linux-kernel - Re: [PATCH v2] net: phylink: add missing supported link modes for the fixed-link

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Message-ID: <0baed1d4-65f0-409e-8284-d6af44bf7beb@bootlin.com>
Date: Mon, 17 Nov 2025 16:24:01 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>,
 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,
 imx@...ts.linux.dev, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net: phylink: add missing supported link modes for the
 fixed-link

Hi,

On 17/11/2025 16:17, Russell King (Oracle) wrote:
> On Sun, Nov 16, 2025 at 10:38:23AM +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 is because
>> it was already set before the blame commit. Moreover, Russell provides
>> a very good explanation of why it needs to be set in the thread [1].
>>
>> [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>
> 
> Even though discussion is still going on, we do need to fix this
> regression. So:
> 
> Reviewed-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>

I already reviewed the patch, but I agree with Russell. Being the author
of the blamed commit, I can say that this change in behavior was not
intentional :(

Maxime