linux-kernel - Re: sata_svw data corruption, strange problems

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Date:	Mon, 23 Jun 2008 09:37:53 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Pavel Machek <pavel@...e.cz>
CC:	kernel list <linux-kernel@...r.kernel.org>,
	benh@...nel.crashing.org, jgarzik@...ox.com,
	IDE/ATA development list <linux-ide@...r.kernel.org>
Subject: Re: sata_svw data corruption, strange problems

Hello,

Pavel Machek wrote:
> I see strange problems on machine with sata_svw. The machine seems to
> corrupt data every few days (ext3 error, dir index corrupted), and has
> some other very strange problems (keyboard misbehaves, pulling out
> SATA disk cures it, see
> https://bugzilla.novell.com/show_bug.cgi?id=400772 ).
> 
> Then I got to the comment 
> 
>         writeb(dmactl | ATA_DMA_START, mmio + ATA_DMA_CMD);
>         /* There is a race condition in certain SATA controllers
> that can be seen when the r/w command is given to the controller
> before the host DMA is started. On a Read command, the controller
> would initiate the command to the drive even before it sees the DMA
> start. When there are very fast drives connected to the controller,
> or when the data request hits in the drive cache, there is the
> possibility that the drive returns a part or all of the requested
> data to the controller before the DMA start is issued.  In this
> case, the controller would become confused as to what to do with the
> data.  In the worst case when all the data is returned back to the
> controller, the controller could hang. In other cases it could
> return partial data returning in data corruption. This problem has
> been seen in PPC systems and can also appear on an system with very
> fast disks, where the SATA controller is sitting behind a number of
> bridges, and hence there is significant latency between the r/w
> command and the start command. */
>         /* issue r/w command if the access is to ATA*/
>         if (qc->tf.protocol == ATA_PROT_DMA)
> 
> ...and that would certainly explain what we are seeing. Are
> serverworks controllers broken by design?

The comment looks like a warning to me as the DMA engine is started
before the command is issued to the drive as explained in the next
comment.

-- 
tejun
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