net: phy: realtek: Add rtl8211e rx/tx delays config

There are two chip pins named TXDLY and RXDLY which actually adds the 2ns
delays to TXC and RXC for TXD/RXD latching. Alas this is the only
documented info regarding the RGMII timing control configurations the PHY
provides. It turns out the same settings can be setup via MDIO registers
hidden in the extension pages layout. Particularly the extension page 0xa4
provides a register 0x1c, which bits 1 and 2 control the described delays.
They are used to implement the "rgmii-{id,rxid,txid}" phy-mode.

The hidden RGMII configs register utilization was found in the rtl8211e
U-boot driver:
https://elixir.bootlin.com/u-boot/v2019.01/source/drivers/net/phy/realtek.c#L99

There is also a freebsd-folks discussion regarding this register:
https://reviews.freebsd.org/D13591

It confirms that the register bits field must control the so called
configuration pins described in the table 12-13 of the official PHY
datasheet:
8:6 = PHY Address
5:4 = Auto-Negotiation
3 = Interface Mode Select
2 = RX Delay
1 = TX Delay
0 = SELRGV
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/realtek.c

@@ -23,11 +23,15 @@
 
 #define RTL821x_INSR				0x13
 
+#define RTL821x_EXT_PAGE_SELECT			0x1e
 #define RTL821x_PAGE_SELECT			0x1f
 
 #define RTL8211F_INSR				0x1d
 
 #define RTL8211F_TX_DELAY			BIT(8)
+#define RTL8211E_TX_DELAY			BIT(1)
+#define RTL8211E_RX_DELAY			BIT(2)
+#define RTL8211E_MODE_MII_GMII			BIT(3)
 
 #define RTL8201F_ISR				0x1e
 #define RTL8201F_IER				0x13
@@ -167,6 +171,52 @@ static int rtl8211f_config_init(struct phy_device *phydev)
 	return phy_modify_paged(phydev, 0xd08, 0x11, RTL8211F_TX_DELAY, val);
 }
 
+static int rtl8211e_config_init(struct phy_device *phydev)
+{
+	int ret = 0, oldpage;
+	u16 val;
+
+	/* enable TX/RX delay for rgmii-* modes, and disable them for rgmii. */
+	switch (phydev->interface) {
+	case PHY_INTERFACE_MODE_RGMII:
+		val = 0;
+		break;
+	case PHY_INTERFACE_MODE_RGMII_ID:
+		val = RTL8211E_TX_DELAY | RTL8211E_RX_DELAY;
+		break;
+	case PHY_INTERFACE_MODE_RGMII_RXID:
+		val = RTL8211E_RX_DELAY;
+		break;
+	case PHY_INTERFACE_MODE_RGMII_TXID:
+		val = RTL8211E_TX_DELAY;
+		break;
+	default: /* the rest of the modes imply leaving delays as is. */
+		return 0;
+	}
+
+	/* According to a sample driver there is a 0x1c config register on the
+	 * 0xa4 extension page (0x7) layout. It can be used to disable/enable
+	 * the RX/TX delays otherwise controlled by RXDLY/TXDLY pins. It can
+	 * also be used to customize the whole configuration register:
+	 * 8:6 = PHY Address, 5:4 = Auto-Negotiation, 3 = Interface Mode Select,
+	 * 2 = RX Delay, 1 = TX Delay, 0 = SELRGV (see original PHY datasheet
+	 * for details).
+	 */
+	oldpage = phy_select_page(phydev, 0x7);
+	if (oldpage < 0)
+		goto err_restore_page;
+
+	ret = phy_write(phydev, RTL821x_EXT_PAGE_SELECT, 0xa4);
+	if (ret)
+		goto err_restore_page;
+
+	ret = phy_modify(phydev, 0x1c, RTL8211E_TX_DELAY | RTL8211E_RX_DELAY,
+			 val);
+
+err_restore_page:
+	return phy_restore_page(phydev, oldpage, ret);
+}
+
 static int rtl8211b_suspend(struct phy_device *phydev)
 {
 	phy_write(phydev, MII_MMD_DATA, BIT(9));
@@ -239,6 +289,7 @@ static struct phy_driver realtek_drvs[] = {
 	}, {
 		PHY_ID_MATCH_EXACT(0x001cc915),
 		.name		= "RTL8211E Gigabit Ethernet",
+		.config_init	= &rtl8211e_config_init,
 		.ack_interrupt	= &rtl821x_ack_interrupt,
 		.config_intr	= &rtl8211e_config_intr,
 		.suspend	= genphy_suspend,