linux-kernel - Re: [PATCH net-next 1/3] net/usb/r8152: support aggregation

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Date:	Tue, 13 Aug 2013 17:17:10 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	hayeswang <hayeswang@...ltek.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net/usb/r8152: support aggregation

On Tue, 2013-08-13 at 20:32 +0800, hayeswang wrote:
>  Oliver Neukum [mailto:oneukum@...e.de] 
> > Sent: Tuesday, August 13, 2013 4:49 PM
> > To: Hayeswang
> > Cc: netdev@...r.kernel.org; linux-kernel@...r.kernel.org; 
> > linux-usb@...r.kernel.org
> > Subject: Re: [PATCH net-next 1/3] net/usb/r8152: support aggregation
> > 
> [...]
> > > +               len_used = 0;
> > > +               rx_desc = agg->head;
> > > +               rx_data = agg->head;
> > > +               smp_wmb();
> > > +               pkt_len = le32_to_cpu(rx_desc->opts1) & RX_LEN_MASK;
> > > +               len_used += sizeof(struct rx_desc) + pkt_len;
> > > +
> > > +               while (urb->actual_length >= len_used) {
> > > +                       if (pkt_len < ETH_ZLEN)
> > > +                               break;
> > > +
> > > +                       pkt_len -= 4; /* CRC */
> > > +                       rx_data += sizeof(struct rx_desc);
> > > +
> > > +                       skb = netdev_alloc_skb_ip_align(netdev,
> > > pkt_len);
> > > +                       if (!skb) {
> > > +                               stats->rx_dropped++;
> > > +                               break;
> > > +                       }
> > > +                       memcpy(skb->data, rx_data, pkt_len);
> > > +                       skb_put(skb, pkt_len);
> > > +                       skb->protocol = eth_type_trans(skb, netdev);
> > > +                       netif_rx(skb);
> > > +                       stats->rx_packets++;
> > > +                       stats->rx_bytes += pkt_len;
> > > +
> > > +                       rx_data = rx_agg_align(rx_data + 
> > pkt_len + 4);
> > > +                       rx_desc = (struct rx_desc *)rx_data;
> > > +                       smp_wmb();
> > 
> > Against what is the memory barrier?
> 
> Excuse me. I don't understand your question. Do you mean the function should not
> be used here?

I don't understand what problem the function is supposed to fix. As long
as I don't understand it I cannot say for sure whether it is correct.
There seems no obvious reason for a memory barrier, but there may be a
hidden reason I don't see.

	Regards
		Oliver


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