[PATCH v2] usb: gadget: uvc: queue empty isoc requests if no video buffer is available

[Avichal Rakesh <arakesh@google.com>]

ISOC transfers expect a certain cadence of requests being queued. Not
keeping up with the expected rate of requests results in missed ISOC
transfers (EXDEV). The application layer is not required to produce video
frames to match this expectation, so uvc gadget driver must not rely
on data from application layer to maintain the ISOC cadence.

Currently, uvc gadget driver waits for new video buffer to become available
before queuing up usb requests. With this patch the gadget driver queues up
0 length usb requests whenever there are no video buffers available. The
USB controller's complete callback is used as the limiter for how quickly
the 0 length packets will be queued. Video buffers are still queued as
soon as they become available.

Link: https://lore.kernel.org/CAMHf4WKbi6KBPQztj9FA4kPvESc1fVKrC8G73-cs6tTeQby9=w@mail.gmail.com/
Signed-off-by: Avichal Rakesh <arakesh@google.com>
---
Changelog:
v2:
  - Updated commit message to make it clear that userspace application is not
    required to match the ISOC rate.
  - Styling and comment revision based on review


 drivers/usb/gadget/function/uvc_video.c | 50 +++++++++++++++++++------
 1 file changed, 39 insertions(+), 11 deletions(-)

diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
index dd1c6b2ca7c6..91af3b1ef0d4 100644
--- a/drivers/usb/gadget/function/uvc_video.c
+++ b/drivers/usb/gadget/function/uvc_video.c
@@ -382,9 +382,12 @@ static void uvcg_video_pump(struct work_struct *work)
 {
 	struct uvc_video *video = container_of(work, struct uvc_video, pump);
 	struct uvc_video_queue *queue = &video->queue;
+	/* video->max_payload_size is only set when using bulk transfer */
+	bool is_bulk = video->max_payload_size;
 	struct usb_request *req = NULL;
 	struct uvc_buffer *buf;
 	unsigned long flags;
+	bool buf_done;
 	int ret;

 	while (video->ep->enabled) {
@@ -408,20 +411,47 @@ static void uvcg_video_pump(struct work_struct *work)
 		 */
 		spin_lock_irqsave(&queue->irqlock, flags);
 		buf = uvcg_queue_head(queue);
-		if (buf == NULL) {
+
+		if (buf != NULL) {
+			video->encode(req, video, buf);
+			buf_done = buf->state == UVC_BUF_STATE_DONE;
+		} else if (!(queue->flags & UVC_QUEUE_DISCONNECTED) && !is_bulk) {
+			/*
+			 * No video buffer available; the queue is still connected and
+			 * we're transferring over ISOC. Queue a 0 length request to
+			 * prevent missed ISOC transfers.
+			 */
+			req->length = 0;
+			buf_done = false;
+		} else {
+			/*
+			 * Either the queue has been disconnected or no video buffer
+			 * available for bulk transfer. Either way, stop processing
+			 * further.
+			 */
 			spin_unlock_irqrestore(&queue->irqlock, flags);
 			break;
 		}

-		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.
+		 * With USB3 handling more requests at a higher speed, we can't
+		 * afford to generate an interrupt for every request. Decide to
+		 * interrupt:
+		 *
+		 * - When no more requests are available in the free queue, as
+		 *   this may be our last chance to refill the endpoint's
+		 *   request queue.
+		 *
+		 * - When this is request is the last request for the video
+		 *   buffer, as we want to start sending the next video buffer
+		 *   ASAP in case it doesn't get started already in the next
+		 *   iteration of this loop.
+		 *
+		 * - Four times over the length of the requests queue (as
+		 *   indicated by video->uvc_num_requests), as a trade-off
+		 *   between latency and interrupt load.
 		 */
-		if (list_empty(&video->req_free) ||
-		    buf->state == UVC_BUF_STATE_DONE ||
+		if (list_empty(&video->req_free) || buf_done ||
 		    !(video->req_int_count %
 		       DIV_ROUND_UP(video->uvc_num_requests, 4))) {
 			video->req_int_count = 0;
@@ -441,8 +471,7 @@ static void uvcg_video_pump(struct work_struct *work)

 		/* Endpoint now owns the request */
 		req = NULL;
-		if (buf->state != UVC_BUF_STATE_DONE)
-			video->req_int_count++;
+		video->req_int_count++;
 	}

 	if (!req)
@@ -527,4 +556,3 @@ int uvcg_video_init(struct uvc_video *video, struct uvc_device *uvc)
 			V4L2_BUF_TYPE_VIDEO_OUTPUT, &video->mutex);
 	return 0;
 }
-
--
2.41.0.rc0.172.g3f132b7071-goog