linux-kernel - Re: [PATCH 1/3] PCI: allow P2P bridge windows starting at PCI bus address zero

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Message-ID: <CAE9FiQW-6x_HrZQLmfw22Temvma7rd12qR3QfKj-Eh1tqWHHNQ@mail.gmail.com>
Date:	Mon, 9 Jul 2012 14:11:35 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	linux-pci@...r.kernel.org, Daniel Yeisley <dan.yeisley@...sys.com>,
	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 1/3] PCI: allow P2P bridge windows starting at PCI bus
 address zero

On Mon, Jul 9, 2012 at 1:32 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> cd81e1ea1a4c added checks that prevent us from using P2P bridge windows
> that start at PCI bus address zero.  The reason was to "prevent us from
> overwriting resources that are unassigned."
>
> But generic code should allow address zero in both BARs and bridge
> windows, so I think that commit was a mistake.
>
> Windows at bus address zero are legal and likely to exist on machines with
> an offset between bus addresses and CPU addresses.  For example, in the
> following hypothetical scenario, the bridge at 00:01.0 has a window at bus
> address zero and the device at 01:00.0 has a BAR at bus address zero, and
> I think both are perfectly valid:
>
>     PCI host bridge to bus 0000:00
>     pci_bus 0000:00: root bus resource [mem 0x100000000-0x1ffffffff] (bus address [0x00000000-0xffffffff])
>     pci 0000:00:01.0: PCI bridge to [bus 01]
>     pci 0000:00:01.0:   bridge window [mem 0x100000000-0x100ffffff]
>     pci 0000:01:00.0: reg 10: [mem 0x100000000-0x100ffffff]
>
> CC: Yinghai Lu <yinghai@...nel.org>
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>

Acked-by: Yinghai Lu <yinghai@...nel.org>
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