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

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Message-ID: <20210715114908.ripblpevmdujkf2m@skbuf>
Date:   Thu, 15 Jul 2021 14:49:08 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     Andrew Lunn <andrew@...n.ch>, 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

On Thu, Jul 15, 2021 at 01:16:12PM +0200, Lino Sanfilippo wrote:
> Sure, I will test this solution. But I think NETIF_F_FRAGLIST should also be
> cleared in this case, right?

Hmm, interesting question. I think only hns3 makes meaningful use of
NETIF_F_FRAGLIST, right? I'm looking at hns3_fill_skb_to_desc().
Other drivers seem to set it for ridiculous reasons - looking at commit
66aa0678efc2 ("ibmveth: Support to enable LSO/CSO for Trunk VEA.") -
they set NETIF_F_FRAGLIST and then linearize the skb chain anyway. The
claimed 4x throughput benefit probably has to do with less skbs
traversing the stack? I don't know.

Anyway, it is hard to imagine all the things that could go wrong with
chains of IP fragments on a DSA interface, precisely because I have so
few examples to look at. I would say, header taggers are probably fine,
tail taggers not so much, so apply the same treatment as for NETIF_F_SG?