linux-kernel - Re: [PATCH net v2] net: dsa: tag_rtl4

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Date:   Tue, 31 Oct 2023 00:20:35 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
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 10:50:31PM +0100, Linus Walleij wrote:
> > you said that what increments is Dot1dTpPortInDiscards
> No this was a coincidence, we can rule this out.

I see commit 86dd9868b878 ("net: dsa: tag_rtl4_a: Support also egress tags")
also mentions: "Qingfang came up with the solution: we need to pad the
ethernet frame to 60 bytes using eth_skb_pad(), then the switch will
happily accept frames with custom tags.". So the __skb_put_padto() was
something very empirical in the first place.

Since it's all problematic, would you mind removing the __skb_put_padto()
altogether from rtl4a_tag_xmit(), and let me know what is the output for
the following sweep through packet sizes? I truly wonder if it's just
for small and large packets that we see packet drops, or if it's something
repetitive throughout the range as well.

for size in $(seq 0 1476); do if ping 10.0.0.56 -s $size -W 1 -c 1 -q >/dev/null; then echo "$((size + 42)): OK"; else echo "$((size + 42)): NOK"; fi; done