media: uvcvideo: queue: Support asynchronous buffer handling

The buffer queue interface currently operates sequentially, processing
buffers after they have fully completed.

In preparation for supporting parallel tasks operating on the buffers,
we will need to support buffers being processed on multiple CPUs.

Adapt the uvc_queue_next_buffer() such that a reference count tracks the
active use of the buffer, returning the buffer to the VB2 stack at
completion.
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/usb/uvc/uvc_queue.c

@@ -142,6 +142,7 @@ static void uvc_buffer_queue(struct vb2_buffer *vb)
 
 	spin_lock_irqsave(&queue->irqlock, flags);
 	if (likely(!(queue->flags & UVC_QUEUE_DISCONNECTED))) {
+		kref_init(&buf->ref);
 		list_add_tail(&buf->queue, &queue->irqqueue);
 	} else {
 		/* If the device is disconnected return the buffer to userspace
@@ -459,28 +460,66 @@ struct uvc_buffer *uvc_queue_get_current_buffer(struct uvc_video_queue *queue)
 	return nextbuf;
 }
 
-struct uvc_buffer *uvc_queue_next_buffer(struct uvc_video_queue *queue,
+/*
+ * uvc_queue_buffer_requeue: Requeue a buffer on our internal irqqueue
+ *
+ * Reuse a buffer through our internal queue without the need to 'prepare'.
+ * The buffer will be returned to userspace through the uvc_buffer_queue call if
+ * the device has been disconnected.
+ */
+static void uvc_queue_buffer_requeue(struct uvc_video_queue *queue,
 		struct uvc_buffer *buf)
 {
-	struct uvc_buffer *nextbuf;
-	unsigned long flags;
-
-	if ((queue->flags & UVC_QUEUE_DROP_CORRUPTED) && buf->error) {
 	buf->error = 0;
 	buf->state = UVC_BUF_STATE_QUEUED;
 	buf->bytesused = 0;
 	vb2_set_plane_payload(&buf->buf.vb2_buf, 0, 0);
-		return buf;
+
+	uvc_buffer_queue(&buf->buf.vb2_buf);
+}
+
+static void uvc_queue_buffer_complete(struct kref *ref)
+{
+	struct uvc_buffer *buf = container_of(ref, struct uvc_buffer, ref);
+	struct vb2_buffer *vb = &buf->buf.vb2_buf;
+	struct uvc_video_queue *queue = vb2_get_drv_priv(vb->vb2_queue);
+
+	if ((queue->flags & UVC_QUEUE_DROP_CORRUPTED) && buf->error) {
+		uvc_queue_buffer_requeue(queue, buf);
+		return;
 	}
 
+	buf->state = buf->error ? UVC_BUF_STATE_ERROR : UVC_BUF_STATE_DONE;
+	vb2_set_plane_payload(&buf->buf.vb2_buf, 0, buf->bytesused);
+	vb2_buffer_done(&buf->buf.vb2_buf, VB2_BUF_STATE_DONE);
+}
+
+/*
+ * Release a reference on the buffer. Complete the buffer when the last
+ * reference is released.
+ */
+void uvc_queue_buffer_release(struct uvc_buffer *buf)
+{
+	kref_put(&buf->ref, uvc_queue_buffer_complete);
+}
+
+/*
+ * Remove this buffer from the queue. Lifetime will persist while async actions
+ * are still running (if any), and uvc_queue_buffer_release will give the buffer
+ * back to VB2 when all users have completed.
+ */
+struct uvc_buffer *uvc_queue_next_buffer(struct uvc_video_queue *queue,
+		struct uvc_buffer *buf)
+{
+	struct uvc_buffer *nextbuf;
+	unsigned long flags;
+
 	spin_lock_irqsave(&queue->irqlock, flags);
 	list_del(&buf->queue);
 	nextbuf = __uvc_queue_get_current_buffer(queue);
 	spin_unlock_irqrestore(&queue->irqlock, flags);
 
-	buf->state = buf->error ? UVC_BUF_STATE_ERROR : UVC_BUF_STATE_DONE;
-	vb2_set_plane_payload(&buf->buf.vb2_buf, 0, buf->bytesused);
-	vb2_buffer_done(&buf->buf.vb2_buf, VB2_BUF_STATE_DONE);
+	uvc_queue_buffer_release(buf);
 
 	return nextbuf;
 }

drivers/media/usb/uvc/uvcvideo.h

@@ -413,6 +413,9 @@ struct uvc_buffer {
 	unsigned int bytesused;
 
 	u32 pts;
+
+	/* Asynchronous buffer handling. */
+	struct kref ref;
 };
 
 #define UVC_QUEUE_DISCONNECTED		(1 << 0)
@@ -729,6 +732,7 @@ void uvc_queue_cancel(struct uvc_video_queue *queue, int disconnect);
 struct uvc_buffer *uvc_queue_next_buffer(struct uvc_video_queue *queue,
 					 struct uvc_buffer *buf);
 struct uvc_buffer *uvc_queue_get_current_buffer(struct uvc_video_queue *queue);
+void uvc_queue_buffer_release(struct uvc_buffer *buf);
 int uvc_queue_mmap(struct uvc_video_queue *queue,
 		   struct vm_area_struct *vma);
 __poll_t uvc_queue_poll(struct uvc_video_queue *queue, struct file *file,