Re: [libusb] less data length in async bulk transfer with 2 cameras

[zheng r. <zhe...@gm...>]

Tim Roberts  wrote:

> Would you please provide the part name of the external FIFO chip?
>
>
> I'll see if I can find that out.  It was 10 years ago...
>

Thanks.


> By far, I haven't get the isoc transfer work, even for one camera.
>
> The bulk transfer looks very tidy.
> ...
> But the data length of the isoc transfer is messy even for one camera,
> which makes me confused.
>
>
> That's the way isochronous works.  You need to read about this in the USB
> spec, because isochronous buffering is quite different from bulk and
> interrupt..  With bulk and interrupt pipes, things work in units of
> transfers.  You ask for 360,960 bytes, and the packets transfer until that
> packets fills.  But with isochronous, everything happens one packet at a
> time.  There is no concept of larger "transfers".  You'll provide a buffer
> that is some multiple of 8x3072 bytes, divided into individual 3072 byte
> packets.  That's why you have to prepare packet descriptors.
>
> The result you get back is not one contiguous buffer.  You have to look at
> the packet descriptors to determine how much valid data is in each packet.
> If you got all three transfers in a microframe, there will be 3072 bytes in
> that packet.  If you only got one transfer in a microframe, there will be
> 1024 bytes of data and 2048 bytes of garbage.  If you missed a whole
> microframe, there will 0 bytes of data in that packet.  You have to
> reassemble the frame from those packets.
>

 It seems I have to get more basic knowledge from USB specs. Thanks for
your instruction.

Best,
Zheng