[RFC PATCH v1] media: uvcvideo: Cache URB header data before processing

[Keiichi Watanabe]

On some platforms with non-coherent DMA (e.g. ARM), USB drivers use
uncached memory allocation methods. In such situations, it sometimes
takes a long time to access URB buffers.  This can be a cause of video
flickering problems if a resolution is high and a USB controller has
a very tight time limit. (e.g. dwc2) To avoid this problem, we copy
header data from (uncached) URB buffer into (cached) local buffer.

This change should make the elapsed time of the interrupt handler
shorter on platforms with non-coherent DMA. We measured the elapsed
time of each callback of uvc_video_complete without/with this patch
while capturing Full HD video in
https://webrtc.github.io/samples/src/content/getusermedia/resolution/.
I tested it on the top of Kieran Bingham's Asynchronous UVC series
https://www.mail-archive.com/linux-media@xxxxxxxxxxxxxxx/msg128359.html.
The test device was Jerry Chromebook (RK3288) with Logitech Brio 4K.
I collected data for 5 seconds. (There were around 480 callbacks in
this case.) The following result shows that this patch makes
uvc_video_complete about 2x faster.

           | average | median  | min     | max     | standard deviation
w/o caching| 45319ns | 40250ns | 33834ns | 142625ns| 16611ns
w/  caching| 20620ns | 19250ns | 12250ns | 56583ns | 6285ns

In addition, we confirmed that this patch doesn't make it worse on
coherent DMA architecture by performing the same measurements on a
Broadwell Chromebox with the same camera.

           | average | median  | min     | max     | standard deviation
w/o caching| 21026ns | 21424ns | 12263ns | 23956ns | 1932ns
w/  caching| 20728ns | 20398ns |  8922ns | 45120ns | 3368ns

Signed-off-by: Keiichi Watanabe <keiichiw@xxxxxxxxxxxx>
---

After applying 6 patches in
https://www.mail-archive.com/linux-media@xxxxxxxxxxxxxxx/msg128359.html,
I measured elapsed time by adding the following code to
/drivers/media/usb/uvc/uvc_video.c

@@ -XXXX,6 +XXXX,9 @@ static void uvc_video_complete(struct urb *urb)
        struct uvc_video_queue *queue = &stream->queue;
 	struct uvc_buffer *buf = NULL;
 	int ret;
+	ktime_t start, end;
+	int elapsed_time;
+	start = ktime_get();
 	switch (urb->status) {
 	case 0:

@@ -XXXX,6 +XXXX,10 @@ static void uvc_video_complete(struct urb *urb)

 	INIT_WORK(&uvc_urb->work, uvc_video_copy_data_work);
 	queue_work(stream->async_wq, &uvc_urb->work);
+
+	end = ktime_get();
+	elapsed_time = ktime_to_ns(ktime_sub(end, start));
+	pr_err("elapsed time: %d ns", elapsed_time);
 }

 /*


 drivers/media/usb/uvc/uvc_video.c | 92 +++++++++++++++----------------
 1 file changed, 43 insertions(+), 49 deletions(-)
diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index a88b2e51a666..ff2eddc55530 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -391,36 +391,15 @@ static inline ktime_t uvc_video_get_time(void)

 static void
 uvc_video_clock_decode(struct uvc_streaming *stream, struct uvc_buffer *buf,
-		       const u8 *data, int len)
+		       const u8 *data, int len, unsigned int header_size,
+		       bool has_pts, bool has_scr)
 {
 	struct uvc_clock_sample *sample;
-	unsigned int header_size;
-	bool has_pts = false;
-	bool has_scr = false;
 	unsigned long flags;
 	ktime_t time;
 	u16 host_sof;
 	u16 dev_sof;

