248 lines
8.1 KiB
ReStructuredText
248 lines
8.1 KiB
ReStructuredText
======================
|
|
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
|