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+======================
+Kconfig macro language
+======================
+
+Concept
+-------
+
+The basic idea was inspired by Make. When we look at Make, we notice sort of
+two languages in one. One language describes dependency graphs consisting of
+targets and prerequisites. The other is a macro language for performing textual
+substitution.
+
+There is clear distinction between the two language stages. For example, you
+can write a makefile like follows::
+
+    APP := foo
+    SRC := foo.c
+    CC := gcc
+
+    $(APP): $(SRC)
+            $(CC) -o $(APP) $(SRC)
+
+The macro language replaces the variable references with their expanded form,
+and handles as if the source file were input like follows::
+
+    foo: foo.c
+            gcc -o foo foo.c
+
+Then, Make analyzes the dependency graph and determines the targets to be
+updated.
+
+The idea is quite similar in Kconfig - it is possible to describe a Kconfig
+file like this::
+
+    CC := gcc
+
+    config CC_HAS_FOO
+            def_bool $(shell, $(srctree)/scripts/gcc-check-foo.sh $(CC))
+
+The macro language in Kconfig processes the source file into the following
+intermediate::
+
+    config CC_HAS_FOO
+            def_bool y
+
+Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
+dependency as explained in kconfig-language.txt.
+
+
+Variables
+---------
+
+Like in Make, a variable in Kconfig works as a macro variable.  A macro
+variable is expanded "in place" to yield a text string that may then be
+expanded further. To get the value of a variable, enclose the variable name in
+$( ). The parentheses are required even for single-letter variable names; $X is
+a syntax error. The curly brace form as in ${CC} is not supported either.
+
+There are two types of variables: simply expanded variables and recursively
+expanded variables.
+
+A simply expanded variable is defined using the := assignment operator. Its
+righthand side is expanded immediately upon reading the line from the Kconfig
+file.
+
+A recursively expanded variable is defined using the = assignment operator.
+Its righthand side is simply stored as the value of the variable without
+expanding it in any way. Instead, the expansion is performed when the variable
+is used.
+
+There is another type of assignment operator; += is used to append text to a
+variable. The righthand side of += is expanded immediately if the lefthand
+side was originally defined as a simple variable. Otherwise, its evaluation is
+deferred.
+
+The variable reference can take parameters, in the following form::
+
+  $(name,arg1,arg2,arg3)
+
+You can consider the parameterized reference as a function. (more precisely,
+"user-defined function" in contrast to "built-in function" listed below).
+
+Useful functions must be expanded when they are used since the same function is
+expanded differently if different parameters are passed. Hence, a user-defined
+function is defined using the = assignment operator. The parameters are
+referenced within the body definition with $(1), $(2), etc.
+
+In fact, recursively expanded variables and user-defined functions are the same
+internally. (In other words, "variable" is "function with zero argument".)
+When we say "variable" in a broad sense, it includes "user-defined function".
+
+
+Built-in functions
+------------------
+
+Like Make, Kconfig provides several built-in functions. Every function takes a
+particular number of arguments.
+
+In Make, every built-in function takes at least one argument. Kconfig allows
+zero argument for built-in functions, such as $(fileno), $(lineno). You could
+consider those as "built-in variable", but it is just a matter of how we call
+it after all. Let's say "built-in function" here to refer to natively supported
+functionality.
+
+Kconfig currently supports the following built-in functions.
+
+ - $(shell,command)
+
+  The "shell" function accepts a single argument that is expanded and passed
+  to a subshell for execution. The standard output of the command is then read
+  and returned as the value of the function. Every newline in the output is
+  replaced with a space. Any trailing newlines are deleted. The standard error
+  is not returned, nor is any program exit status.
+
+ - $(info,text)
+
+  The "info" function takes a single argument and prints it to stdout.
+  It evaluates to an empty string.
+
+ - $(warning-if,condition,text)
+
+  The "warning-if" function takes two arguments. If the condition part is "y",
+  the text part is sent to stderr. The text is prefixed with the name of the
+  current Kconfig file and the current line number.
