Eric Lee / smarc-fsl-linux-kernel

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Documentation/networking/driver.txt 2.7 KB
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98766fbe6   Randy Dunlap   [PATCH] kernel Do... 
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  Document about softnet driver issues
1da177e4c   Linus Torvalds   Linux-2.6.12-rc2 
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  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)
  	{
b74ca3a89   Wang Chen   netdevice: Kill n... 
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  		struct drv *dp = netdev_priv(dev);
1da177e4c   Linus Torvalds   Linux-2.6.12-rc2 
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  		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!
  ",
  			       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);
  		...
  	}
d6bc8ac9e   Matt LaPlante   Fix typos in Docu... 
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     And then at the end of your TX reclamation event handling:
1da177e4c   Linus Torvalds   Linux-2.6.12-rc2 
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  	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.
ce3ba1399   Matti Linnanvuori   Documentation: ad... 
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  3) A hard_start_xmit method must not modify the shared parts of a
     cloned SKB.
  
  4) Do not forget that once you return 0 from your hard_start_xmit
1da177e4c   Linus Torvalds   Linux-2.6.12-rc2 
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     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.