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Commit e34fac1c authored by Ben Hutchings's avatar Ben Hutchings Committed by David S. Miller
Browse files

doc, net: Update ndo_start_xmit return type and values 



Commit dc1f8bf6 ('netdev: change
transmit to limited range type') changed the required return type and
9a1654ba ('net: Optimize
hard_start_xmit() return checking') changed the valid numerical
return values.

Signed-off-by: default avatarBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent de7aca16

Documentation/networking/driver.txt

+12 −10
Original line number Diff line number Diff line
@@ -2,15 +2,15 @@ Document about softnet driver issues 


Transmit path guidelines:



1) The ndo_start_xmit method must never return '1' under any

   normal circumstances.  It is considered a hard error unless

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 int drv_hard_start_xmit(struct sk_buff *skb,

	static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,

					       struct net_device *dev)

	{

		struct drv *dp = netdev_priv(dev);
@@ -23,7 +23,7 @@ Transmit path guidelines: 
			unlock_tx(dp);

			printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",

			       dev->name);

			return 1;

			return NETDEV_TX_BUSY;

		}



		... queue packet to card ...
@@ -35,6 +35,7 @@ Transmit path guidelines: 
		...

		unlock_tx(dp);

		...

		return NETDEV_TX_OK;

	}



   And then at the end of your TX reclamation event handling:
@@ -61,9 +62,9 @@ Transmit path guidelines: 
2) An ndo_start_xmit method must not modify the shared parts of a

   cloned SKB.



3) Do not forget that once you return 0 from your ndo_start_xmit

   method, it is your driver's responsibility to free up the SKB

   and in some finite amount of time.

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
@@ -71,8 +72,9 @@ Transmit path guidelines: 
   This error can deadlock sockets waiting for send buffer room

   to be freed up.



   If you return 1 from the ndo_start_xmit method, you must not keep

   any reference to that SKB and you must not attempt to free it 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: