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: <063D6719AE5E284EB5DD2968C1650D6D455602@AcuExch.aculab.com>
Date:	Wed, 8 Jan 2014 16:51:15 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Alan Stern' <stern@...land.harvard.edu>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
CC:	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

> 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.

	David

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