netdev - [PATCH net-next v3 02/23] docs: ethtool: document that rx_buf

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Message-ID: <353e0195a0f44800c0b5aa4a6d751d3655d9842b.1755499376.git.asml.silence@gmail.com>
Date: Mon, 18 Aug 2025 14:57:18 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Jakub Kicinski <kuba@...nel.org>,
	netdev@...r.kernel.org
Cc: asml.silence@...il.com,
	Eric Dumazet <edumazet@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	andrew+netdev@...n.ch,
	horms@...nel.org,
	davem@...emloft.net,
	sdf@...ichev.me,
	almasrymina@...gle.com,
	dw@...idwei.uk,
	michael.chan@...adcom.com,
	dtatulea@...dia.com,
	ap420073@...il.com,
	linux-kernel@...r.kernel.org,
	io-uring@...r.kernel.org
Subject: [PATCH net-next v3 02/23] 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 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.

Signed-off-by: Jakub Kicinski <kuba@...nel.org>
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 ab20c644af24..cae372f719d1 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