[PATCH v4 2/2] media: uvcvideo: Fix sequence number when no EOF

[Ricardo Ribalda]

If the driver could not detect the EOF, the sequence number is increased
twice:
 1) When we enter uvc_video_decode_start() with the old buffer and FID has
   fliped => We return -EAGAIN and last_fid is not flipped
 2) When we enter uvc_video_decode_start() with the new buffer.

Fix this issue by moving the new frame detection logic earlier in
uvc_video_decode_start().

Cc: stable@xxxxxxxxxx
Fixes: 650b95feee35 ("[media] uvcvideo: Generate discontinuous sequence numbers when frames are lost")
Reported-by: Hans de Goede <hansg@xxxxxxxxxx>
Closes: https://lore.kernel.org/linux-media/CANiDSCuj4cPuB5_v2xyvAagA5FjoN8V5scXiFFOeD3aKDMqkCg@xxxxxxxxxxxxxx/T/#me39fb134e8c2c085567a31548c3403eb639625e4
Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
---
 drivers/media/usb/uvc/uvc_video.c | 48 ++++++++++++++++++++-------------------
 1 file changed, 25 insertions(+), 23 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index 9e06b1d0f0f9..2218e4d8e564 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -1168,6 +1168,31 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
 	header_len = data[0];
 	fid = data[1] & UVC_STREAM_FID;
 
+	/*
+	 * Mark the buffer as done if we're at the beginning of a new frame.
+	 * End of frame detection is better implemented by checking the EOF
+	 * bit (FID bit toggling is delayed by one frame compared to the EOF
+	 * bit), but some devices don't set the bit at end of frame (and the
+	 * last payload can be lost anyway). We thus must check if the FID has
+	 * been toggled.
+	 *
+	 * stream->last_fid is initialized to -1, so the first isochronous
+	 * frame will never trigger an end of frame detection.
+	 *
+	 * Empty buffers (bytesused == 0) don't trigger end of frame detection
+	 * as it doesn't make sense to return an empty buffer. This also
+	 * avoids detecting end of frame conditions at FID toggling if the
+	 * previous payload had the EOF bit set.
+	 */
+	if (stream->last_fid != -1 && fid != stream->last_fid &&
+	    buf && buf->bytesused != 0) {
+		uvc_dbg(stream->dev, FRAME,
+			"Frame complete (FID bit toggled)\n");
+		buf->state = UVC_BUF_STATE_READY;
+
+		return -EAGAIN;
+	}
+
 	/*
 	 * Increase the sequence number regardless of any buffer states, so
 	 * that discontinuous sequence numbers always indicate lost frames.
@@ -1234,29 +1259,6 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
 		buf->state = UVC_BUF_STATE_ACTIVE;
 	}
 
-	/*
-	 * Mark the buffer as done if we're at the beginning of a new frame.
-	 * End of frame detection is better implemented by checking the EOF
-	 * bit (FID bit toggling is delayed by one frame compared to the EOF
-	 * bit), but some devices don't set the bit at end of frame (and the
-	 * last payload can be lost anyway). We thus must check if the FID has
-	 * been toggled.
-	 *
-	 * stream->last_fid is initialized to -1, so the first isochronous
-	 * frame will never trigger an end of frame detection.
-	 *
-	 * Empty buffers (bytesused == 0) don't trigger end of frame detection
-	 * as it doesn't make sense to return an empty buffer. This also
-	 * avoids detecting end of frame conditions at FID toggling if the
-	 * previous payload had the EOF bit set.
-	 */
-	if (fid != stream->last_fid && buf->bytesused != 0) {
-		uvc_dbg(stream->dev, FRAME,
-			"Frame complete (FID bit toggled)\n");
-		buf->state = UVC_BUF_STATE_READY;
-		return -EAGAIN;
-	}
-
 	/*
 	 * Some cameras, when running two parallel streams (one MJPEG alongside
 	 * another non-MJPEG stream), are known to lose the EOF packet for a frame.

-- 
2.53.0.959.g497ff81fa9-goog