net: phy: dp83867: implement configurability for SGMII in-band auto-negotiation

Implement the inband_caps() and config_inband() PHY driver methods, to
allow working with PCS devices that do not support or want in-band to be
used.

There is a complication due to existing logic from commit c76acfb7e19d
("net: phy: dp83867: retrigger SGMII AN when link change") which might
re-enable what dp83867_config_inband() has disabled. So we need to
modify dp83867_link_change_notify() to use phy_modify_changed() when
temporarily disabling in-band autoneg. If the return code is 0, it means
the original in-band was disabled and we need to keep it disabled.
If the return code is 1, the original was enabled and we need to
re-enable it. If negative, there was an error, which was silent before,
and remains silent now.

dp83867_config_inband() and dp83867_link_change_notify() are serialized
by the phydev->lock.

Signed-off-by: Vladimir Oltean <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/phy/dp83867.c

···
 	 * whenever there is a link change.
 	 */
 	if (phydev->interface == PHY_INTERFACE_MODE_SGMII) {
-		int val = 0;
+		int val;
 
-		val = phy_clear_bits(phydev, DP83867_CFG2,
+		val = phy_modify_changed(phydev, DP83867_CFG2,
+					 DP83867_SGMII_AUTONEG_EN, 0);
+
+		/* Keep the in-band setting made by dp83867_config_inband() */
+		if (val != 0)
+			phy_set_bits(phydev, DP83867_CFG2,
 				     DP83867_SGMII_AUTONEG_EN);
-		if (val < 0)
-			return;
-
-		phy_set_bits(phydev, DP83867_CFG2,
-			     DP83867_SGMII_AUTONEG_EN);
 	}
 }
 
···
 			  DP83867_LED_POLARITY(index), polarity);
 }
 
+static unsigned int dp83867_inband_caps(struct phy_device *phydev,
+					phy_interface_t interface)
+{
+	if (interface == PHY_INTERFACE_MODE_SGMII)
+		return LINK_INBAND_ENABLE | LINK_INBAND_DISABLE;
+
+	return 0;
+}
+
+static int dp83867_config_inband(struct phy_device *phydev, unsigned int modes)
+{
+	int val = 0;
+
+	if (modes == LINK_INBAND_ENABLE)
+		val = DP83867_SGMII_AUTONEG_EN;
+
+	return phy_modify(phydev, DP83867_CFG2, DP83867_SGMII_AUTONEG_EN, val);
+}
+
 static struct phy_driver dp83867_driver[] = {
 	{
 		.phy_id		= DP83867_PHY_ID,
···
 		.led_hw_control_set = dp83867_led_hw_control_set,
 		.led_hw_control_get = dp83867_led_hw_control_get,
 		.led_polarity_set = dp83867_led_polarity_set,
+
+		.inband_caps	= dp83867_inband_caps,
+		.config_inband	= dp83867_config_inband,
 	},
 };
 module_phy_driver(dp83867_driver);