scsi: libsas: Reset num_scatter if libata marks qc as NODATA

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()). 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 the following 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"

Update libsas to handle ATA non-data commands separately so num_scatter and
total_xfer_len remain 0.

Link: https://lore.kernel.org/r/20210318225632.2481291-1-jollys@google.com
Fixes: 53de092 ("scsi: libsas: Set data_dir as DMA_NONE if libata marks qc as NODATA")
Tested-by: Luo Jiaxing <luojiaxing@huawei.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jolly Shah <jollys@google.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: Jolly Shah

drivers/scsi/libsas/sas_ata.c

@@ -201,18 +201,17 @@ static unsigned int sas_ata_qc_issue(struct ata_queued_cmd *qc)
 		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;
-	}
-
-	if (qc->tf.protocol == ATA_PROT_NODATA)
-		task->data_dir = DMA_NONE;
-	else
 		task->data_dir = qc->dma_dir;
+	}
 	task->scatter = qc->sg;
 	task->ata_task.retry_count = 1;
 	task->task_state_flags = SAS_TASK_STATE_PENDING;