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

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Date:	Mon, 18 Nov 2013 15:03:23 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	Alan Stern <stern@...land.harvard.edu>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	<netdev@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] usb: xhci: Link TRB must not occur with a USB payload
 burst.

On Mon, 2013-11-18 at 09:48 +0000, David Laight wrote:
> > > But the minimum fragment size is (probably) 4k.
> > > For the network stack an OUT transfer might have a lot (and I mean lots)
> > > of fragments (there may be constraints, and linearising the skb is a option).
> > [...]
> > 
> > The maximum number of fragments in the skb is going to be 17 (including
> > the 'head' area).  (I'm ignoring NETIF_F_FRAGLIST which is not normally
> > supported by physical device drivers.)
> > 
> > I don't know how many fragments that can end up as, at the USB level.
> 
> If you assume that every fragment crosses a 64k boundary that would be 34.
> OTOH I've not seen a fragment of a 64k TSO send crossing a 32k
> boundary, and I think the 'head' area is constrained to be part of
> a single (4k or larger) page.

I don't know that it's possible at the moment, but I wouldn't recommend
relying on that.

> Isn't there something odd about skb merged by receive offload?
> I've not entirely sorted out the full structure of skb.

There has been some work to allow for using both the frags array and
frag list, but a driver will not see such an skb if it does not
advertise the NETIF_F_FRAGLIST feature.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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