netdev - Re: [PATCH v4 net-next] net: ftmac100: support mtu

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Date:   Wed, 19 Oct 2022 21:42:03 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Sergei Antonov <saproj@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, andrew@...n.ch
Subject: Re: [PATCH v4 net-next] net: ftmac100: support mtu > 1500

On Wed, Oct 19, 2022 at 09:36:21PM +0300, Sergei Antonov wrote:
> On Wed, 19 Oct 2022 at 19:55, Vladimir Oltean <olteanv@...il.com> wrote:
> >
> > On Wed, Oct 19, 2022 at 07:20:58PM +0300, Sergei Antonov wrote:
> > > The ftmac100 controller considers some packets FTL (frame
> > > too long) and drops them. An example of a dropped packet:
> > > 6 bytes - dst MAC
> > > 6 bytes - src MAC
> > > 2 bytes - EtherType IPv4 (0800)
> > > 1504 bytes - IPv4 packet
> >
> > Why do you insist writing these confusing messages?
> 
> Yes, I see now it is not good. And I have a question.
> By comparing the packet sent from the sending computer and the packet
> received by ftmac100 I found that 4 bytes get appended to the packet:
> 80 02 00 00
> I guess it is what 88E6060 switch adds. But it is not 0x8100. Then
> what it is? I looked into include/linux/if_vlan.h but still don't get
> it.

See trailer_rcv() in net/dsa/tag_trailer.c. The switch is telling you
that this is a packet received from source port 2 via a tail tag
(trailer, i.e. added to the end of the packet, meaning right before the FCS).
I know that mv88e6060 uses this tagging protocol because this is what
mv88e6060_get_tag_protocol() returns. Still nothing to do with if_vlan.h.

Also see if Documentation/networking/dsa/dsa.rst answers some of the
other questions.