netdev - Re: [PATCH net-next 2/2] net: dsa: Set the master device's MTU to account for DSA overheads

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Message-ID: <2cb7d5ba-5769-ad42-f4f1-e0828e2017f1@gmail.com>
Date:   Thu, 6 Dec 2018 12:21:31 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>, David Miller <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Vivien Didelot <vivien.didelot@...il.com>
Subject: Re: [PATCH net-next 2/2] net: dsa: Set the master device's MTU to
 account for DSA overheads

On 12/6/18 2:36 AM, Andrew Lunn wrote:
> DSA tagging of frames sent over the master interface to the switch
> increases the size of the frame. Such frames can then be bigger than
> the normal MTU of the master interface, and it may drop them. Use the
> overhead information from the tagger to set the MTU of the master
> device to include this overhead.
> 
> Signed-off-by: Andrew Lunn <andrew@...n.ch>
> ---
>  net/dsa/master.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/net/dsa/master.c b/net/dsa/master.c
> index c90ee3227dea..42f525bc68e2 100644
> --- a/net/dsa/master.c
> +++ b/net/dsa/master.c
> @@ -158,8 +158,24 @@ static void dsa_master_ethtool_teardown(struct net_device *dev)
>  	cpu_dp->orig_ethtool_ops = NULL;
>  }
>  
> +void dsa_master_set_mtu(struct net_device *dev, struct dsa_port *cpu_dp)
> +{
> +	unsigned int mtu = ETH_DATA_LEN + cpu_dp->tag_ops->overhead;
> +	int err;
> +
> +	rtnl_lock();
> +	if (mtu <= dev->max_mtu) {
> +		err = dev_set_mtu(dev, mtu);
> +		if (err)
> +			netdev_dbg(dev, "Unable to set MTU to include for DSA overheads\n");
> +	}

Would it make sense to warn the user that there might be
transmit/receive issues with the DSA tagging protocol if either
dev_set_mtu() fails or mtu > dev->max_mtu?

> +	rtnl_unlock();
> +}
> +
>  int dsa_master_setup(struct net_device *dev, struct dsa_port *cpu_dp)
>  {
> +	dsa_master_set_mtu(dev,  cpu_dp);

Not that I think it matters too much to people because unbinding the
switch driver and expecting the CPU port to continue operating is
wishful thinking, but we should probably unwind that operation in
dsa_master_teardown(), right?

> +
>  	/* If we use a tagging format that doesn't have an ethertype
>  	 * field, make sure that all packets from this point on get
>  	 * sent to the tag format's receive function.
> 


-- 
Florian