From mboxrd@z Thu Jan  1 00:00:00 1970
From: Johann Baudy <johaahn@gmail.com>
Subject: RE: [PATCH] Packet socket: mmapped IO: PACKET_TX_RING
Date: Tue, 04 Nov 2008 23:45:43 +0100
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Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Evgeniy Polyakov <zbr@ioremap.net>
To: "Lovich, Vitali" <vlovich@qualcomm.com>
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Hi Vitali,

> Here's the code I came up with to determine the offset - anyone see any problems?
> 
> union {
>    tpacket2_hdr *t2;
>    u8 *raw;
> }
> 
> struct page *first_frag;
> u32 page_offset;
> 
> first_frag = skb_shinfo(skb)->frags[0].page;
> page_offset = skb_shinfo(skb)->frags[0].page_offset;
> 
> page_offset -= (skb_headlen(skb) + sizeof(struct tpacket2_hdr));
> if (unlikely(page_offset < 0)) {
> 	first_frag--;
> 	page_offset += PAGE_SIZE;
> }
> frame.raw = page_address(first_frag) + page_offset;
> 

This won't work if your network card doesn't support Scatter/Gather IO.
Indeed, when SG is not supported, fragmented packets are linearized
after dev_queue_xmit() call. Thus addresses of pages are then lost.

Best regards,
Johann