sata_mv: warn on PIO with multiple DRQs

Chip errata sometimes prevents reliable use of PIO commands which involve
more than a single DRQ (data request).  In normal operation, libata should
not generate such PIO commands (uses DMA instead), but they could be sent
in via SG_IO from userspace.

A full workaround might be to break up such commands into sequences
of single DRQ ones, but that's just way too complex for something
that doesn't normally happen in real life.

So, allow the attempt (it often works, despite the errata),
but log the event for reference when somebody screams.
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/sata_mv.c

@@ -1595,6 +1595,24 @@ static unsigned int mv_qc_issue(struct ata_queued_cmd *qc)
 
 	if ((qc->tf.protocol != ATA_PROT_DMA) &&
 	    (qc->tf.protocol != ATA_PROT_NCQ)) {
+		static int limit_warnings = 10;
+		/*
+		 * Errata SATA#16, SATA#24: warn if multiple DRQs expected.
+		 *
+		 * Someday, we might implement special polling workarounds
+		 * for these, but it all seems rather unnecessary since we
+		 * normally use only DMA for commands which transfer more
+		 * than a single block of data.
+		 *
+		 * Much of the time, this could just work regardless.
+		 * So for now, just log the incident, and allow the attempt.
+		 */
+		if (limit_warnings && (qc->nbytes / qc->sect_size) > 1) {
+			--limit_warnings;
+			ata_link_printk(qc->dev->link, KERN_WARNING, DRV_NAME
+					": attempting PIO w/multiple DRQ: "
+					"this may fail due to h/w errata\n");
+		}
 		/*
 		 * We're about to send a non-EDMA capable command to the
 		 * port.  Turn off EDMA so there won't be problems accessing