From owner-freebsd-usb@FreeBSD.ORG  Fri Sep 18 10:58:07 2009
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From: Hans Petter Selasky <hselasky@c2i.net>
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Date: Fri, 18 Sep 2009 12:58:34 +0200
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Subject: Re: Transfer size limits
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On Friday 18 September 2009 05:09:22 Trevor Blackwell wrote:
> What's the largest practical bulk transfer size in the -8 USB stack?
> I'm currently transferring frames through a UE_BULK  pipe from a
> Mightex camera (it uses bulk, not isochronous.)

You have to check what busdma supports. USB uses 32-bit transfer lenghts. 
Typically 64KByte blocks will give you a decent performance.

What is your performance requirement?

--HPS