libata: IORDY handling

I believe this version meets all Sergei's objections

Correct the logic for when we issue a set features for transfer mode

- If the device has IORDY and the controller has IORDY - set the mode
- If the device has IORDY and the controller does not - turn IORDY off
- If neither has IORDY do nothing
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/libata-core.c

@@ -4373,7 +4373,14 @@ static unsigned int ata_dev_set_xfermode(struct ata_device *dev)
 	tf.feature = SETFEATURES_XFER;
 	tf.flags |= ATA_TFLAG_ISADDR | ATA_TFLAG_DEVICE | ATA_TFLAG_POLLING;
 	tf.protocol = ATA_PROT_NODATA;
-	tf.nsect = dev->xfer_mode;
+	/* If we are using IORDY we must send the mode setting command */
+ 	if (ata_pio_need_iordy(dev))
+ 		tf.nsect = dev->xfer_mode;
+	/* If the device has IORDY and the controller does not - turn it off */
+ 	else if (ata_id_has_iordy(dev->id))
+ 		tf.nsect = 0x01;
+	else /* In the ancient relic department - skip all of this */
+		return 0;
 
 	err_mask = ata_exec_internal(dev, &tf, NULL, DMA_NONE, NULL, 0, 0);