Re: [PATCH net 1/2] net: dsa: don't error out when drivers return ETH_DATA_LEN in .port_max_mtu()

[Simon Horman]

On Tue, Mar 14, 2023 at 08:24:04PM +0200, Vladimir Oltean wrote:
> Currently, when dsa_slave_change_mtu() is called on a user port where
> dev->max_mtu is 1500 (as returned by ds->ops->port_max_mtu()), the code
> will stumble upon this check:
> 
> 	if (new_master_mtu > mtu_limit)
> 		return -ERANGE;
> 
> because new_master_mtu is adjusted for the tagger overhead but mtu_limit
> is not.
> 
> But it would be good if the logic went through, for example if the DSA
> master really depends on an MTU adjustment to accept DSA-tagged frames.
> 
> To make the code pass through the check, we need to adjust mtu_limit for
> the overhead as well, if the minimum restriction was caused by the DSA
> user port's MTU (dev->max_mtu). A DSA user port MTU and a DSA master MTU
> are always offset by the protocol overhead.
> 
> Currently no drivers return 1500 .port_max_mtu(), but this is only
> temporary and a bug in itself - mv88e6xxx should have done that, but
> since commit b9c587fed61c ("dsa: mv88e6xxx: Include tagger overhead when
> setting MTU for DSA and CPU ports") it no longer does. This is a
> preparation for fixing that.
> 
> Fixes: bfcb813203e6 ("net: dsa: configure the MTU for switch ports")
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>

Reviewed-by: Simon Horman <simon.horman@xxxxxxxxxxxx>