From owner-freebsd-usb@FreeBSD.ORG  Thu Mar 19 07:58:54 2009
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From: Hans Petter Selasky <hselasky@c2i.net>
To: Weongyo Jeong <weongyo@freebsd.org>
Date: Thu, 19 Mar 2009 09:01:23 +0100
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Cc: Sam Leffler <sam@freebsd.org>, freebsd-usb@freebsd.org,
	Andrew Thompson <thompsa@freebsd.org>
Subject: Re: q: USB_SET_TIMEOUT in ugen.
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On Thursday 19 March 2009, Weongyo Jeong wrote:
> ugen_default_read_callback:384: actlen=0, aframes=0
> ugen_default_read_callback:384: actlen=0, aframes=0
> ugen_read_clear_stall_callback:477: f=0xc4d5b000: stall cleared

One difference from the old ugen implementation is that a stall error does not 
cause any error to be returned to userland!

You could try to return a ZLP on errors. Try this patch:

http://perforce.freebsd.org/chv.cgi?CH=159423

If you need to distinguish a ZLP from a STALL, then you have to use the new 
libusb! Ugen is not meant to be a replacement for libusb!

--HPS