netdev - Re: [PATCH v4 net-next 1/5] ethtool: Add support for configuring tx_push_buf

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Date:   Sun, 12 Mar 2023 14:41:39 +0200
From:   Gal Pressman <gal@...dia.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Shay Agroskin <shayagr@...zon.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        "Woodhouse, David" <dwmw@...zon.com>,
        "Machulsky, Zorik" <zorik@...zon.com>,
        "Matushevsky, Alexander" <matua@...zon.com>,
        Saeed Bshara <saeedb@...zon.com>,
        "Wilson, Matt" <msw@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        "Bshara, Nafea" <nafea@...zon.com>,
        "Belgazal, Netanel" <netanel@...zon.com>,
        "Saidi, Ali" <alisaidi@...zon.com>,
        "Herrenschmidt, Benjamin" <benh@...zon.com>,
        "Kiyanovski, Arthur" <akiyano@...zon.com>,
        "Dagan, Noam" <ndagan@...zon.com>,
        "Arinzon, David" <darinzon@...zon.com>,
        "Itzko, Shahar" <itzko@...zon.com>,
        "Abboud, Osama" <osamaabb@...zon.com>
Subject: Re: [PATCH v4 net-next 1/5] ethtool: Add support for configuring
 tx_push_buf_len

On 10/03/2023 8:53, Jakub Kicinski wrote:
> On Thu, 9 Mar 2023 19:15:43 +0200 Gal Pressman wrote:
>> I know Jakub prefers the new parameter, but the description of this
>> still sounds extremely similar to TX copybreak to me..
>> TX copybreak was traditionally used to copy packets to preallocated DMA
>> buffers, but this could be implemented as copying the packet to the
>> (preallocated) WQE's inline part. That usually means DMA memory, but
>> could also be device memory in this ENA LLQ case.
>>
>> Are we drawing a line that TX copybreak is the threshold for DMA memory
>> and tx_push_buf_len is the threshold for device memory?
> 
> Pretty much, yes. Not an amazing distinction but since TX copybreak can
> already mean two different things (inline or DMA buf) I'd err on 
> the side of not overloading it with another one. 

Can we document that please?

> And Pensando needed a similar knob? Maybe I'm misremembering now.
> 
>> Are they mutually exclusive?
> 
> Not sure.

Me neither, but it's not clear who takes precedent when both are set.