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

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

> Tim Roberts  wrote:
>
>> 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.
>>
>
> Without "large transfer", we have no way to separate the individual image
> frames from the isoc packets if we don't add any marks between the frames,
> right?
>
> Best,
> Zheng
>