linux-kernel - Re: [PATCH net-next 2/2] firewire: net: set initial MTU = 1500 unconditionally, fix IPv6 on some CardBus cards

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Message-Id: <20161026.172915.2201926740859755129.davem@davemloft.net>
Date:   Wed, 26 Oct 2016 17:29:15 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     stefanr@...6.in-berlin.de
Cc:     netdev@...r.kernel.org, linux1394-devel@...ts.sourceforge.net,
        jarod@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/2] firewire: net: set initial MTU = 1500
 unconditionally, fix IPv6 on some CardBus cards

From: Stefan Richter <stefanr@...6.in-berlin.de>
Date: Sun, 23 Oct 2016 16:30:56 +0200

> 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>

Applied.