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SubjectRE: [RFC v2 10/39] gpio: add HAS_IOPORT dependencies
On Wed, 4 May 2022, David Laight wrote:

> > Well, one can implement a pluggable PCI/e expansion card with a PCI-ISA
> > bridge on it and a backplane to plug ISA cards into. Without support for
> > issuing I/O cycles to PCI from the host however you won't be able to make
> > use of the ISA backplane except maybe for some ancient ISA memory cards.
> > So logically I think CONFIG_ISA should depend on CONFIG_HAS_IOPORT and
> > CONFIG_HAS_IOPORT ought to be selected by platform configurations.
>
> But generating a PCI(e) I/O cycle doesn't need the cpu to be able to
> generate an I/O cycle on its local bus interface.
> All that required is for the PCI(e) host bridge to determine that it
> needs to relevant kind of cycle on the target bus.
> This can easily be based on the physical address.

Sure, you can encode address spaces however you like (there are no
special machine instructions either for PCI/e configuration space access
that I would know of in any CPU architecture), but the host bridge must be
willing to issue those PCI/e I/O cycles in the first place (see my other
message on POWER9 in this thread).

> What you should probably be doing is (outside of 'platform' code)
> change the drivers to use ioread8() instead of inb().
> Then adding in the required calls to get the correct 'token' to
> pass to ioread8() to perform an I/O cycle on the correct target bus.

Yes, probably.

> It is really the attachment of the driver that can't succeed, not the
> compilation.

Except it makes no sense to offer those drivers for platforms known not
to provide for port I/O on PCI/e.

Maciej

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