netdev - Re: [PATCH v2 net] net: mscc: ocelot: properly account for VLAN header length when setting MRU

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Date:   Mon, 09 Mar 2020 18:58:45 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     olteanv@...il.com
Cc:     horatiu.vultur@...rochip.com, alexandre.belloni@...tlin.com,
        andrew@...n.ch, f.fainelli@...il.com, vivien.didelot@...il.com,
        joergen.andreasen@...rochip.com, allan.nielsen@...rochip.com,
        claudiu.manoil@....com, netdev@...r.kernel.org,
        UNGLinuxDriver@...rochip.com, alexandru.marginean@....com,
        xiaoliang.yang_1@....com, yangbo.lu@....com
Subject: Re: [PATCH v2 net] net: mscc: ocelot: properly account for VLAN
 header length when setting MRU

From: Vladimir Oltean <olteanv@...il.com>
Date: Tue, 10 Mar 2020 03:28:18 +0200

> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> What the driver writes into MAC_MAXLEN_CFG does not actually represent
> VLAN_ETH_FRAME_LEN but instead ETH_FRAME_LEN + ETH_FCS_LEN. Yes they are
> numerically equal, but the difference is important, as the switch treats
> VLAN-tagged traffic specially and knows to increase the maximum accepted
> frame size automatically. So it is always wrong to account for VLAN in
> the MAC_MAXLEN_CFG register.
> 
> Unconditionally increase the maximum allowed frame size for
> double-tagged traffic. Accounting for the additional length does not
> mean that the other VLAN membership checks aren't performed, so there's
> no harm done.
> 
> Also, stop abusing the MTU name for configuring the MRU. There is no
> support for configuring the MRU on an interface at the moment.
> 
> Fixes: a556c76adc05 ("net: mscc: Add initial Ocelot switch support")
> Fixes: fa914e9c4d94 ("net: mscc: ocelot: create a helper for changing the port MTU")
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> ---
> Changes in v2:
> Rebased on net tree.

Applied and queued up for -stable, thank you.