kconfig: document use of HAVE_*

It has been discussed on lkml several times but we need
it documented as this is new stuff.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt
index a43fcc68e171ab96491599959314c7039a7b2525..649cb87998900e235afe6b83c5e9f10077be2085 100644 (file)
--- a/Documentation/kbuild/kconfig-language.txt
+++ b/Documentation/kbuild/kconfig-language.txt
@@ -330,6 +330,42 @@ 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.
 
 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
 Build as module only
 ~~~~~~~~~~~~~~~~~~~~
 To restrict a component build to module-only, qualify its config symbol