[RFC v2 02/24] docs: ethtool: document that rx_buf_len must control payload lengths

[Pavel Begunkov]

From: Jakub Kicinski <kuba@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
 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