netdev - Re: [PATCH net v2] net: dsa: tag_rtl4

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Date: Tue, 31 Oct 2023 23:44:58 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Vladimir Oltean <olteanv@...il.com>
Cc: Andrew Lunn <andrew@...n.ch>, Florian Fainelli <f.fainelli@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2] net: dsa: tag_rtl4_a: Bump min packet size

On Mon, Oct 30, 2023 at 3:16 PM Vladimir Oltean <olteanv@...il.com> wrote:

>         if (skb->len >= 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 tag).
> 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