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

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Date:   Thu, 15 Jul 2021 15:08:53 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, woojung.huh@...rochip.com,
        UNGLinuxDriver@...rochip.com, vivien.didelot@...il.com,
        f.fainelli@...il.com, davem@...emloft.net, kuba@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] net: dsa: tag_ksz: dont let the hardware process the
 layer 4 checksum

> - 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