From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261332AbVFAW3i (ORCPT ); Wed, 1 Jun 2005 18:29:38 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261349AbVFAW3b (ORCPT ); Wed, 1 Jun 2005 18:29:31 -0400 Received: from smtpq2.home.nl ([213.51.128.197]:57273 "EHLO smtpq2.home.nl") by vger.kernel.org with ESMTP id S261334AbVFAW1G (ORCPT ); Wed, 1 Jun 2005 18:27:06 -0400 Message-ID: <429E359D.8090701@keyaccess.nl> Date: Thu, 02 Jun 2005 00:24:29 +0200 From: Rene Herman User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8a6) Gecko/20050111 X-Accept-Language: en-us, en MIME-Version: 1.0 To: David Brownell CC: Petr Vandrovec , Mark Lord , Bartlomiej Zolnierkiewicz , Linux Kernel Subject: Re: External USB2 HDD affects speed hda References: <429BA001.2030405@keyaccess.nl> <429E0965.1090809@vc.cvut.cz> <429E1049.20903@keyaccess.nl> <200506011337.29656.david-b@pacbell.net> In-Reply-To: <200506011337.29656.david-b@pacbell.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-AtHome-MailScanner-Information: Neem contact op met support@home.nl voor meer informatie X-AtHome-MailScanner: Found to be clean Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org David Brownell wrote: >>Added EHCI maintainer to this one as well. If possible, this looks like >>a good candidate for a /proc or /sys knob? > > > No, it's based on a mis-understanding of the hardware. > > The controller should only be doing DMA when some driver has submitted > an URB and that URB hasn't yet completed. Pretty much like any other > hardware, like a disk or network controller. Okay, thanks, and okay, crap. Sounded like a nice, plausible explanation... > For periodic transfers -- interrupt, isochronous, neither used for > disk I/O -- the driver issuing the transfer always has control over > the polling period. But that's mostly related to the USB activity; > if a periodic transfer is active, then the current segment of the > periodic schedule has to be scanned (by DMA) every microframe (8x/msec). > If that segment is empty, that's just one word (32 bits). If there > are transfers, it's got to read them and maybe perform them. I see. Well, sort of at least. "Even if the HDD were using periodic transfers, which it isn't, it would be DMAing 32-bits 8x per msec while idle, which certainly isn't going to cost 8MB/s bus bandwidth". Right? Rene.