netdev - Re: question about eth

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Message-ID: <CAKoUArnRnx0txGWhNWpZAFBijWmtzdOe-D7Qb62-5AvBCfgHJQ@mail.gmail.com>
Date:	Tue, 20 Nov 2012 16:51:03 +0200
From:	Rami Rosen <roszenrami@...il.com>
To:	Dmitry Kravkov <dmitry@...adcom.com>
Cc:	netdev@...r.kernel.org
Subject: Re: question about eth_header

Hi,
I think that you should call skb_reset_mac_header()
before calling eth_hdr() and that skb_reset_mac_header() should not be
inside eth_hdr(). Following is explanation:

In the RX path, when we get a packet in the driver, what essentially
happens is that at this point, we have a pointer to
the packet buffer in skb->data, and its length in skb->len.

With ethernet packets, the driver will call eth_type_trans(); please
look at ethernet drivers code.
eth_type_trans() indeed sets the MAC header pointer by calling
skb_reset_mac_header() on the skb.

Afterwards, the eth_type_trans() will advance skb->data by
14, the size of the Ethernet header, and decrease skb->len
by calling skb_pull_inline(skb, ETH_HLEN).

In case you are wondering why the skb_reset_mac_header() is not inside
the eth_hdr(), I think that the reason is that, once
skb_reset_mac_header() on the skb was called, there are cases when we
want to make several calls eth_hdr() without each time again resetting
the mac header, for example, in the bridging code.  There is no reason
to doing it in terms of performance.

rgs,
Rami Rosen

http://ramirose.wix.com/ramirosen

On Tue, Nov 20, 2012 at 3:44 PM, Dmitry Kravkov <dmitry@...adcom.com> wrote:
> Hi,
>
> I am trying to use eth_hdr() but it looks like it doesn't point to mac
> header (mac header is present at skb->data). Shouldn't
> skb_reset_mac_header() be called inside eth_header()?
>
> Thanks.
>
>
>
>
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