netdev - Re: 8139cp dma-debug warning.

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Message-ID: <20090813192840.GA12838@redhat.com>
Date:	Thu, 13 Aug 2009 15:28:40 -0400
From:	Dave Jones <davej@...hat.com>
To:	Francois Romieu <romieu@...zoreil.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: 8139cp dma-debug warning.

On Thu, Aug 13, 2009 at 09:23:26PM +0200, Francois Romieu wrote:
 
 > > 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.

Ah, now I get it. Thanks for the explanation :)

	Dave

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