Commit 1c199f28 authored by Luis R. Rodriguez's avatar Luis R. Rodriguez Committed by Michal Marek

kbuild: document recursive dependency limitation / resolution

Recursive dependency issues with kconfig are unavoidable due to
some limitations with kconfig, since these issues are recurring
provide a hint to the user how they can resolve these dependency
issues and also document why such limitation exists.

While at it also document a bit of future prospects of ways to
enhance Kconfig, including providing formal semantics and evaluation
of use of a SAT solver. If you're interested in this work or prospects
of it check out the kconfig-sat project wiki [0] and mailing list [1].


Cc: Geert Uytterhoeven <>
Cc: James Bottomley <>
Cc: Josh Triplett <>
Cc: Paul Bolle <>
Cc: Herbert Xu <>
Cc: Takashi Iwai <>
Cc: "Yann E. MORIN" <>
Cc: Jonathan Corbet <>
Cc: Mate Soos <>
Signed-off-by: default avatarLuis R. Rodriguez <>
Signed-off-by: default avatarMichal Marek <>
parent 6ff33f39
# Simple Kconfig recursive issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Test with:
# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
# This Kconfig file has a simple recursive dependency issue. In order to
# understand why this recursive dependency issue occurs lets consider what
# Kconfig needs to address. We iterate over what Kconfig needs to address
# by stepping through the questions it needs to address sequentially.
# * What values are possible for CORE?
# CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values
# that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y',
# CORE must be 'y' too.
# * What influences CORE_BELL_A_ADVANCED ?
# As the name implies CORE_BELL_A_ADVANCED is an advanced feature of
# CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y'
# we know CORE_BELL_A_ADVANCED can be 'y' too.
# * What influences CORE_BELL_A ?
# CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A.
# But that is a problem, because this means that in order to determine
# what values are possible for CORE we ended up needing to address questions
# regarding possible values of CORE itself again. Answering the original
# question of what are the possible values of CORE would make the kconfig
# tools run in a loop. When this happens Kconfig exits and complains about
# the "recursive dependency detected" error.
# Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be
# obvious that an easy to solution to this problem should just be the removal
# of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
# since CORE_BELL_A depends on CORE. Recursive dependency issues are not always
# so trivial to resolve, we provide another example below of practical
# implications of this recursive issue where the solution is perhaps not so
# easy to understand. Note that matching semantics on the dependency on
# CORE also consist of a solution to this recursive problem.
mainmenu "Simple example to demo kconfig recursive dependency issue"
config CORE
config CORE_BELL_A
depends on CORE
depends on CORE_BELL_A
select CORE
# 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/
# 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]
mainmenu "Simple example to demo cumulative kconfig recursive dependency implication"
config CORE
config CORE_BELL_A
depends on CORE
select CORE_BELL_A
config CORE_BELL_B
depends on !CORE_BELL_A
select CORE
# Select broken dependency issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Test with:
# make KBUILD_KCONFIG=Documentation/kbuild/ menuconfig
# kconfig will not complain and enable this layout for configuration. This is
# currently a feature of kconfig, given select was designed to be heavy handed.
# Kconfig currently does not check the list of symbols listed on a symbol's
# "select" list, this is done on purpose to help load a set of known required
# symbols. Because of this use of select should be used with caution. An
# example of this issue is below.
# The option B and C are clearly contradicting with respect to A.
# However, when A is set, C can be set as well because Kconfig does not
# visit the dependencies of the select target (in this case B). And since
# Kconfig does not visit the dependencies, it breaks the dependencies of B
# (!A).
mainmenu "Simple example to demo kconfig select broken dependency issue"
config A
bool "CONFIG A"
config B
bool "CONFIG B"
depends on !A
config C
bool "CONFIG C"
depends on A
select B
......@@ -393,3 +393,164 @@ config FOO
depends on BAR && m
limits FOO to module (=m) or disabled (=n).
Kconfig recursive dependency limitations
If you've hit the Kconfig error: "recursive dependency detected" you've run
into a recursive dependency issue with Kconfig, a recursive dependency can be
summarized as a circular dependency. The kconfig tools need to ensure that
Kconfig files comply with specified configuration requirements. In order to do
that kconfig must determine the values that are possible for all Kconfig
symbols, this is currently not possible if there is a circular relation
between two or more Kconfig symbols. For more details refer to the "Simple
Kconfig recursive issue" subsection below. Kconfig does not do recursive
dependency resolution; this has a few implications for Kconfig file writers.
We'll first explain why this issues exists and then provide an example
technical limitation which this brings upon Kconfig developers. Eager
developers wishing to try to address this limitation should read the next
Simple Kconfig recursive issue
Read: Documentation/kbuild/Kconfig.recursion-issue-01
Test with:
make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
Cumulative Kconfig recursive issue
Read: Documentation/kbuild/Kconfig.recursion-issue-02
Test with:
