netdev - Re: [RFC PATCH 1/2] net: Add new network device function to allow for MMIO batching

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Date:	Fri, 13 Jul 2012 18:18:32 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	jeffrey.t.kirsher@...el.com, edumazet@...gle.com,
	bhutchings@...arflare.com, therbert@...gle.com,
	alexander.duyck@...il.com
Subject: Re: [RFC PATCH 1/2] net: Add new network device function to allow
 for MMIO batching

On Fri, 2012-07-13 at 08:49 -0700, Alexander Duyck wrote:
> On 07/13/2012 12:19 AM, Eric Dumazet wrote:
> > On Wed, 2012-07-11 at 17:26 -0700, Alexander Duyck wrote:
> >
> >> +static inline void netdev_complete_xmit(struct netdev_queue *txq)
> >> +{
> >> +	struct net_device *dev = txq->dev;
> >> +	const struct net_device_ops *ops = dev->netdev_ops;
> >> +
> >> +	if (txq->dispatch_pending < txq->dispatch_limit) {
> >> +		if (netif_tx_queue_delayed(txq)) {
> >> +			txq->dispatch_pending++;
> >> +			return;
> >> +		}
> >> +
> >> +		/* start of delayed write sequence */
> >> +		netif_tx_delay_queue(txq);
> > 	I dont understand this part. Isnt a return missing here ?
> >
> >> +	}
> >> +
> >> +	txq->dispatch_pending = 0;
> >> +
> >> +	ops->ndo_complete_xmit(dev, txq - &dev->_tx[0]);
> >> +}
> >> +
> >
> There is intentionally no return there.  The idea is that the first
> packet always gets through.  It is what is going to later force the
> interrupt that will force the final flush if it is needed.  That is one
> of the ways I am helping to reduce the latency of things such as TSO
> which will only be using one or two frames per interrupt anyway.


So for a single packet, we only trigger TX softirq do do nothing at all,
or worse the ndo_complete_xmit() is done twice ?

It looks like you need to add comments, because if I dont understand
this code, who will ?



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