Merge git://git.infradead.org/mtd-2.6

[~andy/linux] / Documentation / kbuild / kconfig-language.txt

diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt
index 53ca12f7999fdaf2137336584adead3c5adeb6e9..c412c245848f9116fb05361bb78e5189b43cfd51 100644 (file)
--- a/Documentation/kbuild/kconfig-language.txt
+++ b/Documentation/kbuild/kconfig-language.txt
@@ -104,14 +104,15 @@ applicable everywhere (see syntax).
   Reverse dependencies can only be used with boolean or tristate
   symbols.
   Note:
   Reverse dependencies can only be used with boolean or tristate
   symbols.
   Note:

-       select is evil.... select will by brute force set a symbol
-       equal to 'y' without visiting the dependencies. So abusing
-       select you are able to select a symbol FOO even if FOO depends
-       on BAR that is not set. In general use select only for
-       non-visible symbols (no prompts anywhere) and for symbols with
-       no dependencies. That will limit the usefulness but on the
-       other hand avoid the illegal configurations all over. kconfig
-       should one day warn about such things.
+       select should be used with care. select will force
+       a symbol to a value without visiting the dependencies.
+       By abusing select you are able to select a symbol FOO even
+       if FOO depends on BAR that is not set.
+       In general use select only for non-visible symbols
+       (no prompts anywhere) and for symbols with no dependencies.
+       That will limit the usefulness but on the other hand avoid
+       the illegal configurations all over.
+       kconfig should one day warn about such things.

 
 - numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
   This allows to limit the range of possible input values for int
 
 - numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
   This allows to limit the range of possible input values for int


@@ -127,6 +128,27 @@ applicable everywhere (see syntax).
   used to help visually separate configuration logic from help within
   the file as an aid to developers.
 
   used to help visually separate configuration logic from help within
   the file as an aid to developers.
 

+- misc options: "option" <symbol>[=<value>]
+  Various less common options can be defined via this option syntax,
+  which can modify the behaviour of the menu entry and its config
+  symbol. These options are currently possible:
+
+  - "defconfig_list"
+    This declares a list of default entries which can be used when
+    looking for the default configuration (which is used when the main
+    .config doesn't exists yet.)
+
+  - "modules"
+    This declares the symbol to be used as the MODULES symbol, which
+    enables the third modular state for all config symbols.
+
+  - "env"=<value>
+    This imports the environment variable into Kconfig. It behaves like
+    a default, except that the value comes from the environment, this
+    also means that the behaviour when mixing it with normal defaults is
+    undefined at this point. The symbol is currently not exported back
+    to the build environment (if this is desired, it can be done via
+    another symbol).

 
 Menu dependencies
 -----------------
 
 Menu dependencies
 -----------------


@@ -309,37 +331,49 @@ 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.
 

-Build as module only
-~~~~~~~~~~~~~~~~~~~~
-To restrict a component build to module-only, qualify its config symbol
-with "depends on m".  E.g.:
+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.

 
 

-config FOO
-       depends on BAR && m
+We would in lib/Kconfig see:

 
 

-limits FOO to module (=m) or disabled (=n).
+# Generic IOMAP is used to ...
+config HAVE_GENERIC_IOMAP

 
 

+config GENERIC_IOMAP
+       depends on HAVE_GENERIC_IOMAP && FOO

 
 

-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:
+And in lib/Makefile we would see:
+obj-$(CONFIG_GENERIC_IOMAP) += iomap.o

 
 

-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.)
+For each architecture using the generic IOMAP functionality we would see:

 
 

-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 X86
+       select ...
+       select HAVE_GENERIC_IOMAP
+       select ...

 
 

-config C
-       depends on A = y || A = B
+Note: we use the existing config option and avoid creating a new
+config variable to select HAVE_GENERIC_IOMAP.

 
 

-or for real examples, use this command in a kernel tree:
+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'.

 
 

-$ find . -name Kconfig\* | xargs grep -ns "depends on.*=.*||.*=" | grep -v orig
+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).