linux-kernel - Re: [PATCH net-next v2 1/1] net: dsa: microchip: ksz9477: implement MTU configuration

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Date:   Tue, 8 Mar 2022 11:06:44 +0100
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Woojung Huh <woojung.huh@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vivien Didelot <vivien.didelot@...il.com>,
        kernel@...gutronix.de, Jakub Kicinski <kuba@...nel.org>,
        UNGLinuxDriver@...rochip.com,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v2 1/1] net: dsa: microchip: ksz9477: implement
 MTU configuration

On Fri, Feb 25, 2022 at 06:35:43PM +0200, Vladimir Oltean wrote:
> On Fri, Feb 25, 2022 at 01:54:30PM +0100, Oleksij Rempel wrote:
> > > No bridge, why create a bridge? And even if you do, why add lan5 to it?
> > > The expectation is that standalone ports still remain functional when
> > > other ports join a bridge.
> > 
> > No, lan5 is not added to the bridge, but stops functioning after creating
> > br with lan1 or any other lanX
> 
> Please take time to investigate the problem and fix it.

ack

> > > I was saying:
> > > 
> > > ip link set lan1 up
> > > ip link add link lan1 name lan1.5 type vlan id 5
> > > ip addr add 172.17.0.2/24 dev lan1.5 && ip link set lan1.5 up
> > > iperf3 -c 172.17.0.10
> > 
> > It works.
> 
> This is akin to saying that without any calls to ksz9477_change_mtu(),
> just writing VLAN_ETH_FRAME_LEN + ETH_FCS_LEN into REG_SW_MTU__2 is
> sufficient to get VLAN-tagged MTU-sized packets to pass through the CPU
> port and the lan1 user port.
> 
> So my question is: is this necessary?
> 
> 	if (dsa_is_cpu_port(ds, port))
> 		new_mtu += KSZ9477_INGRESS_TAG_LEN;
> 

No.

I did some extra tests with following results: REG_SW_MTU__2 should be
configured to 1518 to pass 1514 frame. Independent if the frame is
passed between external ports or external to CPU port. So, I assume,
ETH_FRAME_LEN + ETH_FCS_LEN should be used instead of VLAN_ETH_FRAME_LEN
+ ETH_FCS_LEN. Correct?

Regards,
Oleksij
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