[PATCH v4 6/6] media: uvcvideo: Move decode processing to process context

[Kieran Bingham]

Newer high definition cameras, and cameras with multiple lenses such as
the range of stereo-vision cameras now available have ever increasing
data rates.

The inclusion of a variable length packet header in URB packets mean
that we must memcpy the frame data out to our destination 'manually'.
This can result in data rates of up to 2 gigabits per second being
processed.

To improve efficiency, and maximise throughput, handle the URB decode
processing through a work queue to move it from interrupt context, and
allow multiple processors to work on URBs in parallel.

Signed-off-by: Kieran Bingham <kieran.bingham@xxxxxxxxxxxxxxxx>

---
v2:
 - Lock full critical section of usb_submit_urb()

v3:
 - Fix race on submitting uvc_video_decode_data_work() to work queue.
 - Rename uvc_decode_op -> uvc_copy_op (Generic to encode/decode)
 - Rename decodes -> copy_operations
 - Don't queue work if there is no async task
 - obtain copy op structure directly in uvc_video_decode_data()
 - uvc_video_decode_data_work() -> uvc_video_copy_data_work()

v4:
 - Provide for_each_uvc_urb()
 - Simplify fix for shutdown race to flush queue before freeing URBs
 - Rebase to v4.16-rc4 (linux-media/master) adjusting for metadata
   conflicts.

 drivers/media/usb/uvc/uvc_video.c | 107 ++++++++++++++++++++++++-------
 drivers/media/usb/uvc/uvcvideo.h  |  28 ++++++++-
 2 files changed, 111 insertions(+), 24 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index 7dd0dcb457f3..a62e8caf367c 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -1042,21 +1042,54 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
 	return data[0];
 }
 
-static void uvc_video_decode_data(struct uvc_streaming *stream,
+/*
+ * uvc_video_decode_data_work: Asynchronous memcpy processing
+ *
+ * Perform memcpy tasks in process context, with completion handlers
+ * to return the URB, and buffer handles.
+ */
+static void uvc_video_copy_data_work(struct work_struct *work)
+{
+	struct uvc_urb *uvc_urb = container_of(work, struct uvc_urb, work);
+	unsigned int i;
+	int ret;
+
+	for (i = 0; i < uvc_urb->async_operations; i++) {
+		struct uvc_copy_op *op = &uvc_urb->copy_operations[i];
+
+		memcpy(op->dst, op->src, op->len);
+
+		/* Release reference taken on this buffer */
+		uvc_queue_buffer_release(op->buf);
+	}
+
+	ret = usb_submit_urb(uvc_urb->urb, GFP_ATOMIC);
+	if (ret < 0)
+		uvc_printk(KERN_ERR, "Failed to resubmit video URB (%d).\n",
+			   ret);
+}
+
+static void uvc_video_decode_data(struct uvc_urb *uvc_urb,
 		struct uvc_buffer *buf, const u8 *data, int len)
 {
-	unsigned int maxlen, nbytes;
-	void *mem;
+	unsigned int active_op = uvc_urb->async_operations;
+	struct uvc_copy_op *decode = &uvc_urb->copy_operations[active_op];
+	unsigned int maxlen;
 
 	if (len <= 0)
 		return;
 
-	/* Copy the video data to the buffer. */
 	maxlen = buf->length - buf->bytesused;
-	mem = buf->mem + buf->bytesused;
-	nbytes = min((unsigned int)len, maxlen);
-	memcpy(mem, data, nbytes);
-	buf->bytesused += nbytes;
+
+	/* Take a buffer reference for async work */
+	kref_get(&buf->ref);
+
+	decode->buf = buf;
+	decode->src = data;
+	decode->dst = buf->mem + buf->bytesused;
+	decode->len = min_t(unsigned int, len, maxlen);
+
+	buf->bytesused += decode->len;
 
