linux-kernel - Re: [PATCH -v2] x86, pci: Handle fallout pci devices with peer root bus

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Date:	Mon, 14 Jun 2010 18:56:17 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Bjorn Helgaas <bjorn.helgaas@...com>
CC:	Yinghai Lu <yinghai.lu@...cle.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Graham Ramsey <ramsey.graham@...world.com>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	Robert Richter <robert.richter@....com>,
	Harald Welte <HaraldWelte@...tech.com>,
	Joseph Chan <JosephChan@....com.tw>,
	Jiri Slaby <jslaby@...e.cz>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dominik Brodowski <linux@...inikbrodowski.net>,
	Myron Stowe <myron.stowe@...com>
Subject: Re: [PATCH -v2] x86, pci: Handle fallout pci devices with peer root
 bus

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...

	-hpa

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

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