ahci: implement skip_host_reset parameter

Under certain circumstances (SSP turned off by the BIOS) and for
debugging purposes, skipping global controller reset is helpful.  Add
a kernel parameter for it.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/ahci.c

@@ -49,6 +49,10 @@
 #define DRV_NAME	"ahci"
 #define DRV_VERSION	"3.0"
 
+static int ahci_skip_host_reset;
+module_param_named(skip_host_reset, ahci_skip_host_reset, int, 0444);
+MODULE_PARM_DESC(skip_host_reset, "skip global host reset (0=don't skip, 1=skip)");
+
 static int ahci_enable_alpm(struct ata_port *ap,
 		enum link_pm policy);
 static void ahci_disable_alpm(struct ata_port *ap);
@@ -1088,6 +1092,7 @@ static int ahci_reset_controller(struct ata_host *host)
 	ahci_enable_ahci(mmio);
 
 	/* global controller reset */
+	if (!ahci_skip_host_reset) {
 		tmp = readl(mmio + HOST_CTL);
 		if ((tmp & HOST_RESET) == 0) {
 			writel(tmp | HOST_RESET, mmio + HOST_CTL);
@@ -1109,8 +1114,13 @@ static int ahci_reset_controller(struct ata_host *host)
 		/* turn on AHCI mode */
 		ahci_enable_ahci(mmio);
 
-	/* some registers might be cleared on reset.  restore initial values */
+		/* Some registers might be cleared on reset.  Restore
+		 * initial values.
+		 */
 		ahci_restore_initial_config(host);
+	} else
+		dev_printk(KERN_INFO, host->dev,
+			   "skipping global host reset\n");
 
 	if (pdev->vendor == PCI_VENDOR_ID_INTEL) {
 		u16 tmp16;