 	/* Complete the current frame if the buffer size was exceeded. */
 	if (len > maxlen) {
@@ -1064,6 +1097,8 @@ static void uvc_video_decode_data(struct uvc_streaming *stream,
 		buf->error = 1;
 		buf->state = UVC_BUF_STATE_READY;
 	}
+
+	uvc_urb->async_operations++;
 }
 
 static void uvc_video_decode_end(struct uvc_streaming *stream,
@@ -1272,7 +1307,7 @@ static void uvc_video_decode_isoc(struct uvc_urb *uvc_urb,
 		uvc_video_decode_meta(stream, meta_buf, mem, ret);
 
 		/* Decode the payload data. */
-		uvc_video_decode_data(stream, buf, mem + ret,
+		uvc_video_decode_data(uvc_urb, buf, mem + ret,
 			urb->iso_frame_desc[i].actual_length - ret);
 
 		/* Process the header again. */
@@ -1334,9 +1369,9 @@ static void uvc_video_decode_bulk(struct uvc_urb *uvc_urb,
 	 * sure buf is never dereferenced if NULL.
 	 */
 
-	/* Process video data. */
+	/* Prepare video data for processing. */
 	if (!stream->bulk.skip_payload && buf != NULL)
-		uvc_video_decode_data(stream, buf, mem, len);
+		uvc_video_decode_data(uvc_urb, buf, mem, len);
 
 	/* Detect the payload end by a URB smaller than the maximum size (or
 	 * a payload size equal to the maximum) and process the header again.
@@ -1422,7 +1457,7 @@ static void uvc_video_complete(struct urb *urb)
 		uvc_printk(KERN_WARNING, "Non-zero status (%d) in video "
 			"completion handler.\n", urb->status);
 		/* fall through */
-	case -ENOENT:		/* usb_kill_urb() called. */
+	case -ENOENT:		/* usb_poison_urb() called. */
 		if (stream->frozen)
 			return;
 		/* fall through */
@@ -1436,6 +1471,9 @@ static void uvc_video_complete(struct urb *urb)
 
 	buf = uvc_queue_get_current_buffer(queue);
 
+	/* Re-initialise the URB async work. */
+	uvc_urb->async_operations = 0;
+
 	if (vb2_qmeta) {
 		spin_lock_irqsave(&qmeta->irqlock, flags);
 		if (!list_empty(&qmeta->irqqueue))
@@ -1444,12 +1482,24 @@ static void uvc_video_complete(struct urb *urb)
 		spin_unlock_irqrestore(&qmeta->irqlock, flags);
 	}
 
+	/*
+	 * Process the URB headers, and optionally queue expensive memcpy tasks
+	 * to be deferred to a work queue.
+	 */
 	stream->decode(uvc_urb, buf, buf_meta);
 
-	if ((ret = usb_submit_urb(urb, GFP_ATOMIC)) < 0) {
-		uvc_printk(KERN_ERR, "Failed to resubmit video URB (%d).\n",
-			ret);
+	/* If no async work is needed, resubmit the URB immediately. */
+	if (!uvc_urb->async_operations) {
+		ret = usb_submit_urb(uvc_urb->urb, GFP_ATOMIC);
+		if (ret < 0)
+			uvc_printk(KERN_ERR,
+				   "Failed to resubmit video URB (%d).\n",
+				   ret);
+		return;
 	}
+
+	INIT_WORK(&uvc_urb->work, uvc_video_copy_data_work);
+	queue_work(stream->async_wq, &uvc_urb->work);
 }
 
 /*
@@ -1544,25 +1594,29 @@ static int uvc_alloc_urb_buffers(struct uvc_streaming *stream,
  */
 static void uvc_uninit_video(struct uvc_streaming *stream, int free_buffers)
 {
-	struct urb *urb;
-	unsigned int i;
+	struct uvc_urb *uvc_urb;
 
 	uvc_video_stats_stop(stream);
 
