linux-kernel - Re: [PATCH 08/23] kconfig: add 'macro' keyword to support user-defined function

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Date:   Sat, 17 Feb 2018 00:51:38 +0100
From:   Ulf Magnusson <ulfalizer@...il.com>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kbuild@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/23] kconfig: add 'macro' keyword to support
 user-defined function

On Fri, Feb 16, 2018 at 02:49:31PM -0500, Nicolas Pitre wrote:
> On Sat, 17 Feb 2018, Masahiro Yamada wrote:
> 
> > Now, we got a basic ability to test compiler capability in Kconfig.
> > 
> > config CC_HAS_STACKPROTECTOR
> >         bool
> >         default $(shell $CC -Werror -fstack-protector -c -x c /dev/null -o /dev/null)
> > 
> > This works, but it is ugly to repeat this long boilerplate.
> > 
> > We want to describe like this:
> > 
> > config CC_HAS_STACKPROTECTOR
> >         bool
> >         default $(cc-option -fstack-protector)
> > 
> > It is straight-forward to implement a new function, but I do not like
> > to hard-code specialized functions like this.  Hence, here is another
> > feature to add functions from Kconfig files.
> > 
> > A user-defined function can be defined as a string type symbol with
> > a special keyword 'macro'.  It can be referenced in the same way as
> > built-in functions.  This feature was also inspired by Makefile where
> > user-defined functions are referenced by $(call func-name, args...),
> > but I omitted the 'call' to makes it shorter.
> > 
> > The macro definition can contain $(1), $(2), ... which will be replaced
> > with arguments from the caller.
> > 
> > Example code:
> > 
> >   config cc-option
> >           string
> >           macro $(shell $CC -Werror $(1) -c -x c /dev/null -o /dev/null)
> 
> I think this syntax for defining a macro shouldn't start with the 
> "config" keyword, unless you want it to be part of the config symbol 
> space and land it in .config. And typing it as a "string" while it 
> actually returns y/n (hence a bool) is also strange.
> 
> What about this instead:
> 
> macro cc-option
> 	bool $(shell $CC -Werror $(1) -c -x c /dev/null -o /dev/null)
> 
> This makes it easier to extend as well if need be.
> 
> 
> Nicolas

I haven't gone over the patchset in detail yet and might be missing
something here, but if this is just meant to be a textual shorthand,
then why give it a type at all?

Do you think a simpler syntax like this would make sense?

	macro cc-option "$(shell $CC -Werror $(1) -c -x c /dev/null -o /dev/null)"

That's the most general version, where you could use it for other stuff
besides $(shell ...) as well, just to keep parity.

You could then always just expand $() as a string, and maybe spit out
"n" and "y" in the cases Linus suggested for $(shell ...). The existing
logic for constant symbols should then take care of converting that into
a tristate value where appropriate.

If you go with that and want to support $() outside quotes, then

	$(foo)

would just be a shorthand for

	"$(foo)"

Are there any cases where something more advanced than that might be
warranted (e.g., macros that expand to complete expressions)? It seems
pretty nice and nonmagical otherwise.

Cheers,
Ulf