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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Fri, 6 May 2022 16:56:56 +0200
Message-ID: <CAMuHMdVsjRUKYqS0nkKsT08A4a4ipuhn7AG+ZqcHBqZvVX02OQ@mail.gmail.com>
Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it
 as necessary
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc:     David Laight <David.Laight@aculab.com>,
        Arnd Bergmann <arnd@kernel.org>, Rich Felker <dalias@libc.org>,
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Hi Maciej,

On Fri, May 6, 2022 at 4:44 PM Maciej W. Rozycki <macro@orcam.me.uk> wrote:
> On Fri, 6 May 2022, David Laight wrote:
> > >  It was retrofitted in that x86 systems already existed for ~15 years when
> > > PCI came into picture.  Therefore the makers of the CPU ISA couldn't have
> > > envisaged the need for config access instructions like they did for memory
> > > and port access.
> >
> > Rev 2.0 of the PCI spec (1993) defines two mechanisms for config cycles.
> > #2 is probably the first one and maps all of PCI config space into
> > 4k of IO space (PCI bridges aren't supported).
>
>  This one is even more horrid than #1 in that it requires two separate
> preparatory I/O writes rather than just one, one to the Forward Register
> (at 0xcfa) to set the bus number, and another to the Configuration Space
> Enable Register (at 0xcf8) to set the function number, before you can
> issue a configuration read or write to a device.  So you need MP locking
> too.
>
>  NB only peer bridges aren't supported with this mechanism, normal PCI-PCI
> bridges are, via the Forward Register.
>
> > #1 requires a pair of accesses (and SMP locking).
> >
> > Neither is really horrid.
>
>  Both are.  First neither is MP-safe and second both are indirect in that
> you need to poke at some chipset registers before you can issue the actual
> read or write.
>
>  Sane access would require a single CPU instruction to read or write from
> the configuration space.  To access the conventional PCI configuration
> space in a direct linear manner you need 256 * 21 * 8 * 256 = 10.5MiB of
> address space.  Such amount of address space seems affordable even with
> 32-bit systems.

Won't have fit in the legacy 1 MiB space ("640 KiB...").

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds