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1.. SPDX-License-Identifier: GPL-2.02 3=====================4Softnet Driver Issues5=====================6 7Probing guidelines8==================9 10Address validation11------------------12 13Any hardware layer address you obtain for your device should14be verified. For example, for ethernet check it with15linux/etherdevice.h:is_valid_ether_addr()16 17Close/stop guidelines18=====================19 20Quiescence21----------22 23After the ndo_stop routine has been called, the hardware must24not receive or transmit any data. All in flight packets must25be aborted. If necessary, poll or wait for completion of26any reset commands.27 28Auto-close29----------30 31The ndo_stop routine will be called by unregister_netdevice32if device is still UP.33 34Transmit path guidelines35========================36 37Stop queues in advance38----------------------39 40The ndo_start_xmit method must not return NETDEV_TX_BUSY under41any normal circumstances. It is considered a hard error unless42there is no way your device can tell ahead of time when its43transmit function will become busy.44 45Instead it must maintain the queue properly. For example,46for a driver implementing scatter-gather this means:47 48.. code-block:: c49 50 static u32 drv_tx_avail(struct drv_ring *dr)51 {52 u32 used = READ_ONCE(dr->prod) - READ_ONCE(dr->cons);53 54 return dr->tx_ring_size - (used & bp->tx_ring_mask);55 }56 57 static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,58 struct net_device *dev)59 {60 struct drv *dp = netdev_priv(dev);61 struct netdev_queue *txq;62 struct drv_ring *dr;63 int idx;64 65 idx = skb_get_queue_mapping(skb);66 dr = dp->tx_rings[idx];67 txq = netdev_get_tx_queue(dev, idx);68 69 //...70 /* This should be a very rare race - log it. */71 if (drv_tx_avail(dr) <= skb_shinfo(skb)->nr_frags + 1) {72 netif_stop_queue(dev);73 netdev_warn(dev, "Tx Ring full when queue awake!\n");74 return NETDEV_TX_BUSY;75 }76 77 //... queue packet to card ...78 79 netdev_tx_sent_queue(txq, skb->len);80 81 //... update tx producer index using WRITE_ONCE() ...82 83 if (!netif_txq_maybe_stop(txq, drv_tx_avail(dr),84 MAX_SKB_FRAGS + 1, 2 * MAX_SKB_FRAGS))85 dr->stats.stopped++;86 87 //...88 return NETDEV_TX_OK;89 }90 91And then at the end of your TX reclamation event handling:92 93.. code-block:: c94 95 //... update tx consumer index using WRITE_ONCE() ...96 97 netif_txq_completed_wake(txq, cmpl_pkts, cmpl_bytes,98 drv_tx_avail(dr), 2 * MAX_SKB_FRAGS);99 100Lockless queue stop / wake helper macros101~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~102 103.. kernel-doc:: include/net/netdev_queues.h104 :doc: Lockless queue stopping / waking helpers.105 106No exclusive ownership107----------------------108 109An ndo_start_xmit method must not modify the shared parts of a110cloned SKB.111 112Timely completions113------------------114 115Do not forget that once you return NETDEV_TX_OK from your116ndo_start_xmit method, it is your driver's responsibility to free117up the SKB and in some finite amount of time.118 119For example, this means that it is not allowed for your TX120mitigation scheme to let TX packets "hang out" in the TX121ring unreclaimed forever if no new TX packets are sent.122This error can deadlock sockets waiting for send buffer room123to be freed up.124 125If you return NETDEV_TX_BUSY from the ndo_start_xmit method, you126must not keep any reference to that SKB and you must not attempt127to free it up.128