netdev - RE: [PATCH 0/1] mv643xx_eth: Disable TSO by default

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Date:	Mon, 3 Nov 2014 14:51:50 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Tawfik Bayouk <tawfik@...vell.com>,
	Lior Amsalem <alior@...vell.com>,
	Nadav Haklai <nadavh@...vell.com>
Subject: RE: [PATCH 0/1] mv643xx_eth: Disable TSO by default

From: Eric Dumazet
> On Sat, 2014-11-01 at 12:30 -0300, Ezequiel Garcia wrote:
> > Several users ([1], [2]) have been reporting data corruption with TSO on
> > Kirkwood platforms (i.e. using the mv643xx_eth driver).
> >
> > Until we manage to find what's causing this, this simple patch will make
> > the TSO path disabled by default. This patch should be queued for stable,
> > fixing the TSO feature introduced in v3.16.
> >
> > The corruption itself is very easy to reproduce: checking md5sum on a mounted
> > NFS directory gives a different result each time. Same tests using the mvneta
> > driver (Armada 370/38x/XP SoC) pass with no issues.
> >
> > Frankly, I'm a bit puzzled about this, and so any ideas or debugging hints
> > are well received.
> 
> lack of barriers maybe ?
> 
> It seems you might need to populate all TX descriptors but delay the
> first, like doing the populate in descending order.
> 
> If you take a look at txq_submit_skb(), you'll see the final
> desc->cmd_sts = cmd_sts (line 959) is done _after_ frags were cooked by
> txq_submit_frag_skb()
> 
> You should kick the nick only when all TX descriptors are ready and
> committed to memory.

Don't forget that the nick might process the first descriptor without
being given a 'kick' - it will read it when it finishes processing the
previous frame.
This also means that you have to be careful about the order of the writes
to the first descriptor.

	David