linux-kernel - Re: [PATCH v3 net] net: phylink: add missing supported link modes for the fixed-link

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Message-ID: <aRwtEVvzuchzBHAu@shell.armlinux.org.uk>
Date: Tue, 18 Nov 2025 08:23:45 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Wei Fang <wei.fang@....com>
Cc: andrew@...n.ch, hkallweit1@...il.com, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	eric@...int.com, maxime.chevallier@...tlin.com, imx@...ts.linux.dev,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 net] net: phylink: add missing supported link modes
 for the fixed-link

On Mon, Nov 17, 2025 at 06:29:43PM +0800, Wei Fang wrote:
> Pause, Asym_Pause and Autoneg bits are not set when pl->supported is
> initialized, so these link modes will not work for the fixed-link. This
> leads to a TCP performance degradation issue observed on the i.MX943
> platform.
> 
> The switch CPU port of i.MX943 is connected to an ENETC MAC, this link
> is a fixed link and the link speed is 2.5Gbps. And one of the switch
> user ports is the RGMII interface, and its link speed is 1Gbps. If the
> flow-control of the fixed link is not enabled, we can easily observe
> the iperf performance of TCP packets is very low. Because the inbound
> rate on the CPU port is greater than the outbound rate on the user port,
> the switch is prone to congestion, leading to the loss of some TCP
> packets and requiring multiple retransmissions.
> 
> Solving this problem should be as simple as setting the Asym_Pause and
> Pause bits. The reason why the Autoneg bit needs to be set, Russell
> has gave a very good explanation in the thread [1], see below.
> 
> "As the advertising and lp_advertising bitmasks have to be non-empty,
> and the swphy reports aneg capable, aneg complete, and AN enabled, then
> for consistency with that state, Autoneg should be set. This is how it
> was prior to the blamed commit."
> 
> [1] https://lore.kernel.org/all/aRjqLN8eQDIQfBjS@shell.armlinux.org.uk/
> 
> Fixes: de7d3f87be3c ("net: phylink: Use phy_caps_lookup for fixed-link configuration")
> Signed-off-by: Wei Fang <wei.fang@....com>
> Reviewed-by: Maxime Chevallier <maxime.chevallier@...tlin.com>

NAK. I give up.

-- 
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