[PATCH v3 3/3] net: phy: micrel: expose KSZ87xx low-loss erratum via PHY tunable

[Fidelio Lawson]

Expose the KSZ87xx low-loss cable erratum control through the PHY
tunable interface.

KSZ87xx switches integrate embedded PHYs whose receiver analog front-end
may require specific equalizer or low-pass filter adjustments when used
with short or low-loss Ethernet cables, as described in Microchip errata
DS80000687C (Module 3).

Implement get_tunable / set_tunable callbacks in the Micrel PHY driver
for KSZ87xx devices, mapping the ETHTOOL_PHY_KSZ87XX_LOW_LOSS tunable
to a vendor-specific Clause 22 PHY register. Accesses are routed through
standard phy_read() / phy_write() operations and translated by the KSZ8
DSA driver into the appropriate internal LinkMD table updates.

The tunable uses a u8 mode selector, allowing userspace to select
between the documented equalizer and LPF bandwidth configurations.

Signed-off-by: Fidelio Lawson <fidelio.lawson@xxxxxxxxxx>
---
 drivers/net/phy/micrel.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/drivers/net/phy/micrel.c b/drivers/net/phy/micrel.c
index c6b011a9d636..7cbca6a7ed84 100644
--- a/drivers/net/phy/micrel.c
+++ b/drivers/net/phy/micrel.c
@@ -287,6 +287,11 @@
 /* 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_LOW_LOSS			0x1c
+#define KSZ87XX_LOW_LOSS_MAX			5
+
 /* 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 +945,38 @@ static int ksz8795_match_phy_device(struct phy_device *phydev,
 	return ksz8051_ksz8795_match_phy_device(phydev, false);
 }
 
+static int ksz87xx_get_tunable(struct phy_device *phydev,
+			       struct ethtool_tunable *tuna, void *data)
+{
+	int ret;
+
+	switch (tuna->id) {
+	case ETHTOOL_PHY_KSZ87XX_LOW_LOSS:
+		ret = phy_read(phydev, MII_KSZ87XX_LOW_LOSS);
+		if (ret < 0)
+			return ret;
+		*(u8 *)data = ret;
+		return 0;
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
+static int ksz87xx_set_tunable(struct phy_device *phydev,
+			       struct ethtool_tunable *tuna, const void *data)
+{
+	u8 val = *(const u8 *)data;
+
+	switch (tuna->id) {
+	case ETHTOOL_PHY_KSZ87XX_LOW_LOSS:
+		if (val > KSZ87XX_LOW_LOSS_MAX)
+			return -EINVAL;
+		return phy_write(phydev, MII_KSZ87XX_LOW_LOSS, val);
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
 static int ksz9021_load_values_from_of(struct phy_device *phydev,
 				       const struct device_node *of_node,
 				       u16 reg,
@@ -6809,6 +6846,8 @@ static struct phy_driver ksphy_driver[] = {
 	/* PHY_BASIC_FEATURES */
 	.config_init	= kszphy_config_init,
 	.match_phy_device = ksz8795_match_phy_device,
+	.get_tunable	= ksz87xx_get_tunable,
+	.set_tunable	= ksz87xx_set_tunable,
 	.suspend	= genphy_suspend,
 	.resume		= genphy_resume,
 }, {

-- 
2.53.0