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

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Message-ID: <20080623090130.GF1850@elf.ucw.cz>
Date:	Mon, 23 Jun 2008 11:01:31 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Andreas Schwab <schwab@...e.de>,
	kernel list <linux-kernel@...r.kernel.org>,
	benh@...nel.crashing.org, jgarzik@...ox.com,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Trivial patch monkey <trivial@...nel.org>
Subject: Re: sata_svw data corruption, strange problems

On Mon 2008-06-23 17:56:32, Tejun Heo wrote:
> Pavel Machek wrote:
> > On Mon 2008-06-23 10:39:40, Andreas Schwab wrote:
> >> Pavel Machek <pavel@...e.cz> writes:
> >>
> >>> +	   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
> >> s/returning/resulting/ ?
> > 
> > Fix thinko in sata_svw comment.
> > 
> > Signed-off-by: Pavel Machek <pavel@...e.cz>
> 
> Please collapse into one patch.  Thanks.

--- 

Clarify comment in sata_svw.c.

Signed-off-by: Pavel Machek <pavel@...e.cz>

diff --git a/drivers/ata/sata_svw.c b/drivers/ata/sata_svw.c
index 16aa683..fb13b82 100644
--- a/drivers/ata/sata_svw.c
+++ b/drivers/ata/sata_svw.c
@@ -253,21 +253,29 @@ static void k2_bmdma_start_mmio(struct a
 	/* start host DMA transaction */
 	dmactl = readb(mmio + ATA_DMA_CMD);
 	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*/
+	/* This works around possible data corruption.
+
+	   On 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)
 		ap->ops->sff_exec_command(ap, &qc->tf);
 }


-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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