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

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Date:   Tue, 8 Mar 2022 12:59:55 +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 Tue, Mar 08, 2022 at 01:21:56PM +0200, Vladimir Oltean wrote:
> On Tue, Mar 08, 2022 at 11:06:44AM +0100, Oleksij Rempel wrote:
> > > > > 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?
> 
> Oleksij, to be clear, I only had an issue with consistency.
> You were adding KSZ9477_INGRESS_TAG_LEN during ksz9477_change_mtu() but
> not during initial setup. That prompted the question: is that particular
> member of the sum needed or not? Either it's needed in both places, or
> in none.
> 
> Then, apart from removing KSZ9477_INGRESS_TAG_LEN, you've also made an
> unsolicited change (subtracted VLAN_HLEN from the value programmed to
> hardware) without a clear confirmation that you understand what this
> does, and without explicitly saying that the iperf3 test above still
> works with this formula applied.
> 
> Since the VLAN header is part of L2, it means that a port configured for
> MTU 1500 must also support VLAN-tagged packets with an L2 payload of
> 1500 octets. 1500 + ETH_HLEN + VLAN_HLEN == 1518 octets.

Ah.. now it solves my brain knot.

> And since you need to add ETH_HLEN + ETH_FCS_LEN, I have an unconfirmed
> hunch that VLAN_HLEN is also needed for the case above.

Yes, you are right.

> So, I'm sorry for being paranoid, but you aren't really giving me a
> choice but to ask again, and again.

Thank you for being paranoid. Sorry, i'll send new patch.

Regards,
Oleksij
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