linux-kernel - RE: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary

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Date:   Fri, 6 May 2022 13:28:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Maciej W. Rozycki'" <macro@...am.me.uk>
CC:     Arnd Bergmann <arnd@...nel.org>, Rich Felker <dalias@...c.org>,
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Subject: RE: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it
 as necessary

From: Maciej W. Rozycki
> Sent: 06 May 2022 14:15
> On Fri, 6 May 2022, David Laight wrote:
> 
> > >  The PCI configuration space was retrofitted into x86 systems (and is
> > > accessed in an awkward manner with them), but with a new design such a
> > > clean approach is most welcome IMHO.  Thank you for your explanation.
> >
> > Actually I think x86 was the initial system for PCI.
> > The PCI config space 'mess' is all about trying to make
> > something that wouldn't break existing memory maps.
> 
>  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).
#1 requires a pair of accesses (and SMP locking).

Neither is really horrid.

For horrid try the ISApnp interface.

	David

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