-	for (i = 0; i < UVC_URBS; ++i) {
-		struct uvc_urb *uvc_urb = &stream->uvc_urb[i];
+	/*
+	 * We must poison the URBs rather than kill them to ensure that even
+	 * after the completion handler returns, any asynchronous workqueues
+	 * will be prevented from resubmitting the URBs
+	 */
+	for_each_uvc_urb(uvc_urb, stream)
+		usb_poison_urb(uvc_urb->urb);
 
-		urb = uvc_urb->urb;
-		if (urb == NULL)
-			continue;
+	flush_workqueue(stream->async_wq);
 
-		usb_kill_urb(urb);
-		usb_free_urb(urb);
+	for_each_uvc_urb(uvc_urb, stream) {
+		usb_free_urb(uvc_urb->urb);
 		uvc_urb->urb = NULL;
 	}
 
 	if (free_buffers)
 		uvc_free_urb_buffers(stream);
+
+	destroy_workqueue(stream->async_wq);
 }
 
 /*
@@ -1720,6 +1774,11 @@ static int uvc_init_video(struct uvc_streaming *stream, gfp_t gfp_flags)
 
 	uvc_video_stats_start(stream);
 
+	stream->async_wq = alloc_workqueue("uvcvideo", WQ_UNBOUND | WQ_HIGHPRI,
+			0);
+	if (!stream->async_wq)
+		return -ENOMEM;
+
 	if (intf->num_altsetting > 1) {
 		struct usb_host_endpoint *best_ep = NULL;
 		unsigned int best_psize = UINT_MAX;
diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
index 112eed49bf50..27c230430eda 100644
--- a/drivers/media/usb/uvc/uvcvideo.h
+++ b/drivers/media/usb/uvc/uvcvideo.h
@@ -485,12 +485,30 @@ struct uvc_stats_stream {
 #define UVC_METATADA_BUF_SIZE 1024
 
 /**
+ * struct uvc_copy_op: Context structure to schedule asynchronous memcpy
+ *
+ * @buf: active buf object for this operation
+ * @dst: copy destination address
+ * @src: copy source address
+ * @len: copy length
+ */
+struct uvc_copy_op {
+	struct uvc_buffer *buf;
+	void *dst;
+	const __u8 *src;
+	size_t len;
+};
+
+/**
  * struct uvc_urb - URB context management structure
  *
  * @urb: the URB described by this context structure
  * @stream: UVC streaming context
  * @buffer: memory storage for the URB
  * @dma: DMA coherent addressing for the urb_buffer
+ * @async_operations: counter to indicate the number of copy operations
+ * @copy_operations: work descriptors for asynchronous copy operations
+ * @work: work queue entry for asynchronous decode
  */
 struct uvc_urb {
 	struct urb *urb;
@@ -498,6 +516,10 @@ struct uvc_urb {
 
 	char *buffer;
 	dma_addr_t dma;
+
+	unsigned int async_operations;
+	struct uvc_copy_op copy_operations[UVC_MAX_PACKETS];
+	struct work_struct work;
 };
 
 struct uvc_streaming {
@@ -530,6 +552,7 @@ struct uvc_streaming {
 	/* Buffers queue. */
 	unsigned int frozen : 1;
 	struct uvc_video_queue queue;
+	struct workqueue_struct *async_wq;
 	void (*decode)(struct uvc_urb *uvc_urb, struct uvc_buffer *buf,
 		       struct uvc_buffer *meta_buf);
 
@@ -583,6 +606,11 @@ struct uvc_streaming {
 	} clock;
 };
 
+#define for_each_uvc_urb(uvc_urb, uvc_streaming) \
+	for (uvc_urb = &uvc_streaming->uvc_urb[0]; \
+	     uvc_urb < &uvc_streaming->uvc_urb[UVC_URBS]; \
+	     ++uvc_urb)
+
 struct uvc_device {
 	struct usb_device *udev;
 	struct usb_interface *intf;
-- 
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