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:

quoted

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.