libata-sff: Don't scan disabled ports when checking for legacy mode.

libata-sff.c checks for legacy mode by testing if both primary and
secondary ports on a controller are in legacy mode and selects legacy
if either one is. However on some south bridge chips (e.g AMD
SB600/SB700) the secondary port is not wired, and when it is disabled
by setting the disable bit in the PCI header it appears as a fixed
legacy port.

Prevent incorrect detection by not testing ports that are marked as
'dummy'

tj: Addressed Sergei's review points.  Other style edits.
Signed-off-by: Darren Stevens <darren@stevens-zone.net>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>

drivers/ata/libata-sff.c

@@ -2428,11 +2428,21 @@ int ata_pci_sff_activate_host(struct ata_host *host,
 		return rc;
 
 	if ((pdev->class >> 8) == PCI_CLASS_STORAGE_IDE) {
-		u8 tmp8, mask;
+		u8 tmp8, mask = 0;
 
-		/* TODO: What if one channel is in native mode ... */
+		/*
+		 * ATA spec says we should use legacy mode when one
+		 * port is in legacy mode, but disabled ports on some
+		 * PCI hosts appear as fixed legacy ports, e.g SB600/700
+		 * on which the secondary port is not wired, so
+		 * ignore ports that are marked as 'dummy' during
+		 * this check
+		 */
 		pci_read_config_byte(pdev, PCI_CLASS_PROG, &tmp8);
-		mask = (1 << 2) | (1 << 0);
+		if (!ata_port_is_dummy(host->ports[0]))
+			mask |= (1 << 0);
+		if (!ata_port_is_dummy(host->ports[1]))
+			mask |= (1 << 2);
 		if ((tmp8 & mask) != mask)
 			legacy_mode = 1;
 	}