64 lines
2.8 KiB
Plaintext
64 lines
2.8 KiB
Plaintext
|
# 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 semantics 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://lore.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
|