[GitHub/mt8127/android_kernel_alcatel_ttab.git] / Documentation / networking / driver.txt

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.
Commit	Line	Data
98766fbe	1	Document about softnet driver issues
1da177e4 LT	2
	3	Transmit path guidelines:
	4
e34fac1c BH	5	1) The ndo_start_xmit method must not return NETDEV_TX_BUSY under
e34fac1c BH	6	any normal circumstances. It is considered a hard error unless
1da177e4 LT	7	there is no way your device can tell ahead of time when it's
	8	transmit function will become busy.
	9
	10	Instead it must maintain the queue properly. For example,
	11	for a driver implementing scatter-gather this means:
	12
e34fac1c BH	13	static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,
e34fac1c BH	14	struct net_device *dev)
1da177e4	15	{
b74ca3a8	16	struct drv *dp = netdev_priv(dev);
1da177e4 LT	17
	18	lock_tx(dp);
	19	...
	20	/* This is a hard error log it. */
	21	if (TX_BUFFS_AVAIL(dp) <= (skb_shinfo(skb)->nr_frags + 1)) {
	22	netif_stop_queue(dev);
	23	unlock_tx(dp);
	24	printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
	25	dev->name);
e34fac1c	26	return NETDEV_TX_BUSY;
1da177e4 LT	27	}
	28
	29	... queue packet to card ...
	30	... update tx consumer index ...
	31
	32	if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
	33	netif_stop_queue(dev);
	34
	35	...
	36	unlock_tx(dp);
	37	...
e34fac1c	38	return NETDEV_TX_OK;
1da177e4 LT	39	}
1da177e4 LT	40
d6bc8ac9	41	And then at the end of your TX reclamation event handling:
1da177e4 LT	42
	43	if (netif_queue_stopped(dp->dev) &&
	44	TX_BUFFS_AVAIL(dp) > (MAX_SKB_FRAGS + 1))
	45	netif_wake_queue(dp->dev);
	46
	47	For a non-scatter-gather supporting card, the three tests simply become:
	48
	49	/* This is a hard error log it. */
	50	if (TX_BUFFS_AVAIL(dp) <= 0)
	51
	52	and:
	53
	54	if (TX_BUFFS_AVAIL(dp) == 0)
	55
	56	and:
	57
	58	if (netif_queue_stopped(dp->dev) &&
	59	TX_BUFFS_AVAIL(dp) > 0)
	60	netif_wake_queue(dp->dev);
	61
de7aca16	62	2) An ndo_start_xmit method must not modify the shared parts of a
ce3ba139 ML	63	cloned SKB.
ce3ba139 ML	64
e34fac1c BH	65	3) Do not forget that once you return NETDEV_TX_OK from your
	66	ndo_start_xmit method, it is your driver's responsibility to free
	67	up the SKB and in some finite amount of time.
1da177e4 LT	68
	69	For example, this means that it is not allowed for your TX
	70	mitigation scheme to let TX packets "hang out" in the TX
	71	ring unreclaimed forever if no new TX packets are sent.
	72	This error can deadlock sockets waiting for send buffer room
	73	to be freed up.
	74
e34fac1c BH	75	If you return NETDEV_TX_BUSY from the ndo_start_xmit method, you
	76	must not keep any reference to that SKB and you must not attempt
	77	to free it up.
1da177e4 LT	78
	79	Probing guidelines:
	80
	81	1) Any hardware layer address you obtain for your device should
	82	be verified. For example, for ethernet check it with
	83	linux/etherdevice.h:is_valid_ether_addr()
	84
	85	Close/stop guidelines:
	86
b3cf6545	87	1) After the ndo_stop routine has been called, the hardware must
1da177e4 LT	88	not receive or transmit any data. All in flight packets must
	89	be aborted. If necessary, poll or wait for completion of
	90	any reset commands.
	91
b3cf6545	92	2) The ndo_stop routine will be called by unregister_netdevice
1da177e4	93	if device is still UP.