linux-kernel - Re: [PATCH net v1 2/2] net: phy: microchip: limit 100M workaround to link-down events on LAN88xx

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Message-ID: <a6c2059c-487b-4e00-9399-b98d867168b5@lunn.ch>
Date: Wed, 9 Jul 2025 16:17:52 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Oleksij Rempel <o.rempel@...gutronix.de>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Woojung Huh <woojung.huh@...rochip.com>,
	Arun Ramadoss <arun.ramadoss@...rochip.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Russell King <linux@...linux.org.uk>,
	Yuiko Oshino <yuiko.oshino@...rochip.com>, kernel@...gutronix.de,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	UNGLinuxDriver@...rochip.com, Phil Elwell <phil@...pberrypi.org>
Subject: Re: [PATCH net v1 2/2] net: phy: microchip: limit 100M workaround to
 link-down events on LAN88xx

On Wed, Jul 09, 2025 at 03:07:53PM +0200, Oleksij Rempel wrote:
> Restrict the 100Mbit forced-mode workaround to link-down transitions
> only, to prevent repeated link reset cycles in certain configurations.
> 
> The workaround was originally introduced to improve signal reliability
> when switching cables between long and short distances. It temporarily
> forces the PHY into 10 Mbps before returning to 100 Mbps.
> 
> However, when used with autonegotiating link partners (e.g., Intel i350),
> executing this workaround on every link change can confuse the partner
> and cause constant renegotiation loops. This results in repeated link
> down/up transitions and the PHY never reaching a stable state.
> 
> Limit the workaround to only run during the PHY_NOLINK state. This ensures
> it is triggered only once per link drop, avoiding disruptive toggling
> while still preserving its intended effect.
> 
> Note: I am not able to reproduce the original issue that this workaround
> addresses. I can only confirm that 100 Mbit mode works correctly in my
> test setup. Based on code inspection, I assume the workaround aims to
> reset some internal state machine or signal block by toggling speeds.
> However, a PHY reset is already performed earlier in the function via
> phy_init_hw(), which may achieve a similar effect. Without a reproducer,
> I conservatively keep the workaround but restrict its conditions.
> 
> Fixes: e57cf3639c32 ("net: lan78xx: fix accessing the LAN7800's internal phy specific registers from the MAC driver")
> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew