netdev - Re: 8139cp dma-debug warning.

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Date:	Thu, 13 Aug 2009 21:23:26 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	Dave Jones <davej@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: 8139cp dma-debug warning.

Dave Jones <davej@...hat.com> :
[...]
>  > @@ -549,8 +549,7 @@ rx_status_loop:
>  >  			pr_debug("%s: rx slot %d status 0x%x len %d\n",
>  >  			       dev->name, rx_tail, status, len);
>  >  
>  > -		buflen = cp->rx_buf_sz + NET_IP_ALIGN;
>  > -		new_skb = netdev_alloc_skb(dev, buflen);
>  > +		new_skb = netdev_alloc_skb(dev, buflen + NET_IP_ALIGN);
>  >  		if (!new_skb) {
>  >  			dev->stats.rx_dropped++;
>  >  			goto rx_next;
> 
> Below this, we're still doing an skb_reserve(NET_IP_ALIGN) on new_skb.
> Although the mapping is now constantly sized, aren't we still wastefully
> bumping the data/tail of the skb twice ?

$ grep -n NET_IP_ALIGN drivers/net/8139cp.c
552:		new_skb = netdev_alloc_skb(dev, buflen + NET_IP_ALIGN);
558:		skb_reserve(new_skb, NET_IP_ALIGN);
1059:		skb = netdev_alloc_skb(dev, cp->rx_buf_sz + NET_IP_ALIGN);
1063:		skb_reserve(skb, NET_IP_ALIGN);

I do not get it : netdev_alloc_skb allocates but it does not "bump" as
skb_reserve does (and skb_reserve does not allocate). Where would the
double bump come from ?

The mapping is constantly sized but we want the ethernet header following
IP data to be evenly aligned and thus the mapping to be oddly aligned.

-- 
Ueimor
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