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.
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: James Bottomley <jbottomley@odin.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Michal Marek <mmarek@suse.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-kbuild@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt
index 350f733..7e0510d 100644
--- a/Documentation/kbuild/kconfig-language.txt
+++ b/Documentation/kbuild/kconfig-language.txt
@@ -393,3 +393,25 @@
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. Kconfig does not do recursive
+dependency resolution, this has a few implications for Kconfig file writers. In
+practice it means that for instance if a driver A selects a few kconfig symbols
+another driver B which selects any of these symbols cannot negate any of the
+symbols the driver A selected. Because of this current limitation developers
+who run into this type of recursive dependency issue have two diverging
+options:
+
+ a) Either swap all "select FOO" to "depends on FOO" or,
+ b) Change the offending "depends on FOO" to "select FOO"
+
+Kconfig's limitations can be addressed by implementing a SAT solver for it,
+but until then, Kconfig is limitted to require developers to use one of
+the above two mechanisms to address recursive dependency issues. For more
+details you can refer to this thread and discussion:
+
+http://lkml.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com
diff --git a/scripts/kconfig/symbol.c b/scripts/kconfig/symbol.c
index 70c5ee1..4d61b74 100644
--- a/scripts/kconfig/symbol.c
+++ b/scripts/kconfig/symbol.c
@@ -1117,6 +1117,8 @@
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, "section \"Kconfig recursive dependency limitations\"\n");
if (stack->expr) {
fprintf(stderr, "%s:%d:\tsymbol %s %s value contains %s\n",
prop->file->name, prop->lineno,
