From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 8 May 2008 11:22:08 +0800
> On Wed, May 07, 2008 at 03:48:06PM +0200, Michael Buesch wrote:
> >
> > So there's no way to actually fail in a TX handler? Drivers
> > are doomed to drop the packet, if they cannot handle it due to
> > ring overflow?
>
> You're supposed to stop the queue before the ring overflows.
Right, and this is why drivers choose a TX wakeup threshold such
that they can accept an arbitrarily sized TSO frame.
For example, from drivers/net/tg3.c's ->hard_start_xmit():
if (unlikely(tg3_tx_avail(tp) <= (MAX_SKB_FRAGS + 1))) {
netif_stop_queue(dev);
if (tg3_tx_avail(tp) > TG3_TX_WAKEUP_THRESH(tp))
netif_wake_queue(tp->dev);
}
The driver is responsible for stopping the queue _before_ it
enters a state where there is not enough space in the queue
to accept a packet.
This is why most drivers make the following kind of BUG check
at the start of their ->hard_start_xmit()
if (unlikely(tg3_tx_avail(tp) <= (skb_shinfo(skb)->nr_frags + 1))) {
if (!netif_queue_stopped(dev)) {
netif_stop_queue(dev);
/* This is a hard error, log it. */
printk(KERN_ERR PFX "%s: BUG! Tx Ring full when "
"queue awake!\n", dev->name);
}
return NETDEV_TX_BUSY;
}
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