "select FW_LOADER" [0], in the end the simple alternative solution to this +# problem consisted on matching semantics with newly introduced features. +# +# [0] https://lkml.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com + +mainmenu "Simple example to demo cumulative kconfig recursive dependency implication" + +config CORE + tristate + +config CORE_BELL_A + tristate + depends on CORE + +config CORE_BELL_A_ADVANCED + tristate + select CORE_BELL_A + +config CORE_BELL_B + tristate + depends on !CORE_BELL_A + select CORE |