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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Fri, 6 May 2022 14:53:49 +0200
Message-ID: <CAK8P3a0EMK0gHOmb-jvtfVLb1dun72kYUMKpb11T_GgXiuR9Mw@mail.gmail.com>
Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it
 as necessary
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc:     Bjorn Helgaas <helgaas@kernel.org>,
        Niklas Schnelle <schnelle@linux.ibm.com>,
        Arnd Bergmann <arnd@arndb.de>,
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On Fri, May 6, 2022 at 2:27 PM Maciej W. Rozycki <macro@orcam.me.uk> wrote:
>
> On Fri, 6 May 2022, Arnd Bergmann wrote:
>
> > >  If this is PCI/PCIe indeed, then an I/O access is just a different bit
> > > pattern put on the bus/in the TLP in the address phase.  So what is there
> > > inherent to the s390 architecture that prevents that different bit pattern
> > > from being used?
> >
> > The hardware design for PCI on s390 is very different from any other
> > architecture, and more abstract. Rather than implementing MMIO register
> > access as pointer dereference, this is a separate CPU instruction that
> > takes a device/bar plus offset as arguments rather than a pointer, and
> > Linux encodes this back into a fake __iomem token.
>
>  OK, that seems to me like a reasonable and quite a clean design (on the
> hardware side).
>
>  So what happens if the instruction is given an I/O rather than memory BAR
> as the relevant argument?  Is the address space indicator bit (bit #0)
> simply ignored or what?

Not sure. My best guess is that it would actually work as you'd expect,
but is deliberately left out of the architecture specification so they don't
have to to validate the correctness.  Note that only a small number of
PCIe cards are actually supported by IBM, and I think the firmware
only passes devices to the OS if they are whitelisted.

        Arnd