usb: gadget: uvc_video: add worker to handle the frame pumping

This patch changes the function uvc_video_pump to be a separate
scheduled worker. This way the completion handler of each usb request
and every direct caller of the pump has only to schedule the worker
instead of doing the request handling by itself.

Moving the request handling to one thread solves the locking problems
between the three queueing cases in the completion handler, v4l2_qbuf
and video_enable.

Many drivers handle the completion handlers directly in their interrupt
handlers. This patch also reduces the workload on each interrupt.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/gadget/function/uvc.h

@@ -77,6 +77,8 @@ struct uvc_video {
 	struct uvc_device *uvc;
 	struct usb_ep *ep;
 
+	struct work_struct pump;
+
 	/* Frame parameters */
 	u8 bpp;
 	u32 fcc;

drivers/usb/gadget/function/uvc_v4l2.c

@@ -169,7 +169,9 @@ uvc_v4l2_qbuf(struct file *file, void *fh, struct v4l2_buffer *b)
 	if (ret < 0)
 		return ret;
 
-	return uvcg_video_pump(video);
+	schedule_work(&video->pump);
+
+	return ret;
 }
 
 static int

drivers/usb/gadget/function/uvc_video.c

@@ -142,44 +142,12 @@ static int uvcg_video_ep_queue(struct uvc_video *video, struct usb_request *req)
 	return ret;
 }
 
-/*
- * I somehow feel that synchronisation won't be easy to achieve here. We have
- * three events that control USB requests submission:
- *
- * - USB request completion: the completion handler will resubmit the request
- *   if a video buffer is available.
- *
- * - USB interface setting selection: in response to a SET_INTERFACE request,
- *   the handler will start streaming if a video buffer is available and if
- *   video is not currently streaming.
- *
- * - V4L2 buffer queueing: the driver will start streaming if video is not
- *   currently streaming.
- *
- * Race conditions between those 3 events might lead to deadlocks or other
- * nasty side effects.
- *
- * The "video currently streaming" condition can't be detected by the irqqueue
- * being empty, as a request can still be in flight. A separate "queue paused"
- * flag is thus needed.
- *
- * The paused flag will be set when we try to retrieve the irqqueue head if the
- * queue is empty, and cleared when we queue a buffer.
- *
- * The USB request completion handler will get the buffer at the irqqueue head
- * under protection of the queue spinlock. If the queue is empty, the streaming
- * paused flag will be set. Right after releasing the spinlock a userspace
- * application can queue a buffer. The flag will then cleared, and the ioctl
- * handler will restart the video stream.
- */
 static void
 uvc_video_complete(struct usb_ep *ep, struct usb_request *req)
 {
 	struct uvc_video *video = req->context;
 	struct uvc_video_queue *queue = &video->queue;
-	struct uvc_buffer *buf;
 	unsigned long flags;
-	int ret;
 
 	switch (req->status) {
 	case 0:
@@ -188,39 +156,20 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req)
 	case -ESHUTDOWN:	/* disconnect from host. */
 		uvcg_dbg(&video->uvc->func, "VS request cancelled.\n");
 		uvcg_queue_cancel(queue, 1);
-		goto requeue;
+		break;
 
 	default:
 		uvcg_info(&video->uvc->func,
 			  "VS request completed with status %d.\n",
 			  req->status);
 		uvcg_queue_cancel(queue, 0);
-		goto requeue;
 	}
 
-	spin_lock_irqsave(&video->queue.irqlock, flags);
-	buf = uvcg_queue_head(&video->queue);
-	if (buf == NULL) {
-		spin_unlock_irqrestore(&video->queue.irqlock, flags);
-		goto requeue;
-	}
-
-	video->encode(req, video, buf);
-
-	ret = uvcg_video_ep_queue(video, req);
-	spin_unlock_irqrestore(&video->queue.irqlock, flags);
-
-	if (ret < 0) {
-		uvcg_queue_cancel(queue, 0);
-		goto requeue;
-	}
-
-	return;
-
-requeue:
 	spin_lock_irqsave(&video->req_lock, flags);
 	list_add_tail(&req->list, &video->req_free);
 	spin_unlock_irqrestore(&video->req_lock, flags);
+
+	schedule_work(&video->pump);
 }
 
 static int
@@ -294,26 +243,23 @@ uvc_video_alloc_requests(struct uvc_video *video)
  * This function fills the available USB requests (listed in req_free) with
  * video data from the queued buffers.
  */
-int uvcg_video_pump(struct uvc_video *video)
+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;
 	struct usb_request *req;
 	struct uvc_buffer *buf;
 	unsigned long flags;
 	int ret;
 
-	/* FIXME TODO Race between uvcg_video_pump and requests completion
-	 * handler ???
-	 */
-
 	while (1) {
 		/* Retrieve the first available USB request, protected by the
 		 * request lock.
 		 */
 		spin_lock_irqsave(&video->req_lock, flags);
 		if (list_empty(&video->req_free)) {
 			spin_unlock_irqrestore(&video->req_lock, flags);
-			return 0;
+			return;
 		}
 		req = list_first_entry(&video->req_free, struct usb_request,
 					list);
@@ -345,7 +291,7 @@ int uvcg_video_pump(struct uvc_video *video)
 	spin_lock_irqsave(&video->req_lock, flags);
 	list_add_tail(&req->list, &video->req_free);
 	spin_unlock_irqrestore(&video->req_lock, flags);
-	return 0;
+	return;
 }
 
 /*
@@ -363,6 +309,9 @@ int uvcg_video_enable(struct uvc_video *video, int enable)
 	}
 
 	if (!enable) {
+		cancel_work_sync(&video->pump);
+		uvcg_queue_cancel(&video->queue, 0);
+
 		for (i = 0; i < UVC_NUM_REQUESTS; ++i)
 			if (video->req[i])
 				usb_ep_dequeue(video->ep, video->req[i]);
@@ -384,7 +333,9 @@ int uvcg_video_enable(struct uvc_video *video, int enable)
 	} else
 		video->encode = uvc_video_encode_isoc;
 
-	return uvcg_video_pump(video);
+	schedule_work(&video->pump);
+
+	return ret;
 }
 
 /*
@@ -394,6 +345,7 @@ int uvcg_video_init(struct uvc_video *video, struct uvc_device *uvc)
 {
 	INIT_LIST_HEAD(&video->req_free);
 	spin_lock_init(&video->req_lock);
+	INIT_WORK(&video->pump, uvcg_video_pump);
 
 	video->uvc = uvc;
 	video->fcc = V4L2_PIX_FMT_YUYV;

drivers/usb/gadget/function/uvc_video.h

@@ -14,8 +14,6 @@
 
 struct uvc_video;
 
-int uvcg_video_pump(struct uvc_video *video);
-
 int uvcg_video_enable(struct uvc_video *video, int enable);
 
 int uvcg_video_init(struct uvc_video *video, struct uvc_device *uvc);