I narrowed the issue a little bit more. It looks like a bandwidth/
throughput issue. The sequence is like this:
Timestamp(seconds)
101.995432 Queue 0
101.995705 Queue 1
101.995978 Queue 2
101.996250 Queue 3
103.129925 Frame 0 complete
103.165819 Frame 1 complete
103.201672 Frame 2 complete
103.237764 Frame 3 complete
103.239244 Dequeue 0
103.241105 Queue 0
103.241153 Dequeue 1
103.243283 Queue 1
103.243333 Dequeue 2
103.245113 Queue 2
103.245161 Dequeue 3
103.247259 Queue 3
103.296360 Frame 0 complete
....
In this case, the frame interval is: 33ms-33ms-33ms-67ms.
So in this case, effectively 1 frame is dropped at the driver level.
One thing worth noting is that the user space application has a loop
like this (in pseudo code):
Stream on
......
while(true) {
Dequeue buffer;
Queue buffer;
}
Is the frame drop due to the isochronous nature of the UVC protocol?
In other words, because of the multiple Dequeue & Queue operations
happening at the 103.239244 timestamp, the next frame doesn't have
enough time/processing bandwidth to finish. Hence 1 frame is dropped.
Does this make sense?
-Shuzhen
--
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
--
> -----Original Message-----
> From: linux-uvc-devel-boun...@lists.berlios.de [mailto:linux-uvc-devel-
> boun...@lists.berlios.de] On Behalf Of Shuzhen Wang
> Sent: Thursday, December 16, 2010 4:46 PM
> To: linux-uvc-devel@lists.berlios.de
> Subject: [Linux-uvc-devel] uvc_queue_next_buffer called at an interval
> greater than framerate.
>
> Hello,
>
> We are using UVC driver with a Chicony USB camera. The camera runs at
> 30fps
> fixed frame rate.
>
> We see some jittering in the video stream. When we look at dmesg, it
> shows
> that sometimes the interval between 2 uvc_queue_next_buffer calls are
> about
> 65 ms. Most of the time it's around 30-33ms. Obviously frame drop is
> happening here.
>
> My understanding is that uvc_queue_next_buffer is called in interrupt
> contexts. So it's unlikely it's scheduling issue. I can think of 2
> possibilities:
> 1. Problem with camera firmware: the frame stream coming from the
> camera
> module is already problematic.
> 2. Memory/Cache performance issue causing the frame processing takes
> too
> long.
>
> How can I narrow down the issue furthuer? Your comment is appreciated.
>
> -Shuzhen
>
> _______________________________________________
> Linux-uvc-devel mailing list
> Linux-uvc-devel@lists.berlios.de
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