Implement get_mse_capability() and get_mse_snapshot() for the DP83TD510E
to expose its Mean Square Error (MSE) register via the new PHY MSE
UAPI.

The DP83TD510E does not document any peak MSE values; it only exposes
a single average MSE register used internally to derive SQI. This
implementation therefore advertises only PHY_MSE_CAP_AVG, along with
LINK and channel-A selectors. Scaling is fixed to 0xFFFF, and the
refresh interval/number of symbols are estimated from 10BASE-T1L
symbol rate (7.5 MBd) and typical diagnostic intervals (~1 ms).

For 10BASE-T1L deployments, SQI is a reliable indicator of link
modulation quality once the link is established, but it does not
indicate whether autonegotiation pulses will be correctly received
in marginal conditions. MSE provides a direct measurement of slicer
error rate that can be used to evaluate if autonegotiation is likely
to succeed under a given cable length and condition. In practice,
testing such scenarios often requires forcing a fixed-link setup to
isolate MSE behaviour from the autonegotiation process.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/net/phy/dp83td510.c | 61 +++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/drivers/net/phy/dp83td510.c b/drivers/net/phy/dp83td510.c
index 23af1ac194fa..6875f418fa78 100644
--- a/drivers/net/phy/dp83td510.c
+++ b/drivers/net/phy/dp83td510.c
@@ -61,6 +61,7 @@
 #define DP83TD510E_MASTER_SLAVE_RESOL_FAIL	BIT(15)
 
 #define DP83TD510E_MSE_DETECT			0xa85
+#define DP83TD510E_MSE_MAX			U16_MAX
 
 #define DP83TD510_SQI_MAX	7
 
@@ -249,6 +250,63 @@ struct dp83td510_priv {
 #define DP83TD510E_ALCD_COMPLETE			BIT(15)
 #define DP83TD510E_ALCD_CABLE_LENGTH			GENMASK(10, 0)
 
+static int dp83td510_get_mse_capability(struct phy_device *phydev,
+					struct phy_mse_capability *cap)
+{
+	/* DP83TD510E documents only a single (average) MSE register
+	 * (used to derive SQI); no peak or worst-peak counters are
+	 * described. Advertise only PHY_MSE_CAP_AVG.
+	 */
+	cap->supported_caps = PHY_MSE_CAP_AVG;
+	/* 10BASE-T1L is a single-pair medium, so there are no B/C/D channels.
+	 * We still advertise PHY_MSE_CAP_CHANNEL_A to indicate that the PHY
+	 * can attribute the measurement to a specific pair (the only one),
+	 * rather than exposing it only as a link-aggregate.
+	 *
+	 * Rationale:
+	 *  - Keeps the ethtool MSE_GET selection logic consistent: per-channel
+	 *    (A/B/C/D) is preferred over WORST/LINK, so userspace receives a
+	 *    CHANNEL_A nest instead of LINK.
+	 *  - Signals to tools that "per-pair" data is available (even if there's
+	 *    just one pair), avoiding the impression that only aggregate values
+	 *    are supported.
+	 *  - Remains compatible with multi-pair PHYs and uniform UI handling.
+	 *
+	 * Note: WORST and other channels are not advertised on 10BASE-T1L.
+	 */
+	cap->supported_caps |= PHY_MSE_CHANNEL_A | PHY_MSE_CAP_LINK;
+	cap->max_average_mse = DP83TD510E_MSE_MAX;
+
+	/* The datasheet does not specify the refresh rate or symbol count,
+	 * but based on similar PHYs and standards, we can assume a common
+	 * value. For 10BASE-T1L, the symbol rate is 7.5 MBd. A common
+	 * diagnostic interval is around 1ms.
+	 * 7.5e6 symbols/sec * 0.001 sec = 7500 symbols.
+	 */
+	cap->refresh_rate_ps = 1000000000; /* 1 ms */
+	cap->num_symbols = 7500;
+
+	return 0;
+}
+
+static int dp83td510_get_mse_snapshot(struct phy_device *phydev, u32 channel,
+				      struct phy_mse_snapshot *snapshot)
+{
+	int ret;
+
+	if (channel != PHY_MSE_CHANNEL_LINK &&
+	    channel != PHY_MSE_CHANNEL_A)
+		return -EOPNOTSUPP;
+
+	ret = phy_read_mmd(phydev, MDIO_MMD_VEND2, DP83TD510E_MSE_DETECT);
+	if (ret < 0)
+		return ret;
+
+	snapshot->average_mse = ret;
+
+	return 0;
+}
+
 static int dp83td510_led_brightness_set(struct phy_device *phydev, u8 index,
 					enum led_brightness brightness)
 {
@@ -893,6 +951,9 @@ static struct phy_driver dp83td510_driver[] = {
 	.get_phy_stats	= dp83td510_get_phy_stats,
 	.update_stats	= dp83td510_update_stats,
 
+	.get_mse_capability = dp83td510_get_mse_capability,
+	.get_mse_snapshot = dp83td510_get_mse_snapshot,
+
 	.led_brightness_set = dp83td510_led_brightness_set,
 	.led_hw_is_supported = dp83td510_led_hw_is_supported,
 	.led_hw_control_set = dp83td510_led_hw_control_set,
-- 
2.47.3