1.. SPDX-License-Identifier: GPL-2.0 2 3===================== 4Softnet Driver Issues 5===================== 6 7Probing guidelines 8================== 9 10Address validation 11------------------ 12 13Any hardware layer address you obtain for your device should 14be verified. For example, for ethernet check it with 15linux/etherdevice.h:is_valid_ether_addr() 16 17Close/stop guidelines 18===================== 19 20Quiescence 21---------- 22 23After the ndo_stop routine has been called, the hardware must 24not receive or transmit any data. All in flight packets must 25be aborted. If necessary, poll or wait for completion of 26any reset commands. 27 28Auto-close 29---------- 30 31The ndo_stop routine will be called by unregister_netdevice 32if device is still UP. 33 34Transmit path guidelines 35======================== 36 37Stop queues in advance 38---------------------- 39 40The ndo_start_xmit method must not return NETDEV_TX_BUSY under 41any normal circumstances. It is considered a hard error unless 42there is no way your device can tell ahead of time when its 43transmit 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:: c 49 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 & dr->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_tx_stop_queue(txq); 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:: c 94 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 macros 101~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 102 103.. kernel-doc:: include/net/netdev_queues.h 104 :doc: Lockless queue stopping / waking helpers. 105 106The standard macros like netif_txq_maybe_stop(), netif_txq_try_stop() etc. 107are well tested, prefer them over local synchronization schemes. 108 109No exclusive ownership 110---------------------- 111 112An ndo_start_xmit method must not modify the shared parts of a 113cloned SKB. 114 115Timely completions 116------------------ 117 118Do not forget that once you return NETDEV_TX_OK from your 119ndo_start_xmit method, it is your driver's responsibility to free 120up the SKB and in some finite amount of time. 121 122For example, this means that it is not allowed for your TX 123mitigation scheme to let TX packets "hang out" in the TX 124ring unreclaimed forever if no new TX packets are sent. 125This error can deadlock sockets waiting for send buffer room 126to be freed up. 127 128If you return NETDEV_TX_BUSY from the ndo_start_xmit method, you 129must not keep any reference to that SKB and you must not attempt 130to free it up. 131 132Error message reporting 133======================= 134 135A number of driver configuration interfaces pass a Netlink extended ACK 136(``extack``) object to the driver (either directly as an argument or 137as a member of a parameter struct). The drivers should try to report 138most errors via the ``extack`` object. System level exceptions, 139indicating that system or device is misbehaving or is in bad state, 140should continue to be reported to system logs. 141 142Messages should be passed **either** via ``extack`` **or** to system logs. 143Drivers should not try to report the same information to both. 144