Re: [PATCH -v2] x86, pci: Handle fallout pci devices with peer rootbus

From: H. Peter Anvin
Date: Mon Jun 14 2010 - 21:57:51 EST

On 06/14/2010 06:49 PM, Bjorn Helgaas wrote:

>> Invisible PCI bridges have been known to occur in pure PCI space, too.
> Are you talking about PCI host bridges that don't appear in PCI config
> space? I suppose those could be described as "invisible," but since
> host bridges aren't architected and their primary interface isn't PCI,
> it seems only natural that we'd discover them by a non-PCI mechanism.
> They're invisible in PCI terms, but obviously perfectly discoverable
> and configurable via ACPI.

I mean invisible PCI-PCI bridges. Yes, they exist.

> If you ask me, it's weird that most x86 chipsets put PCI host bridge
> configuration in PCI config space -- it may be convenient in some ways,
> but still architecturally strange.

It is only strange because they are non-bridge devices. PCI-Express
fixes that to some degree with the whole "root complex" notion, but
really a PCI host bridge should have been a bridge device from the start.

> I suppose one could argue that there's a non-standard P2P bridge
> from bus 00 to bus 80, but I can't imagine anybody doing that.

Ah, ye of little imagination.

> An OS would have to have vendor-specific code just to do PCI
> resource management, and that really misses the point of PCI.

This really misses the point of HT...

> It seems more likely to me that one of the VIA host bridges leads
> to bus 80. PCI host bridges are not architected, so if this bridge
> lives on HT chain 00, and we can think of HT as "not quite PCI,"
> then it seems natural that the host bridge would be VIA-specific,
> just like it was in pre-HT days.

I think the best word for it is "incompetent braindamage", but that's
just me...


H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.

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