-	switch (data[1] & (UVC_STREAM_PTS | UVC_STREAM_SCR)) {
-	case UVC_STREAM_PTS | UVC_STREAM_SCR:
-		header_size = 12;
-		has_pts = true;
-		has_scr = true;
-		break;
-	case UVC_STREAM_PTS:
-		header_size = 6;
-		has_pts = true;
-		break;
-	case UVC_STREAM_SCR:
-		header_size = 8;
-		has_scr = true;
-		break;
-	default:
-		header_size = 2;
-		break;
-	}
-
 	/* Check for invalid headers. */
 	if (len < header_size)
 		return;
@@ -717,11 +696,10 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
  */

 static void uvc_video_stats_decode(struct uvc_streaming *stream,
-		const u8 *data, int len)
+				   const u8 *data, int len,
+				   unsigned int header_size, bool has_pts,
+				   bool has_scr)
 {
-	unsigned int header_size;
-	bool has_pts = false;
-	bool has_scr = false;
 	u16 uninitialized_var(scr_sof);
 	u32 uninitialized_var(scr_stc);
 	u32 uninitialized_var(pts);
@@ -730,25 +708,6 @@ static void uvc_video_stats_decode(struct uvc_streaming *stream,
 	    stream->stats.frame.nb_packets == 0)
 		stream->stats.stream.start_ts = ktime_get();

-	switch (data[1] & (UVC_STREAM_PTS | UVC_STREAM_SCR)) {
-	case UVC_STREAM_PTS | UVC_STREAM_SCR:
-		header_size = 12;
-		has_pts = true;
-		has_scr = true;
-		break;
-	case UVC_STREAM_PTS:
-		header_size = 6;
-		has_pts = true;
-		break;
-	case UVC_STREAM_SCR:
-		header_size = 8;
-		has_scr = true;
-		break;
-	default:
-		header_size = 2;
-		break;
-	}
-
 	/* Check for invalid headers. */
 	if (len < header_size || data[0] < header_size) {
 		stream->stats.frame.nb_invalid++;
@@ -957,10 +916,41 @@ static void uvc_video_stats_stop(struct uvc_streaming *stream)
  * to be called with a NULL buf parameter. uvc_video_decode_data and
  * uvc_video_decode_end will never be called with a NULL buffer.
  */
+static void uvc_video_decode_header_size(const u8 *data, int *header_size,
+					 bool *has_pts, bool *has_scr)
+{
+	switch (data[1] & (UVC_STREAM_PTS | UVC_STREAM_SCR)) {
+	case UVC_STREAM_PTS | UVC_STREAM_SCR:
+		*header_size = 12;
+		*has_pts = true;
+		*has_scr = true;
+		break;
+	case UVC_STREAM_PTS:
+		*header_size = 6;
+		*has_pts = true;
+		break;
+	case UVC_STREAM_SCR:
+		*header_size = 8;
+		*has_scr = true;
+		break;
+	default:
+		*header_size = 2;
+	}
+}
+
 static int uvc_video_decode_start(struct uvc_streaming *stream,
-		struct uvc_buffer *buf, const u8 *data, int len)
+				  struct uvc_buffer *buf, const u8 *urb_data,
+				  int len)
 {
 	u8 fid;
+	u8 data[12];
+	unsigned int header_size;
+	bool has_pts = false, has_scr = false;
+
+	/* Cache the header since urb_data is uncached memory. The size of
+	 * header is at most 12 bytes.
+	 */
+	memcpy(data, urb_data, min(len, 12));

 	/* Sanity checks:
 	 * - packet must be at least 2 bytes long
@@ -983,8 +973,12 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
 			uvc_video_stats_update(stream);
 	}

-	uvc_video_clock_decode(stream, buf, data, len);
-	uvc_video_stats_decode(stream, data, len);
+	uvc_video_decode_header_size(data, &header_size, &has_pts, &has_scr);
+
+	uvc_video_clock_decode(stream, buf, data, len, header_size, has_pts,
+			       has_scr);
+	uvc_video_stats_decode(stream, data, len, header_size, has_pts,
+			       has_scr);

 	/* Store the payload FID bit and return immediately when the buffer is
 	 * NULL.
--
2.18.0.rc2.346.g013aa6912e-goog