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Date:   Fri, 6 May 2022 13:27:20 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@orcam.me.uk>
To:     Arnd Bergmann <arnd@kernel.org>
cc:     Bjorn Helgaas <helgaas@kernel.org>,
        Niklas Schnelle <schnelle@linux.ibm.com>,
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Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select
 it as necessary
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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?

> >  But that has nothing to do with the presence or absence of any specific
> > processor instructions.  It's just a limitation of bus glue.  So I guess
> > it's just that all PCI/PCIe glue logic implementations for s390 have such
> > a limitation, right?
> 
> There are separate instructions for PCI memory and config space, but
> no instructions for I/O space, or for non-PCI MMIO that it could be mapped
> into.

 The PCI configuration space was retrofitted into x86 systems (and is 
accessed in an awkward manner with them), but with a new design such a 
clean approach is most welcome IMHO.  Thank you for your explanation.

  Maciej