libata: beef up iterators

There currently are the following looping constructs.

* __ata_port_for_each_link() for all available links
* ata_port_for_each_link() for edge links
* ata_link_for_each_dev() for all devices
* ata_link_for_each_dev_reverse() for all devices in reverse order

Now there's a need for looping construct which is similar to
__ata_port_for_each_link() but iterates over PMP links before the host
link.  Instead of adding another one with long name, do the following
cleanup.

* Implement and export ata_link_next() and ata_dev_next() which take
  @mode parameter and can be used to build custom loop.
* Implement ata_for_each_link() and ata_for_each_dev() which take
  looping mode explicitly.

The following iteration modes are implemented.

* ATA_LITER_EDGE		: loop over edge links
* ATA_LITER_HOST_FIRST		: loop over all links, host link first
* ATA_LITER_PMP_FIRST		: loop over all links, PMP links first

* ATA_DITER_ENABLED		: loop over enabled devices
* ATA_DITER_ENABLED_REVERSE	: loop over enabled devices in reverse order
* ATA_DITER_ALL			: loop over all devices
* ATA_DITER_ALL_REVERSE		: loop over all devices in reverse order

This change removes exlicit device enabledness checks from many loops
and makes it clear which ones are iterated over in which direction.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/pata_pdc2027x.c

@@ -406,23 +406,20 @@ static int pdc2027x_set_mode(struct ata_link *link, struct ata_device **r_failed
 	if (rc < 0)
 		return rc;
 
-	ata_link_for_each_dev(dev, link) {
-		if (ata_dev_enabled(dev)) {
+	ata_for_each_dev(dev, link, ENABLED) {
+		pdc2027x_set_piomode(ap, dev);
 
-			pdc2027x_set_piomode(ap, dev);
+		/*
+		 * Enable prefetch if the device support PIO only.
+		 */
+		if (dev->xfer_shift == ATA_SHIFT_PIO) {
+			u32 ctcr1 = ioread32(dev_mmio(ap, dev, PDC_CTCR1));
+			ctcr1 |= (1 << 25);
+			iowrite32(ctcr1, dev_mmio(ap, dev, PDC_CTCR1));
 
-			/*
-			 * Enable prefetch if the device support PIO only.
-			 */
-			if (dev->xfer_shift == ATA_SHIFT_PIO) {
-				u32 ctcr1 = ioread32(dev_mmio(ap, dev, PDC_CTCR1));
-				ctcr1 |= (1 << 25);
-				iowrite32(ctcr1, dev_mmio(ap, dev, PDC_CTCR1));
-
-				PDPRINTK("Turn on prefetch\n");
-			} else {
-				pdc2027x_set_dmamode(ap, dev);
-			}
+			PDPRINTK("Turn on prefetch\n");
+		} else {
+			pdc2027x_set_dmamode(ap, dev);
 		}
 	}
 	return 0;