pkt_sched: sch_generic: Add generic qdisc->ops->peek() implementation.

With feedback from Patrick McHardy.

Signed-off-by: Jarek Poplawski <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sched/sch_api.c

···
    requeues once dequeued packet. It is used for non-standard or
    just buggy devices, which can defer output even if netif_queue_stopped()=0.
 
+   ---peek
+
+   like dequeue but without removing a packet from the queue
+
    ---reset
 
    returns qdisc to initial state: purge all buffers, clear all
···
 		qops->enqueue = noop_qdisc_ops.enqueue;
 	if (qops->requeue == NULL)
 		qops->requeue = noop_qdisc_ops.requeue;
+	if (qops->peek == NULL) {
+		if (qops->dequeue == NULL) {
+			qops->peek = noop_qdisc_ops.peek;
+		} else {
+			rc = -EINVAL;
+			goto out;
+		}
+	}
 	if (qops->dequeue == NULL)
 		qops->dequeue = noop_qdisc_ops.dequeue;
 

net/sched/sch_generic.c

···
 	.priv_size	=	0,
 	.enqueue	=	noop_enqueue,
 	.dequeue	=	noop_dequeue,
+	.peek		=	noop_dequeue,
 	.requeue	=	noop_requeue,
 	.owner		=	THIS_MODULE,
 };
···
 	.priv_size	=	0,
 	.enqueue	=	noop_enqueue,
 	.dequeue	=	noop_dequeue,
+	.peek		=	noop_dequeue,
 	.requeue	=	noop_requeue,
 	.owner		=	THIS_MODULE,
 };
···
 	return NULL;
 }
 
+static struct sk_buff *pfifo_fast_peek(struct Qdisc* qdisc)
+{
+	int prio;
+	struct sk_buff_head *list = qdisc_priv(qdisc);
+
+	for (prio = 0; prio < PFIFO_FAST_BANDS; prio++) {
+		if (!skb_queue_empty(list + prio))
+			return skb_peek(list + prio);
+	}
+
+	return NULL;
+}
+
 static int pfifo_fast_requeue(struct sk_buff *skb, struct Qdisc* qdisc)
 {
 	qdisc->q.qlen++;
···
 	.priv_size	=	PFIFO_FAST_BANDS * sizeof(struct sk_buff_head),
 	.enqueue	=	pfifo_fast_enqueue,
 	.dequeue	=	pfifo_fast_dequeue,
+	.peek		=	pfifo_fast_peek,
 	.requeue	=	pfifo_fast_requeue,
 	.init		=	pfifo_fast_init,
 	.reset		=	pfifo_fast_reset,