linux-kernel - 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 09:24:09 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, woojung.huh@...rochip.com,
        UNGLinuxDriver@...rochip.com, vivien.didelot@...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

On 7/15/21 6:08 AM, Andrew Lunn wrote:
>> - 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.

Yes, the SYSTEMPORT MAC as well as bgmac to some extent will
automagically work, but they have to be told to expect a Broadcom tag in
order to generate an appropriate checksum on receive. This is why we
have this:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/broadcom/bcmsysport.c#n144

Likewise, for TX:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/broadcom/bcmsysport.c#n172
-- 
Florian