linux-kernel - Re: [PATCH net-next] net: phy: micrel: Add support for lan8842

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Message-ID: <d4bbca28-5ba9-403a-8389-da712602af68@lunn.ch>
Date: Wed, 23 Jul 2025 16:28:29 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Oleksij Rempel <o.rempel@...gutronix.de>
Cc: Horatiu Vultur <horatiu.vultur@...rochip.com>, hkallweit1@...il.com,
	linux@...linux.org.uk, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: phy: micrel: Add support for lan8842

> # FLF Behavior (Immediate Failure)
> An FLF-enabled PHY is designed to report link instability almost
> immediately (~1 ms). Instead of trying to recover silently, it
> immediately reports a hard link failure to the operating system.
> 
> # The Disadvantage in a Single-Link System
> 
> For a system with only one link, this "fail-fast" approach can be a
> disadvantage. Consider a short, recoverable noise burst:
> 
> - Without FLF: The PHY uses its 750 ms grace period to recover. The
> link stays up, and the service interruption is limited to brief packet
> loss.
> 
> - With FLF: The PHY reports "link down" after ~1 ms. The operating
> system tears down the network interface. Even if the hardware recovers
> quickly, the OS has to bring the interface back up, re-run DHCP, and
> re-establish all application connections. This system-level recovery
> often takes much longer than the original glitch.
> 
> In short, FLF can turn a minor, recoverable physical-layer glitch into a
> more disruptive, longer-lasting outage at the application level when
> there is no backup link to switch to.

Fast link down can be a useful feature to have, but the PHY should
default to what 802.3 says. There is however:

ETHTOOL_PHY_FAST_LINK_DOWN

which two drivers support.

	Andrew