2018-04-15 17:08 GMT+09:00 Ulf Magnusson <ulfali...@gmail.com>: > On Fri, Apr 13, 2018 at 7:06 AM, Masahiro Yamada > <yamada.masah...@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.masah...@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)." >
OK. We do not need two ways to do the same thing. I will consider it although supporting single-letter variable is not costly. -- Best Regards Masahiro Yamada
