Re: Kconfig: '+config' valid syntax?

Paul Bolle
2015-07-02T09:01:20Z

[Dropped Yann. You already know Yann disappeared.]

On Thu, 2015-07-02 at 10:08 +0200, Valentin Rothberg wrote:
> commit ed013214afa7 ("ACPI / init: Make it possible to override _REV")
> is in today's linux-next tree (i.e., next-20150702) adding the
> following hunk to drivers/acpi/Kconfig:
>
> --- a/drivers/acpi/Kconfig
> +++ b/drivers/acpi/Kconfig
> @@ -428,6 +428,26 @@ config XPOWER_PMIC_OPREGION
> help
> This config adds ACPI operation region support for XPower
> AXP288 PMIC.
>
> ++config ACPI_REV_OVERRIDE_POSSIBLE

(Odd. Botched conflict resolution?)

> + bool "Allow supported ACPI revision to be overriden"
> + depends on X86
> + default y
> [...]
>
> By having a close look at the first added line, we can see that
> '+config ACPI_...' is added. To my great surprise, it's valid Kconfig
> syntax.

I played a bit with this. It seems you can basically add a '+' anywhere
you like and kconfig will just ignore it.

> How is that possible? IMHO it's an invalid token, such that
> Kconfig should complain about it. Or do I miss something?

Welcome to the wonders of lex and yacc!

I try to spend as little time as possible looking at the lex rules, so
I'm just guessing here. Anyhow, you might start by looking at this
snippet in zconf.l:
. {
unput(yytext[0]);
BEGIN(COMMAND);
}

<COMMAND>{
{n}+ {
[...]
}
.
\n {
BEGIN(INITIAL);
current_file->lineno++;
return T_EOL;
}
}

Which perhaps translates to:
- ignore unknown stuff for now and go in COMMAND state;
- do something if we encounter some text ({n} = [A-Za-z0-9_]);
- go in INITIAL state if we encounter newlines or unknown stuff.

At the end of which we're back where we started before encountering
the'+'. But there are more references to '.' in the lex rules so it's
probably more complicated.

Hope this helps,

Paul Bolle