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

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Message-ID:
 <PAXPR04MB851052762C78966B59B96AA388D6A@PAXPR04MB8510.eurprd04.prod.outlook.com>
Date: Tue, 18 Nov 2025 02:05:05 +0000
From: Wei Fang <wei.fang@....com>
To: Russell King <linux@...linux.org.uk>
CC: "andrew@...n.ch" <andrew@...n.ch>, "hkallweit1@...il.com"
	<hkallweit1@...il.com>, "davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "kuba@...nel.org"
	<kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>, "eric@...int.com"
	<eric@...int.com>, "maxime.chevallier@...tlin.com"
	<maxime.chevallier@...tlin.com>, "imx@...ts.linux.dev" <imx@...ts.linux.dev>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] net: phylink: add missing supported link modes for the
 fixed-link

> 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://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore
> > .kernel.org%2Fall%2FaRjqLN8eQDIQfBjS%40shell.armlinux.org.uk%2F&data=
> 0
> >
> 5%7C02%7Cwei.fang%40nxp.com%7C046af7619cb34d31ce1f08de25ec6ac4%7
> C686ea
> >
> 1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C638989894529904535%7CUnkn
> own%7CT
> >
> WFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4
> zMiI
> >
> sIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=C3b%2FDWqT8
> yliu6m
> > tGDX0JtrdDevoTik74gZwn%2F1%2BJXk%3D&reserved=0
> >
> > 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>
> 
> Thanks!

Thanks, based on Andrew's comments, I have sent a v3 patch.

https://lore.kernel.org/all/20251117102943.1862680-1-wei.fang@nxp.com/