taprio: don't reject same mqprio settings

The taprio qdisc allows to set mqprio setting but only once. In case
if mqprio settings are provided next time the error is returned as
it's not allowed to change traffic class mapping in-flignt and that
is normal. But if configuration is absolutely the same - no need to
return error. It allows to provide same command couple times,
changing only base time for instance, or changing only scheds maps,
but leaving mqprio setting w/o modification. It more corresponds the
message: "Changing the traffic mapping of a running schedule is not
supported", so reject mqprio if it's really changed.

Also corrected TC_BITMASK + 1 for consistency, as proposed.

Fixes: a3d43c0d56f1 ("taprio: Add support adding an admin schedule")
Reviewed-by: Vladimir Oltean <[e-mail]>
Tested-by: Vladimir Oltean <[e-mail]>
Acked-by: Vinicius Costa Gomes <[e-mail]>
Signed-off-by: Ivan Khoronzhuk <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sched/sch_taprio.c

···
 	}
 
 	/* Verify priority mapping uses valid tcs */
-	for (i = 0; i < TC_BITMASK + 1; i++) {
+	for (i = 0; i <= TC_BITMASK; i++) {
 		if (qopt->prio_tc_map[i] >= qopt->num_tc) {
 			NL_SET_ERR_MSG(extack, "Invalid traffic class in priority to traffic class mapping");
 			return -EINVAL;
···
 	return err;
 }
 
+static int taprio_mqprio_cmp(const struct net_device *dev,
+			     const struct tc_mqprio_qopt *mqprio)
+{
+	int i;
+
+	if (!mqprio || mqprio->num_tc != dev->num_tc)
+		return -1;
+
+	for (i = 0; i < mqprio->num_tc; i++)
+		if (dev->tc_to_txq[i].count != mqprio->count[i] ||
+		    dev->tc_to_txq[i].offset != mqprio->offset[i])
+			return -1;
+
+	for (i = 0; i <= TC_BITMASK; i++)
+		if (dev->prio_tc_map[i] != mqprio->prio_tc_map[i])
+			return -1;
+
+	return 0;
+}
+
 static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 			 struct netlink_ext_ack *extack)
 {
···
 	oper = rcu_dereference(q->oper_sched);
 	admin = rcu_dereference(q->admin_sched);
 	rcu_read_unlock();
+
+	/* no changes - no new mqprio settings */
+	if (!taprio_mqprio_cmp(dev, mqprio))
+		mqprio = NULL;
 
 	if (mqprio && (oper || admin)) {
 		NL_SET_ERR_MSG(extack, "Changing the traffic mapping of a running schedule is not supported");
···
 					    mqprio->offset[i]);
 
 		/* Always use supplied priority mappings */
-		for (i = 0; i < TC_BITMASK + 1; i++)
+		for (i = 0; i <= TC_BITMASK; i++)
 			netdev_set_prio_tc_map(dev, i,
 					       mqprio->prio_tc_map[i]);
 	}