From: Stefan Eichenberger <stefan.eichenberger@toradex.com>

Add a property to keep the preamble before the SFD is sent from the PHY
to the Ethernet controller. By default the preamble is removed, this
property allows to keep it.

This allows to workaround broken Ethernet controllers as found on the
NXP i.MX8MP. Specifically, errata ERR050694 that states:
ENET_QOS: MAC incorrectly discards the received packets when Preamble
Byte does not precede SFD or SMD.

The bit which disables this feature is not documented in the datasheet
from Micrel, but has been found by NXP and Micrel following this
discussion:
https://community.nxp.com/t5/i-MX-Processors/iMX8MP-eqos-not-working-for-10base-t/m-p/2151032

It has been tested on Verdin iMX8MP from Toradex by forcing the PHY to
10MBit. Withouth this property set, no packets are received. With this
property set, reception works fine.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
---
 drivers/net/phy/micrel.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/drivers/net/phy/micrel.c b/drivers/net/phy/micrel.c
index 05de68b9f7191..0bcc380d73653 100644
--- a/drivers/net/phy/micrel.c
+++ b/drivers/net/phy/micrel.c
@@ -101,6 +101,14 @@
 #define LAN8814_CABLE_DIAG_VCT_DATA_MASK	GENMASK(7, 0)
 #define LAN8814_PAIR_BIT_SHIFT			12
 
+/* KSZ9x31 remote loopback register */
+#define KSZ9x31_REMOTE_LOOPBACK			0x11
+/* This is an undocumented bit of the KSZ9131RNX.
+ * It was reported by NXP in cooperation with Micrel.
+ */
+#define KSZ9x31_REMOTE_LOOPBACK_KEEP_PREAMBLE	BIT(2)
+#define KSZ9x31_REMOTE_LOOPBACK_EN		BIT(8)
+
 #define LAN8814_SKUS				0xB
 
 #define LAN8814_WIRE_PAIR_MASK			0xF
@@ -455,6 +463,7 @@ struct kszphy_priv {
 	bool rmii_ref_clk_sel_val;
 	bool clk_enable;
 	bool is_ptp_available;
+	bool keep_preamble_before_sfd;
 	u64 stats[ARRAY_SIZE(kszphy_hw_stats)];
 	struct kszphy_phy_stats phy_stats;
 };
@@ -1452,6 +1461,7 @@ static int ksz9131_config_init(struct phy_device *phydev)
 		"txd2-skew-psec", "txd3-skew-psec"
 	};
 	char *control_skews[2] = {"txen-skew-psec", "rxdv-skew-psec"};
+	struct kszphy_priv *priv = phydev->priv;
 	const struct device *dev_walker;
 	int ret;
 
@@ -1500,6 +1510,17 @@ static int ksz9131_config_init(struct phy_device *phydev)
 	if (ret < 0)
 		return ret;
 
+	if (priv->keep_preamble_before_sfd) {
+		ret = phy_read(phydev, KSZ9x31_REMOTE_LOOPBACK);
+		if (ret < 0)
+			return ret;
+
+		ret |= KSZ9x31_REMOTE_LOOPBACK_KEEP_PREAMBLE;
+		ret = phy_write(phydev, KSZ9x31_REMOTE_LOOPBACK, ret);
+		if (ret < 0)
+			return ret;
+	}
+
 	return 0;
 }
 
@@ -2683,6 +2704,14 @@ static int kszphy_probe(struct phy_device *phydev)
 		priv->rmii_ref_clk_sel_val = true;
 	}
 
+	/* Keep the preamble before the SFD (Start Frame Delimiter). This might
+	 * be needed on some boards with a broken Ethernet controller like the
+	 * eqos used in the NXP i.MX8MP.
+	 */
+	if (phydev_id_compare(phydev, PHY_ID_KSZ9131))
+		priv->keep_preamble_before_sfd = of_property_read_bool(np,
+								       "micrel,keep-preamble-before-sfd");
+
 	return 0;
 }
 
-- 
2.51.0