usb: gadget: Introduce usb_request->is_last

To take advantage of DWC3 internal TRB prefetch and cache for
performance, inform the controller the last request with stream_id
before switching to a different stream transfer. This allows the
controller to maintain its transfer burst within the stream ID.

Introduce the usb-request is_last field to help inform the DWC3
controller of this.
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

include/linux/usb/gadget.h

@@ -42,6 +42,8 @@ struct usb_ep;
  * @num_mapped_sgs: number of SG entries mapped to DMA (internal)
  * @length: Length of that data
  * @stream_id: The stream id, when USB3.0 bulk streams are being used
+ * @is_last: Indicates if this is the last request of a stream_id before
+ *	switching to a different stream (required for DWC3 controllers).
  * @no_interrupt: If true, hints that no completion irq is needed.
  *	Helpful sometimes with deep request queues that are handled
  *	directly by DMA controllers.
@@ -104,6 +106,7 @@ struct usb_request {
 	unsigned		num_mapped_sgs;
 
 	unsigned		stream_id:16;
+	unsigned		is_last:1;
 	unsigned		no_interrupt:1;
 	unsigned		zero:1;
 	unsigned		short_not_ok:1;