ahci: retry enabling AHCI a few times before spitting out WARN_ON()

Some chips need AHCI_EN set more than once to actually set it.  Try a
few times before giving up and spitting out WARN_ON().

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Peer Chen <pchen@nvidia.com>
Cc: Volker Armin Hemmann <volker.armin.hemmann@tu-clausthal.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/ahci.c

@@ -556,16 +556,27 @@ static inline void __iomem *ahci_port_base(struct ata_port *ap)
 
 static void ahci_enable_ahci(void __iomem *mmio)
 {
+	int i;
 	u32 tmp;
 
 	/* turn on AHCI_EN */
 	tmp = readl(mmio + HOST_CTL);
-	if (!(tmp & HOST_AHCI_EN)) {
+	if (tmp & HOST_AHCI_EN)
+		return;
+
+	/* Some controllers need AHCI_EN to be written multiple times.
+	 * Try a few times before giving up.
+	 */
+	for (i = 0; i < 5; i++) {
 		tmp |= HOST_AHCI_EN;
 		writel(tmp, mmio + HOST_CTL);
 		tmp = readl(mmio + HOST_CTL);	/* flush && sanity check */
-		WARN_ON(!(tmp & HOST_AHCI_EN));
+		if (tmp & HOST_AHCI_EN)
+			return;
+		msleep(10);
 	}
+
+	WARN_ON(1);
 }
 
 /**