netdev - Re: [PATCH net-next v4 4/4] net: dsa: microchip: ksz8: add MTU configuration support

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Date:   Thu, 10 Nov 2022 15:03:25 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Woojung Huh <woojung.huh@...rochip.com>,
        UNGLinuxDriver@...rochip.com, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Arun Ramadoss <arun.ramadoss@...rochip.com>,
        kernel@...gutronix.de, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next v4 4/4] net: dsa: microchip: ksz8: add MTU
 configuration support

On Thu, Nov 10, 2022 at 01:22:25PM +0100, Oleksij Rempel wrote:
> Make MTU configurable on KSZ87xx and KSZ88xx series of switches.
> 
> Before this patch, pre-configured behavior was different on different
> switch series, due to opposite meaning of the same bit:
> - KSZ87xx: Reg 4, Bit 1 - if 1, max frame size is 1532; if 0 - 1514
> - KSZ88xx: Reg 4, Bit 1 - if 1, max frame size is 1514; if 0 - 1532
> 
> Since the code was telling "... SW_LEGAL_PACKET_DISABLE, true)", I
> assume, the idea was to set max frame size to 1532.
> 
> With this patch, by setting MTU size 1500, both switch series will be
> configured to the 1532 frame limit.
> 
> This patch was tested on KSZ8873.
> 
> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> Acked-by: Arun Ramadoss <arun.ramadoss@...rochip.com>
> ---

Reviewed-by: Vladimir Oltean <olteanv@...il.com>

As an extension to this patch set, you might also want to set
ds->mtu_enforcement_ingress = true, to activate the bridge MTU
normalization logic, since you have one MTU global to the entire switch.