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: 05b35e7eb9a1 ("smsc95xx: add phylib support")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: Andre Edich <andre.edich@microchip.com>
---
 drivers/net/phy/smsc.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/drivers/net/phy/smsc.c b/drivers/net/phy/smsc.c
index ad9a3d91bb8a..b6489da5cfcd 100644
--- a/drivers/net/phy/smsc.c
+++ b/drivers/net/phy/smsc.c
@@ -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);
 }
 
-- 
2.39.5

On Thu, Jul 03, 2025 at 01:49:41PM +0200, Oleksij Rempel wrote:
> 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.

That is not good. Somebody got that badly wrong. They should operate
independently.

> 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.

The text implies it, rather than states it, but the code supports
setting a fixed mode before turning autoneg off. So user space does
have full control if needed.

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

    Andrew