Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary

Maciej W. Rozycki
2022-05-06T12:28:32Z

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