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

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Message-ID: <4789B2F8.5010800@garzik.org>
Date:	Sun, 13 Jan 2008 01:43:04 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Matthew Wilcox <matthew@....cx>
CC:	Arjan van de Ven <arjan@...radead.org>, tcamuso@...hat.com,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Greg KH <greg@...ah.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.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

Matthew Wilcox wrote:
> On Sat, Jan 12, 2008 at 08:42:48PM -0800, Arjan van de Ven wrote:
>> Wanne bet there'll be devices that screw this up? THere's devices that even screwed
>> up the 64-256 region after all.
> 
> I don't know if they 'screwed it up'.  There are devices that misbehave
> when registers are read from pci config space.  But this was never
> guaranteed to be a safe thing to do; it gradualy became clear that
> people expected to be able to read random registers and manufacturers
> responded accordingly, but I don't think you were ever guaranteed to be
> able to peek at bits of config space arbitrarily.

Quite correct...  Reading registers can have all sorts of side effects, 
for example clearing chip conditions.

	Jeff



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