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

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Date:   Thu, 16 Mar 2023 13:42:44 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Gal Pressman <gal@...dia.com>
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 Thu, 16 Mar 2023 13:57:26 +0200 Gal Pressman wrote:
> On 14/03/2023 17:38, Shay Agroskin wrote:
> >> 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 don't understand what it means to implement both.

If skb head is large you can copy part into the iomem, part into 
a pre-mapped space.

> It's confusing because both parameters result in skipping the DMA map
> operation, but their usage motivation is different?
> What are we instructing our customers? Use copybreak when your iommu is
> slow, but when should they use this new parameter?

Your customers? Is mlx5 going to implement the iomem based push which
needs explicit slot size control?

> IMO, a reasonable way to use both would be to only account for the
> tx_push_buf_len when tx_push is enabled, otherwise use copybreak.

I thought Shay already agreed. Let's get the v5 out.