Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756850AbaAHR0b (ORCPT ); Wed, 8 Jan 2014 12:26:31 -0500 Received: from mx0.aculab.com ([213.249.233.131]:36466 "HELO mx0.aculab.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1756115AbaAHR01 convert rfc822-to-8bit (ORCPT ); Wed, 8 Jan 2014 12:26:27 -0500 From: David Laight To: "'Alan Stern'" 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 Thread-Topic: [PATCH 3.12 033/118] usb: xhci: Link TRB must not occur within a USB payload burst Thread-Index: AQHPDAsx5d97FnYUxUSD3GhphkgfRJp7CFoAgAABzdCAAAfrgIAAApgg Date: Wed, 8 Jan 2014 17:24:42 +0000 Message-ID: <063D6719AE5E284EB5DD2968C1650D6D45567F@AcuExch.aculab.com> References: <063D6719AE5E284EB5DD2968C1650D6D455602@AcuExch.aculab.com> In-Reply-To: Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.202.99.200] Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: Alan Stern > 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? Anything can be changed :-) But it is a bit pervasive. Padding out with nops isolated the change. David -- 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/