usb: gadget: uvc: decrease the interrupt load to a quarter

With usb3 we handle many more requests. Decrease the interrupt load by
only enabling the interrupt every quarter of the allocated requests.

Reviewed-by: Paul Elder <[e-mail]>
Signed-off-by: Michael Grzeschik <[e-mail]>

--
v1 -> v2: - edited patch description
- removed extra parantheses
- added a comment for the logic
- using unsigned int instead of int
- reinitializing req_int_count in uvcg_video_enable
v2 -> v3: -
Link: https://lore.kernel.org/r/[e-mail]

Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/function/uvc.h

···
 	struct list_head req_free;
 	spinlock_t req_lock;
 
+	unsigned int req_int_count;
+
 	void (*encode) (struct usb_request *req, struct uvc_video *video,
 			struct uvc_buffer *buf);
 

drivers/usb/gadget/function/uvc_video.c

···
 
 		video->encode(req, video, buf);
 
+		/* With usb3 we have more requests. This will decrease the
+		 * interrupt load to a quarter but also catches the corner
+		 * cases, which needs to be handled */
+		if (list_empty(&video->req_free) ||
+		    buf->state == UVC_BUF_STATE_DONE ||
+		    !(video->req_int_count %
+		       DIV_ROUND_UP(video->uvc_num_requests, 4))) {
+			video->req_int_count = 0;
+			req->no_interrupt = 0;
+		} else {
+			req->no_interrupt = 1;
+		}
+
 		/* Queue the USB request */
 		ret = uvcg_video_ep_queue(video, req);
 		spin_unlock_irqrestore(&queue->irqlock, flags);
···
 			uvcg_queue_cancel(queue, 0);
 			break;
 		}
+		video->req_int_count++;
 	}
 
 	spin_lock_irqsave(&video->req_lock, flags);
···
 	} else
 		video->encode = video->queue.use_sg ?
 			uvc_video_encode_isoc_sg : uvc_video_encode_isoc;
+
+	video->req_int_count = 0;
 
 	schedule_work(&video->pump);