[PATCH net-next v2] net: stmmac: dwmac: Add a fixup for the Micrel KSZ9131 PHY

[Stefan Eichenberger <eichest@gmail.com>]

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

Add a fixup to the stmmac driver to keep the preamble before the SFD
(Start Frame Delimiter) on the Micrel KSZ9131 PHY when the driver is
used on an NXP i.MX8MP SoC.

This allows to workaround errata ERR050694 of the NXP i.MX8MP 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. Without bit 2 being set in the remote loopback register, no
packets are received. With the bit set, reception works fine.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
---
Changes in v2:
- Use phy_register_fixup_for_uid() instead of adding a new device tree
  property
- I will send the conversion of the micrel.txt binding as a separate
  patch series
- Link to v1: https://lore.kernel.org/all/20251212084657.29239-1-eichest@gmail.com/

 .../net/ethernet/stmicro/stmmac/dwmac-imx.c   | 56 ++++++++++++++++++-
 1 file changed, 55 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-imx.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-imx.c
index db288fbd5a4df..23bc917d3f0bd 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-imx.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-imx.c
@@ -19,6 +19,7 @@
 #include <linux/regmap.h>
 #include <linux/slab.h>
 #include <linux/stmmac.h>
+#include <linux/micrel_phy.h>
 
 #include "stmmac_platform.h"
 
@@ -39,6 +40,12 @@
 #define RMII_RESET_SPEED		(0x3 << 14)
 #define CTRL_SPEED_MASK			GENMASK(15, 14)
 
+/* Undocumented bit of the KSZ9131RNX in the remote loopback register to keep
+ * the preamble before sfd. It was reported by NXP in cooperation with Micrel.
+ */
+#define KSZ9x31_REMOTE_LOOPBACK			0x11
+#define KSZ9x31_REMOTE_LOOPBACK_KEEP_PREAMBLE	BIT(2)
+
 struct imx_priv_data;
 
 struct imx_dwmac_ops {
@@ -282,6 +289,30 @@ imx_dwmac_parse_dt(struct imx_priv_data *dwmac, struct device *dev)
 	return err;
 }
 
+static int imx8mp_dwmac_phy_micrel_fixup(struct phy_device *phydev)
+{
+	struct device *mdio_bus_dev = phydev->mdio.dev.parent;
+	struct device_node *mac_node;
+
+	if (!mdio_bus_dev || !mdio_bus_dev->parent)
+		return 0;
+
+	mac_node = mdio_bus_dev->parent->of_node;
+	if (!mac_node)
+		return 0;
+
+	if (!of_device_is_compatible(mac_node, "nxp,imx8mp-dwmac-eqos"))
+		return 0;
+
+	/* Keep the preamble before the SFD (Start Frame Delimiter) for the
+	 * Micrel KSZ9131. This is required on the i.MX8MP because of errata
+	 * ERR050694.
+	 */
+	return phy_modify_changed(phydev, KSZ9x31_REMOTE_LOOPBACK,
+				  KSZ9x31_REMOTE_LOOPBACK_KEEP_PREAMBLE,
+				  KSZ9x31_REMOTE_LOOPBACK_KEEP_PREAMBLE);
+}
+
 static int imx_dwmac_probe(struct platform_device *pdev)
 {
 	struct plat_stmmacenet_data *plat_dat;
@@ -389,7 +420,30 @@ static struct platform_driver imx_dwmac_driver = {
 		.of_match_table = imx_dwmac_match,
 	},
 };
-module_platform_driver(imx_dwmac_driver);
+
+static int __init imx_dwmac_init(void)
+{
+	int ret;
+
+	if (of_machine_is_compatible("fsl,imx8mp")) {
+		ret = phy_register_fixup_for_uid(PHY_ID_KSZ9131, MICREL_PHY_ID_MASK,
+						 imx8mp_dwmac_phy_micrel_fixup);
+		if (ret)
+			return ret;
+	}
+
+	return platform_driver_register(&imx_dwmac_driver);
+}
+module_init(imx_dwmac_init);
+
+static void __exit imx_dwmac_exit(void)
+{
+	if (of_machine_is_compatible("fsl,imx8mp"))
+		phy_unregister_fixup_for_uid(PHY_ID_KSZ9131, MICREL_PHY_ID_MASK);
+
+	platform_driver_unregister(&imx_dwmac_driver);
+}
+module_exit(imx_dwmac_exit);
 
 MODULE_AUTHOR("NXP");
 MODULE_DESCRIPTION("NXP imx8 DWMAC Specific Glue layer");
-- 
2.51.0