Re: [PATCH] scsi: libsas: Reset num_scatter if libata mark qc as NODATA

[John Garry]

On 18/03/2021 00:24, Jolly Shah wrote:
> Hi John,
>
> Thanks for the review.
>
>
> On Wed, Mar 17, 2021 at 4:44 AM John Garry <john.garry@xxxxxxxxxx> wrote:
> > On 16/03/2021 19:39, Jolly Shah wrote:
> > > When the cache_type for the scsi device is changed, the scsi layer
> > > issues a MODE_SELECT command. The caching mode details are communicated
> > > via a request buffer associated with the scsi command with data
> > > direction set as DMA_TO_DEVICE (scsi_mode_select). When this command
> > > reaches the libata layer, as a part of generic initial setup, libata
> > > layer sets up the scatterlist for the command using the scsi command
> > > (ata_scsi_qc_new). This command is then translated by the libata layer
> > > into ATA_CMD_SET_FEATURES (ata_scsi_mode_select_xlat). The libata layer
> > > treats this as a non data command (ata_mselect_caching), since it only
> > > needs an ata taskfile to pass the caching on/off information to the
> > > device. It does not need the scatterlist that has been setup, so it does
> > > not perform dma_map_sg on the scatterlist (ata_qc_issue).
> >
> > So if we don't perform the dma_map_sg() on the sgl at this point, then
> > it seems to me that we should not perform for_each_sg() on it either,
> > right? That is still what happens in sas_ata_qc_issue() in this case.

Hi Jolly Shah,

> Yes that's right. To avoid that, I can add elseif block for
> ATA_PROT_NODATA before for_each_sg() is performed. Currently there's
> existing code block for ATA_PROT_NODATA after for_each_sg()  is
> performed,
> reused that to reset num_scatter. Please suggest.

How about just combine the 2x if-else statements into 1x if-elif-else 
statement, like:


if (ata_is_atapi(qc->tf.protocol)) {
	memcpy(task->ata_task.atapi_packet, qc->cdb, qc->dev->cdb_len);
	task->total_xfer_len = qc->nbytes;
	task->num_scatter = qc->n_elem;
	task->data_dir = qc->dma_dir;
} else if (qc->tf.protocol == ATA_PROT_NODATA) {
	task->data_dir = DMA_NONE;
} else {
	for_each_sg(qc->sg, sg, qc->n_elem, si)
		xfer += sg_dma_len(sg);

	task->total_xfer_len = xfer;
	task->num_scatter = si;
	task->data_dir = qc->dma_dir;
}

> > > Unfortunately,
> > > when this command reaches the libsas layer(sas_ata_qc_issue), libsas
> > > layer sees it as a non data command with a scatterlist. It cannot
> > > extract the correct dma length, since the scatterlist has not been
> > > mapped with dma_map_sg for a DMA operation. When this partially
> > > constructed SAS task reaches pm80xx LLDD, it results in below warning.
> > >
> > > "pm80xx_chip_sata_req 6058: The sg list address
> > > start_addr=0x0000000000000000 data_len=0x0end_addr_high=0xffffffff
> > > end_addr_low=0xffffffff has crossed 4G boundary"
> > >
> > > This patch assigns appropriate value to  num_sectors for ata non data
> > > commands.
> > >
> > > Signed-off-by: Jolly Shah <jollys@xxxxxxxxxx>
> > > ---
> > >    drivers/scsi/libsas/sas_ata.c | 6 ++++--
> > >    1 file changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/scsi/libsas/sas_ata.c b/drivers/scsi/libsas/sas_ata.c
> > > index 024e5a550759..94ec08cebbaa 100644
> > > --- a/drivers/scsi/libsas/sas_ata.c
> > > +++ b/drivers/scsi/libsas/sas_ata.c
> > > @@ -209,10 +209,12 @@ static unsigned int sas_ata_qc_issue(struct ata_queued_cmd *qc)
> > >                task->num_scatter = si;
> > >        }
> > >
> > > -     if (qc->tf.protocol == ATA_PROT_NODATA)
> > > +     if (qc->tf.protocol == ATA_PROT_NODATA) {
> > >                task->data_dir = DMA_NONE;
> > > -     else
> > > +             task->num_scatter = 0;
> >
> > task->num_scatter has already been set in this function. Best not set it
> > twice.
>
> Sure. Please suggest if I should update patch to above suggested
> approach. That will avoid setting num_scatter twice.

Thanks,
John

BTW, could we add a fixes tag?