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Document about softnet driver issues
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Transmit path guidelines:
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1) The hard_start_xmit method must never return '1' under any
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normal circumstances. It is considered a hard error unless
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there is no way your device can tell ahead of time when it's
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transmit function will become busy.
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Instead it must maintain the queue properly. For example,
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for a driver implementing scatter-gather this means:
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static int drv_hard_start_xmit(struct sk_buff *skb,
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struct net_device *dev)
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{
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struct drv *dp = dev->priv;
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lock_tx(dp);
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...
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/* This is a hard error log it. */
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if (TX_BUFFS_AVAIL(dp) <= (skb_shinfo(skb)->nr_frags + 1)) {
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netif_stop_queue(dev);
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unlock_tx(dp);
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printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
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dev->name);
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return 1;
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}
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... queue packet to card ...
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... update tx consumer index ...
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if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
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netif_stop_queue(dev);
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...
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unlock_tx(dp);
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...
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}
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And then at the end of your TX reclaimation event handling:
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if (netif_queue_stopped(dp->dev) &&
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TX_BUFFS_AVAIL(dp) > (MAX_SKB_FRAGS + 1))
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netif_wake_queue(dp->dev);
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For a non-scatter-gather supporting card, the three tests simply become:
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/* This is a hard error log it. */
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if (TX_BUFFS_AVAIL(dp) <= 0)
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and:
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if (TX_BUFFS_AVAIL(dp) == 0)
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and:
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if (netif_queue_stopped(dp->dev) &&
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TX_BUFFS_AVAIL(dp) > 0)
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netif_wake_queue(dp->dev);
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2) Do not forget to update netdev->trans_start to jiffies after
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each new tx packet is given to the hardware.
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3) Do not forget that once you return 0 from your hard_start_xmit
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method, it is your driver's responsibility to free up the SKB
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and in some finite amount of time.
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For example, this means that it is not allowed for your TX
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mitigation scheme to let TX packets "hang out" in the TX
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ring unreclaimed forever if no new TX packets are sent.
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This error can deadlock sockets waiting for send buffer room
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to be freed up.
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If you return 1 from the hard_start_xmit method, you must not keep
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any reference to that SKB and you must not attempt to free it up.
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Probing guidelines:
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1) Any hardware layer address you obtain for your device should
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be verified. For example, for ethernet check it with
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linux/etherdevice.h:is_valid_ether_addr()
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Close/stop guidelines:
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1) After the dev->stop routine has been called, the hardware must
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not receive or transmit any data. All in flight packets must
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be aborted. If necessary, poll or wait for completion of
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any reset commands.
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2) The dev->stop routine will be called by unregister_netdevice
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if device is still UP.
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