net: phy: micrel: Add loopback support

The KSZ9031 PHYs requires full duplex for loopback mode. Add PHY
specific set_loopback() to ensure this.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250312203010.47429-4-gerhard@engleder-embedded.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/phy/micrel.c

@@ -1032,6 +1032,29 @@ static int ksz9021_config_init(struct phy_device *phydev)
 #define MII_KSZ9031RN_EDPD		0x23
 #define MII_KSZ9031RN_EDPD_ENABLE	BIT(0)
 
+static int ksz9031_set_loopback(struct phy_device *phydev, bool enable,
+				int speed)
+{
+	u16 ctl = BMCR_LOOPBACK;
+	int val;
+
+	if (!enable)
+		return genphy_loopback(phydev, enable, 0);
+
+	if (speed == SPEED_10 || speed == SPEED_100 || speed == SPEED_1000)
+		phydev->speed = speed;
+	else if (speed)
+		return -EINVAL;
+	phydev->duplex = DUPLEX_FULL;
+
+	ctl |= mii_bmcr_encode_fixed(phydev->speed, phydev->duplex);
+
+	phy_write(phydev, MII_BMCR, ctl);
+
+	return phy_read_poll_timeout(phydev, MII_BMSR, val, val & BMSR_LSTATUS,
+				     5000, 500000, true);
+}
+
 static int ksz9031_of_load_skew_values(struct phy_device *phydev,
 				       const struct device_node *of_node,
 				       u16 reg, size_t field_sz,
@@ -5565,6 +5588,7 @@ static struct phy_driver ksphy_driver[] = {
 	.resume		= kszphy_resume,
 	.cable_test_start	= ksz9x31_cable_test_start,
 	.cable_test_get_status	= ksz9x31_cable_test_get_status,
+	.set_loopback	= ksz9031_set_loopback,
 }, {
 	.phy_id		= PHY_ID_LAN8814,
 	.phy_id_mask	= MICREL_PHY_ID_MASK,