+
+mainmenu:
+
+ "mainmenu" <prompt>
+
+This sets the config program's title bar if the config program chooses
+to use it.
+
+
+Kconfig hints
+-------------
+This is a collection of Kconfig tips, most of which aren't obvious at
+first glance and most of which have become idioms in several Kconfig
+files.
+
+Adding common features and make the usage configurable
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It is a common idiom to implement a feature/functionality that are
+relevant for some architectures but not all.
+The recommended way to do so is to use a config variable named HAVE_*
+that is defined in a common Kconfig file and selected by the relevant
+architectures.
+An example is the generic IOMAP functionality.
+
+We would in lib/Kconfig see:
+
+# Generic IOMAP is used to ...
+config HAVE_GENERIC_IOMAP
+
+config GENERIC_IOMAP
+ depends on HAVE_GENERIC_IOMAP && FOO
+
+And in lib/Makefile we would see:
+obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
+
+For each architecture using the generic IOMAP functionality we would see:
+
+config X86
+ select ...
+ select HAVE_GENERIC_IOMAP
+ select ...
+
+Note: we use the existing config option and avoid creating a new
+config variable to select HAVE_GENERIC_IOMAP.
+
+Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
+introduced to overcome the limitation of select which will force a
+config option to 'y' no matter the dependencies.
+The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
+situation where select forces a symbol equals to 'y'.
+
+Build as module only
+~~~~~~~~~~~~~~~~~~~~
+To restrict a component build to module-only, qualify its config symbol
+with "depends on m". E.g.:
+
+config FOO
+ depends on BAR && m
+
+limits FOO to module (=m) or disabled (=n).
+
+
+Build limited by a third config symbol which may be =y or =m
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A common idiom that we see (and sometimes have problems with) is this:
+
+When option C in B (module or subsystem) uses interfaces from A (module
+or subsystem), and both A and B are tristate (could be =y or =m if they
+were independent of each other, but they aren't), then we need to limit
+C such that it cannot be built statically if A is built as a loadable
+module. (C already depends on B, so there is no dependency issue to
+take care of here.)
+
+If A is linked statically into the kernel image, C can be built
+statically or as loadable module(s). However, if A is built as loadable
+module(s), then C must be restricted to loadable module(s) also. This
+can be expressed in kconfig language as:
+
+config C
+ depends on A = y || A = B
+
+or for real examples, use this command in a kernel tree:
+
+$ find . -name Kconfig\* | xargs grep -ns "depends on.*=.*||.*=" | grep -v orig
+