[PATCH] ahci: enable prefetching for PACKET commands

Turn on AHCI_CMD_PREFETCH for PACKET commands.  This hints the
controller that it can prefetch the CDB and the PRD entries.  This
patch is originally from Jeff Garzik.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/scsi/ahci.c

@@ -66,6 +66,7 @@ enum {
 	AHCI_IRQ_ON_SG		= (1 << 31),
 	AHCI_CMD_ATAPI		= (1 << 5),
 	AHCI_CMD_WRITE		= (1 << 6),
+	AHCI_CMD_PREFETCH	= (1 << 7),
 	AHCI_CMD_RESET		= (1 << 8),
 	AHCI_CMD_CLR_BUSY	= (1 << 10),
 
@@ -631,7 +632,7 @@ static void ahci_qc_prep(struct ata_queued_cmd *qc)
 	if (qc->tf.flags & ATA_TFLAG_WRITE)
 		opts |= AHCI_CMD_WRITE;
 	if (is_atapi)
-		opts |= AHCI_CMD_ATAPI;
+		opts |= AHCI_CMD_ATAPI | AHCI_CMD_PREFETCH;
 
 	ahci_fill_cmd_slot(pp, opts);
 }