usb: gadget: uvc: only enqueue zero length requests in potential underrun

The complete handler will at least be called after 16 requests have
completed, but will still handle all finisher requests. Since we have
to maintain a costant filling in the isoc queue we ensure this by
adding zero length requests.

By counting the amount enqueued requests we can ensure that the queue is
never underrun and only need to get active if the queue is running
critical. This patch is setting 32 as the critical level, which
is twice the request amount that is needed to create interrupts.

To properly solve the amount of zero length requests that needs to
be held in the hardware after one interrupt needs to be measured
and depends on the runtime of the first enqueue run after the interrupt
triggered. For now we just use twice the amount of requests between an
interrupt.

Signed-off-by: Michael Grzeschik <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/function/uvc.h

···
 
 #define UVCG_REQUEST_HEADER_LEN			12
 
+#define UVCG_REQ_MAX_INT_COUNT			16
+#define UVCG_REQ_MAX_ZERO_COUNT			(2 * UVCG_REQ_MAX_INT_COUNT)
+
 /* ------------------------------------------------------------------------
  * Structures
  */
···
 
 	struct work_struct pump;
 	struct workqueue_struct *async_wq;
+
+	atomic_t queued;
 
 	/* Frame parameters */
 	u8 bpp;

drivers/usb/gadget/function/uvc_video.c

···
 		}
 	}
 
+	atomic_inc(&video->queued);
+
 	return ret;
 }
 
···
 		 */
 		if (list_empty(&video->req_free) || ureq->last_buf ||
 			!(video->req_int_count %
-			DIV_ROUND_UP(video->uvc_num_requests, 4))) {
+			min(DIV_ROUND_UP(video->uvc_num_requests, 4), UVCG_REQ_MAX_INT_COUNT))) {
 			video->req_int_count = 0;
 			req->no_interrupt = 0;
 		} else {
···
 	int ret = 0;
 
 	spin_lock_irqsave(&video->req_lock, flags);
+	atomic_dec(&video->queued);
 	if (!video->is_enabled) {
 		/*
 		 * When is_enabled is false, uvcg_video_disable() ensures
···
 		 * happen.
 		 */
 		queue_work(video->async_wq, &video->pump);
+	} else if (atomic_read(&video->queued) > UVCG_REQ_MAX_ZERO_COUNT) {
+		list_add_tail(&to_queue->list, &video->req_free);
+		/*
+		 * There is a new free request - wake up the pump.
+		 */
+		queue_work(video->async_wq, &video->pump);
+
+		spin_unlock_irqrestore(&video->req_lock, flags);
+
+		return;
 	}
 	/*
 	 * Queue to the endpoint. The actual queueing to ep will
···
 			uvc_video_encode_isoc_sg : uvc_video_encode_isoc;
 
 	video->req_int_count = 0;
+
+	atomic_set(&video->queued, 0);
 
 	uvc_video_ep_queue_initial_requests(video);
 	queue_work(video->async_wq, &video->pump);