net: dsa: mv88e6xxx: prevent interrupt storm caused by mv88e6390x_port_set_cmode

When debugging another issue I faced an interrupt storm in this
driver (88E6390, port 9 in SGMII mode), consisting of alternating
link-up / link-down interrupts. Analysis showed that the driver
wanted to set a cmode that was set already. But so far
mv88e6390x_port_set_cmode() doesn't check this and powers down
SERDES, what causes the link to break, and eventually results in
the described interrupt storm.

Fix this by checking whether the cmode actually changes. We want
that the very first call to mv88e6390x_port_set_cmode() always
configures the registers, therefore initialize port.cmode with
a value that is different from any supported cmode value.
We have to take care that we only init the ports cmode once
chip->info->num_ports is set.

v2:
- add small helper and init the number of actual ports only

Fixes: 364e9d7776 ("net: dsa: mv88e6xxx: Power on/off SERDES on cmode change")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -4595,6 +4595,14 @@ static int mv88e6xxx_smi_init(struct mv88e6xxx_chip *chip,
 	return 0;
 }
 
+static void mv88e6xxx_ports_cmode_init(struct mv88e6xxx_chip *chip)
+{
+	int i;
+
+	for (i = 0; i < mv88e6xxx_num_ports(chip); i++)
+		chip->ports[i].cmode = MV88E6XXX_PORT_STS_CMODE_INVALID;
+}
+
 static enum dsa_tag_protocol mv88e6xxx_get_tag_protocol(struct dsa_switch *ds,
 							int port)
 {
@@ -4631,6 +4639,8 @@ static const char *mv88e6xxx_drv_probe(struct device *dsa_dev,
 	if (err)
 		goto free;
 
+	mv88e6xxx_ports_cmode_init(chip);
+
 	mutex_lock(&chip->reg_lock);
 	err = mv88e6xxx_switch_reset(chip);
 	mutex_unlock(&chip->reg_lock);

drivers/net/dsa/mv88e6xxx/port.c

@@ -398,6 +398,10 @@ int mv88e6390x_port_set_cmode(struct mv88e6xxx_chip *chip, int port,
 		cmode = 0;
 	}
 
+	/* cmode doesn't change, nothing to do for us */
+	if (cmode == chip->ports[port].cmode)
+		return 0;
+
 	lane = mv88e6390x_serdes_get_lane(chip, port);
 	if (lane < 0)
 		return lane;

drivers/net/dsa/mv88e6xxx/port.h

@@ -52,6 +52,7 @@
 #define MV88E6185_PORT_STS_CMODE_1000BASE_X	0x0005
 #define MV88E6185_PORT_STS_CMODE_PHY		0x0006
 #define MV88E6185_PORT_STS_CMODE_DISABLED	0x0007
+#define MV88E6XXX_PORT_STS_CMODE_INVALID	0xff
 
 /* Offset 0x01: MAC (or PCS or Physical) Control Register */
 #define MV88E6XXX_PORT_MAC_CTL				0x01