ahci: Turn off DMA engines when there's no device attached

According to section 10.3.1 of the AHCI spec, PxCMD.ST must not be set
unless there's a device attached. Following this saves us a measurable
quantity of power and does not impair hotplug support. Based on a patch
by Kristen Carlson Accardi.
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/libahci.c

@@ -437,11 +437,29 @@ static int ahci_scr_write(struct ata_link *link, unsigned int sc_reg, u32 val)
 	return -EINVAL;
 }
 
+static int ahci_is_device_present(void __iomem *port_mmio)
+{
+	u8 status = readl(port_mmio + PORT_TFDATA) & 0xff;
+
+	/* Make sure PxTFD.STS.BSY and PxTFD.STS.DRQ are 0 */
+	if (status & (ATA_BUSY | ATA_DRQ))
+		return 0;
+
+	/* Make sure PxSSTS.DET is 3h */
+	status = readl(port_mmio + PORT_SCR_STAT) & 0xf;
+	if (status != 3)
+		return 0;
+	return 1;
+}
+
 void ahci_start_engine(struct ata_port *ap)
 {
 	void __iomem *port_mmio = ahci_port_base(ap);
 	u32 tmp;
 
+	if (!ahci_is_device_present(port_mmio))
+		return;
+
 	/* start DMA */
 	tmp = readl(port_mmio + PORT_CMD);
 	tmp |= PORT_CMD_START;