[PATCH net-next] net: phy: marvell: Fix 88e1510 downshift counter errata

[Rohan G Thomas via B4 Relay <devnull+rohan.g.thomas.altera.com@kernel.org>]

From: Rohan G Thomas <rohan.g.thomas@altera.com>

The 88e1510 PHY has an erratum where the phy downshift counter is not
cleared on a link power down/up. 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 <rohan.g.thomas@altera.com>
Reviewed-by: Matthew Gerlach <matthew.gerlach@altera.com>
---
 drivers/net/phy/marvell.c | 39 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/drivers/net/phy/marvell.c b/drivers/net/phy/marvell.c
index 623292948fa706a2b0d8b98919ead8b609bbd949..4c3d5fbcfda0a960f6c1284f07f16061d9fa0229 100644
--- a/drivers/net/phy/marvell.c
+++ b/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
+ * during a link power down/up. 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,

---
base-commit: 2fd4161d0d2547650d9559d57fc67b4e0a26a9e3
change-id: 20250902-marvell_fix-3f2a3d5e0fca

Best regards,
-- 
Rohan G Thomas <rohan.g.thomas@altera.com>