Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756679AbYC0EL6 (ORCPT ); Thu, 27 Mar 2008 00:11:58 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750846AbYC0ELs (ORCPT ); Thu, 27 Mar 2008 00:11:48 -0400 Received: from ns2.suse.de ([195.135.220.15]:36227 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750769AbYC0ELr (ORCPT ); Thu, 27 Mar 2008 00:11:47 -0400 Date: Wed, 26 Mar 2008 21:13:06 -0700 From: Greg KH To: Alan Stern Cc: mark gross , lkml , linux-rt-users@vger.kernel.org, linux-usb@vger.kernel.org Subject: Re: Real time USB2Serial devices and behaivor Message-ID: <20080327041306.GA10095@kroah.com> References: <20080326232419.GC15468@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.16 (2007-06-09) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1408 Lines: 34 On Wed, Mar 26, 2008 at 10:58:37PM -0400, Alan Stern wrote: > 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? I don't know, but the person who tried this a while ago said it wasn't really "real-time" enough for their application (robot arm movement). > 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. It's all about bounding the longest latency. Sometimes, under heavy loads, latency can be pretty big. But now that we have the -rt kernel, it might be a lot better than before, so that might be possible now, haven't tried it... good luck, greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/