netdev - Layer 2 acceleration vs GSO

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Date:	Fri, 10 Jan 2014 20:05:39 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	John Fastabend <john.r.fastabend@...el.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	Neil Horman <nhorman@...driver.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	netdev <netdev@...r.kernel.org>
Subject: Layer 2 acceleration vs GSO

What happens when an skb to be sent through ndo_dfwd_start_xmit()
requires software GSO?

dev_hard_start_xmit() will segment it and then submit each segment, and
then:

> gso:
> 	do {
[...]
> 		if (accel_priv)
> 			rc = ops->ndo_dfwd_start_xmit(nskb, dev, accel_priv);
> 		else
> 			rc = ops->ndo_start_xmit(nskb, dev);
> 		trace_net_dev_xmit(nskb, rc, dev, skb_len);
[...]
> 		txq_trans_update(txq);

Oops, txq is NULL.  And once we add the obvious condition to that,

> 		if (unlikely(netif_xmit_stopped(txq) && skb->next))
> 			return NETDEV_TX_BUSY;

How can we tell if the hardware transmit queue filled up?

It seems like this feature currently relies on the driver returning
NETDEV_TX_BUSY when the TX queue is already full, like ixgbe does.  But
that is exactly what drivers are *not* supposed to do.

Ben.

> 	} while (skb->next);

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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