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

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Message-ID: <pj41zlbkkv2v6z.fsf@u570694869fb251.ant.amazon.com>
Date:   Tue, 14 Mar 2023 17:38:23 +0200
From:   Shay Agroskin <shayagr@...zon.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     Gal Pressman <gal@...dia.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


Jakub Kicinski <kuba@...nel.org> writes:

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> On Sun, 12 Mar 2023 14:41:39 +0200 Gal Pressman wrote:
>> 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?
>
> Shay, could you add a paragraph in the docs regarding copybreak 
> in v5?

Document that tx_copybreak defines the threshold below which the 
packet is copied into a preallocated DMA'ed buffer and that 
tx_push_buf defines the same but for device memory?
Are we sure we want to make this distinction ? While the meaning 
of both params can overlap in their current definition, the 
motivation to use them is pretty different.
A driver can implement both for different purposes (and still copy 
both into the device).

I'll modify the documentation in next version