netdev - RE: [PATCH] usb: xhci: Link TRB must not occur with a USB payload burst.

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Message-ID: <A2CA0424C0A6F04399FB9E1CD98E030458E28EB7@US01WEMBX2.internal.synopsys.com>
Date:	Wed, 20 Nov 2013 21:11:37 +0000
From:	Paul Zimmerman <Paul.Zimmerman@...opsys.com>
To:	David Laight <David.Laight@...LAB.COM>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
CC:	Alan Stern <stern@...land.harvard.edu>,
	Ben Hutchings <bhutchings@...arflare.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: RE: [PATCH] usb: xhci: Link TRB must not occur with a USB payload
 burst.

> From: linux-usb-owner@...r.kernel.org [mailto:linux-usb-owner@...r.kernel.org] On Behalf Of David Laight
> Sent: Wednesday, November 20, 2013 9:16 AM
> 
> > Ok, so the networking layer should be fine.  However, with the current
> > patch, if the mass storage driver sends down a scatter-gather list
> > that's bigger than a ring segment, or needs to be split up so it doesn't
> > cross 64K boundaries, then the URB submission will fail.  We don't want
> > that to happen.
> 
> My suspicion is that long SG lists are unusual - otherwise the
> ring expansion code would have been needed much earlier.
> 
> Can anyone remember whether that was needed because of long SG lists
> or because of large numbers of outstanding requests?
> 
> I've seen it for network cards - but only because usbnet sends
> down far too many tx buffers.

usb-storage limits the maximum transfer size to 120K. That is a max of
31 page-size segments if my math is right. That's probably why mass-storage
never saw a problem.

-- 
Paul

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