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Date:   Mon, 24 Oct 2016 23:05:14 -0400
From:   Jarod Wilson <jarod@...hat.com>
To:     Stefan Richter <stefanr@...6.in-berlin.de>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux1394-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/2 v2] firewire: net: set initial MTU = 1500
 unconditionally, fix IPv6 on some CardBus cards

On Mon, Oct 24, 2016 at 02:26:13PM +0200, Stefan Richter wrote:
> firewire-net, like the older eth1394 driver, reduced the initial MTU to
> less than 1500 octets if the local link layer controller's asynchronous
> packet reception limit was lower.
> 
> This is bogus, since this reception limit does not have anything to do
> with the transmission limit.  Neither did this reduction affect the TX
> path positively, nor could it prevent link fragmentation at the RX path.
> 
> Many FireWire CardBus cards have a max_rec of 9, causing an initial MTU
> of 1024 - 16 = 1008.  RFC 2734 and RFC 3146 allow a minimum max_rec = 8,
> which would result in an initial MTU of 512 - 16 = 496.  On such cards,
> IPv6 could only be employed if the MTU was manually increased to 1280 or
> more, i.e. IPv6 would not work without intervention from userland.
> 
> We now always initialize the MTU to 1500, which is the default according
> to RFC 2734 and RFC 3146.
> 
> On a VIA VT6316 based CardBus card which was affected by this, changing
> the MTU from 1008 to 1500 also increases TX bandwidth by 6 %.
> RX remains unaffected.
> 
> CC: netdev@...r.kernel.org
> CC: linux1394-devel@...ts.sourceforge.net
> CC: Jarod Wilson <jarod@...hat.com>
> Signed-off-by: Stefan Richter <stefanr@...6.in-berlin.de>
> ---
> v2: use ETH_DATA_LEN, add comment

Acked-by: Jarod Wilson <jarod@...hat.com>

-- 
Jarod Wilson
jarod@...hat.com

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