libata: clear TF before IDENTIFYing

Some devices chock if Feature is not clear when IDENTIFY is issued.
Set ATA_TFLAG_ISADDR | ATA_TFLAG_DEVICE for IDENTIFY such that whole
TF is cleared when reading ID data.

Kudos to Art Haas for testing various futile patches over several
months and Mark Lord for pointing out the fix.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Art Haas <ahaas@airmail.net>
Cc: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/libata-core.c

@@ -1410,7 +1410,16 @@ int ata_dev_read_id(struct ata_device *dev, unsigned int *p_class,
 	}
 
 	tf.protocol = ATA_PROT_PIO;
-	tf.flags |= ATA_TFLAG_POLLING; /* for polling presence detection */
+
+	/* Some devices choke if TF registers contain garbage.  Make
+	 * sure those are properly initialized.
+	 */
+	tf.flags |= ATA_TFLAG_ISADDR | ATA_TFLAG_DEVICE;
+
+	/* Device presence detection is unreliable on some
+	 * controllers.  Always poll IDENTIFY if available.
+	 */
+	tf.flags |= ATA_TFLAG_POLLING;
 
 	err_mask = ata_exec_internal(dev, &tf, NULL, DMA_FROM_DEVICE,
 				     id, sizeof(id[0]) * ATA_ID_WORDS);