linux-kernel - Re: Real time USB2Serial devices and behaivor

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Message-ID: <Pine.LNX.4.44L0.0803262255320.21714-100000@netrider.rowland.org>
Date:	Wed, 26 Mar 2008 22:58:37 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Greg KH <greg@...ah.com>
cc:	mark gross <mgross@...ux.intel.com>,
	lkml <linux-kernel@...r.kernel.org>,
	<linux-rt-users@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: Re: Real time USB2Serial devices and behaivor

On Wed, 26 Mar 2008, Greg KH wrote:

> > Is there any reason to think that if I created my own isochronous
> > USB2Serial adapter and iso-usb-driver that I couldn't get determinism?
> 
> I strongly doubt it as others have tried and failed in the past.

I don't understand.  Isochronous transfers have pretty strict 
transfer-time guarantees.  Why wouldn't this work?

One reason I can think of is that Iso transfers aren't reliable.  But
then regular RS232-type serial transfers aren't reliable either.

The only other reason is that the USB stack itself has an unpredictable 
amount of overhead.  However I think it should fall within an 
acceptable range for RT applications.

Alan Stern

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