usb: host: xhci-ring: don't need to clear interrupt pending for MSI enabled hcd

According to xHCI spec Figure 30: Interrupt Throttle Flow Diagram

	If PCI Message Signaled Interrupts (MSI or MSI-X) are enabled,
       	then the assertion of the Interrupt Pending (IP) flag in Figure 30
       	generates a PCI Dword write. The IP flag is automatically cleared
       	by the completion of the PCI write.

the MSI enabled HCs don't need to clear interrupt pending bit, but
hcd->irq = 0 doesn't equal to MSI enabled HCD. At some Dual-role
controller software designs, it sets hcd->irq as 0 to avoid HCD
requesting interrupt, and they want to decide when to call usb_hcd_irq
by software.
Signed-off-by: NPeter Chen <peter.chen@nxp.com>
Signed-off-by: NMathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-ring.c

@@ -2707,12 +2707,9 @@ irqreturn_t xhci_irq(struct usb_hcd *hcd)
 	 */
 	status |= STS_EINT;
 	writel(status, &xhci->op_regs->status);
-	/* FIXME when MSI-X is supported and there are multiple vectors */
-	/* Clear the MSI-X event interrupt status */
 
-	if (hcd->irq) {
+	if (!hcd->msi_enabled) {
 		u32 irq_pending;
-		/* Acknowledge the PCI interrupt */
 		irq_pending = readl(&xhci->ir_set->irq_pending);
 		irq_pending |= IMAN_IP;
 		writel(irq_pending, &xhci->ir_set->irq_pending);

drivers/usb/host/xhci.c

@@ -359,9 +359,10 @@ static int xhci_try_enable_msi(struct usb_hcd *hcd)
 		/* fall back to msi*/
 		ret = xhci_setup_msi(xhci);
 
-	if (!ret)
-		/* hcd->irq is 0, we have MSI */
+	if (!ret) {
+		hcd->msi_enabled = 1;
 		return 0;
+	}
 
 	if (!pdev->irq) {
 		xhci_err(xhci, "No msi-x/msi found and no IRQ in BIOS\n");

include/linux/usb/hcd.h

@@ -148,6 +148,7 @@ struct usb_hcd {
 	unsigned		rh_registered:1;/* is root hub registered? */
 	unsigned		rh_pollable:1;	/* may we poll the root hub? */
 	unsigned		msix_enabled:1;	/* driver has MSI-X enabled? */
+	unsigned		msi_enabled:1;	/* driver has MSI enabled? */
 	unsigned		remove_phy:1;	/* auto-remove USB phy */
 
 	/* The next flag is a stopgap, to be removed when all the HCDs