Commit 9dfe110c authored Jul 03, 2025 by Oleksij Rempel Committed by Jakub Kicinski Jul 08, 2025

net: phy: smsc: Fix link failure in forced mode with Auto-MDIX



Force a fixed MDI-X mode when auto-negotiation is disabled to prevent
link instability.

When forcing the link speed and duplex on a LAN9500 PHY (e.g., with
`ethtool -s eth0 autoneg off ...`) while leaving MDI-X control in auto
mode, the PHY fails to establish a stable link. This occurs because the
PHY's Auto-MDIX algorithm is not designed to operate when
auto-negotiation is disabled. In this state, the PHY continuously
toggles the TX/RX signal pairs, which prevents the link partner from
synchronizing.

This patch resolves the issue by detecting when auto-negotiation is
disabled. If the MDI-X control mode is set to 'auto', the driver now
forces a specific, stable mode (ETH_TP_MDI) to prevent the pair
toggling. This choice of a fixed MDI mode mirrors the behavior the
hardware would exhibit if the AUTOMDIX_EN strap were configured for a
fixed MDI connection.

Fixes: 05b35e7e ("smsc95xx: add phylib support")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: Andre Edich <andre.edich@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250703114941.3243890-4-o.rempel@pengutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent 0713e555

drivers/net/phy/smsc.c

+22 −3

Original line number	Diff line number	Diff line
		@@ -155,10 +155,29 @@ static int smsc_phy_reset(struct phy_device *phydev)

		static int lan87xx_config_aneg(struct phy_device *phydev)
		{
		int rc;
		u8 mdix_ctrl;
		int val;
		int rc;

		/* When auto-negotiation is disabled (forced mode), the PHY's
		* Auto-MDIX will continue toggling the TX/RX pairs.
		*
		* To establish a stable link, we must select a fixed MDI mode.
		* If the user has not specified a fixed MDI mode (i.e., mdix_ctrl is
		* 'auto'), we default to ETH_TP_MDI. This choice of a ETH_TP_MDI mode
		* mirrors the behavior the hardware would exhibit if the AUTOMDIX_EN
		* strap were configured for a fixed MDI connection.
		*/
		if (phydev->autoneg == AUTONEG_DISABLE) {
		if (phydev->mdix_ctrl == ETH_TP_MDI_AUTO)
		mdix_ctrl = ETH_TP_MDI;
		else
		mdix_ctrl = phydev->mdix_ctrl;
		} else {
		mdix_ctrl = phydev->mdix_ctrl;
		}

		switch (phydev->mdix_ctrl) {
		switch (mdix_ctrl) {
		case ETH_TP_MDI:
		val = SPECIAL_CTRL_STS_OVRRD_AMDIX_;
		break;
		@@ -184,7 +203,7 @@ static int lan87xx_config_aneg(struct phy_device *phydev)
		rc \|= val;
		phy_write(phydev, SPECIAL_CTRL_STS, rc);

		phydev->mdix = phydev->mdix_ctrl;
		phydev->mdix = mdix_ctrl;
		return genphy_config_aneg(phydev);
		}