usb: dwc2: Add dwc2_gadget_dec_frame_num_by_one() function

Added dwc2_gadget_dec_frame_num_by_one() function in gadget.c.
This function will be used to calculate descriptor frame number field
value. For service interval mode frame number in descriptor should point
to last (u)frame in the interval.
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc2/gadget.c

@@ -122,6 +122,24 @@ static inline void dwc2_gadget_incr_frame_num(struct dwc2_hsotg_ep *hs_ep)
 	}
 }
 
+/**
+ * dwc2_gadget_dec_frame_num_by_one - Decrements the targeted frame number
+ *                                    by one.
+ * @hs_ep: The endpoint.
+ *
+ * This function used in service interval based scheduling flow to calculate
+ * descriptor frame number filed value. For service interval mode frame
+ * number in descriptor should point to last (u)frame in the interval.
+ *
+ */
+static inline void dwc2_gadget_dec_frame_num_by_one(struct dwc2_hsotg_ep *hs_ep)
+{
+	if (hs_ep->target_frame)
+		hs_ep->target_frame -= 1;
+	else
+		hs_ep->target_frame = DSTS_SOFFN_LIMIT;
+}
+
 /**
  * dwc2_hsotg_en_gsint - enable one or more of the general interrupt
  * @hsotg: The device state