Add support for the KSZ87xx low-loss cable PHY tunables in the Micrel
PHY driver by implementing get_tunable and set_tunable callbacks.

These callbacks expose vendor-specific PHY tunables used to control the
KSZ87xx embedded PHY receiver behavior when operating with short or
low-loss Ethernet cables. The tunables provide:

- a boolean short-cable preset applying known good settings;
- an integer LPF bandwidth control;
- an integer DSP EQ initial value control.

The Micrel PHY driver forwards these tunables via standard phy_read() /
phy_write() operations, which are virtualized by the KSZ8 DSA driver and
translated into the appropriate indirect switch register accesses.

Reviewed-by: Marek Vasut <marex@nabladev.com>
Signed-off-by: Fidelio Lawson <fidelio.lawson@exotec.com>
---
 drivers/net/phy/micrel.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/drivers/net/phy/micrel.c b/drivers/net/phy/micrel.c
index e211a523c258..bb8f3054e257 100644
--- a/drivers/net/phy/micrel.c
+++ b/drivers/net/phy/micrel.c
@@ -287,6 +287,12 @@
 /* PHY Control 2 / PHY Control (if no PHY Control 1) */
 #define MII_KSZPHY_CTRL_2			0x1f
 #define MII_KSZPHY_CTRL				MII_KSZPHY_CTRL_2
+
+/* Vendor-specific Clause 22 register, virtualized by KSZ87xx embedded PHYs DSA driver */
+#define MII_KSZ87XX_SHORT_CABLE			0x1a
+#define MII_KSZ87XX_LPF_BW				0x1b
+#define MII_KSZ87XX_EQ_INIT				0x1c
+
 /* bitmap of PHY register to set interrupt mode */
 #define KSZ8081_CTRL2_HP_MDIX			BIT(15)
 #define KSZ8081_CTRL2_MDI_MDI_X_SELECT		BIT(14)
@@ -940,6 +946,52 @@ static int ksz8795_match_phy_device(struct phy_device *phydev,
 	return ksz8051_ksz8795_match_phy_device(phydev, false);
 }
 
+static int ksz8795_get_tunable(struct phy_device *phydev,
+			       struct ethtool_tunable *tuna, void *data)
+{
+	int ret;
+
+	switch (tuna->id) {
+	case ETHTOOL_PHY_SHORT_CABLE_PRESET:
+		ret = phy_read(phydev, MII_KSZ87XX_SHORT_CABLE);
+		if (ret < 0)
+			return ret;
+		*(u8 *)data = ret;
+		return 0;
+	case ETHTOOL_PHY_LPF_BW:
+		ret = phy_read(phydev, MII_KSZ87XX_LPF_BW);
+		if (ret < 0)
+			return ret;
+		*(u8 *)data = ret;
+		return 0;
+	case ETHTOOL_PHY_DSP_EQ_INIT_VALUE:
+		ret = phy_read(phydev, MII_KSZ87XX_EQ_INIT);
+		if (ret < 0)
+			return ret;
+		*(u8 *)data = ret;
+		return 0;
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
+static int ksz8795_set_tunable(struct phy_device *phydev,
+			       struct ethtool_tunable *tuna, const void *data)
+{
+	u8 val = *(const u8 *)data;
+
+	switch (tuna->id) {
+	case ETHTOOL_PHY_SHORT_CABLE_PRESET:
+		return phy_write(phydev, MII_KSZ87XX_SHORT_CABLE, val);
+	case ETHTOOL_PHY_LPF_BW:
+		return phy_write(phydev, MII_KSZ87XX_LPF_BW, val);
+	case ETHTOOL_PHY_DSP_EQ_INIT_VALUE:
+		return phy_write(phydev, MII_KSZ87XX_EQ_INIT, val);
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
 static int ksz9021_load_values_from_of(struct phy_device *phydev,
 				       const struct device_node *of_node,
 				       u16 reg,
@@ -6961,6 +7013,8 @@ static struct phy_driver ksphy_driver[] = {
 	/* PHY_BASIC_FEATURES */
 	.config_init	= kszphy_config_init,
 	.match_phy_device = ksz8795_match_phy_device,
+	.get_tunable	= ksz8795_get_tunable,
+	.set_tunable	= ksz8795_set_tunable,
 	.suspend	= genphy_suspend,
 	.resume		= genphy_resume,
 }, {

-- 
2.54.0