[PATCH 4/4] media: uvcvideo: Do not add clock samples with small sof delta

[Ricardo Ribalda <ribalda@chromium.org>]

Some UVC 1.1 cameras running in fast isochronous mode tend to spam the
USB host with a lot of empty packets. These packets contain clock
information and are added to the clock buffer but do not add any
accuracy to the calculation. In fact, it is quite the opposite, in our
calculations, only the first and the last timestamp is used, and we only
have 32 slots.

Ignore the samples that will produce less than MIN_HW_TIMESTAMP_DIFF
data.

Fixes: 141270bd95d4 ("media: uvcvideo: Refactor clock circular buffer")
Cc: stable@vger.kernel.org
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
---
 drivers/media/usb/uvc/uvc_video.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index dcbc0941ffe6..e1a4e84d6841 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -544,6 +544,19 @@ static void uvc_video_clock_add_sample(struct uvc_clock *clock,
 	spin_unlock_irqrestore(&clock->lock, flags);
 }
 
+static inline u16 sof_diff(u16 a, u16 b)
+{
+	u32 aux;
+
+	a &= 2047;
+	b &= 2047;
+	if (a >= b)
+		return a - b;
+
+	aux = a + 2048;
+	return (u16)(aux - b);
+}
+
 static void
 uvc_video_clock_decode(struct uvc_streaming *stream, struct uvc_buffer *buf,
 		       const u8 *data, int len)
@@ -664,12 +677,13 @@ uvc_video_clock_decode(struct uvc_streaming *stream, struct uvc_buffer *buf,
 	sample.dev_sof = (sample.dev_sof + stream->clock.sof_offset) & 2047;
 
 	/*
-	 * To limit the amount of data, drop SCRs with an SOF identical to the
+	 * To limit the amount of data, drop SCRs with an SOF similar to the
 	 * previous one. This filtering is also needed to support UVC 1.5, where
 	 * all the data packets of the same frame contains the same SOF. In that
 	 * case only the first one will match the host_sof.
 	 */
-	if (sample.dev_sof == stream->clock.last_sof)
+	if (sof_diff(sample.dev_sof, stream->clock.last_sof) <=
+	    (MIN_HW_TIMESTAMP_DIFF / stream->clock.size))
 		return;
 
 	uvc_video_clock_add_sample(&stream->clock, &sample);

-- 
2.53.0.959.g497ff81fa9-goog