netdev - Re: [PATCH v3 net] net: ftmac100: support frames with DSA tag

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Message-ID: <CABikg9ziSUJ5_DWro_TgCDMRYmgBfQNaEjKrCFQjaoUhkwRZOg@mail.gmail.com>
Date:   Wed, 19 Oct 2022 16:57:05 +0300
From:   Sergei Antonov <saproj@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH v3 net] net: ftmac100: support frames with DSA tag

On Mon, 17 Oct 2022 at 18:55, Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Thu, Oct 13, 2022 at 06:57:24PM +0300, Sergei Antonov wrote:
> > Fixes the problem when frames coming from DSA were discarded.
> > DSA tag might make frame size >1518. Such frames are discarded
> > by the controller when FTMAC100_MACCR_RX_FTL is not set in the
> > MAC Control Register, see datasheet [1].
> >
> > Set FTMAC100_MACCR_RX_FTL in the MAC Control Register.
> > For received packets marked with FTMAC100_RXDES0_FTL check if packet
> > length (with FCS excluded) is within expected limits, that is not
> > greater than netdev->mtu + 14 (Ethernet headers). Otherwise trigger
> > an error. In the presence of DSA netdev->mtu is 1504 to accommodate
> > for VLAN tag.
>
> Which VLAN tag do you mean? Earlier you mentioned a tail tag as can be
> seen and parsed by net/dsa/tag_trailer.c. Don't conflate the two, one
> has nothing to do with the other.

I used print_hex_dump_bytes() in the ftmac100 driver to see what is
inside the received packet. Here is how the 1518 byte long packet
looks:
6 bytes - dst MAC
6 bytes - src MAC
4 bytes - 08 00 45 10
2 bytes - 0x5dc - data length (1500)
1500 bytes - data

I am not sure what is the proper name for these 08 00 45 10 bytes.
Tell me the correct name to use in the next version of the patch :).
Thanks for your feedback. I will make an improved version of the patch
targeted for net-next.