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+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Softnet Driver Issues
+=====================
+
+Probing guidelines
+==================
+
+Address validation
+------------------
+
+Any hardware layer address you obtain for your device should
+be verified. For example, for ethernet check it with
+linux/etherdevice.h:is_valid_ether_addr()
+
+Close/stop guidelines
+=====================
+
+Quiescence
+----------
+
+After the ndo_stop routine has been called, the hardware must
+not receive or transmit any data. All in flight packets must
+be aborted. If necessary, poll or wait for completion of
+any reset commands.
+
+Auto-close
+----------
+
+The ndo_stop routine will be called by unregister_netdevice
+if device is still UP.
+
+Transmit path guidelines
+========================
+
+Stop queues in advance
+----------------------
+
+The ndo_start_xmit method must not return NETDEV_TX_BUSY under
+any normal circumstances. It is considered a hard error unless
+there is no way your device can tell ahead of time when its
+transmit function will become busy.
+
+Instead it must maintain the queue properly. For example,
+for a driver implementing scatter-gather this means:
+
+.. code-block:: c
+
+ static u32 drv_tx_avail(struct drv_ring *dr)
+ {
+ u32 used = READ_ONCE(dr->prod) - READ_ONCE(dr->cons);
+
+ return dr->tx_ring_size - (used & bp->tx_ring_mask);
+ }
+
+ static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,
+ struct net_device *dev)
+ {
+ struct drv *dp = netdev_priv(dev);
+ struct netdev_queue *txq;
+ struct drv_ring *dr;
+ int idx;
+
+ idx = skb_get_queue_mapping(skb);
+ dr = dp->tx_rings[idx];
+ txq = netdev_get_tx_queue(dev, idx);
+
+ //...
+ /* This should be a very rare race - log it. */
+ if (drv_tx_avail(dr) <= skb_shinfo(skb)->nr_frags + 1) {
+ netif_stop_queue(dev);
+ netdev_warn(dev, "Tx Ring full when queue awake!\n");
+ return NETDEV_TX_BUSY;
+ }
+
+ //... queue packet to card ...
+
+ netdev_tx_sent_queue(txq, skb->len);
+
+ //... update tx producer index using WRITE_ONCE() ...
+
+ if (!netif_txq_maybe_stop(txq, drv_tx_avail(dr),
+ MAX_SKB_FRAGS + 1, 2 * MAX_SKB_FRAGS))
+ dr->stats.stopped++;
+
+ //...
+ return NETDEV_TX_OK;
+ }
+
+And then at the end of your TX reclamation event handling:
+
+.. code-block:: c
+
+ //... update tx consumer index using WRITE_ONCE() ...
+
+ netif_txq_completed_wake(txq, cmpl_pkts, cmpl_bytes,
+ drv_tx_avail(dr), 2 * MAX_SKB_FRAGS);
+
+Lockless queue stop / wake helper macros
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: include/net/netdev_queues.h
+ :doc: Lockless queue stopping / waking helpers.
+
+No exclusive ownership
+----------------------
+
+An ndo_start_xmit method must not modify the shared parts of a
+cloned SKB.
+
+Timely completions
+------------------
+
+Do not forget that once you return NETDEV_TX_OK from your
+ndo_start_xmit method, it is your driver's responsibility to free
+up the SKB and in some finite amount of time.
+
+For example, this means that it is not allowed for your TX
+mitigation scheme to let TX packets "hang out" in the TX
+ring unreclaimed forever if no new TX packets are sent.
+This error can deadlock sockets waiting for send buffer room
+to be freed up.
+
+If you return NETDEV_TX_BUSY from the ndo_start_xmit method, you
+must not keep any reference to that SKB and you must not attempt
+to free it up.