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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Wed, 11 Dec 2013 10:03:21 +1100
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Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH v2 10/14] pci: allow 0 address
 for PCI IO regions
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>, QEMU Developers <qemu-devel@nongnu.org>, Mike Day <ncmike@ncultra.org>, Paul Mackerras <paulus@samba.org>, tyreld@linux.vnet.ibm.com, nfont@linux.vnet.ibm.com, "qemu-ppc@nongnu.org" <qemu-ppc@nongnu.org>

On Tue, 2013-12-10 at 22:14 +0000, Peter Maydell wrote:

> Googling again brought up this mailing list thread:
> 
> http://www.pcisig.com/reflector/msg00459.html
> 
> which includes what is supposedly a quote from the PCI 2.1 spec:
> 
> # "Note: A Base Address register does not contain a valid
> # address when it is equal to "0""
> 
> (I don't have access to the 2.1 version to check.)
> 
> This text seems to have been removed from the 2.2 spec.

I have seen practical cases of both:

 - Systems where the FW sets up a BAR to 0 and considers it valid

 - Adapters that treat a BAR set to 0 as disabled

There's no win here. However it makes sense for qemu not to treat 0
as a special value, it's not necessary.

Cheers,
Ben.