usb: dwc3: gadget: Handle TRB index 0 when full or empty

If the trb->enqueue == trb->dequeue, then it could be full or empty.
This could also happen at TRB index 0, so modify the check to handle
that condition. At index 0, the previous TRB is the one just before the
link TRB.
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -842,6 +842,25 @@ static void dwc3_prepare_one_trb(struct dwc3_ep *dep,
 	trace_dwc3_prepare_trb(dep, trb);
 }
 
+/**
+ * dwc3_ep_prev_trb() - Returns the previous TRB in the ring
+ * @dep: The endpoint with the TRB ring
+ * @index: The index of the current TRB in the ring
+ *
+ * Returns the TRB prior to the one pointed to by the index. If the
+ * index is 0, we will wrap backwards, skip the link TRB, and return
+ * the one just before that.
+ */
+static struct dwc3_trb *dwc3_ep_prev_trb(struct dwc3_ep *dep, u8 index)
+{
+	if (!index)
+		index = DWC3_TRB_NUM - 2;
+	else
+		index = dep->trb_enqueue - 1;
+
+	return &dep->trb_pool[index];
+}
+
 static u32 dwc3_calc_trbs_left(struct dwc3_ep *dep)
 {
 	struct dwc3_trb		*tmp;
@@ -855,12 +874,9 @@ static u32 dwc3_calc_trbs_left(struct dwc3_ep *dep)
 	 * more transfers in our ring.
 	 */
 	if (dep->trb_enqueue == dep->trb_dequeue) {
-		/* If we're full, enqueue/dequeue are > 0 */
-		if (dep->trb_enqueue) {
-			tmp = &dep->trb_pool[dep->trb_enqueue - 1];
+		tmp = dwc3_ep_prev_trb(dep, dep->trb_enqueue);
 		if (tmp->ctrl & DWC3_TRB_CTRL_HWO)
 			return 0;
-		}
 
 		return DWC3_TRB_NUM - 1;
 	}