netdev - Re: [PATCH v3 net-next 2/2] net: deprecate eth_change

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Message-Id: <20161017.132553.1015830249902227215.davem@davemloft.net>
Date:   Mon, 17 Oct 2016 13:25:53 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     kubakici@...pl
Cc:     jarod@...hat.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v3 net-next 2/2] net: deprecate eth_change_mtu, remove
 usage

From: Jakub Kicinski <kubakici@...pl>
Date: Mon, 17 Oct 2016 18:20:49 +0100

> Hm.  I must be missing something really obvious.  I just booted
> net-next an hour ago and couldn't set MTU to anything larger than 1500
> on either nfp or igb.  As far as I can read the code it will set the
> max_mtu to 1500 in setup_ether() but none of the jumbo-capable drivers
> had been touched by Jarod so far...

Indeed.

Jarod, this doesn't work.

I guess the idea was that if the driver overrides ndo_change_mtu and
enforeced it's limits there, the driver would still work after your
changes.

But that isn't what is happening, look at the IGB example.

It uses ether_setup(), which sets those new defaults, but now when
the MTU is changed you enforce those default min/max before the
driver's ->ndo_change_mtu() has a change to step in front and make
the decision on it's own.

This means your changes pretty much did indeed break a lot of
drivers's ability to set larger than a 1500 byte MTU.