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

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Date:   Fri, 25 Feb 2022 13:54:30 +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 01:58:02PM +0200, Vladimir Oltean wrote:
> On Fri, Feb 25, 2022 at 12:47:40PM +0100, Oleksij Rempel wrote:
> > On Thu, Feb 24, 2022 at 11:46:57AM +0200, Vladimir Oltean wrote:
> >  
> > > So where is it failing exactly? Here I guess, because mtu_limit will be
> > > negative?
> > > 
> > > 	mtu_limit = min_t(int, master->max_mtu, dev->max_mtu);
> > > 	old_master_mtu = master->mtu;
> > > 	new_master_mtu = largest_mtu + dsa_tag_protocol_overhead(cpu_dp->tag_ops);
> > > 	if (new_master_mtu > mtu_limit)
> > > 		return -ERANGE;
> > > 
> > > I don't think we can work around it in DSA, it's garbage in, garbage out.
> > > 
> > > In principle, I don't have such a big issue with writing the MTU
> > > register as part of the switch initialization, especially if it's global
> > > and not per port. But tell me something else. You pre-program the MTU
> > > with VLAN_ETH_FRAME_LEN + ETH_FCS_LEN, but in the MTU change procedure,
> > > you also add KSZ9477_INGRESS_TAG_LEN (2) to that. Is that needed at all?
> > > I expect that if it's needed, it's needed in both places. Can you
> > > sustain an iperf3 tcp session over a VLAN upper of a ksz9477 port?
> > > I suspect that the missing VLAN_HLEN is masking a lack of KSZ9477_INGRESS_TAG_LEN.
> > 
> > Hm... I assume I need to do something like this:
> > - build kernel with BRIDGE_VLAN_FILTERING
> > - |
> >   ip l a name br0 type bridge vlan_filtering 1
> >   ip l s dev br0 up
> >   ip l s lan1 master br0
> >   ip l s dev lan1 up
> >   bridge vlan add dev lan1 vid 5 pvid untagged
> >   ip link add link br0 name vlan5 type vlan id 5
> > 
> > I use lan5@ksz for net boot. As soon as i link lan1@ksz to the br0 with
> > vlan_filtering enabled, the nfs on lan5 will be broken. Currently I have
> > no time to investigate it. I'll try to fix VLAN support in a separate
> > task. What will is acceptable way to proceed with MTU patch?
> 
> 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

> 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.

Regards,
Oleksij
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