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

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Message-ID: <CAK8P3a2j-OFUUp+haHoV4PyL-On4EASZ9+59SDqNqmL8Gv_k7Q@mail.gmail.com>
Date:   Tue, 28 Dec 2021 22:41:18 -0500
From:   Arnd Bergmann <arnd@...nel.org>
To:     Michael Schmitz <schmitzmic@...il.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        John Garry <john.garry@...wei.com>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Chris Zankel <chris@...kel.net>,
        Karol Gugala <kgugala@...micro.com>,
        Jeff Dike <jdike@...toit.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Brian Cain <bcain@...eaurora.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Vineet Gupta <vgupta@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
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        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
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        Thomas Gleixner <tglx@...utronix.de>,
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        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-csky@...r.kernel.org,
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        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        Greg Ungerer <gerg@...ux-m68k.org>
Subject: Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it as necessary

On Tue, Dec 28, 2021 at 8:20 PM Michael Schmitz <schmitzmic@...il.com> wrote:
> Am 28.12.2021 um 23:08 schrieb Geert Uytterhoeven:
> > On Mon, Dec 27, 2021 at 5:44 PM Niklas Schnelle <schnelle@...ux.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@...nel.org>
> >> Signed-off-by: Arnd Bergmann <arnd@...nel.org>
> >> Signed-off-by: Niklas Schnelle <schnelle@...ux.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