Re: submit urb question (isochronous mode)

[Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>]

On Mon, 10 Sep 2007, Philip Rakity wrote:

> 
> Hi Alan,
> 
> compressed video works just fine at full speed with 1ms timing.
> 
> another question if you do not mind.
> 
> a) is a urb coupled to a USB transaction ?  
> 
> if I assume say 1ms transfer of say 800 bytes and I submit a urb with
> say 10 buffers of 800 bytes each.  What happens ?  are 9 of the
> buffers wasted ?  or are things split into 10 usb transactions each
> 1ms apart.

For Isochronours URBs, each buffer corresponds to a single, separate
transaction.  So there would be 10 transactions, each of 800 bytes, 
lasting for 10 ms total.

For other types of URBs, there is only one buffer and it corresponds to 
as many transactions as needed (roughly speaking, the buffer size 
divided by the endpoint's max-packet size).

Alan Stern


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