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

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Date:	Sat, 16 Nov 2013 01:38:24 +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 Wed, 2013-11-13 at 16:58 +0000, David Laight wrote:
[...]
> > > > > You can split a bulk TD on a 1k boundary and the target won't know the
> > > > > difference.
> > > >
> > > > The target won't know the difference, but the host will.  Here's an
> > > > example: Suppose the driver submits two URBs, each for a data-in
> > > > transfer of 32 KB.  You split each URB up into two 16-KB TDs; let's
> > > > call them A, B, C, and D (where A and B make up the first URB, and C
> > > > and D make up the second URB).
> > >
> > > I was thinking about OUT transfers, IN ones are unlikely to be badly
> > > fragmented.
> > 
> > Maybe not for the network stack, but OUT and IN end up equally
> > fragmented for the storage stack.
> 
> 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.

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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