Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756903AbaAHROa (ORCPT ); Wed, 8 Jan 2014 12:14:30 -0500 Received: from iolanthe.rowland.org ([192.131.102.54]:59156 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1756730AbaAHRO0 (ORCPT ); Wed, 8 Jan 2014 12:14:26 -0500 Date: Wed, 8 Jan 2014 12:14:25 -0500 (EST) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: David Laight cc: Sarah Sharp , walt , Greg Kroah-Hartman , Linux Kernel , "stable@vger.kernel.org" , "linux-usb@vger.kernel.org" , "linux-scsi@vger.kernel.org" Subject: RE: [PATCH 3.12 033/118] usb: xhci: Link TRB must not occur within a USB payload burst In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6D455602@AcuExch.aculab.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 8 Jan 2014, David Laight wrote: > > From: Alan Stern > > > > This may be a foolish question, but why is xhci-hcd using no-op TRBs in > > the first place? > > Because it can't write in a link TRB because other parts of the > code use link TRBs to detect the end of the ring. > > The problem is that it can't put a link TRB in the middle of > a chain of data fragments unless it is at a 'suitable' offset > from the start of the data TD. Given arbitrary input fragmentation > this means that you can't put a link TRB in the middle of a TD. > (The documented alignment might be as high as 16kB.) > > If the rest of the code used a 'ring end pointer' then a link TRB > could be used instead. I see. Sounds like a poor design decision in hindsight. Can it be changed? Alan Stern -- 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/