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

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Message-ID: <20260114221743.tb56lxfi6eqoh2rw@skbuf>
Date: Thu, 15 Jan 2026 00:17:43 +0200
From: Vladimir Oltean <olteanv@...il.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Daniel Golle <daniel@...rotopia.org>, 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>,
	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

On Tue, Jan 13, 2026 at 03:00:18PM +0100, Andrew Lunn wrote:
> On Tue, Jan 13, 2026 at 03:11:54AM +0000, Daniel Golle wrote:
> > MediaTek's Ethernet Frame Engine is tailored for use with their
> > switches. This broke checksum and VLAN offloading when attaching a
> > DSA switch which does not use MediaTek special tag format.
> 
> This has been seen before. The Freescale FEC has similar problems when
> combined with a Marvell switch, it cannot find the IP header, and so
> checksum offloading does not work.
> 
> I thought we solved this be modifying the ndev->feature of the conduit
> interface to disable such offloads. But i don't see such code. So i
> must be remembering wrongly.
> 
> This is assuming the frame engine respects these flags:
> 
> /usr/sbin/ethtool -k enp2s0
> Features for enp2s0:
> rx-checksumming: on
> tx-checksumming: on
> 	tx-checksum-ipv4: on
> 	tx-checksum-ip-generic: off [fixed]
> 	tx-checksum-ipv6: on
> 	tx-checksum-fcoe-crc: off [fixed]
> 	tx-checksum-sctp: off [fixed]
> 
> When you combine a Marvell Ethernet interface with a Marvell switch
> offloading works of course. So it probably does require some logic in
> the MAC driver to determine if the switch is of the same vendor or
> not.

I don't know about FEC, but we did end up documenting this:

If the DSA conduit driver still uses the legacy NETIF_F_IP_CSUM
or NETIF_F_IPV6_CSUM in vlan_features, the offload might only work if the
offload hardware already expects that specific tag (perhaps due to matching
vendors). DSA user ports inherit those flags from the conduit, and it is up to
the driver to correctly fall back to software checksum when the IP header is not
where the hardware expects.

I would suggest searching for skb_checksum_help() in xmit() implementations.
For example c2945c435c99 ("net: stmmac: Prevent DSA tags from breaking COE").

This is just at a first glance having read the commit message and the
discussion, not necessarily the code.