Re: [PATCH 17/30] Documentation: kconfig: document a new Kconfig macro language

[Ulf Magnusson <ulfalizer@gmail.com>]

On Fri, Apr 13, 2018 at 7:06 AM, Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
> Add a document for the macro language introduced to Kconfig.
>
> The motivation of this work is to move the compiler option tests to
> Kconfig from Makefile.  A number of kernel features require the
> compiler support.  Enabling such features blindly in Kconfig ends up
> with a lot of nasty build-time testing in Makefiles.  If a chosen
> feature turns out unsupported by the compiler, what the build system
> can do is either to disable it (silently!) or to forcibly break the
> build, despite Kconfig has let the user to enable it.
>
> This change was strongly prompted by Linus Torvalds.  You can find
> his suggestions [1] [2] in ML.  The original idea was to add a new
> 'option', but I found generalized text expansion would make Kconfig
> more powerful and lovely.  While polishing up the implementation, I
> noticed sort of similarity between Make and Kconfig.  This might be
> too immature to be called 'language', but anyway here it is.  All
> ideas are from Make (you can even say it is addicted), so people
> will easily understand how it works.
>
> [1]: https://lkml.org/lkml/2016/12/9/577
> [2]: https://lkml.org/lkml/2018/2/7/527
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> ---
>
> Changes in v3: None
> Changes in v2: None
>
>  Documentation/kbuild/kconfig-macro-language.txt | 179 ++++++++++++++++++++++++
>  MAINTAINERS                                     |   2 +-
>  2 files changed, 180 insertions(+), 1 deletion(-)
>  create mode 100644 Documentation/kbuild/kconfig-macro-language.txt
>
> diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt
> new file mode 100644
> index 0000000..1f6281b
> --- /dev/null
> +++ b/Documentation/kbuild/kconfig-macro-language.txt
> @@ -0,0 +1,179 @@
> +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, which is 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 expanded
> +further. To get the value of a variable, enclose the variable name in $( ).
> +As a special case, single-letter variable names can omit the parentheses and is
> +simply referenced like $X. Unlike Make, Kconfig does not support curly braces
> +as in ${CC}.

Do we need single-letter variable names for anything? It looks like
we're deviating
a bit from Make behavior already.

I suspect they're just a side effect of Make having automatic variables like $@.
The Make manual discourages them otherwise:

"A dollar sign followed by a character other than a dollar sign,
open-parenthesis or
open-brace treats that single character as the variable name. Thus, you could
reference the variable x with `$x'. However, this practice is strongly
discouraged,
except in the case of the automatic variables (see section Automatic
Variables)."

Cheers,
Ulf