usb: dwc3: gadget: always enable CSP

CSP bit of TRB Control is useful for protocols such
CDC EEM/ECM/NCM where we're transferring in blocks
of MTU-sized requests (usually MTU is 1500 bytes).

We know we will always have a short packet after two
(for HS) wMaxPacketSize packets and, usually, we
will have a long(-ish) queue of requests (for our
g_ether gadget, we have at least 10
requests).

Instead of always stopping the queue processing to
interrupt, giveback and restart, let's tell dwc3 to
interrupt but continue processing following request
if we have anything already pending in the queue.

This gave me a considerable improvement of 40% on my
test setup.
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -726,6 +726,9 @@ static void dwc3_prepare_one_trb(struct dwc3_ep *dep,
 			trb->ctrl = DWC3_TRBCTL_ISOCHRONOUS_FIRST;
 		else
 			trb->ctrl = DWC3_TRBCTL_ISOCHRONOUS;
+
+		/* always enable Interrupt on Missed ISOC */
+		trb->ctrl |= DWC3_TRB_CTRL_ISP_IMI;
 		break;
 
 	case USB_ENDPOINT_XFER_BULK:
@@ -740,15 +743,14 @@ static void dwc3_prepare_one_trb(struct dwc3_ep *dep,
 		BUG();
 	}
 
-	if (!req->request.no_interrupt && !chain)
-		trb->ctrl |= DWC3_TRB_CTRL_IOC;
+	/* always enable Continue on Short Packet */
+	trb->ctrl |= DWC3_TRB_CTRL_CSP;
 
-	if (usb_endpoint_xfer_isoc(dep->endpoint.desc)) {
-		trb->ctrl |= DWC3_TRB_CTRL_ISP_IMI;
-		trb->ctrl |= DWC3_TRB_CTRL_CSP;
-	} else if (last) {
+	if (!req->request.no_interrupt)
+		trb->ctrl |= DWC3_TRB_CTRL_IOC | DWC3_TRB_CTRL_ISP_IMI;
+
+	if (last)
 		trb->ctrl |= DWC3_TRB_CTRL_LST;
-	}
 
 	if (chain)
 		trb->ctrl |= DWC3_TRB_CTRL_CHN;