Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it as necessary

From: Arnd Bergmann <arnd@kernel.org>
To: Michael Schmitz <schmitzmic@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
	Niklas Schnelle <schnelle@linux.ibm.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	John Garry <john.garry@huawei.com>,
	Nick Hu <nickhu@andestech.com>, Greentime Hu <green.hu@gmail.com>,
	Vincent Chen <deanbo422@gmail.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>, Guo Ren <guoren@kernel.org>,
	Dinh Nguyen <dinguyen@kernel.org>,
	Chris Zankel <chris@zankel.net>,
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	Jeff Dike <jdike@addtoit.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
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	Heiko Carstens <hca@linux.ibm.com>,
	Richard Henderson <rth@twiddle.net>,
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	Greg Ungerer <gerg@linux-m68k.org>
Subject: Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it as necessary
Date: Tue, 28 Dec 2021 22:41:18 -0500	[thread overview]
Message-ID: <CAK8P3a2j-OFUUp+haHoV4PyL-On4EASZ9+59SDqNqmL8Gv_k7Q@mail.gmail.com> (raw)
In-Reply-To: <d406b93a-0f76-d056-3380-65d459d05ea9@gmail.com>

On Tue, Dec 28, 2021 at 8:20 PM Michael Schmitz <schmitzmic@gmail.com> wrote:
> Am 28.12.2021 um 23:08 schrieb Geert Uytterhoeven:
> > On Mon, Dec 27, 2021 at 5:44 PM Niklas Schnelle <schnelle@linux.ibm.com> wrote:
> >> We introduce a new HAS_IOPORT Kconfig option to gate support for
> >> I/O port access. In a future patch HAS_IOPORT=n will disable compilation
> >> of the I/O accessor functions inb()/outb() and friends on architectures
> >> which can not meaningfully support legacy I/O spaces. On these platforms
> >> inb()/outb() etc are currently just stubs in asm-generic/io.h which when
> >> called will cause a NULL pointer access which some compilers actually
> >> detect and warn about.
> >>
> >> The dependencies on HAS_IOPORT in drivers as well as ifdefs for
> >> HAS_IOPORT specific sections will be added in subsequent patches on
> >> a per subsystem basis. Then a final patch will ifdef the I/O access
> >> functions on HAS_IOPORT thus turning any use not gated by HAS_IOPORT
> >> into a compile-time warning.
> >>
> >> Link: https://lore.kernel.org/lkml/CAHk-=wg80je=K7madF4e7WrRNp37e3qh6y10Svhdc7O8SZ_-8g@mail.gmail.com/
> >> Co-developed-by: Arnd Bergmann <arnd@kernel.org>
> >> Signed-off-by: Arnd Bergmann <arnd@kernel.org>
> >> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> >
> > Thanks for your patch!
> >
> >> --- a/arch/m68k/Kconfig
> >> +++ b/arch/m68k/Kconfig
> >> @@ -16,6 +16,7 @@ config M68K
> >>         select GENERIC_CPU_DEVICES
> >>         select GENERIC_IOMAP
> >>         select GENERIC_IRQ_SHOW
> >> +       select HAS_IOPORT
> >>         select HAVE_AOUT if MMU
> >>         select HAVE_ASM_MODVERSIONS
> >>         select HAVE_DEBUG_BUGVERBOSE
> >
> > This looks way too broad to me: most m68k platform do not have I/O
> > port access support.
> >
> > My gut feeling says:
> >
> >     select HAS_IOPORT if PCI || ISA
> >
> > but that might miss some intricate details...
>
> In particular, this misses the Atari ROM port ISA adapter case -
>
>         select HAS_IOPORT if PCI || ISA || ATARI_ROM_ISA
>
> might do instead.

Right, makes sense. I had suggested to go the easy way and assume that
each architecture would select HAS_IOPORT if any configuration supports
it, but it looks like for m68k there is a clearly defined set of platforms that
do.

Note that for the platforms that don't set any of the three symbols, the
fallback makes inb() an alias for readb() with a different argument type,
so there may be m68k specific drivers that rely on this, but those would
already be broken if ATARI_ROM_ISA is set.

          Arnd