1 files changed, 105 insertions, 6 deletions

diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt
index 616043a6da99..649cb8799890 100644
--- a/Documentation/kbuild/kconfig-language.txt
+++ b/Documentation/kbuild/kconfig-language.txt
@@ -24,7 +24,7 @@ visible if its parent entry is also visible.
 Menu entries
 ------------
 
-Most entries define a config option, all other entries help to organize
+Most entries define a config option; all other entries help to organize
 them. A single configuration option is defined like this:
 
 config MODVERSIONS
@@ -50,7 +50,7 @@ applicable everywhere (see syntax).
 
 - type definition: "bool"/"tristate"/"string"/"hex"/"int"
   Every config option must have a type. There are only two basic types:
-  tristate and string, the other types are based on these two. The type
+  tristate and string; the other types are based on these two. The type
   definition optionally accepts an input prompt, so these two examples
   are equivalent:
 
@@ -108,7 +108,7 @@ applicable everywhere (see syntax).
         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 promts anywhere) and for symbols with
+        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.
@@ -127,6 +127,27 @@ applicable everywhere (see syntax).
   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
 -----------------
@@ -162,9 +183,9 @@ An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
 respectively for calculations). A menu entry becomes visible when it's
 expression evaluates to 'm' or 'y'.
 
-There are two types of symbols: constant and nonconstant symbols.
-Nonconstant symbols are the most common ones and are defined with the
-'config' statement. Nonconstant symbols consist entirely of alphanumeric
+There are two types of symbols: constant and non-constant symbols.
+Non-constant symbols are the most common ones and are defined with the
+'config' statement. Non-constant symbols consist entirely of alphanumeric
 characters or underscores.
 Constant symbols are only part of expressions. Constant symbols are
 always surrounded by single or double quotes. Within the quote, any
@@ -301,3 +322,81 @@ mainmenu:
 
 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
+