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- Document about softnet driver issues
- Transmit path guidelines:
- 1) The hard_start_xmit method must never return '1' under any
- normal circumstances. It is considered a hard error unless
- there is no way your device can tell ahead of time when it's
- transmit function will become busy.
- Instead it must maintain the queue properly. For example,
- for a driver implementing scatter-gather this means:
- static int drv_hard_start_xmit(struct sk_buff *skb,
- struct net_device *dev)
- {
- struct drv *dp = dev->priv;
- lock_tx(dp);
- ...
- /* This is a hard error log it. */
- if (TX_BUFFS_AVAIL(dp) <= (skb_shinfo(skb)->nr_frags + 1)) {
- netif_stop_queue(dev);
- unlock_tx(dp);
- printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
- dev->name);
- return 1;
- }
- ... queue packet to card ...
- ... update tx consumer index ...
- if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
- netif_stop_queue(dev);
- ...
- unlock_tx(dp);
- ...
- }
- And then at the end of your TX reclamation event handling:
- if (netif_queue_stopped(dp->dev) &&
- TX_BUFFS_AVAIL(dp) > (MAX_SKB_FRAGS + 1))
- netif_wake_queue(dp->dev);
- For a non-scatter-gather supporting card, the three tests simply become:
- /* This is a hard error log it. */
- if (TX_BUFFS_AVAIL(dp) <= 0)
- and:
- if (TX_BUFFS_AVAIL(dp) == 0)
- and:
- if (netif_queue_stopped(dp->dev) &&
- TX_BUFFS_AVAIL(dp) > 0)
- netif_wake_queue(dp->dev);
- 2) Do not forget to update netdev->trans_start to jiffies after
- each new tx packet is given to the hardware.
- 3) Do not forget that once you return 0 from your hard_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 1 from the hard_start_xmit method, you must not keep
- any reference to that SKB and you must not attempt to free it up.
- Probing guidelines:
- 1) 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:
- 1) After the dev->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.
- 2) The dev->stop routine will be called by unregister_netdevice
- if device is still UP.
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