From: Dave Jones <davej@redhat.com>
To: Francois Romieu <romieu@fr.zoreil.com>
Cc: David Miller <davem@davemloft.net>, netdev@vger.kernel.org
Subject: Re: 8139cp dma-debug warning.
Date: Thu, 13 Aug 2009 15:28:40 -0400 [thread overview]
Message-ID: <20090813192840.GA12838@redhat.com> (raw)
In-Reply-To: <20090813192326.GA32435@electric-eye.fr.zoreil.com>
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
prev parent reply other threads:[~2009-08-13 19:28 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-08-06 21:57 8139cp dma-debug warning Dave Jones
2009-08-12 17:13 ` Dave Jones
2009-08-13 5:20 ` David Miller
2009-08-13 13:45 ` Dave Jones
2009-08-13 19:23 ` Francois Romieu
2009-08-13 19:28 ` Dave Jones [this message]
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