RE: [RFC v2 10/39] gpio: add HAS_IOPORT dependencies

["Maciej W. Rozycki" <macro@orcam.me.uk>]

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