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Date: Thu, 27 Aug 2009 09:37:01 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Robert Hancock <hancockrwd@gmail.com>
Cc: Tejun Heo <htejun@gmail.com>, Andrew Morton <akpm@linux-foundation.org>,
       Georgi Chulkov <g.chulkov@jacobs-university.de>,
       linux-kernel@vger.kernel.org, linux-ide@vger.kernel.org,
       Mark Lord <liml@rtr.ca>
Subject: Re: ATA device reset, shoud I be concerned?
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> Nvidia CK804 SATA controllers appear to also explode on reading TF 
> registers after media errors in certain cases. (They tend to either 
> lockup the machine or throw HyperTransport timeout machine check 

You must halt any running DMA activity before reading them. This is true
for pretty much all task file style controllers although the results of
not doing so vary by device somewhat.