taprio: Fix enabling offload with wrong number of traffic classes

If the driver implementing taprio offloading depends on the value of
the network device number of traffic classes (dev->num_tc) for
whatever reason, it was going to receive the value zero. The value was
only set after the offloading function is called.

So, moving setting the number of traffic classes to before the
offloading function is called fixes this issue. This is safe because
this only happens when taprio is instantiated (we don't allow this
configuration to be changed without first removing taprio).

Fixes: 9c66d15 ("taprio: Add support for hardware offloading")
Reported-by: Po Liu <po.liu@nxp.com>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Acked-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_taprio.c

@@ -1444,6 +1444,19 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 
 	taprio_set_picos_per_byte(dev, q);
 
+	if (mqprio) {
+		netdev_set_num_tc(dev, mqprio->num_tc);
+		for (i = 0; i < mqprio->num_tc; i++)
+			netdev_set_tc_queue(dev, i,
+					    mqprio->count[i],
+					    mqprio->offset[i]);
+
+		/* Always use supplied priority mappings */
+		for (i = 0; i <= TC_BITMASK; i++)
+			netdev_set_prio_tc_map(dev, i,
+					       mqprio->prio_tc_map[i]);
+	}
+
 	if (FULL_OFFLOAD_IS_ENABLED(taprio_flags))
 		err = taprio_enable_offload(dev, mqprio, q, new_admin, extack);
 	else
@@ -1471,19 +1484,6 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 		q->advance_timer.function = advance_sched;
 	}
 
-	if (mqprio) {
-		netdev_set_num_tc(dev, mqprio->num_tc);
-		for (i = 0; i < mqprio->num_tc; i++)
-			netdev_set_tc_queue(dev, i,
-					    mqprio->count[i],
-					    mqprio->offset[i]);
-
-		/* Always use supplied priority mappings */
-		for (i = 0; i <= TC_BITMASK; i++)
-			netdev_set_prio_tc_map(dev, i,
-					       mqprio->prio_tc_map[i]);
-	}
-
 	if (FULL_OFFLOAD_IS_ENABLED(taprio_flags)) {
 		q->dequeue = taprio_dequeue_offload;
 		q->peek = taprio_peek_offload;