linux-kernel - Re: [PATCH 2.6.28 1/3] PCI-quirks: Unhide MCH5/6 memory controller configuration device

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Message-Id: <200901061221.18512.jbarnes@virtuousgeek.org>
Date:	Tue, 6 Jan 2009 12:21:18 -0800
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Bjorn Helgaas <bjorn.helgaas@...com>
Cc:	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	bluesmoke-devel@...ts.sourceforge.net
Subject: Re: [PATCH 2.6.28 1/3] PCI-quirks: Unhide MCH5/6 memory controller configuration device

On Tuesday, January 6, 2009 11:02 am Bjorn Helgaas wrote:
> On Monday 05 January 2009 01:30:06 pm Michał Mirosław wrote:
> > Some BIOSes hide 'overflow' device (dev #6) for i82875P/PE chipsets.
> > The same happens for i82865P/PE. Add a quirk to enable this device.
> > This allows i82875 EDAC driver to bind to chipset's dev #6 and not
> > dev #0 as the latter is used by AGP driver.
> >
> > After testing this patch for couple of days on my laptop (i82856P)
> > it looks like something is resetting device 0 (MCH) config register
> > 0xF4 to zero and effectively disabling the device again. The delay
> > looks random to me.
>
> The BIOS left the device hidden.  When you enable it with the quirk,
> the fact that it mysteriously gets disabled later seems like a pretty
> clear indication that something else we don't know about is using the
> device.  Since there's no synchronization between the "something else"
> and the i82875p_edac.c driver, it seems like you're introducing the
> possibility for problems.
>
> I don't know anything about the EDAC driver.  Is the value it provides
> really worth the possible problems with this approach?  Maybe it is,
> but I don't want to be the one to debug a random interaction that
> causes a problem.

Yeah, there's some uncertainty there for sure so I've dropped the patch.  If 
it really provides a compelling feature we can always add it again with a 
config option or runtime option to enable it.

-- 
Jesse Barnes, Intel Open Source Technology Center
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