[PATCH net 1/1] net: stmmac: Prevent DSA tags from breaking COE

[Maxime Chevallier]

Hi Vlad,

+ Linus Walleij

On Tue, 19 Dec 2023 14:20:34 +0200
Vladimir Oltean <olteanv at gmail.com> wrote:

> Hi Romain,
> 
> On Mon, Dec 18, 2023 at 05:23:23PM +0100, Romain Gantois wrote:
> > Some stmmac cores have Checksum Offload Engines that cannot handle DSA tags
> > properly. These cores find the IP/TCP headers on their own and end up
> > computing an incorrect checksum when a DSA tag is inserted between the MAC
> > header and IP header.
> > 
> > Add an additional check on stmmac link up so that COE is deactivated
> > when the stmmac device is used as a DSA conduit.
> > 
> > Add a new dma_feature flag to allow cores to signal that their COEs can't
> > handle DSA tags on TX.
> > 
> > Fixes: 6b2c6e4a938f ("net: stmmac: propagate feature flags to vlan")
> > Cc: stable at vger.kernel.org
> > Reported-by: Richard Tresidder <rtresidd at electromag.com.au>
> > Closes: https://lore.kernel.org/netdev/e5c6c75f-2dfa-4e50-a1fb-6bf4cdb617c2@electromag.com.au/
> > Reported-by: Romain Gantois <romain.gantois at bootlin.com>
> > Closes: https://lore.kernel.org/netdev/c57283ed-6b9b-b0e6-ee12-5655c1c54495@bootlin.com/
> > Signed-off-by: Romain Gantois <romain.gantois at bootlin.com>
> > ---  
> 
> DSA_TAG_PROTO_LAN9303, DSA_TAG_PROTO_SJA1105 and DSA_TAG_PROTO_SJA1110
> construct tags with ETH_P_8021Q as EtherType. Do you still think it
> would be correct to say that all DSA tags break COE on the stmmac, as
> this patch assumes?
> 
> The NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM convention is not about
> statically checking whether the interface using DSA, but about looking
> at each packet before deciding whether to use the offload engine or to
> call skb_checksum_help().

So it looks like an acceptable solution would be something along the
lines of what Linus is suggesting here :

https://lore.kernel.org/netdev/20231216-new-gemini-ethernet-regression-v2-2-64c269413dfa@linaro.org/

If so, maybe it's worth adding a new helper for that check ?

Maxime