linux-kernel - RE: [PATCH 3.12 033/118] usb: xhci: Link TRB must not occur within a USB payload burst

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Message-ID: <Pine.LNX.4.44L0.1401081210140.1659-100000@iolanthe.rowland.org>
Date:	Wed, 8 Jan 2014 12:14:25 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	David Laight <David.Laight@...LAB.COM>
cc:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	walt <w41ter@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: RE: [PATCH 3.12 033/118] usb: xhci: Link TRB must not occur within
 a USB payload burst

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

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