libata: ahci_start_engine compliant to AHCI spec

At the end of section 10.1 of AHCI spec (rev 1.3), it states

Software shall not set PxCMD.ST to 1 until it is determined that
a functoinal device is present on the port as determined by
PxTFD.STS.BSY=0, PxTFD.STS.DRQ=0 and PxSSTS.DET=3h

Even though most AHCI host controller works without this check,
specific controller will fail under this condition.
Signed-off-by: NJian Peng <jipeng2005@gmail.com>
Signed-off-by: NJeff Garzik <jgarzik@pobox.com>

drivers/ata/libahci.c

@@ -561,6 +561,27 @@ void ahci_start_engine(struct ata_port *ap)
 {
 	void __iomem *port_mmio = ahci_port_base(ap);
 	u32 tmp;
+	u8 status;
+
+	status = readl(port_mmio + PORT_TFDATA) & 0xFF;
+
+	/*
+	 * At end of section 10.1 of AHCI spec (rev 1.3), it states
+	 * Software shall not set PxCMD.ST to 1 until it is determined
+	 * that a functoinal device is present on the port as determined by
+	 * PxTFD.STS.BSY=0, PxTFD.STS.DRQ=0 and PxSSTS.DET=3h
+	 *
+	 * Even though most AHCI host controllers work without this check,
+	 * specific controller will fail under this condition
+	 */
+	if (status & (ATA_BUSY | ATA_DRQ))
+		return;
+	else {
+		ahci_scr_read(&ap->link, SCR_STATUS, &tmp);
+
+		if ((tmp & 0xf) != 0x3)
+			return;
+	}
 
 	/* start DMA */
 	tmp = readl(port_mmio + PORT_CMD);