linux-kernel - Re: [PATCH] ARM: use "depends on" for SoC configs instead of "if" after prompt

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Message-ID: <20151116110622.GL8644@n2100.arm.linux.org.uk>
Date:	Mon, 16 Nov 2015 11:06:22 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	yamada.masahiro@...ionext.com
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Subject: Re: [PATCH] ARM: use "depends on" for SoC configs instead of "if"
 after prompt

On Mon, Nov 16, 2015 at 10:32:51AM +0000, yamada.masahiro@...ionext.com wrote:
> Hi Arnd,
> 
>  
> > On Monday 16 November 2015 12:06:10 Masahiro Yamada wrote:
> > > Many ARM sub-architectures use prompts followed by "if" conditional,
> > > but it is wrong.
> > >
> > > Please notice the difference between
> > >
> > >     config ARCH_FOO
> > >             bool "Foo SoCs" if ARCH_MULTI_V7
> > >
> > > and
> > >
> > >     config ARCH_FOO
> > >             bool "Foo SoCs"
> > >             depends on ARCH_MULTI_V7
> > >
> > > These two are *not* equivalent!
> > >
> > > In the former statement, it is not ARCH_FOO, but its prompt that
> > > depends on ARCH_MULTI_V7.  So, it is completely valid that ARCH_FOO is
> > > selected by another, but ARCH_MULTI_V7 is still disabled. As it is not
> > > unmet dependency, Kconfig never warns.  This is probably not what you
> > > want.
> > 
> > Did you encounter a case where someone actually did a 'select' on one of
> > those symbols? I probably introduced a lot of them and did not expect that
> > to happen.
> 
> No, for ARM sub-architectures.
> But, yes for the ARM core part.
> 
> 
> For example, the following entry in arch/arm/Kconfig is suspicous.
> 
> config PCI
>         bool "PCI support" if MIGHT_HAVE_PCI
>         help
>           Find out whether you have a PCI motherboard. PCI is the name of a
>           bus system, i.e. the way the CPU talks to the other stuff inside
>           your box. Other bus systems are ISA, EISA, MicroChannel (MCA) or
>           VESA. If you have PCI, say Y, otherwise N.
> 
> 
> 
> 
> Try "make ARCH=arm footbridge_defconfig" and check the .config file.
> 
> It defines CONFIG_PCI=y, but not CONFIG_MIGHT_HAVE_PCI.
> I am not sure this is a sane .config or not.

It's correct.  "MIGHT_HAVE_PCI" is used by platforms which _might_ _have_
_PCI_, not by platforms which _do_ _have_ _PCI_.  Platforms which _do_
_have_ _PCI_ select PCI directly, and because "MIGHT_HAVE_PCI" is not
set, users are not offered an option that they can never disable.

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