linux-kernel - Re: [PATCH] kconfig: make 'imply' obey the direct dependency

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Message-ID: <nycvar.YSQ.7.76.2002191046520.1559@knanqh.ubzr>
Date:   Wed, 19 Feb 2020 11:15:57 -0500 (EST)
From:   Nicolas Pitre <nico@...xnic.net>
To:     Masahiro Yamada <masahiroy@...nel.org>
cc:     linux-kbuild@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Richard Cochran <richardcochran@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mark Brown <broonie@...nel.org>,
        Ulf Magnusson <ulfalizer@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Jonathan Corbet <corbet@....net>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kconfig: make 'imply' obey the direct dependency

On Wed, 19 Feb 2020, Masahiro Yamada wrote:

> The 'imply' statement may create unmet direct dependency when the
> implied symbol depends on m.
> 
> [Test Code]
> 
>   config FOO
>           tristate "foo"
>           imply BAZ
> 
>   config BAZ
>           tristate "baz"
>           depends on BAR
> 
>   config BAR
>           def_tristate m
> 
>   config MODULES
>           def_bool y
>           option modules
> 
> If you set FOO=y, BAZ is also promoted to y, which results in the
> following .config file:
> 
>   CONFIG_FOO=y
>   CONFIG_BAZ=y
>   CONFIG_BAR=m
>   CONFIG_MODULES=y
> 
> This ignores the dependency "BAZ depends on BAR".
> 
> Unlike 'select', what is worse, Kconfig never shows the
> "WARNING: unmet direct dependencies detected for ..." for this case.
> 
> Because 'imply' should be weaker than 'depends on', Kconfig should
> take the direct dependency into account.
> 
> Describe this case in Documentation/kbuild/kconfig-language.rst for
> clarification.
> 
> Commit 237e3ad0f195 ("Kconfig: Introduce the "imply" keyword") says that
> a symbol implied by y is restricted to y or n, excluding m.
> 
> As for the combination of FOO=y and BAR=m, the case of BAZ=m is excluded
> by the 'imply', and BAZ=y is also excluded by 'depends on'. So, only the
> possible value is BAZ=n.

I don't think this is right. The imply keyword provide influence over 
another symbol but it should not impose any restrictions. If BAR=m then 
BAZ should still be allowed to be m or n.

> @@ -174,6 +174,9 @@ applicable everywhere (see syntax).
>  	n		y		n		N/m/y
>  	m		y		m		M/y/n
>  	y		y		y		Y/n
> +	n		m		n		N/m
> +	m		m		m		M/n
> +	y		m		n		N

Here the last line shoule be y m n N/m.

Generally speaking, the code enabled by FOO may rely on functionalities 
provided by BAZ only when BAZ >= FOO. This is accomplished with 
IS_REACHABLE():

	foo_init()
	{
		if (IS_REACHABLE(CONFIG_BAZ))
			baz_register(&foo);
		...
	}

So if FOO=y and BAZ=m then IS_REACHABLE(CONFIG_BAZ) will be false. Maybe 
adding a note to that effect linked to the "y m n N/m" line in the table 
would be a good idea.


Nicolas