netdev - [PATCH] dsa: tag_rtl4_a: Bump min packet size

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Message-Id: <20231027-fix-rtl8366rb-v1-1-d565d905535a@linaro.org>
Date: Fri, 27 Oct 2023 22:21:39 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Andrew Lunn <andrew@...n.ch>, Florian Fainelli <f.fainelli@...il.com>, 
 Vladimir Oltean <olteanv@...il.com>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH] dsa: tag_rtl4_a: Bump min packet size

It was reported that the "LuCI" web UI was not working properly
with a device using the RTL8366RB switch. Disabling the egress
port tagging code made the switch work again, but this is not
a good solution as we want to be able to direct traffic to a
certain port.

It turns out that sometimes, but not always, small packets are
dropped by the switch for no reason.

If we pad the ethernet frames to a minimum of ETH_FRAME_LEN + FCS
(1518 bytes) everything starts working fine.

As we completely lack datasheet or any insight into how this
switch works, this trial-and-error solution is the best we
can do.

I have tested smaller sizes, ETH_FRAME_LEN doesn't work for
example.

Fixes: 0e90dfa7a8d8 ("net: dsa: tag_rtl4_a: Fix egress tags")
Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
---
 net/dsa/tag_rtl4_a.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/dsa/tag_rtl4_a.c b/net/dsa/tag_rtl4_a.c
index c327314b95e3..8b1e81a6377b 100644
--- a/net/dsa/tag_rtl4_a.c
+++ b/net/dsa/tag_rtl4_a.c
@@ -41,8 +41,11 @@ static struct sk_buff *rtl4a_tag_xmit(struct sk_buff *skb,
 	u8 *tag;
 	u16 out;

-	/* Pad out to at least 60 bytes */
-	if (unlikely(__skb_put_padto(skb, ETH_ZLEN, false)))
+	/* We need to pad out to at least ETH_FRAME_LEN + FCS bytes. This was
+	 * found by trial-and-error. Sometimes smaller frames work, but sadly
+	 * not always.
+	 */
+	if (unlikely(__skb_put_padto(skb, ETH_FRAME_LEN + ETH_FCS_LEN, false)))
 		return NULL;

 	netdev_dbg(dev, "add realtek tag to package to port %d\n",

---
base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
change-id: 20231027-fix-rtl8366rb-e752bd5901ca

Best regards,
-- 
Linus Walleij <linus.walleij@...aro.org>