netdev - Re: [PATCH net v5 1/2] net: ethernet: cortina: Drop software checksum and TSO

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Date: Sat, 6 Jan 2024 00:35:35 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Vladimir Oltean <olteanv@...il.com>
Cc: Hans Ulli Kroll <ulli.kroll@...glemail.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Household Cang <canghousehold@....com>, Romain Gantois <romain.gantois@...tlin.com>, 
	netdev@...r.kernel.org
Subject: Re: [PATCH net v5 1/2] net: ethernet: cortina: Drop software checksum
 and TSO

On Fri, Jan 5, 2024 at 12:32 PM Vladimir Oltean <olteanv@...il.com> wrote:

> So are you taking back the statement that "Hardware offloaded
> checksumming isn't working on frames bigger than 1514 bytes"?

Yes, the correct statement is that it isn't working in frames
bigger than 1514 bytes, if they have a custom DSA ethernet
tag.

The previous workaround has made the driver work fine
with the device that has a Realtek DSA switch with custom
ethertype, but it broke the driver for devices that have a
PHY connected directly to the ethernet block.

(I blame manual testing...)

> Have you increased the interface MTU beyond 1500, and tested with plain
> TCP (no DSA) on top of it? Who will provide the TCP checksum for them now?
>
> I don't understand why you remove the skb_checksum_help() call.
> It doesn't play nice with skb_is_gso() packets, agreed, but you removed
> the TSO netdev feature.

You're right, I was stuck there and larger MTU would not work.

Simply dropping the TSO and leaving the SW checksum in place
make it all work nicely!

Thank you so much Vladimir for pointing this out!

Yours,
Linus Walleij