net: phy: marvell: Fix 88e1510 downshift counter errata

The 88e1510 PHY has an erratum where the phy downshift counter is not
cleared after phy being suspended(BMCR_PDOWN set) and then later
resumed(BMCR_PDOWN cleared). This can cause the gigabit link to
intermittently downshift to a lower speed.

Disabling and re-enabling the downshift feature clears the counter,
allowing the PHY to retry gigabit link negotiation up to the programmed
retry count times before downshifting. This behavior has been observed
on copper links.

Signed-off-by: Rohan G Thomas <[email protected]>
Reviewed-by: Matthew Gerlach <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: rohangt07

drivers/net/phy/marvell.c

@@ -1902,6 +1902,43 @@ static int marvell_resume(struct phy_device *phydev)
 	return err;
 }
 
+/* m88e1510_resume
+ *
+ * The 88e1510 PHY has an erratum where the phy downshift counter is not cleared
+ * after phy being suspended(BMCR_PDOWN set) and then later resumed(BMCR_PDOWN
+ * cleared). This can cause the link to intermittently downshift to a lower speed.
+ *
+ * Disabling and re-enabling the downshift feature clears the counter, allowing
+ * the PHY to retry gigabit link negotiation up to the programmed retry count
+ * before downshifting. This behavior has been observed on copper links.
+ */
+static int m88e1510_resume(struct phy_device *phydev)
+{
+	int err;
+	u8 cnt = 0;
+
+	err = marvell_resume(phydev);
+	if (err < 0)
+		return err;
+
+	/* read downshift counter value */
+	err = m88e1011_get_downshift(phydev, &cnt);
+	if (err < 0)
+		return err;
+
+	if (cnt) {
+		/* downshift disabled */
+		err = m88e1011_set_downshift(phydev, 0);
+		if (err < 0)
+			return err;
+
+		/* downshift enabled, with previous counter value */
+		err = m88e1011_set_downshift(phydev, cnt);
+	}
+
+	return err;
+}
+
 static int marvell_aneg_done(struct phy_device *phydev)
 {
 	int retval = phy_read(phydev, MII_M1011_PHY_STATUS);
@@ -3923,7 +3960,7 @@ static struct phy_driver marvell_drivers[] = {
 		.handle_interrupt = marvell_handle_interrupt,
 		.get_wol = m88e1318_get_wol,
 		.set_wol = m88e1318_set_wol,
-		.resume = marvell_resume,
+		.resume = m88e1510_resume,
 		.suspend = marvell_suspend,
 		.read_page = marvell_read_page,
 		.write_page = marvell_write_page,