Override the hardware strap configuration for MDI-X mode to ensure a
predictable initial state for the driver. The initial mode of the LAN87xx
PHY is determined by the AUTOMDIX_EN strap pin, but the driver has no
documented way to read its latched status.

This unpredictability means the driver cannot know if the PHY has
initialized with Auto-MDIX enabled or disabled, preventing it from
providing a reliable interface to the user.

This patch introduces a `config_init` hook that forces the PHY into a
known state by explicitly enabling Auto-MDIX.

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 | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/drivers/net/phy/smsc.c b/drivers/net/phy/smsc.c
index adf12d7108b5..ad9a3d91bb8a 100644
--- a/drivers/net/phy/smsc.c
+++ b/drivers/net/phy/smsc.c
@@ -262,6 +262,33 @@ int lan87xx_read_status(struct phy_device *phydev)
 }
 EXPORT_SYMBOL_GPL(lan87xx_read_status);
 
+static int lan87xx_phy_config_init(struct phy_device *phydev)
+{
+	int rc;
+
+	/* The LAN87xx PHY's initial MDI-X mode is determined by the AUTOMDIX_EN
+	 * hardware strap, but the driver cannot read the strap's status. This
+	 * creates an unpredictable initial state.
+	 *
+	 * To ensure consistent and reliable behavior across all boards,
+	 * override the strap configuration on initialization and force the PHY
+	 * into a known state with Auto-MDIX enabled, which is the expected
+	 * default for modern hardware.
+	 */
+	rc = phy_modify(phydev, SPECIAL_CTRL_STS,
+			SPECIAL_CTRL_STS_OVRRD_AMDIX_ |
+			SPECIAL_CTRL_STS_AMDIX_ENABLE_ |
+			SPECIAL_CTRL_STS_AMDIX_STATE_,
+			SPECIAL_CTRL_STS_OVRRD_AMDIX_ |
+			SPECIAL_CTRL_STS_AMDIX_ENABLE_);
+	if (rc < 0)
+		return rc;
+
+	phydev->mdix_ctrl = ETH_TP_MDI_AUTO;
+
+	return smsc_phy_config_init(phydev);
+}
+
 static int lan874x_phy_config_init(struct phy_device *phydev)
 {
 	u16 val;
@@ -696,7 +723,7 @@ static struct phy_driver smsc_phy_driver[] = {
 
 	/* basic functions */
 	.read_status	= lan87xx_read_status,
-	.config_init	= smsc_phy_config_init,
+	.config_init	= lan87xx_phy_config_init,
 	.soft_reset	= smsc_phy_reset,
 	.config_aneg	= lan87xx_config_aneg,
 
-- 
2.39.5

On Thu,  3 Jul 2025 13:49:40 +0200
Oleksij Rempel <o.rempel@pengutronix.de> wrote:

> Override the hardware strap configuration for MDI-X mode to ensure a
> predictable initial state for the driver. The initial mode of the LAN87xx
> PHY is determined by the AUTOMDIX_EN strap pin, but the driver has no
> documented way to read its latched status.
> 
> This unpredictability means the driver cannot know if the PHY has
> initialized with Auto-MDIX enabled or disabled, preventing it from
> providing a reliable interface to the user.
> 
> This patch introduces a `config_init` hook that forces the PHY into a
> known state by explicitly enabling Auto-MDIX.
> 
> Fixes: 05b35e7eb9a1 ("smsc95xx: add phylib support")
> Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> Cc: Andre Edich <andre.edich@microchip.com>

The patch looks good to me, but I have a few questions.  As this
overrides some configuration on existing HW, and I'm not utra familiar
with auto-mdix, is there any chance this could cause regressions ?

Especially regarding your patch 3, is there any chance that the PHY is
strapped in a fixed MDIX mode to address the broken autoneg off mode ? 

I'm not saying that strapping is a good solution for that ofc :) it's a
shame we can't read the strap config :/

Maxime

> The patch looks good to me, but I have a few questions.  As this
> overrides some configuration on existing HW, and I'm not utra familiar
> with auto-mdix, is there any chance this could cause regressions ?
> 
> Especially regarding your patch 3, is there any chance that the PHY is
> strapped in a fixed MDIX mode to address the broken autoneg off mode ? 
> 
> I'm not saying that strapping is a good solution for that ofc :) it's a
> shame we can't read the strap config :/

This might technically be considered a behaviour change. However there
are few systems which don't support it. It is also independent of
auto-neg, so autoneg off should not be an issue.

I think this is reasonably safe, and if somebody does report a
regression, we can revert the patch.

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

    Andrew