linux-kernel - Re: [RFC v2 25/39] pcmcia: add HAS

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Date:   Wed, 4 May 2022 12:22:01 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [RFC v2 25/39] pcmcia: add HAS_IOPORT dependencies

On Wed, May 04, 2022 at 03:24:39PM +0100, Maciej W. Rozycki wrote:
> On Wed, 4 May 2022, Arnd Bergmann wrote:
> 
> > >  POWER9 is another architecture with no port I/O space[1]:
> > 
> > POWER9 is just an implementation of the power architecture
> > that has a particular PCI host bridge. I would assume that
> > arch/powerpc/ would continue to set HAS_IOPORT because
> > it knows how to access I/O ports at compile-time.
> 
>  Well, yes, except I would expect POWER9_CPU (and any higher versions we 
> eventually get) to clear HAS_IOPORT.  Generic configurations (GENERIC_CPU) 
> would set HAS_IOPORT of course, as would any lower architecture variants 
> that do or may support port I/O (it's not clear to me if there are any 
> that do not).  Ideally a generic configuration would not issue accesses to 
> random MMIO locations for port I/O accesses via `inb'/`outb', etc. for 
> systems that do not support port I/O (which it now does, or at least used 
> to until recently).

It would seem weird to me that a module would build and run on a
generic kernel running on POWER9 (with some safe way of handling
inb/outb that don't actually work), but not on a kernel built
specifically for POWER9_CPU.

It sounds like inb/outb in a generic kernel on POWER9 may not
currently do something sensible, but that's fixable, e.g., make inb()
return 0xff and outb() a no-op.  I would naively expect the same
behavior in a POWER9_CPU kernel.

Bjorn