make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
Practical solutions to kconfig recursive issue
Developers who run into the recursive Kconfig issue have three options
at their disposal. We document them below and also provide a list of
historical issues resolved through these different solutions.
a) Remove any superfluous "select FOO" or "depends on FOO"
b) Match dependency semantics:
b1) Swap all "select FOO" to "depends on FOO" or,
b2) Swap all "depends on FOO" to "select FOO"
The resolution to a) can be tested with the sample Kconfig file
Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
since CORE_BELL_A depends on CORE. At times it may not be possible to remove
some dependency criteria, for such cases you can work with solution b).
The two different resolutions for b) can be tested in the sample Kconfig file
Below is a list of examples of prior fixes for these types of recursive issues;
all errors appear to involve one or more select's and one or more "depends on".
commit fix
====== ===
06b718c01208 select A -> depends on A
c22eacfe82f9 depends on A -> depends on B
6a91e854442c select A -> depends on A
118c565a8f2e select A -> select B
f004e5594705 select A -> depends on A
c7861f37b4c6 depends on A -> (null)
80c69915e5fb select A -> (null) (1)
c2218e26c0d0 select A -> depends on A (1)
d6ae99d04e1c select A -> depends on A
95ca19cf8cbf select A -> depends on A
8f057d7bca54 depends on A -> (null)
8f057d7bca54 depends on A -> select A
a0701f04846e select A -> depends on A
0c8b92f7f259 depends on A -> (null)
e4e9e0540928 select A -> depends on A (2)
7453ea886e87 depends on A > (null) (1)
7b1fff7e4fdf select A -> depends on A
86c747d2a4f0 select A -> depends on A
d9f9ab51e55e select A -> depends on A
0c51a4d8abd6 depends on A -> select A (3)
e98062ed6dc4 select A -> depends on A (3)
91e5d284a7f1 select A -> (null)
(1) Partial (or no) quote of error.
(2) That seems to be the gist of that fix.
(3) Same error.
Future kconfig work
Work on kconfig is welcomed on both areas of clarifying semantics and on
evaluating the use of a full SAT solver for it. A full SAT solver can be
desirable to enable more complex dependency mappings and / or queries,
for instance on possible use case for a SAT solver could be that of handling
the current known recursive dependency issues. It is not known if this would
address such issues but such evaluation is desirable. If support for a full SAT
solver proves too complex or that it cannot address recursive dependency issues
Kconfig should have at least clear and well defined semantics which also
addresses and documents limitations or requirements such as the ones dealing
with recursive dependencies.
Further work on both of these areas is welcomed on Kconfig. We elaborate
on both of these in the next two subsections.
Semantics of Kconfig
The use of Kconfig is broad, Linux is now only one of Kconfig's users:
one study has completed a broad analysis of Kconfig use in 12 projects [0].
Despite its widespread use, and although this document does a reasonable job
in documenting basic Kconfig syntax a more precise definition of Kconfig
semantics is welcomed. One project deduced Kconfig semantics through
the use of the xconfig configurator [1]. Work should be done to confirm if
the deduced semantics matches our intended Kconfig design goals.
Having well defined semantics can be useful for tools for practical
evaluation of depenencies, for instance one such use known case was work to
express in boolean abstraction of the inferred semantics of Kconfig to
translate Kconfig logic into boolean formulas and run a SAT solver on this to
find dead code / features (always inactive), 114 dead features were found in
Linux using this methodology [1] (Section 8: Threats to validity).
Confirming this could prove useful as Kconfig stands as one of the the leading
industrial variability modeling languages [1] [2]. Its study would help
evaluate practical uses of such languages, their use was only theoretical
and real world requirements were not well understood. As it stands though
only reverse engineering techniques have been used to deduce semantics from
variability modeling languages such as Kconfig [3].
Full SAT solver for Kconfig
Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in
the previous subsection, work has been done however to express in boolean
abstraction the inferred semantics of Kconfig to translate Kconfig logic into
boolean formulas and run a SAT solver on it [1]. Another known related project
is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has
been introduced first with [5]. The basic concept of undertaker is to exract
variability models from Kconfig, and put them together with a propositional
formula extracted from CPP #ifdefs and build-rules into a SAT solver in order
to find dead code, dead files, and dead symbols. If using a SAT solver is
desirable on Kconfig one approach would be to evaluate repurposing such efforts
somehow on Kconfig. There is enough interest from mentors of existing projects
to not only help advise how to integrate this work upstream but also help
maintain it long term. Interested developers should visit:
......@@ -1116,6 +1116,8 @@ static void sym_check_print_recursive(struct symbol *last_sym)
if (stack->sym == last_sym)
fprintf(stderr, "%s:%d:error: recursive dependency detected!\n",
prop->file->name, prop->lineno);
fprintf(stderr, "For a resolution refer to Documentation/kbuild/kconfig-language.txt\n");
fprintf(stderr, "subsection \"Kconfig recursive dependency limitations\"\n");
if (stack->expr) {
fprintf(stderr, "%s:%d:\tsymbol %s %s value contains %s\n",
prop->file->name, prop->lineno,
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