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

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[ Upstream commit 9dfe110cc0f6ef42af8e81ce52aef34a647d0b8a ]

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: 05b35e7eb9 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Oleksij Rempel
2025-07-03 13:49:41 +02:00
committed by Greg Kroah-Hartman
parent 59d8f05aa8
commit 964c7ce785
1 changed files with 22 additions and 3 deletions
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25
drivers/net/phy/smsc.c
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@@ -120,10 +120,29 @@ static int lan911x_config_init(struct phy_device *phydev)
static int lan87xx_config_aneg(struct phy_device *phydev)
{
int rc;
u8 mdix_ctrl;
int val;
int rc;
switch (phydev->mdix_ctrl) {
/* 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 (mdix_ctrl) {
case ETH_TP_MDI:
val = SPECIAL_CTRL_STS_OVRRD_AMDIX_;
break;
@@ -149,7 +168,7 @@ 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);
}