netdev - Re: [RFC net-next 01/22] docs: ethtool: document that rx_buf

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Message-ID: <aAfyg5fXv4Q11nEF@LQ3V64L9R2>
Date: Tue, 22 Apr 2025 12:48:19 -0700
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
	pabeni@...hat.com, andrew+netdev@...n.ch, horms@...nel.org,
	donald.hunter@...il.com, sdf@...ichev.me, almasrymina@...gle.com,
	dw@...idwei.uk, asml.silence@...il.com, ap420073@...il.com,
	dtatulea@...dia.com, michael.chan@...adcom.com
Subject: Re: [RFC net-next 01/22] docs: ethtool: document that rx_buf_len
 must control payload lengths

On Mon, Apr 21, 2025 at 03:28:06PM -0700, Jakub Kicinski wrote:
> Document the semantics of the rx_buf_len ethtool ring param.
> Clarify its meaning in case of HDS, where driver may have
> two separate buffer pools.

FWIW the docs added below don't explicitly mention HDS, but I
suppose that is implied from the multiple memory pools? Not sure if
it's worth elucidating that.
 
> The various zero-copy TCP Rx schemes we have suffer from memory
> management overhead. Specifically applications aren't too impressed
> with the number of 4kB buffers they have to juggle. Zero-copy
> TCP makes most sense with larger memory transfers so using
> 16kB or 32kB buffers (with the help of HW-GRO) feels more
> natural.

I think adding something about the above into the docs could be
helpful, but then again I am always in favor of more words.

See below for a minor spelling nit.

> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
>  Documentation/networking/ethtool-netlink.rst | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/networking/ethtool-netlink.rst b/Documentation/networking/ethtool-netlink.rst
> index b6e9af4d0f1b..eaa9c17a3cb1 100644
> --- a/Documentation/networking/ethtool-netlink.rst
> +++ b/Documentation/networking/ethtool-netlink.rst
> @@ -957,7 +957,6 @@ Kernel checks that requested ring sizes do not exceed limits reported by
>  driver. Driver may impose additional constraints and may not support all
>  attributes.
>  
> -
>  ``ETHTOOL_A_RINGS_CQE_SIZE`` specifies the completion queue event size.
>  Completion queue events (CQE) are the events posted by NIC to indicate the
>  completion status of a packet when the packet is sent (like send success or
> @@ -971,6 +970,11 @@ completion queue size can be adjusted in the driver if CQE size is modified.
>  header / data split feature. If a received packet size is larger than this
>  threshold value, header and data will be split.
>  
> +``ETHTOOL_A_RINGS_RX_BUF_LEN`` controls the size of the buffer chunks driver
> +uses to receive packets. If the device uses different memory polls for headers
                                                         pools?  ^^