linux-kernel - [PATCH net-next v4 02/24] docs: ethtool: document that rx_buf

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Message-ID: <e685eeccc2a46d8ebfde77209e505a77d14f7e8a.1760364551.git.asml.silence@gmail.com>
Date: Mon, 13 Oct 2025 15:54:04 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: netdev@...r.kernel.org
Cc: Andrew Lunn <andrew@...n.ch>,
	Jakub Kicinski <kuba@...nel.org>,
	davem@...emloft.net,
	Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Donald Hunter <donald.hunter@...il.com>,
	Michael Chan <michael.chan@...adcom.com>,
	Pavan Chebbi <pavan.chebbi@...adcom.com>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Joshua Washington <joshwash@...gle.com>,
	Harshitha Ramamurthy <hramamurthy@...gle.com>,
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	Salil Mehta <salil.mehta@...wei.com>,
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	Alexander Duyck <alexanderduyck@...com>,
	kernel-team@...a.com,
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	Joe Damato <joe@...a.to>,
	David Wei <dw@...idwei.uk>,
	Willem de Bruijn <willemb@...gle.com>,
	Mina Almasry <almasrymina@...gle.com>,
	Pavel Begunkov <asml.silence@...il.com>,
	Breno Leitao <leitao@...ian.org>,
	Dragos Tatulea <dtatulea@...dia.com>,
	linux-kernel@...r.kernel.org,
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	linux-rdma@...r.kernel.org,
	Jonathan Corbet <corbet@....net>
Subject: [PATCH net-next v4 02/24] docs: ethtool: document that rx_buf_len must control payload lengths

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

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.

The various zero-copy TCP Rx schemes 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.

Signed-off-by: Jakub Kicinski <kuba@...nel.org>
Reviewed-by: Mina Almasry <almasrymina@...gle.com>
Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 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 b270886c5f5d..392a359a9cab 100644
--- a/Documentation/networking/ethtool-netlink.rst
+++ b/Documentation/networking/ethtool-netlink.rst
@@ -966,7 +966,6 @@ 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
@@ -980,6 +979,11 @@ 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 buffers driver
+uses to receive packets. If the device uses different buffer pools for
+headers and payload (due to HDS, HW-GRO etc.) this setting must
+control the size of the payload buffers.
+
 CHANNELS_GET
 ============
 
-- 
2.49.0