netdev - Re: [PATCH net-next v3 3/7] net: ethtool: add support for configuring tcp-data-split-thresh

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Message-ID: <CAMArcTX0sD9T2qhoKEswVp3CNVjOchZyEqypBcjMNtQRHBfk5w@mail.gmail.com>
Date: Fri, 4 Oct 2024 04:33:38 +0900
From: Taehee Yoo <ap420073@...il.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	edumazet@...gle.com, netdev@...r.kernel.org, linux-doc@...r.kernel.org, 
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Subject: Re: [PATCH net-next v3 3/7] net: ethtool: add support for configuring tcp-data-split-thresh

On Fri, Oct 4, 2024 at 3:25 AM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Thu, Oct 3, 2024 at 9:07 AM Taehee Yoo <ap420073@...il.com> wrote:
> >
> > The tcp-data-split-thresh option configures the threshold value of
> > the tcp-data-split.
> > If a received packet size is larger than this threshold value, a packet
> > will be split into header and payload.
>
> Why do you need this? devmem TCP will always not work with unsplit
> packets. Seems like you always want to set thresh to 0 to support
> something like devmem TCP.
>
> Why would the user ever want to configure this? I can't think of a
> scenario where the user wouldn't want packets under X bytes to be
> unsplit.

I totally understand what you mean,
Yes, tcp-data-split is zerocopy friendly option but as far as I know,
this option is not only for the zerocopy usecase.
So, If users enable tcp-data-split, they would assume threshold is 0.
But there are already NICs that have been supporting tcp-data-split
enabled by default.
bnxt_en's default value is 256bytes.
If we just assume the tcp-data-split-threshold to 0 for all cases,
it would change the default behavior of bnxt_en driver(maybe other drivers too)
for the not zerocopy case.
Jakub pointed out the generic case, not only for zerocopy usecase
in the v1 and I agree with that opinion.
https://lore.kernel.org/netdev/20240906183844.2e8226f3@kernel.org/