USB: xhci - fix math in xhci_get_endpoint_interval()

When parsing exponent-expressed intervals we subtract 1 from the
value and then expect it to match with original + 1, which is
highly unlikely, and we end with frequent spew:

	usb 3-4: ep 0x83 - rounding interval to 512 microframes

Also, parsing interval for fullspeed isochronous endpoints was
incorrect - according to USB spec they use exponent-based
intervals (but xHCI spec claims frame-based intervals). I trust
USB spec more, especially since USB core agrees with it.

This should be queued for stable kernels back to 2.6.31.
Reviewed-by: Micah Elizabeth Scott <micah@vmware.com>
Signed-off-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@kernel.org

drivers/usb/host/xhci-mem.c

@@ -974,6 +974,47 @@ int xhci_setup_addressable_virt_dev(struct xhci_hcd *xhci, struct usb_device *ud
 	return 0;
 }
 
+/*
+ * Convert interval expressed as 2^(bInterval - 1) == interval into
+ * straight exponent value 2^n == interval.
+ *
+ */
+static unsigned int xhci_parse_exponent_interval(struct usb_device *udev,
+		struct usb_host_endpoint *ep)
+{
+	unsigned int interval;
+
+	interval = clamp_val(ep->desc.bInterval, 1, 16) - 1;
+	if (interval != ep->desc.bInterval - 1)
+		dev_warn(&udev->dev,
+			 "ep %#x - rounding interval to %d microframes\n",
+			 ep->desc.bEndpointAddress,
+			 1 << interval);
+
+	return interval;
+}
+
+/*
+ * Convert bInterval expressed in frames (in 1-255 range) to exponent of
+ * microframes, rounded down to nearest power of 2.
+ */
+static unsigned int xhci_parse_frame_interval(struct usb_device *udev,
+		struct usb_host_endpoint *ep)
+{
+	unsigned int interval;
+
+	interval = fls(8 * ep->desc.bInterval) - 1;
+	interval = clamp_val(interval, 3, 10);
+	if ((1 << interval) != 8 * ep->desc.bInterval)
+		dev_warn(&udev->dev,
+			 "ep %#x - rounding interval to %d microframes, ep desc says %d microframes\n",
+			 ep->desc.bEndpointAddress,
+			 1 << interval,
+			 8 * ep->desc.bInterval);
+
+	return interval;
+}
+
 /* Return the polling or NAK interval.
  *
  * The polling interval is expressed in "microframes".  If xHCI's Interval field
@@ -991,45 +1032,38 @@ static unsigned int xhci_get_endpoint_interval(struct usb_device *udev,
 	case USB_SPEED_HIGH:
 		/* Max NAK rate */
 		if (usb_endpoint_xfer_control(&ep->desc) ||
-				usb_endpoint_xfer_bulk(&ep->desc))
+		    usb_endpoint_xfer_bulk(&ep->desc)) {
 			interval = ep->desc.bInterval;
+			break;
+		}
 		/* Fall through - SS and HS isoc/int have same decoding */
+
 	case USB_SPEED_SUPER:
 		if (usb_endpoint_xfer_int(&ep->desc) ||
 		    usb_endpoint_xfer_isoc(&ep->desc)) {
-			if (ep->desc.bInterval == 0)
-				interval = 0;
-			else
-				interval = ep->desc.bInterval - 1;
-			if (interval > 15)
-				interval = 15;
-			if (interval != ep->desc.bInterval + 1)
-				dev_warn(&udev->dev, "ep %#x - rounding interval to %d microframes\n",
-						ep->desc.bEndpointAddress, 1 << interval);
+			interval = xhci_parse_exponent_interval(udev, ep);
 		}
 		break;
-	/* Convert bInterval (in 1-255 frames) to microframes and round down to
-	 * nearest power of 2.
-	 */
+
 	case USB_SPEED_FULL:
+		if (usb_endpoint_xfer_int(&ep->desc)) {
+			interval = xhci_parse_exponent_interval(udev, ep);
+			break;
+		}
+		/*
+		 * Fall through for isochronous endpoint interval decoding
+		 * since it uses the same rules as low speed interrupt
+		 * endpoints.
+		 */
+
 	case USB_SPEED_LOW:
 		if (usb_endpoint_xfer_int(&ep->desc) ||
 		    usb_endpoint_xfer_isoc(&ep->desc)) {
-			interval = fls(8*ep->desc.bInterval) - 1;
-			if (interval > 10)
-				interval = 10;
-			if (interval < 3)
-				interval = 3;
-			if ((1 << interval) != 8*ep->desc.bInterval)
-				dev_warn(&udev->dev,
-						"ep %#x - rounding interval"
-						" to %d microframes, "
-						"ep desc says %d microframes\n",
-						ep->desc.bEndpointAddress,
-						1 << interval,
-						8*ep->desc.bInterval);
+
+			interval = xhci_parse_frame_interval(udev, ep);
 		}
 		break;
+
 	default:
 		BUG();
 	}