linux-kernel - RE: [PATCH v6 2/4] net: moxa: replace build_skb() with netdev_alloc_skb_ip

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Date:	Tue, 26 Aug 2014 09:10:19 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Arnd Bergmann' <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
CC:	Jonas Jensen <jonas.jensen@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"f.fainelli@...il.com" <f.fainelli@...il.com>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mirqus@...il.com" <mirqus@...il.com>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH v6 2/4] net: moxa: replace build_skb() with
 netdev_alloc_skb_ip_align() / memcpy()

From: Arnd Bergmann
> On Monday 25 August 2014 16:22:22 Jonas Jensen wrote:
> > @@ -226,13 +226,15 @@ static int moxart_rx_poll(struct napi_struct *napi, int budget)
> >                 if (len > RX_BUF_SIZE)
> >                         len = RX_BUF_SIZE;
> >
> > -               skb = build_skb(priv->rx_buf[rx_head], priv->rx_buf_size);
> > +               skb = netdev_alloc_skb_ip_align(ndev, len);
> > +
> >                 if (unlikely(!skb)) {
> > -                       net_dbg_ratelimited("build_skb failed\n");
> > +                       net_dbg_ratelimited("netdev_alloc_skb_ip_align failed\n");
> >                         priv->stats.rx_dropped++;
> >                         priv->stats.rx_errors++;
> >                 }
> >
> > +               memcpy(skb->data, priv->rx_buf[rx_head], len);

Is this memcpy() aligned?
If the hardware can receive to a 4n+2 offset it is probably better to
copy the two bytes before the frame data and to round the length of to
a whole number of words.

> >                 skb_put(skb, len);
> >                 skb->protocol = eth_type_trans(skb, ndev);
> >                 napi_gro_receive(&priv->napi, skb);
> 
> While this seems correct, I wonder why you don't do the normal approach of
> dequeuing the skb from the chain and adding a newly allocated skb to it to
> save the memcpy.

Because the receive buffer area isn't made of skbs.
Post-allocating the skb also reduces the 'true size' of the skb.

	David



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