+
+ - $(error-if,condition,text)
+
+  The "error-if" function is similar to "warning-if", but it terminates the
+  parsing immediately if the condition part is "y".
+
+ - $(filename)
+
+  The 'filename' takes no argument, and $(filename) is expanded to the file
+  name being parsed.
+
+ - $(lineno)
+
+  The 'lineno' takes no argument, and $(lineno) is expanded to the line number
+  being parsed.
+
+
+Make vs Kconfig
+---------------
+
+Kconfig adopts Make-like macro language, but the function call syntax is
+slightly different.
+
+A function call in Make looks like this::
+
+  $(func-name arg1,arg2,arg3)
+
+The function name and the first argument are separated by at least one
+whitespace. Then, leading whitespaces are trimmed from the first argument,
+while whitespaces in the other arguments are kept. You need to use a kind of
+trick to start the first parameter with spaces. For example, if you want
+to make "info" function print "  hello", you can write like follows::
+
+  empty :=
+  space := $(empty) $(empty)
+  $(info $(space)$(space)hello)
+
+Kconfig uses only commas for delimiters, and keeps all whitespaces in the
+function call. Some people prefer putting a space after each comma delimiter::
+
+  $(func-name, arg1, arg2, arg3)
+
+In this case, "func-name" will receive " arg1", " arg2", " arg3". The presence
+of leading spaces may matter depending on the function. The same applies to
+Make - for example, $(subst .c, .o, $(sources)) is a typical mistake; it
+replaces ".c" with " .o".
+
+In Make, a user-defined function is referenced by using a built-in function,
+'call', like this::
+
+    $(call my-func,arg1,arg2,arg3)
+
+Kconfig invokes user-defined functions and built-in functions in the same way.
+The omission of 'call' makes the syntax shorter.
+
+In Make, some functions treat commas verbatim instead of argument separators.
+For example, $(shell echo hello, world) runs the command "echo hello, world".
+Likewise, $(info hello, world) prints "hello, world" to stdout. You could say
+this is _useful_ inconsistency.
+
+In Kconfig, for simpler implementation and grammatical consistency, commas that
+appear in the $( ) context are always delimiters. It means::
+
+  $(shell, echo hello, world)
+
+is an error because it is passing two parameters where the 'shell' function
+accepts only one. To pass commas in arguments, you can use the following trick::
+
+  comma := ,
+  $(shell, echo hello$(comma) world)
+
+
+Caveats
+-------
+
+A variable (or function) cannot be expanded across tokens. So, you cannot use
+a variable as a shorthand for an expression that consists of multiple tokens.
+The following works::
+
+    RANGE_MIN := 1
+    RANGE_MAX := 3
+
+    config FOO
+            int "foo"
+            range $(RANGE_MIN) $(RANGE_MAX)
+
+But, the following does not work::
+
+    RANGES := 1 3
+
+    config FOO
+            int "foo"
+            range $(RANGES)
+
+A variable cannot be expanded to any keyword in Kconfig.  The following does
+not work::
+
+    MY_TYPE := tristate
+
+    config FOO
+            $(MY_TYPE) "foo"
+            default y
+
+Obviously from the design, $(shell command) is expanded in the textual
+substitution phase. You cannot pass symbols to the 'shell' function.
+
+The following does not work as expected::
+
+    config ENDIAN_FLAG
+            string
+            default "-mbig-endian" if CPU_BIG_ENDIAN
+            default "-mlittle-endian" if CPU_LITTLE_ENDIAN
+
+    config CC_HAS_ENDIAN_FLAG
+            def_bool $(shell $(srctree)/scripts/gcc-check-flag ENDIAN_FLAG)
+
+Instead, you can do like follows so that any function call is statically
+expanded::
+
+    config CC_HAS_ENDIAN_FLAG
+            bool
+            default $(shell $(srctree)/scripts/gcc-check-flag -mbig-endian) if CPU_BIG_ENDIAN
+            default $(shell $(srctree)/scripts/gcc-check-flag -mlittle-endian) if CPU_LITTLE_ENDIAN