netdev - A GRO question

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Message-ID: <4E12A3EC.7010803@gmail.com>
Date:	Tue, 05 Jul 2011 13:41:00 +0800
From:	Li Yu <raise.sail@...il.com>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: A GRO question

Hi,

	I have a question about GRO implementation, this indeed confuses me.

	I found that we assume that NAPI_GRO_CB(skb)->frag0 starts 
with a mac/L2 header in compare_ether_header(), which is called in
__napi_gro_receive()

	However, in further dev_gro_receive() -> ptype->gro_receive [inet_gro_receive],
we use same address as IPv4/L3 header, like below:

        off = skb_gro_offset(skb); //it should keep zero until now, in my words.
        hlen = off + sizeof(*iph);      
        iph = skb_gro_header_fast(skb, off);  //just return NAPI_GRO_CB(skb)->frag0 + 0

	So we forget that updating NAPI_GRO_CB(skb)->data_offset here, or I miss sth?

	And, in my understanding against igb source code, if rx_ring->rx_buffer_len < 1024 
(if we used large MTU), then igb driver use header split mode, in such case, the mac header
should be saved in skb->data : skb_put(skb, igb_get_hlen(rx_ring, rx_desc)), the rest data
is loaded by below skb_fill_page_desc() call. so NAPI_GRO_CB(skb)->frag0 should start with
L3 header. 

	Thanks.

Yu
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