Implement the standard ETHTOOL_PHY_DOWNSHIFT tunable for the LAN88xx
PHY. This allows runtime configuration of the auto-downshift feature
via ethtool:

  ethtool --set-phy-tunable eth0 downshift on count 3

The LAN88xx PHY supports downshifting from 1000BASE-T to 100BASE-TX
after 2-5 failed auto-negotiation attempts. Valid count values are
2, 3, 4 and 5.

This is based on an earlier downstream implementation by Phil Elwell.

Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
---
 drivers/net/phy/microchip.c  | 89 ++++++++++++++++++++++++++++++++++++
 include/linux/microchipphy.h |  9 ++++
 2 files changed, 98 insertions(+)

diff --git a/drivers/net/phy/microchip.c b/drivers/net/phy/microchip.c
index dc8634e7bcbe..a044be1ade79 100644
--- a/drivers/net/phy/microchip.c
+++ b/drivers/net/phy/microchip.c
@@ -193,6 +193,93 @@ static void lan88xx_config_TR_regs(struct phy_device *phydev)
 		phydev_warn(phydev, "Failed to Set Register[0x1686]\n");
 }
 
+static int lan88xx_get_downshift(struct phy_device *phydev, u8 *data)
+{
+	int val;
+
+	val = phy_read_paged(phydev, 1, LAN78XX_PHY_CTRL3);
+	if (val < 0)
+		return val;
+
+	if (!(val & LAN78XX_PHY_CTRL3_AUTO_DOWNSHIFT)) {
+		*data = DOWNSHIFT_DEV_DISABLE;
+		return 0;
+	}
+
+	switch (val & LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_MASK) {
+	case LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_2:
+		*data = 2;
+		break;
+	case LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_3:
+		*data = 3;
+		break;
+	case LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_4:
+		*data = 4;
+		break;
+	case LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_5:
+		*data = 5;
+		break;
+	}
+
+	return 0;
+}
+
+static int lan88xx_set_downshift(struct phy_device *phydev, u8 cnt)
+{
+	u32 mask = LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_MASK |
+		   LAN78XX_PHY_CTRL3_AUTO_DOWNSHIFT;
+	u32 val;
+
+	if (cnt == DOWNSHIFT_DEV_DISABLE)
+		return phy_modify_paged(phydev, 1, LAN78XX_PHY_CTRL3,
+					LAN78XX_PHY_CTRL3_AUTO_DOWNSHIFT, 0);
+
+	if (cnt == DOWNSHIFT_DEV_DEFAULT_COUNT)
+		cnt = 2;
+
+	switch (cnt) {
+	case 2:
+		val = LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_2;
+		break;
+	case 3:
+		val = LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_3;
+		break;
+	case 4:
+		val = LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_4;
+		break;
+	case 5:
+		val = LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_5;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return phy_modify_paged(phydev, 1, LAN78XX_PHY_CTRL3, mask,
+				val | LAN78XX_PHY_CTRL3_AUTO_DOWNSHIFT);
+}
+
+static int lan88xx_get_tunable(struct phy_device *phydev,
+			       struct ethtool_tunable *tuna, void *data)
+{
+	switch (tuna->id) {
+	case ETHTOOL_PHY_DOWNSHIFT:
+		return lan88xx_get_downshift(phydev, data);
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
+static int lan88xx_set_tunable(struct phy_device *phydev,
+			       struct ethtool_tunable *tuna, const void *data)
+{
+	switch (tuna->id) {
+	case ETHTOOL_PHY_DOWNSHIFT:
+		return lan88xx_set_downshift(phydev, *(const u8 *)data);
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
 static int lan88xx_probe(struct phy_device *phydev)
 {
 	struct device *dev = &phydev->mdio.dev;
@@ -499,6 +586,8 @@ static struct phy_driver microchip_phy_driver[] = {
 	.set_wol	= lan88xx_set_wol,
 	.read_page	= lan88xx_read_page,
 	.write_page	= lan88xx_write_page,
+	.get_tunable	= lan88xx_get_tunable,
+	.set_tunable	= lan88xx_set_tunable,
 },
 {
 	PHY_ID_MATCH_MODEL(PHY_ID_LAN937X_TX),
diff --git a/include/linux/microchipphy.h b/include/linux/microchipphy.h
index 517288da19fd..a8deae3977e9 100644
--- a/include/linux/microchipphy.h
+++ b/include/linux/microchipphy.h
@@ -61,6 +61,15 @@
 /* Registers specific to the LAN7800/LAN7850 embedded phy */
 #define LAN78XX_PHY_LED_MODE_SELECT		(0x1D)
 
+/* PHY Control 3 register (page 1) */
+#define LAN78XX_PHY_CTRL3			(0x14)
+#define  LAN78XX_PHY_CTRL3_AUTO_DOWNSHIFT	BIT(4)
+#define  LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_MASK	GENMASK(3, 2)
+#define  LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_2	(0 << 2)
+#define  LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_3	(1 << 2)
+#define  LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_4	(2 << 2)
+#define  LAN78XX_PHY_CTRL3_DOWNSHIFT_CTRL_5	(3 << 2)
+
 /* DSP registers */
 #define PHY_ARDENNES_MMD_DEV_3_PHY_CFG		(0x806A)
 #define PHY_ARDENNES_MMD_DEV_3_PHY_CFG_ZD_DLY_EN_	(0x2000)
-- 
2.51.0