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The MTU callbacks are in layer 1 size, so for example 1500 bytes is a normal setting. Cache this size, and only add the layer 2 framing right before choosing the setting. On the CPU port this will however include the DSA tag since this is transmitted from the parent ethernet interface! Add the layer 2 overhead such as ethernet and VLAN framing and FCS before selecting the size in the register. This will make the code easier to understand. The rtl8366rb_max_mtu() callback returns a bogus MTU just subtracting the CPU tag, which is the only thing we should NOT subtract. Return the correct layer 1 max MTU after removing headers and checksum. Signed-off-by: Linus Walleij <linus.walleij@linaro.org> Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk> Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com> Signed-off-by: Paolo Abeni <pabeni@redhat.com>