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Re: [libusb] Issue with Bulk Transfer in Raspberry PI
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From: Abhishek M. <abm...@ya...> - 2014-01-04 15:36:14
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Hi Tim,
We were dropping every other frame. Now, with your suggestion I setup multiple queues at the host level and able to get full frame rate. Thanks for your help.
Regards,
Abhishek Madaan
On Friday, January 3, 2014 12:51 PM, Tim Roberts <ti...@pr...> wrote:
Abhishek Madaan wrote:
>
> The reason we send the control transfer to start the frame is that we want to make sure that we always sync up with the frame. We always make sure that FSYNC goes high and LSYNC goes high then we activate the data. Once the FSYNC goes low we know that we got the full frame and then we terminate the GPIF waveform at the firmware level. But we also tried doing the video streaming but we lost sync with the frame. The way we did it instead of terminating GPIF waveform once we get the first frame, we reset it. GPIF waveform runs forever and continuously sending frame data to the host. Even though we are making sure that host is always receiving in a separate thread, we get only 58 FPS but our camera is running at 60 FPS. I believe this leads to buffer overflow and this causes the frame to go out of phase.
>
You are describing a timing nightmare here. Have you tried to draw
out a timeline on a whiteboard? This is way more delicate than you
think.
So, you transfer one frame, then you process it, then you send a
control request to trigger another. But, given what you are saying,
ALL of that has to be done during the vertical blank interval. If
you take a little too long, or if you get an inconvenient context
switch, your control request will arrive just after the start of the
video frame, in which case you'll drop that frame. Right?
Whereas, if you sent a short packet after end of frame, you could
queue up two or three frame read requests in your application, and
the firmware could send data continuously. You'd always have a
request ready to receive the next frame, and the short packet would
let you get aligned to start-of-frame again. THAT'S how streaming
video is done.
--
Tim Roberts, ti...@pr... Providenza & Boekelheide, Inc.
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