On Fri, Apr 13, 2018 at 7:06 AM, Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> > --- > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
