TX path

Linux v6.6: net/core/dev.c
Linux v6.6: net/sched/sch_generic.c
Linux v6.6: net/ipv4/ip_output.c

/**
 * __dev_queue_xmit() - transmit a buffer
 * @skb:	buffer to transmit
 * @sb_dev:	suboordinate device used for L2 forwarding offload
 *
 * Queue a buffer for transmission to a network device. The caller must
 * have set the device and priority and built the buffer before calling
 * this function. The function can be called from an interrupt.
 *
 * When calling this method, interrupts MUST be enabled. This is because
 * the BH enable code must have IRQs enabled so that it will not deadlock.
 *
 * Regardless of the return value, the skb is consumed, so it is currently
 * difficult to retry a send to this method. (You can bump the ref count
 * before sending to hold a reference for retry if you are careful.)
 *
 * Return:
 * * 0				- buffer successfully transmitted
 * * positive qdisc return code	- NET_XMIT_DROP etc.
 * * negative errno		- other errors
 */
int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
{
	struct net_device *dev = skb->dev;
	struct netdev_queue *txq = NULL;
	struct Qdisc *q;
	int rc = -ENOMEM;
	bool again = false;

	skb_reset_mac_header(skb);
	skb_assert_len(skb);

	if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_SCHED_TSTAMP))
		__skb_tstamp_tx(skb, NULL, NULL, skb->sk, SCM_TSTAMP_SCHED);

	/* Disable soft irqs for various locks below. Also
	 * stops preemption for RCU.
	 */
	rcu_read_lock_bh();

	skb_update_prio(skb);

	qdisc_pkt_len_init(skb);
	tcx_set_ingress(skb, false);
#ifdef CONFIG_NET_EGRESS
	if (static_branch_unlikely(&egress_needed_key)) {
		if (nf_hook_egress_active()) {
			skb = nf_hook_egress(skb, &rc, dev);
			if (!skb)
				goto out;
		}

		netdev_xmit_skip_txqueue(false);

		nf_skip_egress(skb, true);
		skb = sch_handle_egress(skb, &rc, dev);
		if (!skb)
			goto out;
		nf_skip_egress(skb, false);

		if (netdev_xmit_txqueue_skipped())
			txq = netdev_tx_queue_mapping(dev, skb);
	}
#endif
	/* If device/qdisc don't need skb->dst, release it right now while
	 * its hot in this cpu cache.
	 */
	if (dev->priv_flags & IFF_XMIT_DST_RELEASE)
		skb_dst_drop(skb);
	else
		skb_dst_force(skb);

	if (!txq)
		txq = netdev_core_pick_tx(dev, skb, sb_dev);

	q = rcu_dereference_bh(txq->qdisc);

	trace_net_dev_queue(skb);
	if (q->enqueue) {
		rc = __dev_xmit_skb(skb, q, dev, txq);
		goto out;
	}

struct sk_buff *dev_hard_start_xmit(struct sk_buff *first, struct net_device *dev,
				    struct netdev_queue *txq, int *ret)
{
	struct sk_buff *skb = first;
	int rc = NETDEV_TX_OK;

	while (skb) {
		struct sk_buff *next = skb->next;

		skb_mark_not_on_list(skb);
		rc = xmit_one(skb, dev, txq, next != NULL);
		if (unlikely(!dev_xmit_complete(rc))) {
			skb->next = next;
			goto out;
		}

		skb = next;
		if (netif_tx_queue_stopped(txq) && skb) {
			rc = NETDEV_TX_BUSY;
			break;
		}
	}

out:
	*ret = rc;
	return skb;
}