net: phy: mscc: RGMII skew delay configuration

This patch adds support for configuring the RGMII skew delays in Rx and
Tx. The Rx and Tx skews are set based on the interface mode. By default
their configuration is set to the default value in hardware (0.2ns);
this means the driver do not rely anymore on the bootloader
configuration.

Then based on the interface mode being used, a 2ns delay is added:
- RGMII_ID adds it for both Rx and Tx.
- RGMII_RXID adds it for Rx.
- RGMII_TXID adds it for Tx.
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/mscc/mscc.h

@@ -161,6 +161,20 @@ enum rgmii_rx_clock_delay {
 /* Extended Page 2 Registers */
 #define MSCC_PHY_CU_PMD_TX_CNTL		  16
 
+#define MSCC_PHY_RGMII_SETTINGS		  18
+#define RGMII_SKEW_RX_POS		  1
+#define RGMII_SKEW_TX_POS		  4
+
+/* RGMII skew values, in ns */
+#define VSC8584_RGMII_SKEW_0_2		  0
+#define VSC8584_RGMII_SKEW_0_8		  1
+#define VSC8584_RGMII_SKEW_1_1		  2
+#define VSC8584_RGMII_SKEW_1_7		  3
+#define VSC8584_RGMII_SKEW_2_0		  4
+#define VSC8584_RGMII_SKEW_2_3		  5
+#define VSC8584_RGMII_SKEW_2_6		  6
+#define VSC8584_RGMII_SKEW_3_4		  7
+
 #define MSCC_PHY_RGMII_CNTL		  20
 #define RGMII_RX_CLK_DELAY_MASK		  0x0070
 #define RGMII_RX_CLK_DELAY_POS		  4

drivers/net/phy/mscc/mscc_main.c

@@ -1288,6 +1288,32 @@ static bool vsc8584_is_pkg_init(struct phy_device *phydev, bool reversed)
 	return false;
 }
 
+static void vsc8584_rgmii_set_skews(struct phy_device *phydev)
+{
+	u32 skew_rx, skew_tx;
+
+	/* We first set the Rx and Tx skews to their default value in h/w
+	 * (0.2 ns).
+	 */
+	skew_rx = VSC8584_RGMII_SKEW_0_2;
+	skew_tx = VSC8584_RGMII_SKEW_0_2;
+
+	/* We then set the skews based on the interface mode. */
+	if (phydev->interface == PHY_INTERFACE_MODE_RGMII_ID ||
+	    phydev->interface == PHY_INTERFACE_MODE_RGMII_RXID)
+		skew_rx = VSC8584_RGMII_SKEW_2_0;
+	if (phydev->interface == PHY_INTERFACE_MODE_RGMII_ID ||
+	    phydev->interface == PHY_INTERFACE_MODE_RGMII_TXID)
+		skew_tx = VSC8584_RGMII_SKEW_2_0;
+
+	/* Finally we apply the skews configuration. */
+	phy_modify_paged(phydev, MSCC_PHY_PAGE_EXTENDED_2,
+			 MSCC_PHY_RGMII_SETTINGS,
+			 (0x7 << RGMII_SKEW_RX_POS) | (0x7 << RGMII_SKEW_TX_POS),
+			 (skew_rx << RGMII_SKEW_RX_POS) |
+			 (skew_tx << RGMII_SKEW_TX_POS));
+}
+
 static int vsc8584_config_init(struct phy_device *phydev)
 {
 	struct vsc8531_private *vsc8531 = phydev->priv;
@@ -1422,6 +1448,9 @@ static int vsc8584_config_init(struct phy_device *phydev)
 	if (ret)
 		return ret;
 
+	if (phy_interface_is_rgmii(phydev))
+		vsc8584_rgmii_set_skews(phydev);
+
 	ret = genphy_soft_reset(phydev);
 	if (ret)
 		return ret;