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Subject: Re: [PATCH] Gigabit Ethernet driver of Topcliff PCH
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Masayuki Ohtake <masa-korg@dsn.okisemi.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>, Sam Ravnborg <sam@ravnborg.org>,
        Joe Perches <joe@perches.com>, LKML <linux-kernel@vger.kernel.org>,
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        Randy Dunlap <randy.dunlap@oracle.com>,
        "David S. Miller" <davem@davemloft.net>, MeeGo <meego-dev@meego.com>,
        "Wang, Qi" <qi.wang@intel.com>, "Wang, Yong Y" <yong.y.wang@intel.com>,
        Andrew <andrew.chih.howe.khor@intel.com>,
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Date: Thu, 02 Sep 2010 15:40:47 +0200
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Le jeudi 02 septembre 2010 à 21:39 +0900, Masayuki Ohtake a écrit :
> Hi Eric
> 
> Thank you for your comments.
> 
> > I find hard to believe this driver needs to copy all outgoing frames on
> > pre-allocated skbs.
> >
> > +       /* [Header:14][payload] ---> [Header:14][paddong:2][payload]    */
> > +       memcpy(tmp_skb->data, skb->data, ETH_HLEN);
> > +       tmp_skb->data[ETH_HLEN] = 0x00;
> > +       tmp_skb->data[ETH_HLEN + 1] = 0x00;
> > +       tmp_skb->len = skb->len;
> > +       memcpy(&tmp_skb->data[ETH_HLEN + 2], &skb->data[ETH_HLEN],
> > +              (skb->len - ETH_HLEN));
> > +       buffer_info->kernel_skb = skb;
> > +       skb = tmp_skb;
> >
> > Whats the deal here please ?
> 
> This processing depends on hardware specification.
> 
> At the time of transmission.
> Hardware accepts a packet in the following format.
>   [Header: 14octet] + [padding: 2octet] + [payload]
>   Also, it is necessary to align the head of a [Header: 14octet] at 64byte.
> 
> In my knowledge, SKB received by kernel are the following format.
>   [padding: 2octet] + [Header:14octet] + [payload]
>   Also, The head of [payload] has aligned at 16 byte.
> 
> So, it has adjusted with the format of hardware by a copy.

The two bytes padding can be handled by network stack, if you
force in your setup phase :
	dev->hard_header_len = ETH_HLEN + 2;

then, you can do a single memcpy:

memcpy(tmp_skb->data, skb->data, skb->len);

About the 64 byte alignement, it might be a bit more complex :)