net: mdio: Add workaround for Micrel PHYs which are not C45 compatible

After scanning the bus for C22 devices, check if any Micrel PHYs have
been found.  They are known to do bad things if there are C45
transactions on the bus. Prevent the scanning of the bus using C45 if
such a PHY has been detected.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/phy/mdio_bus.c

@@ -19,6 +19,7 @@
 #include <linux/interrupt.h>
 #include <linux/io.h>
 #include <linux/kernel.h>
+#include <linux/micrel_phy.h>
 #include <linux/mii.h>
 #include <linux/mm.h>
 #include <linux/module.h>
@@ -600,6 +601,32 @@ static int mdiobus_scan_bus_c45(struct mii_bus *bus)
 	return 0;
 }
 
+/* There are some C22 PHYs which do bad things when where is a C45
+ * transaction on the bus, like accepting a read themselves, and
+ * stomping over the true devices reply, to performing a write to
+ * themselves which was intended for another device. Now that C22
+ * devices have been found, see if any of them are bad for C45, and if we
+ * should skip the C45 scan.
+ */
+static bool mdiobus_prevent_c45_scan(struct mii_bus *bus)
+{
+	int i;
+
+	for (i = 0; i < PHY_MAX_ADDR; i++) {
+		struct phy_device *phydev;
+		u32 oui;
+
+		phydev = mdiobus_get_phy(bus, i);
+		if (!phydev)
+			continue;
+		oui = phydev->phy_id >> 10;
+
+		if (oui == MICREL_OUI)
+			return true;
+	}
+	return false;
+}
+
 /**
  * __mdiobus_register - bring up all the PHYs on a given bus and attach them to bus
  * @bus: target mii_bus
@@ -617,8 +644,9 @@ static int mdiobus_scan_bus_c45(struct mii_bus *bus)
 int __mdiobus_register(struct mii_bus *bus, struct module *owner)
 {
 	struct mdio_device *mdiodev;
-	int i, err;
 	struct gpio_desc *gpiod;
+	bool prevent_c45_scan;
+	int i, err;
 
 	if (!bus || !bus->name)
 		return -EINVAL;
@@ -691,8 +719,11 @@ int __mdiobus_register(struct mii_bus *bus, struct module *owner)
 			goto error;
 	}
 
-	if (bus->probe_capabilities == MDIOBUS_C45 ||
-	    bus->probe_capabilities == MDIOBUS_C22_C45) {
+	prevent_c45_scan = mdiobus_prevent_c45_scan(bus);
+
+	if (!prevent_c45_scan &&
+	    (bus->probe_capabilities == MDIOBUS_C45 ||
+	     bus->probe_capabilities == MDIOBUS_C22_C45)) {
 		err = mdiobus_scan_bus_c45(bus);
 		if (err)
 			goto error;

include/linux/micrel_phy.h

@@ -8,6 +8,8 @@
 #ifndef _MICREL_PHY_H
 #define _MICREL_PHY_H
 
+#define MICREL_OUI		0x0885
+
 #define MICREL_PHY_ID_MASK	0x00fffff0
 
 #define PHY_ID_KSZ8873MLL	0x000e7237