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Re: submit urb question (isochronous mode)

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On Mon, 10 Sep 2007, Philip Rakity wrote:

> I am trying to understand the difference of the following:
> 
> video camera -- isochronous mode  --- data only comes into the HOST
> 
> polling is done every 1ms
> 
> 
> Now for the question
> 
> I submit say
> 10 urbs each with 10 buffers
> 
> or
> 
> I submit say
> 
> 100 urb's with 1 buffer each
> 
> assume that each buffer is big enough to hold the video frame that is  
> received by the host.

Okay... but the maximum size of a full-speed isochronous transfer is 
only 1023 bytes.  Is that really large enough to hold a video frame?

> so what is the compare and contrast --
> 
> I guess that if every buffer is filled each 1ms then my callback is  
> called every 1ms for the 100 urb case and every 10ms for the 10urb case

That's right.

> anything else ?

The CPU has more interrupt overhead in the 100-URB case, because it 
gets interrupted every ms.  In the 10-URB case, interrupts occur every 
10 ms.

There's more general processing overhead in the 100-URB case, simply 
because it takes a certain amount of work to submit each URB -- and the 
amount of work needed for a 10-buffer URB is less than 10 times the 
work needed for a 1-buffer URB.

Alan Stern


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