netdev - stmmac: zero udp checksum

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Message-ID: <cfc1ede9-4a1c-1a62-bdeb-d1e54c27f2e7@freifunk-rtk.de>
Date:   Mon, 5 Apr 2021 14:53:15 +0200
From:   Julian Labus <julian@...ifunk-rtk.de>
To:     netdev@...r.kernel.org
Cc:     mschiffer@...verse-factory.net
Subject: stmmac: zero udp checksum

Hi all,

in our community mesh network we recently discovered that a TP-Link 
Archer C2600 device is unable to receive IPv6 UDP packets with a zero 
checksum when RX checksum offloading is enabled. The device uses 
ipq806x-gmac-dwmac for its ethernet ports.

According to https://tools.ietf.org/html/rfc2460#section-8.1 this sounds 
like correct behavior as it says a UDP checksum must not be zero for 
IPv6 packets. But this definition was relaxed in 
https://tools.ietf.org/html/rfc6935#section-5 to allow zero checksums in 
tunneling protocols like VXLAN where we discovered the problem.

Can the behavior of the stmmac driver be changed to meet RFC6935 or 
would it be possible to make the (RX) Checksum Offloading Engine 
configurable via a device tree property to disable it in environments 
were it causes problems?

Best regards,
Julian Labus