Re: sata_svw data corruption, strange problems

[Benjamin Herrenschmidt]

On Mon, 2008-06-23 at 11:01 +0200, Pavel Machek wrote:
> 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@xxxxxxx> 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@xxxxxxx>
> > 
> > Please collapse into one patch.  Thanks.

Am I the only one to find Pavel variant almost as obscure as
the original one ? :-)

It should explain precisely what the workaround is. Ie. to start the
DMA there instead of where it normally is started which is the
bmdma_setup() function.

BTW. Tejun, I suppose that usually starting DMA after issuing the
command is a standard practice of legacy/sff type controllers ? Or it's
just because that's how linux did it until now ?

Ben.

> --- 
> 
> Clarify comment in sata_svw.c.
> 
> Signed-off-by: Pavel Machek <pavel@xxxxxxx>
> 
> 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);
>  }
> 
> 

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