Message-ID: <a2f99c07-f9d0-7e35-61bb-cf3bf76ca919@gmail.com>
Date:   Wed, 15 Mar 2023 11:34:18 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.7.1
Subject: Re: [PATCH net 1/2] net: dsa: don't error out when drivers return
 ETH_DATA_LEN in .port_max_mtu()
Content-Language: en-US
To:     Vladimir Oltean <vladimir.oltean@nxp.com>, netdev@vger.kernel.org
Cc:     Andrew Lunn <andrew@lunn.ch>,
        Murali Krishna Policharla <murali.policharla@broadcom.com>,
        Lukasz Majewski <lukma@denx.de>,
        Russell King <linux@armlinux.org.uk>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org
References: <20230314182405.2449898-1-vladimir.oltean@nxp.com>
 <20230314182405.2449898-2-vladimir.oltean@nxp.com>
From:   Florian Fainelli <f.fainelli@gmail.com>
In-Reply-To: <20230314182405.2449898-2-vladimir.oltean@nxp.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 3/14/23 11:24, 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@nxp.com>

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
-- 
Florian