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: 05b35e7eb9a1 ("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>

diff --git a/drivers/net/phy/smsc.c b/drivers/net/phy/smsc.c
index ad9a3d91bb8ae46422690a1207c5e4ffc5b302f1..b6489da5cfcdfb405457058dc88575c0d84d259d 100644 (file)
--- 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);
 }