Re: [PATCH 2/2] net: dsa: tag_ksz: dont let the hardware process the layer 4 checksum

[Andrew Lunn]

> - If we inherit NETIF_F_HW_CSUM from the master for tail taggers, it is
>   actively detrimential to keep this feature enabled, as proven my Lino.
>   As for header taggers, I fail to see how this would be helpful, since
>   the DSA master would always fail to see the real IP header (it has
>   been pushed to the right by the DSA tag), and therefore, the DSA
>   master offload would be effectively bypassed.

The Marvell MACs know about DSA and should be able to perform hardware
checksumming. It is a long time since i looked at how this works, but
i think there is a field in the descriptor which gets set with the
offset to the IP header, so it work for DSA as well as EDSA.

I _think_ Broadcom MACs also know about Broadcom tags and can do the
right thing.

So we need to be a bit careful here to prevent performance regressions
for same vendor MAC+Switch combinations.

    Andrew