usb: dwc3: gadget: lazily map requests for DMA

Some functions might want to have very, very long request queues. We
can't make any assumptions about how many requests we *are* able to
map, so instead of mapping requests early, let's map them late. This
way, functions can queue as many requests as they'd like but we won't
take DMA resources until they are needed.

Also, we can now stop processing requests when we run out of DMA
resources but still keep requests in the queue for late processing.
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -1099,6 +1099,17 @@ static void dwc3_prepare_trbs(struct dwc3_ep *dep)
 	}
 
 	list_for_each_entry_safe(req, n, &dep->pending_list, list) {
+		struct dwc3	*dwc = dep->dwc;
+		int		ret;
+
+		ret = usb_gadget_map_request_by_dev(dwc->sysdev, &req->request,
+						    dep->direction);
+		if (ret)
+			return;
+
+		req->sg			= req->request.sg;
+		req->num_pending_sgs	= req->request.num_mapped_sgs;
+
 		if (req->num_pending_sgs > 0)
 			dwc3_prepare_one_trb_sg(dep, req);
 		else
@@ -1205,7 +1216,7 @@ static void dwc3_gadget_start_isoc(struct dwc3 *dwc,
 static int __dwc3_gadget_ep_queue(struct dwc3_ep *dep, struct dwc3_request *req)
 {
 	struct dwc3		*dwc = dep->dwc;
-	int			ret;
+	int			ret = 0;
 
 	if (!dep->endpoint.desc) {
 		dev_err(dwc->dev, "%s: can't queue to disabled endpoint\n",
@@ -1229,14 +1240,6 @@ static int __dwc3_gadget_ep_queue(struct dwc3_ep *dep, struct dwc3_request *req)
 
 	trace_dwc3_ep_queue(req);
 
-	ret = usb_gadget_map_request_by_dev(dwc->sysdev, &req->request,
-					    dep->direction);
-	if (ret)
-		return ret;
-
-	req->sg			= req->request.sg;
-	req->num_pending_sgs	= req->request.num_mapped_sgs;
-
 	list_add_tail(&req->list, &dep->pending_list);
 
 	/*