Date: Mon, 5 Jan 2004 12:35:09 -0800
From: "David S. Miller" <davem@redhat.com>
To: Andi Kleen <ak@muc.de>
Cc: gibbs@scsiguy.com, linux-kernel@vger.kernel.org,
       linux-scsi@vger.kernel.org
Subject: Re: [BUG] x86_64 pci_map_sg modifies sg list - fails multiple
 map/unmaps
Message-Id: <20040105123509.4bacf670.davem@redhat.com>
In-Reply-To: <m3brpi41q0.fsf@averell.firstfloor.org>
References: <2938942704.1073325455@aslan.btc.adaptec.com>
	<m3brpi41q0.fsf@averell.firstfloor.org>
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On Mon, 05 Jan 2004 20:47:19 +0100
Andi Kleen <ak@muc.de> wrote:

> Actually I disabled merging by default in the latest x86-64 code,
> but it can be still enabled by the user using options (it makes some
> adapters run several percent faster). I would appreciate if you could
> fix the problem anyways.
> 
> I was actually planning to add a BUG() for this. Should do that.
> There is already one that triggers often when the problem occurs.

Andi, you must not modify sg->length in any way shape or form.

The following is legal:

	pci_map_sg(..&sg);
	pci_unmap_sg(...&sg);
	pci_map_sg(..&sg);

If you must modify the length field for DMA, you must have a seperate
dma_length member of the scatterlist structure on your platform, see what
sparc64 does here.

If the documentation states this wrongly, it's a doc bug.
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