Change in coreboot[master]: [RFC] pci_mmio_cfg: Move guard around pci_s_* functions to x86

[Shelley Chen (Code Review)]

Attention is currently required from: Nico Huber, Furquan Shaikh, Tim Wawrzynczak.
Shelley Chen has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/58331 )

Change subject: [RFC] pci_mmio_cfg: Move guard around pci_s_* functions to x86
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Patch Set 2:

(1 comment)

File src/arch/x86/include/arch/pci_ops.h:

https://review.coreboot.org/c/coreboot/+/58331/comment/4080491b_7d5c9d90 
PS2, Line 7: MMCONF_SUPPORT
> I don't think we need a Kconfig option for mmio

ops in general.

Sorry, I'm a little confused by this statement.  Just to clarify, do you imagine:

MMCONF_SUPPORT : enable mmio ops (mostly non-x86 platforms)
MMCONF_ECAM_SUPPORT: enable mmio ops using ECAM (mostly x86 platforms)
NO_MMCONF_SUPPORT: no mmio ops support (revert to io ops)

The reason that we wanted to rename MMCONF was because it seems like there was a lot of confusion around the meaning of MMCONF:
  1. does it mean mmio ops?
  2. does it only cover ECAM?
I think that I actually started out thinking #1, but realized that I was wrong.  That was the reason we wanted to introduce the PCI_MMIO_OPS Kconfig in the first place.  Our proposal was:

PCI_MMIO_OPS : enable mmio ops (mostly non-x86 platforms)
PCI_ECAM: enable mmio ops using ECAM (mostly x86 platforms)
PCI_NO_MMIO_OPS: no mmio ops support (revert to io ops)

I am happy to add all this explanation to the doc (CB:57861) if that is useful.