net: phy: realtek: add support for configuring the RX delay on RTL8211F

On RTL8211F the RX and TX delays (2ns) can be configured in two ways:
- pin strapping (RXD1 for the TX delay and RXD0 for the RX delay, LOW
means "off" and HIGH means "on") which is read during PHY reset
- using software to configure the TX and RX delay registers

So far only the configuration using pin strapping has been supported.
Add support for enabling or disabling the RGMII RX delay based on the
phy-mode to be able to get the RX delay into a known state. This is
important because the RX delay has to be coordinated between the PHY,
MAC and the PCB design (trace length). With an invalid RX delay applied
(for example if both PHY and MAC add a 2ns RX delay) Ethernet may not
work at all.

Also add debug logging when configuring the RX delay (just like the TX
delay) because this is a common source of problems.

Signed-off-by: Martin Blumenstingl <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/phy/realtek.c

···
 #define RTL8211F_INSR				0x1d
 
 #define RTL8211F_TX_DELAY			BIT(8)
+#define RTL8211F_RX_DELAY			BIT(3)
+
 #define RTL8211E_TX_DELAY			BIT(1)
 #define RTL8211E_RX_DELAY			BIT(2)
 #define RTL8211E_MODE_MII_GMII			BIT(3)
···
 static int rtl8211f_config_init(struct phy_device *phydev)
 {
 	struct device *dev = &phydev->mdio.dev;
-	u16 val;
+	u16 val_txdly, val_rxdly;
 	int ret;
 
-	/* enable TX-delay for rgmii-{id,txid}, and disable it for rgmii and
-	 * rgmii-rxid. The RX-delay can be enabled by the external RXDLY pin.
-	 */
 	switch (phydev->interface) {
 	case PHY_INTERFACE_MODE_RGMII:
+		val_txdly = 0;
+		val_rxdly = 0;
+		break;
+
 	case PHY_INTERFACE_MODE_RGMII_RXID:
-		val = 0;
+		val_txdly = 0;
+		val_rxdly = RTL8211F_RX_DELAY;
 		break;
-	case PHY_INTERFACE_MODE_RGMII_ID:
+
 	case PHY_INTERFACE_MODE_RGMII_TXID:
-		val = RTL8211F_TX_DELAY;
+		val_txdly = RTL8211F_TX_DELAY;
+		val_rxdly = 0;
 		break;
+
+	case PHY_INTERFACE_MODE_RGMII_ID:
+		val_txdly = RTL8211F_TX_DELAY;
+		val_rxdly = RTL8211F_RX_DELAY;
+		break;
+
 	default: /* the rest of the modes imply leaving delay as is. */
 		return 0;
 	}
 
 	ret = phy_modify_paged_changed(phydev, 0xd08, 0x11, RTL8211F_TX_DELAY,
-				       val);
+				       val_txdly);
 	if (ret < 0) {
 		dev_err(dev, "Failed to update the TX delay register\n");
 		return ret;
 	} else if (ret) {
 		dev_dbg(dev,
 			"%s 2ns TX delay (and changing the value from pin-strapping RXD1 or the bootloader)\n",
-			val ? "Enabling" : "Disabling");
+			val_txdly ? "Enabling" : "Disabling");
 	} else {
 		dev_dbg(dev,
 			"2ns TX delay was already %s (by pin-strapping RXD1 or bootloader configuration)\n",
-			val ? "enabled" : "disabled");
+			val_txdly ? "enabled" : "disabled");
+	}
+
+	ret = phy_modify_paged_changed(phydev, 0xd08, 0x15, RTL8211F_RX_DELAY,
+				       val_rxdly);
+	if (ret < 0) {
+		dev_err(dev, "Failed to update the RX delay register\n");
+		return ret;
+	} else if (ret) {
+		dev_dbg(dev,
+			"%s 2ns RX delay (and changing the value from pin-strapping RXD0 or the bootloader)\n",
+			val_rxdly ? "Enabling" : "Disabling");
+	} else {
+		dev_dbg(dev,
+			"2ns RX delay was already %s (by pin-strapping RXD0 or bootloader configuration)\n",
+			val_rxdly ? "enabled" : "disabled");
 	}
 
 	return 0;