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

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Message-ID: <CABikg9xi4_i-Qn4MqBJTi1HE9pbUKna+xUydu4e2XwofAnxxVA@mail.gmail.com>
Date:   Wed, 19 Oct 2022 19:25:19 +0300
From:   Sergei Antonov <saproj@...il.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, andrew@...n.ch,
        Vladimir Oltean <olteanv@...il.com>
Subject: Re: [PATCH v4 net-next] net: ftmac100: support mtu > 1500

On Wed, 19 Oct 2022 at 19:21, Sergei Antonov <saproj@...il.com> 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
>
> Do the following to let the driver receive these packets.
> Set FTMAC100_MACCR_RX_FTL when mtu>1500 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.
>
> Fixes: 8d77c036b57c ("net: add Faraday FTMAC100 10/100 Ethernet driver")
> Signed-off-by: Sergei Antonov <saproj@...il.com>
> Suggested-by: Vladimir Oltean <olteanv@...il.com>
> ---

Sorry, forgot the changelog:
v4:
* Set FTMAC100_MACCR_RX_FTL depending on the "mtu > 1500" condition.
* DSA tagging seems unrelated to the issue - updated description and a
code comment accordingly.

v3:
* Corrected the explanation of the problem: datasheet is correct.
* Rewrote the code to use the currently set mtu to handle DSA frames.

v2:
* Typos in description fixed.