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Date:   Fri, 6 May 2022 15:44:19 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@orcam.me.uk>
To:     David Laight <David.Laight@ACULAB.COM>
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Subject: RE: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select
 it as necessary
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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.

  Maciej