net: stmmac: dwmac-meson8b: add support for the RX delay configuration

Configure the PRG_ETH0_ADJ_* bits to enable or disable the RX delay
based on the various RGMII PHY modes. For now the only supported RX
delay settings are:
- disabled, use for example for phy-mode "rgmii-id"
- 0ns - this is treated identical to "disabled", used for example on
  boards where the PHY provides 2ns TX delay and the PCB trace length
  already adds 2ns RX delay
- 2ns - for whenever the PHY cannot add the RX delay and the traces on
  the PCB don't add any RX delay

Disabling the RX delay (in case u-boot enables it, which is the case
for example on Meson8b Odroid-C1) simply means that PRG_ETH0_ADJ_ENABLE,
PRG_ETH0_ADJ_SETUP, PRG_ETH0_ADJ_DELAY and PRG_ETH0_ADJ_SKEW should be
disabled (just disabling PRG_ETH0_ADJ_ENABLE may be enough, since that
disables the whole re-timing logic - but I find it makes more sense to
clear the other bits as well since they depend on that setting).

u-boot on Odroid-C1 uses the following steps to enable a 2ns RX delay:
- enabling enabling the timing adjustment clock
- enabling the timing adjustment logic by setting PRG_ETH0_ADJ_ENABLE
- setting the PRG_ETH0_ADJ_SETUP bit

The documentation for the PRG_ETH0_ADJ_DELAY and PRG_ETH0_ADJ_SKEW
registers indicates that we can even set different RX delays. However,
I could not find out how this works exactly, so for now we only support
a 2ns RX delay using the exact same way that Odroid-C1's u-boot does.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c

@@ -85,6 +85,7 @@ struct meson8b_dwmac {
 	phy_interface_t			phy_mode;
 	struct clk			*rgmii_tx_clk;
 	u32				tx_delay_ns;
+	u32				rx_delay_ns;
 	struct clk			*timing_adj_clk;
 };
 
@@ -284,25 +285,64 @@ static int meson8b_devm_clk_prepare_enable(struct meson8b_dwmac *dwmac,
 
 static int meson8b_init_prg_eth(struct meson8b_dwmac *dwmac)
 {
+	u32 tx_dly_config, rx_dly_config, delay_config;
 	int ret;
-	u8 tx_dly_val = 0;
+
+	tx_dly_config = FIELD_PREP(PRG_ETH0_TXDLY_MASK,
+				   dwmac->tx_delay_ns >> 1);
+
+	if (dwmac->rx_delay_ns == 2)
+		rx_dly_config = PRG_ETH0_ADJ_ENABLE | PRG_ETH0_ADJ_SETUP;
+	else
+		rx_dly_config = 0;
 
 	switch (dwmac->phy_mode) {
 	case PHY_INTERFACE_MODE_RGMII:
+		delay_config = tx_dly_config | rx_dly_config;
+		break;
 	case PHY_INTERFACE_MODE_RGMII_RXID:
-		tx_dly_val = dwmac->tx_delay_ns >> 1;
-		/* fall through */
-
-	case PHY_INTERFACE_MODE_RGMII_ID:
+		delay_config = tx_dly_config;
+		break;
 	case PHY_INTERFACE_MODE_RGMII_TXID:
+		delay_config = rx_dly_config;
+		break;
+	case PHY_INTERFACE_MODE_RGMII_ID:
+	case PHY_INTERFACE_MODE_RMII:
+		delay_config = 0;
+		break;
+	default:
+		dev_err(dwmac->dev, "unsupported phy-mode %s\n",
+			phy_modes(dwmac->phy_mode));
+		return -EINVAL;
+	};
+
+	if (rx_dly_config & PRG_ETH0_ADJ_ENABLE) {
+		if (!dwmac->timing_adj_clk) {
+			dev_err(dwmac->dev,
+				"The timing-adjustment clock is mandatory for the RX delay re-timing\n");
+			return -EINVAL;
+		}
+
+		/* The timing adjustment logic is driven by a separate clock */
+		ret = meson8b_devm_clk_prepare_enable(dwmac,
+						      dwmac->timing_adj_clk);
+		if (ret) {
+			dev_err(dwmac->dev,
+				"Failed to enable the timing-adjustment clock\n");
+			return ret;
+		}
+	}
+
+	meson8b_dwmac_mask_bits(dwmac, PRG_ETH0, PRG_ETH0_TXDLY_MASK |
+				PRG_ETH0_ADJ_ENABLE | PRG_ETH0_ADJ_SETUP |
+				PRG_ETH0_ADJ_DELAY | PRG_ETH0_ADJ_SKEW,
+				delay_config);
+
+	if (phy_interface_mode_is_rgmii(dwmac->phy_mode)) {
 		/* only relevant for RMII mode -> disable in RGMII mode */
 		meson8b_dwmac_mask_bits(dwmac, PRG_ETH0,
 					PRG_ETH0_INVERTED_RMII_CLK, 0);
 
-		meson8b_dwmac_mask_bits(dwmac, PRG_ETH0, PRG_ETH0_TXDLY_MASK,
-					FIELD_PREP(PRG_ETH0_TXDLY_MASK,
-						   tx_dly_val));
-
 		/* Configure the 125MHz RGMII TX clock, the IP block changes
 		 * the output automatically (= without us having to configure
 		 * a register) based on the line-speed (125MHz for Gbit speeds,
@@ -322,24 +362,11 @@ static int meson8b_init_prg_eth(struct meson8b_dwmac *dwmac)
 				"failed to enable the RGMII TX clock\n");
 			return ret;
 		}
-		break;
-
-	case PHY_INTERFACE_MODE_RMII:
+	} else {
 		/* invert internal clk_rmii_i to generate 25/2.5 tx_rx_clk */
 		meson8b_dwmac_mask_bits(dwmac, PRG_ETH0,
 					PRG_ETH0_INVERTED_RMII_CLK,
 					PRG_ETH0_INVERTED_RMII_CLK);
-
-		/* TX clock delay cannot be configured in RMII mode */
-		meson8b_dwmac_mask_bits(dwmac, PRG_ETH0, PRG_ETH0_TXDLY_MASK,
-					0);
-
-		break;
-
-	default:
-		dev_err(dwmac->dev, "unsupported phy-mode %s\n",
-			phy_modes(dwmac->phy_mode));
-		return -EINVAL;
 	}
 
 	/* enable TX_CLK and PHY_REF_CLK generator */
@@ -394,6 +421,18 @@ static int meson8b_dwmac_probe(struct platform_device *pdev)
 				 &dwmac->tx_delay_ns))
 		dwmac->tx_delay_ns = 2;
 
+	/* use 0ns as fallback since this is what most boards actually use */
+	if (of_property_read_u32(pdev->dev.of_node, "amlogic,rx-delay-ns",
+				 &dwmac->rx_delay_ns))
+		dwmac->rx_delay_ns = 0;
+
+	if (dwmac->rx_delay_ns != 0 && dwmac->rx_delay_ns != 2) {
+		dev_err(&pdev->dev,
+			"The only allowed RX delays values are: 0ns, 2ns");
+		ret = -EINVAL;
+		goto err_remove_config_dt;
+	}
+
 	dwmac->timing_adj_clk = devm_clk_get_optional(dwmac->dev,
 						      "timing-adjustment");
 	if (IS_ERR(dwmac->timing_adj_clk)) {