linux-kernel - RE: [RFC v2 10/39] gpio: add HAS

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Date:   Wed, 4 May 2022 14:02:28 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     David Laight <David.Laight@...LAB.COM>
cc:     'Linus Walleij' <linus.walleij@...aro.org>,
        William Breathitt Gray <william.gray@...aro.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: RE: [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