net: dsa: mv88e6xxx: Release mutex between each ATU read

The PTP code needs low latency access to the PTP hardware timestamps.
Reading all the ATU entries in one go adds a lot of latency to the PTP
code. So take and release the reg_lock mutex for each individual MAC
address in the ATU, allowing the PTP thread jump in between.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -1437,7 +1437,9 @@ static int mv88e6xxx_port_db_dump_fid(struct mv88e6xxx_chip *chip,
 	eth_broadcast_addr(addr.mac);
 
 	do {
+		mutex_lock(&chip->reg_lock);
 		err = mv88e6xxx_g1_atu_getnext(chip, fid, &addr);
+		mutex_unlock(&chip->reg_lock);
 		if (err)
 			return err;
 
@@ -1470,7 +1472,10 @@ static int mv88e6xxx_port_db_dump(struct mv88e6xxx_chip *chip, int port,
 	int err;
 
 	/* Dump port's default Filtering Information Database (VLAN ID 0) */
+	mutex_lock(&chip->reg_lock);
 	err = mv88e6xxx_port_get_fid(chip, port, &fid);
+	mutex_unlock(&chip->reg_lock);
+
 	if (err)
 		return err;
 
@@ -1480,7 +1485,9 @@ static int mv88e6xxx_port_db_dump(struct mv88e6xxx_chip *chip, int port,
 
 	/* Dump VLANs' Filtering Information Databases */
 	do {
+		mutex_lock(&chip->reg_lock);
 		err = mv88e6xxx_vtu_getnext(chip, &vlan);
+		mutex_unlock(&chip->reg_lock);
 		if (err)
 			return err;
 
@@ -1500,13 +1507,8 @@ static int mv88e6xxx_port_fdb_dump(struct dsa_switch *ds, int port,
 				   dsa_fdb_dump_cb_t *cb, void *data)
 {
 	struct mv88e6xxx_chip *chip = ds->priv;
-	int err;
-
-	mutex_lock(&chip->reg_lock);
-	err = mv88e6xxx_port_db_dump(chip, port, cb, data);
-	mutex_unlock(&chip->reg_lock);
 
-	return err;
+	return mv88e6xxx_port_db_dump(chip, port, cb, data);
 }
 
 static int mv88e6xxx_bridge_map(struct mv88e6xxx_chip *chip,