Add the rt linux 4.1.3-rt3 as base

[kvmfornfv.git] / kernel / Documentation / networking / driver.txt

diff --git a/kernel/Documentation/networking/driver.txt b/kernel/Documentation/networking/driver.txt
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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.