[libata] preparation for writeback caching support

* bug fix: make sure 'nsect' member of struct ata_queued_cmd is
initialized each time a cmd is re-used.  Only affects PIO data xfers,
which nobody uses.
* slightly change the way a device's flags are printed out.  currently
the only flag is 'lba48', but soon 'wcache' will appear also.
* add WB-cache-related constants and macros to linux/ata.h

drivers/scsi/libata-core.c

@@ -1156,11 +1156,11 @@ static void ata_dev_identify(struct ata_port *ap, unsigned int device)
 		ap->host->max_cmd_len = 16;
 
 		/* print device info to dmesg */
-		printk(KERN_INFO "ata%u: dev %u ATA, max %s, %Lu sectors%s\n",
+		printk(KERN_INFO "ata%u: dev %u ATA, max %s, %Lu sectors: %s\n",
 		       ap->id, device,
 		       ata_udma_string(udma_modes),
 		       (unsigned long long)dev->n_sectors,
-		       dev->flags & ATA_DFLAG_LBA48 ? " (lba48)" : "");
+		       dev->flags & ATA_DFLAG_LBA48 ? " lba48" : "");
 	}
 
 	/* ATAPI-specific feature tests */
@@ -2311,6 +2311,7 @@ struct ata_queued_cmd *ata_qc_new_init(struct ata_port *ap,
 		qc->ap = ap;
 		qc->dev = dev;
 		qc->cursect = qc->cursg = qc->cursg_ofs = 0;
+		qc->nsect = 0;
 
 		ata_tf_init(ap, &qc->tf, dev->devno);
 

include/linux/ata.h

@@ -104,6 +104,8 @@ enum {
 
 	/* ATA device commands */
 	ATA_CMD_EDD		= 0x90,	/* execute device diagnostic */
+	ATA_CMD_FLUSH		= 0xE7,
+	ATA_CMD_FLUSH_EXT	= 0xEA,
 	ATA_CMD_ID_ATA		= 0xEC,
 	ATA_CMD_ID_ATAPI	= 0xA1,
 	ATA_CMD_READ		= 0xC8,
@@ -196,7 +198,9 @@ struct ata_taskfile {
 };
 
 #define ata_id_is_ata(dev)	(((dev)->id[0] & (1 << 15)) == 0)
+#define ata_id_wcache_enabled(dev) ((dev)->id[85] & (1 << 5))
 #define ata_id_has_lba48(dev)	((dev)->id[83] & (1 << 10))
+#define ata_id_has_wcache(dev)	((dev)->id[82] & (1 << 5))
 #define ata_id_has_lba(dev)	((dev)->id[49] & (1 << 8))
 #define ata_id_has_dma(dev)	((dev)->id[49] & (1 << 9))
 #define ata_id_u32(dev,n)	\