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

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Message-ID: <476DEC66.9050701@garzik.org>
Date:	Sun, 23 Dec 2007 00:04:38 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, gregkh@...e.de,
	linux-pci@...ey.karlin.mff.cuni.cz,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [patch] Make MMCONFIG space (extended PCI config space) a driver
 opt-in issue

Linus Torvalds wrote:
> I want to limit that downside. Right now, the easiest way to limit it 
> seems to be to say that those (very very few) drivers that actually care 
> could enable it. That way, we automatically limit it to only those 
> machines that have hardware that cares.

Then let's do it right:  disable mmconfig by default on x86, and enable 
it when passed "pci=mmconfig".

For the rare -- you and I agree its very rare -- case where it is 
REQUIRED, the user can pass pci=mmconfig as instructed by driver 
documentation somewhere.

Let's not bend over backwards and introduce an API for these 
presently-theoretical cases.  Given the complete lack of hw vendor 
testing and potential to confuse userspace, the two choices for a 
computer should be "mmconfig off" or "mmconfig on."

Kernel hackers developing drivers and code for new machines will know 
enough to pass pci=mmconfig if they NEED it.  That practice will only 
become annoying when x86 hardware actually starts to NEED extended 
config space -- at which future time we can revisit, as you describe.

> And yes, if you want the capability following to notice automatically when 
> capabilities really do go into the 0x100+ range, that's fine. I suspect 

Yes, we /must/ do this checking, if we don't already.

	Jeff

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