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

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Message-ID: <CAK8P3a3NqU-3nUZ9ve=QyPPB5Uep3eK+_hicjjSiP8VuL4FYfA@mail.gmail.com>
Date:   Wed, 29 Dec 2021 20:48:23 -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>,
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        Dinh Nguyen <dinguyen@...nel.org>,
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        Jeff Dike <jdike@...toit.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
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        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 11:15 PM Michael Schmitz <schmitzmic@...il.com> wrote:
> Am 29.12.2021 um 16:41 schrieb Arnd Bergmann:
> > On Tue, Dec 28, 2021 at 8:20 PM Michael Schmitz <schmitzmic@...il.com> wrote:
> I'd hope not - we spent some effort to make sure setting ATARI_ROM_ISA
> does not affect other m68k platforms when e.g. building multiplatform
> kernels.

Ok

> Replacing inb() by readb() without any address translation won't do much
> good for m68k though - addresses in the traditional ISA I/O port range
> would hit the (unmapped) zero page.

Correct, this is exactly the problem that Niklas is trying to solve here:
we do have drivers that hit this bug, and on s390 clang actually produces
a compile-time error for drivers that cause a NULL pointer dereference
this way.

What some other architectures do is to rely on inb()/outb() to have a
zero-based offset, and use an io_offset in PCI buses to ensure that a
low port number on the bus gets translated into a pointer value for the
virtual mapping in the kernel, which is then represented as an unsigned
int.

As this is indistinguishable from architectures that just don't have
a base address for I/O ports (we unfortunately picked 0 as the default
PCI_IOBASE value), my suggestion was to start marking architectures
that may have this problem as using HAS_IOPORT in order to keep
the existing behavior unchanged. If m68k does not suffer from this,
making HAS_IOPORT conditional on those config options that actually
need it would of course be best.

         Arnd