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+Document about softnet driver issues
+
+Transmit path guidelines:
+
+1) The ndo_start_xmit method must not return NETDEV_TX_BUSY 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 netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,
+ struct net_device *dev)
+ {
+ struct drv *dp = netdev_priv(dev);
+
+ 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 NETDEV_TX_BUSY;
+ }
+
+ ... queue packet to card ...
+ ... update tx consumer index ...
+
+ if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
+ netif_stop_queue(dev);
+
+ ...
+ unlock_tx(dp);
+ ...
+ return NETDEV_TX_OK;
+ }
+
+ 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) An ndo_start_xmit method must not modify the shared parts of a
+ cloned SKB.
+
+3) Do not forget that once you return NETDEV_TX_OK from your
+ ndo_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 NETDEV_TX_BUSY from the ndo_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 ndo_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 ndo_stop routine will be called by unregister_netdevice
+ if device is still UP.