linux-kernel - Re: [Patch v2] Make PCI extended config space (MMCONFIG) a driver opt-in

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Date:	Mon, 14 Jan 2008 06:46:14 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	tcamuso@...hat.com
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Greg KH <greg@...ah.com>, Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	Jeff Garzik <jeff@...zik.org>,
	linux-pci@...ey.karlin.mff.cuni.cz,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Mares <mj@....cz>, Loic Prylli <loic@...i.com>,
	Prarit Bhargava <prarit@...hat.com>,
	"Chumbalkar, Nagananda" <Nagananda.Chumbalkar@...com>,
	"Schoeller, Patrick (Linux - Houston, TX)" <Patrick.Schoeller@...com>,
	Bhavana Nagendra <bnagendr@...hat.com>
Subject: Re: [Patch v2] Make PCI extended config space (MMCONFIG) a driver
 opt-in

On Mon, 14 Jan 2008 08:01:01 -0500
Tony Camuso <tcamuso@...hat.com> wrote:

> Arjan van de Ven wrote:
> > On Sun, 13 Jan 2008 22:29:23 -0500
> > Tony Camuso <tcamuso@...hat.com> wrote:
> > 
> >> . There is no need to provide different PCI config access
> >>    mechanisms at device granularity, since the PCI config access
> >>    mechanism between the CPU and the Northbridge is opaque to
> >>    the devices. PCI config mechanisms only need to differ at
> >>    the Northbridge level.
> > 
> > This ignores the "lets make it not matter for the 99% of the users"
> > case.
> 
> I don't understand. 

That;s clear :)

> If we're going to differentiate MMCONFIG from
> some other access mechanism, it only needs to be done at the
> Northbridge level. Devices are electrically ignorant of the protocol
> used between CPU and Northbridge to get the Northbridge to assert
> config cycles on the bus.

Again this is about having systems that don't need extended config space not use it. At all.
The only way to do that is have the drivers say they need it, and not use it otherwise.
It has NOTHING to do with how things are wired up. It's pure a kernel level policy decision
about whether to use extended config space AT ALL.



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