Hi Florian,

...

+/* genphy_set_test - Make a PHY enter one of the standard IEEE defined
+ * test modes
+ * @phydev: the PHY device instance
+ * @test: the desired test mode
+ * @data: test specific data (none)
+ *
+ * This function makes the designated @phydev enter the desired standard
+ * 100BaseT2 or 1000BaseT test mode as defined in IEEE 802.3-2012 section TWO
+ * and THREE under 32.6.1.2.1 and 40.6.1.1.2 respectively
+ */
+int genphy_set_test(struct phy_device *phydev,
+		    struct ethtool_phy_test *test, const u8 *data)
+{
+	u16 shift, base, bmcr = 0;
+	int ret;
+
+	/* Exit test mode */
+	if (test->mode == PHY_STD_TEST_MODE_NORMAL) {
+		ret = phy_read(phydev, MII_CTRL1000);
+		if (ret < 0)
+			return ret;
+
+		ret &= ~GENMASK(15, 13);
+
+		return phy_write(phydev, MII_CTRL1000, ret);
+	}
+
+	switch (test->mode) {
+	case PHY_STD_TEST_MODE_100BASET2_1:
+	case PHY_STD_TEST_MODE_100BASET2_2:
+	case PHY_STD_TEST_MODE_100BASET2_3:
+		if (!(phydev->supported & PHY_100BT_FEATURES))
+			return -EOPNOTSUPP;
+
+		shift = 14;
+		base = test->mode - PHY_STD_TEST_MODE_NORMAL;
+		bmcr = BMCR_SPEED100;
+		break;
+
+	case PHY_STD_TEST_MODE_1000BASET_1:
+	case PHY_STD_TEST_MODE_1000BASET_2:
+	case PHY_STD_TEST_MODE_1000BASET_3:
+	case PHY_STD_TEST_MODE_1000BASET_4:
+		if (!(phydev->supported & PHY_1000BT_FEATURES))
+			return -EOPNOTSUPP;
+
+		shift = 13;
+		base = test->mode - PHY_STD_TEST_MODE_100BASET2_MAX;
+		bmcr = BMCR_SPEED1000;
+		break;
+
+	default:
+		/* Let an upper driver deal with additional modes it may
+		 * support
+		 */
+		return -EOPNOTSUPP;
+	}
+
+	/* Force speed and duplex */
+	ret = phy_write(phydev, MII_BMCR, bmcr | BMCR_FULLDPLX);
+	if (ret < 0)
+		return ret;
+
+	/* Set the desired test mode bit */
+	return phy_write(phydev, MII_CTRL1000, (test->mode + base) << shift);
+}

For now, these are for 100B-T2 & 1000B-TX.
But, other speeds such as 802.3bw/bq/cq have very similar format,
how about make  phy_write() to BMCR & CTRL1000 as another function call per speed?

Thanks.
Woojung