pkt_sched: Fix tx queue selection in tc_modify_qdisc

After the recent mq change there is the new select_queue qdisc class
method used in tc_modify_qdisc, but it works OK only for direct child
qdiscs of mq qdisc. Grandchildren always get the first tx queue, which
would give wrong qdisc_root etc. results (e.g. for sch_htb as child of
sch_prio). This patch fixes it by using parent's dev_queue for such
grandchildren qdiscs. The select_queue method's return type is changed
BTW.

With feedback from: Patrick McHardy <kaber@trash.net>

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -81,7 +81,7 @@ struct Qdisc
 struct Qdisc_class_ops
 {
 	/* Child qdisc manipulation */
-	unsigned int		(*select_queue)(struct Qdisc *, struct tcmsg *);
+	struct netdev_queue *	(*select_queue)(struct Qdisc *, struct tcmsg *);
 	int			(*graft)(struct Qdisc *, unsigned long cl,
 					struct Qdisc *, struct Qdisc **);
 	struct Qdisc *		(*leaf)(struct Qdisc *, unsigned long cl);

net/sched/sch_api.c

@@ -1116,12 +1116,16 @@ create_n_graft:
 				 tcm->tcm_parent, tcm->tcm_parent,
 				 tca, &err);
 	else {
-		unsigned int ntx = 0;
+		struct netdev_queue *dev_queue;
 
 		if (p && p->ops->cl_ops && p->ops->cl_ops->select_queue)
-			ntx = p->ops->cl_ops->select_queue(p, tcm);
+			dev_queue = p->ops->cl_ops->select_queue(p, tcm);
+		else if (p)
+			dev_queue = p->dev_queue;
+		else
+			dev_queue = netdev_get_tx_queue(dev, 0);
 
-		q = qdisc_create(dev, netdev_get_tx_queue(dev, ntx), p,
+		q = qdisc_create(dev, dev_queue, p,
 				 tcm->tcm_parent, tcm->tcm_handle,
 				 tca, &err);
 	}

net/sched/sch_mq.c

@@ -125,13 +125,18 @@ static struct netdev_queue *mq_queue_get(struct Qdisc *sch, unsigned long cl)
 	return netdev_get_tx_queue(dev, ntx);
 }
 
-static unsigned int mq_select_queue(struct Qdisc *sch, struct tcmsg *tcm)
+static struct netdev_queue *mq_select_queue(struct Qdisc *sch,
+					    struct tcmsg *tcm)
 {
 	unsigned int ntx = TC_H_MIN(tcm->tcm_parent);
+	struct netdev_queue *dev_queue = mq_queue_get(sch, ntx);
 
-	if (!mq_queue_get(sch, ntx))
-		return 0;
-	return ntx - 1;
+	if (!dev_queue) {
+		struct net_device *dev = qdisc_dev(sch);
+
+		return netdev_get_tx_queue(dev, 0);
+	}
+	return dev_queue;
 }
 
 static int mq_graft(struct Qdisc *sch, unsigned long cl, struct Qdisc *new,