dsa: Fix conversion from host device to mii bus

Commit b4d2394 ("dsa: Replace mii_bus with a generic host device")
replaces mii_bus with a generic host_dev, and introduces
dsa_host_dev_to_mii_bus() to support conversion from host_dev to mii_bus.
However, in some cases it uses to_mii_bus to perform that conversion.
Since host_dev is not the phy bus device but typically a platform device,
this fails and results in a crash with the affected drivers.

BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: [<ffffffff81781d35>] __mutex_lock_slowpath+0x75/0x100
PGD 406783067 PUD 406784067 PMD 0
Oops: 0002 [#1] SMP
...
Call Trace:
[<ffffffff810a538b>] ? pick_next_task_fair+0x61b/0x880
[<ffffffff81781de3>] mutex_lock+0x23/0x37
[<ffffffff81533244>] mdiobus_read+0x34/0x60
[<ffffffff8153b95a>] __mv88e6xxx_reg_read+0x8a/0xa0
[<ffffffff8153b9bc>] mv88e6xxx_reg_read+0x4c/0xa0

Fixes: b4d2394 ("dsa: Replace mii_bus with a generic host device")
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6060.c

@@ -21,8 +21,12 @@
 
 static int reg_read(struct dsa_switch *ds, int addr, int reg)
 {
-	return mdiobus_read(to_mii_bus(ds->master_dev),
-			    ds->pd->sw_addr + addr, reg);
+	struct mii_bus *bus = dsa_host_dev_to_mii_bus(ds->master_dev);
+
+	if (bus == NULL)
+		return -EINVAL;
+
+	return mdiobus_read(bus, ds->pd->sw_addr + addr, reg);
 }
 
 #define REG_READ(addr, reg)					\
@@ -38,8 +42,12 @@ static int reg_read(struct dsa_switch *ds, int addr, int reg)
 
 static int reg_write(struct dsa_switch *ds, int addr, int reg, u16 val)
 {
-	return mdiobus_write(to_mii_bus(ds->master_dev),
-			     ds->pd->sw_addr + addr, reg, val);
+	struct mii_bus *bus = dsa_host_dev_to_mii_bus(ds->master_dev);
+
+	if (bus == NULL)
+		return -EINVAL;
+
+	return mdiobus_write(bus, ds->pd->sw_addr + addr, reg, val);
 }
 
 #define REG_WRITE(addr, reg, val)				\

drivers/net/dsa/mv88e6xxx.c

@@ -75,11 +75,14 @@ int __mv88e6xxx_reg_read(struct mii_bus *bus, int sw_addr, int addr, int reg)
 int mv88e6xxx_reg_read(struct dsa_switch *ds, int addr, int reg)
 {
 	struct mv88e6xxx_priv_state *ps = ds_to_priv(ds);
+	struct mii_bus *bus = dsa_host_dev_to_mii_bus(ds->master_dev);
 	int ret;
 
+	if (bus == NULL)
+		return -EINVAL;
+
 	mutex_lock(&ps->smi_mutex);
-	ret = __mv88e6xxx_reg_read(to_mii_bus(ds->master_dev),
-				   ds->pd->sw_addr, addr, reg);
+	ret = __mv88e6xxx_reg_read(bus, ds->pd->sw_addr, addr, reg);
 	mutex_unlock(&ps->smi_mutex);
 
 	return ret;
@@ -119,11 +122,14 @@ int __mv88e6xxx_reg_write(struct mii_bus *bus, int sw_addr, int addr,
 int mv88e6xxx_reg_write(struct dsa_switch *ds, int addr, int reg, u16 val)
 {
 	struct mv88e6xxx_priv_state *ps = ds_to_priv(ds);
+	struct mii_bus *bus = dsa_host_dev_to_mii_bus(ds->master_dev);
 	int ret;
 
+	if (bus == NULL)
+		return -EINVAL;
+
 	mutex_lock(&ps->smi_mutex);
-	ret = __mv88e6xxx_reg_write(to_mii_bus(ds->master_dev),
-				    ds->pd->sw_addr, addr, reg, val);
+	ret = __mv88e6xxx_reg_write(bus, ds->pd->sw_addr, addr, reg, val);
 	mutex_unlock(&ps->smi_mutex);
 
 	return ret;