Documentation/networking/driver.txt

   1 Document about softnet driver issues
   2
   3 Transmit path guidelines:
   4
   5 1) The ndo_start_xmit method must never return '1' under any
   6    normal circumstances.  It is considered a hard error unless
   7    there is no way your device can tell ahead of time when it's
   8    transmit function will become busy.
   9
  10    Instead it must maintain the queue properly.  For example,
  11    for a driver implementing scatter-gather this means:
  12
  13         static int drv_hard_start_xmit(struct sk_buff *skb,
  14                                        struct net_device *dev)
  15         {
  16                 struct drv *dp = netdev_priv(dev);
  17
  18                 lock_tx(dp);
  19                 ...
  20                 /* This is a hard error log it. */
  21                 if (TX_BUFFS_AVAIL(dp) <= (skb_shinfo(skb)->nr_frags + 1)) {
  22                         netif_stop_queue(dev);
  23                         unlock_tx(dp);
  24                         printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
  25                                dev->name);
  26                         return 1;
  27                 }
  28
  29                 ... queue packet to card ...
  30                 ... update tx consumer index ...
  31
  32                 if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
  33                         netif_stop_queue(dev);
  34
  35                 ...
  36                 unlock_tx(dp);
  37                 ...
  38         }
  39
  40    And then at the end of your TX reclamation event handling:
  41
  42         if (netif_queue_stopped(dp->dev) &&
  43             TX_BUFFS_AVAIL(dp) > (MAX_SKB_FRAGS + 1))
  44                 netif_wake_queue(dp->dev);
  45
  46    For a non-scatter-gather supporting card, the three tests simply become:
  47
  48                 /* This is a hard error log it. */
  49                 if (TX_BUFFS_AVAIL(dp) <= 0)
  50
  51    and:
  52
  53                 if (TX_BUFFS_AVAIL(dp) == 0)
  54
  55    and:
  56
  57         if (netif_queue_stopped(dp->dev) &&
  58             TX_BUFFS_AVAIL(dp) > 0)
  59                 netif_wake_queue(dp->dev);
  60
  61 2) An ndo_start_xmit method must not modify the shared parts of a
  62    cloned SKB.
  63
  64 3) Do not forget that once you return 0 from your ndo_start_xmit
  65    method, it is your driver's responsibility to free up the SKB
  66    and in some finite amount of time.
  67
  68    For example, this means that it is not allowed for your TX
  69    mitigation scheme to let TX packets "hang out" in the TX
  70    ring unreclaimed forever if no new TX packets are sent.
  71    This error can deadlock sockets waiting for send buffer room
  72    to be freed up.
  73
  74    If you return 1 from the ndo_start_xmit method, you must not keep
  75    any reference to that SKB and you must not attempt to free it up.
  76
  77 Probing guidelines:
  78
  79 1) Any hardware layer address you obtain for your device should
  80    be verified.  For example, for ethernet check it with
  81    linux/etherdevice.h:is_valid_ether_addr()
  82
  83 Close/stop guidelines:
  84
  85 1) After the ndo_stop routine has been called, the hardware must
  86    not receive or transmit any data.  All in flight packets must
  87    be aborted. If necessary, poll or wait for completion of
  88    any reset commands.
  89
  90 2) The ndo_stop routine will be called by unregister_netdevice
  91    if device is still UP.