netdev - Re: [net-next PATCH 16/19] net: dsa: qca8k: enable mtu_enforcement

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Date:   Fri, 19 Nov 2021 04:20:08 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Ansuel Smith <ansuelsmth@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [net-next PATCH 16/19] net: dsa: qca8k: enable
 mtu_enforcement_ingress

On Wed, Nov 17, 2021 at 10:04:48PM +0100, Ansuel Smith wrote:
> qca8k have a global MTU. Inform DSA of this as the change MTU port
> function checks the max MTU across all port and sets the max value
> anyway as this switch doesn't support per port MTU.
> 
> Signed-off-by: Ansuel Smith <ansuelsmth@...il.com>
> ---
>  drivers/net/dsa/qca8k.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/net/dsa/qca8k.c b/drivers/net/dsa/qca8k.c
> index c3234988aabf..cae58753bb1f 100644
> --- a/drivers/net/dsa/qca8k.c
> +++ b/drivers/net/dsa/qca8k.c
> @@ -1315,6 +1315,9 @@ qca8k_setup(struct dsa_switch *ds)
>  	/* Set max number of LAGs supported */
>  	ds->num_lag_ids = QCA8K_NUM_LAGS;
>  
> +	/* Global MTU. Inform dsa that per port MTU is not supported */
> +	ds->mtu_enforcement_ingress = true;
> +
>  	return 0;
>  }
>  
> -- 
> 2.32.0
> 

This doesn't do what you think it does. If you want the dev->mtu of all
interfaces to get updated at once, you need to do that yourself. Setting
ds->mtu_enforcement_ingress will only update the MTU for ports belonging
to the same bridge, and for a different reason. Or I'm missing the
reason why you're making this change now.