Documentation: kbuild: explain handling optional dependencies

This problem frequently comes up in randconfig testing, with
drivers failing to link because of a dependency on an optional
feature.

The Kconfig language for this is very confusing, so try to
document it in "Kconfig hints" section.
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/kconfig-language.rst

@@ -573,6 +573,32 @@ above, leading to:
 	bool "Support for foo hardware"
 	depends on ARCH_FOO_VENDOR || COMPILE_TEST
 
+Optional dependencies
+~~~~~~~~~~~~~~~~~~~~~
+
+Some drivers are able to optionally use a feature from another module
+or build cleanly with that module disabled, but cause a link failure
+when trying to use that loadable module from a built-in driver.
+
+The most common way to express this optional dependency in Kconfig logic
+uses the slightly counterintuitive::
+
+  config FOO
+	tristate "Support for foo hardware"
+	depends on BAR || !BAR
+
+This means that there is either a dependency on BAR that disallows
+the combination of FOO=y with BAR=m, or BAR is completely disabled.
+For a more formalized approach if there are multiple drivers that have
+the same dependency, a helper symbol can be used, like::
+
+  config FOO
+	tristate "Support for foo hardware"
+	depends on BAR_OPTIONAL
+
+  config BAR_OPTIONAL
+	def_tristate BAR || !BAR
+
 Kconfig recursive dependency limitations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~