[PATCH net] net: phy: smsc: Skip soft reset when a hardware reset GPIO is provided

[Fabio Estevam <festevam@gmail.com>]

On platforms using the LAN8720 in RMII mode, issuing a soft reset through
genphy_soft_reset() can temporarily disrupt the PHY output clock (REF_CLK).

Boards that source ENET_REF_CLK from the LAN8720 are therefore sensitive
to PHY soft resets, as the MAC receives an unstable or missing RMII clock
during the transition.

When a "reset-gpios" property is present, the MDIO core already performs a
hardware reset using this GPIO before calling the driver's ->reset() hook.
Issuing an additional soft reset in smsc_phy_reset() is redundant and may
result in RX CRC/frame errors, packet loss, and general link instability at
100 Mbps.

Change smsc_phy_reset() so that:

- If reset-gpios is present: rely solely on the hardware reset and skip
the soft reset.
- If reset-gpios is absent: fall back to genphy_soft_reset(), preserving
the existing behavior.

The soft reset to remove the PHY from power down is kept, as this is
a requirement mentioned in the LAN8720 datasheet.

This fixes packet loss observed on i.MX6 platforms using LAN8720 without
breaking boards that rely on the existing soft reset path.

Fixes: fc0f7e3317c5 ("net: phy: smsc: reintroduced unconditional soft reset")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
---
 drivers/net/phy/smsc.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/net/phy/smsc.c b/drivers/net/phy/smsc.c
index 48487149c225..3840b658a996 100644
--- a/drivers/net/phy/smsc.c
+++ b/drivers/net/phy/smsc.c
@@ -54,6 +54,7 @@ struct smsc_phy_priv {
 	unsigned int edpd_mode_set_by_user:1;
 	unsigned int edpd_max_wait_ms;
 	bool wol_arp;
+	bool reset_gpio;
 };
 
 static int smsc_phy_ack_interrupt(struct phy_device *phydev)
@@ -136,6 +137,7 @@ EXPORT_SYMBOL_GPL(smsc_phy_config_init);
 
 static int smsc_phy_reset(struct phy_device *phydev)
 {
+	struct smsc_phy_priv *priv = phydev->priv;
 	int rc = phy_read(phydev, MII_LAN83C185_SPECIAL_MODES);
 	if (rc < 0)
 		return rc;
@@ -147,9 +149,17 @@ static int smsc_phy_reset(struct phy_device *phydev)
 		/* set "all capable" mode */
 		rc |= MII_LAN83C185_MODE_ALL;
 		phy_write(phydev, MII_LAN83C185_SPECIAL_MODES, rc);
+		/* reset the phy */
+		return genphy_soft_reset(phydev);
 	}
 
-	/* reset the phy */
+	/* If the reset-gpios property exists, hardware reset will be
+	 * performed by the MDIO core, so do NOT issue a soft reset here.
+	 */
+	if (priv->reset_gpio)
+		return 0;
+
+	/* No reset GPIO found: fall back to soft reset */
 	return genphy_soft_reset(phydev);
 }
 
@@ -671,6 +681,9 @@ int smsc_phy_probe(struct phy_device *phydev)
 	if (device_property_present(dev, "smsc,disable-energy-detect"))
 		priv->edpd_enable = false;
 
+	if (device_property_present(dev, "reset-gpios"))
+		priv->reset_gpio = true;
+
 	phydev->priv = priv;
 
 	/* Make clk optional to keep DTB backward compatibility. */
-- 
2.34.1