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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