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References: <20231030-fix-rtl8366rb-v2-1-e66e1ef7dbd2@linaro.org> <20231030141623.ufzhb4ttvxi3ukbj@skbuf>
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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Tue, 31 Oct 2023 23:44:58 +0100
Message-ID: <CACRpkdZUYg6APYU4vz5XRUDSp8P0jndgZH-J_q27MUsmZkQODw@mail.gmail.com>
Subject: Re: [PATCH net v2] net: dsa: tag_rtl4_a: Bump min packet size
To:     Vladimir Oltean <olteanv@gmail.com>
Cc:     Andrew Lunn <andrew@lunn.ch>,
        Florian Fainelli <f.fainelli@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Mon, Oct 30, 2023 at 3:16=E2=80=AFPM Vladimir Oltean <olteanv@gmail.com>=
 wrote:

>         if (skb->len >=3D 1496)
>                 __skb_put_padto(skb, 1518, false);
>
>         (...)
>
>         skb_push(skb, 4);
>
> which means that here, skb->len will be 1522, if it was originally 1496.
> So the code adds 26 extra octets, and only 4 of those are legitimate (a t=
ag).
> The rest is absolutely unexplained, which means that until there is a
> valid explanation for them:

Indeed only 4 of them are needed, I tested to add 4 bytes on the tail
for > 1496 and it works.

I'll send a new version.

However that its "tag" is bogus since the extra tail isn't needed before
the paket becomes 1496 bytes, i.e. 1500 bytes including the tag.
It has a logic to it but ... yeah. All I can think of is bad VHDL in the
switch.

Yours,
Linus Walleij