linux-kernel - Re: [PATCH net-next RFC] net: ethernet: mtk_eth_soc: support using non-MediaTek DSA switches

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Message-ID: <CAJq09z727C=9mh2a0BEVcVLKkWcdVOL7oNvVBq12k6zm7nxXZQ@mail.gmail.com>
Date: Tue, 13 Jan 2026 19:38:50 -0300
From: Luiz Angelo Daros de Luca <luizluca@...il.com>
To: Daniel Golle <daniel@...rotopia.org>
Cc: Andrew Lunn <andrew@...n.ch>, Felix Fietkau <nbd@....name>, John Crispin <john@...ozen.org>, 
	Sean Wang <sean.wang@...iatek.com>, Lorenzo Bianconi <lorenzo@...nel.org>, 
	Bc-bocun Chen <bc-bocun.chen@...iatek.com>, Rex Lu <rex.lu@...iatek.com>, 
	Mason-cw Chang <Mason-cw.Chang@...iatek.com>, Andrew Lunn <andrew+netdev@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Vladimir Oltean <olteanv@...il.com>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net-next RFC] net: ethernet: mtk_eth_soc: support using
 non-MediaTek DSA switches

> MediaTek folks also got back to me in a private message, confirming
> the issue and also clarifying that the length of the tag is the
> limiting factor. Every 4-byte tag can work, sizes other than 4 bytes
> cannot. As MediaTek's tag format includes the 802.1Q VLAN as part of
> the tag itself I suspect VLAN offloading will still need some extra
> care to work on non-MTK 4-byte tags (like RealTek 4B, for example)...

My suggestion is to enable it only when sure (mediatek tag) and drop
it otherwise. Something like this:

https://github.com/openwrt/openwrt/commit/2603d6d81d1a088c6ad271fe20a63fa6e3a75124

It is not worth it to risk enabling offload for unknown cases.

As Realtek was mentioned, the 4-byte tag (rtl4_a) is for old SoCs,
hardly paired with a mediatek SoC. The newer tag, rtl8_4, is already
too big (8 bytes).

Regards,

Luiz