From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH] net: smsc911x: fix RX FIFO fastforwarding when
 dropping packets
Date: Thu, 12 Apr 2012 16:01:39 +0200
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Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Steve Glendinning <steve.glendinning@smsc.com>
To: Will Deacon <will.deacon@arm.com>
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On Thu, 2012-04-12 at 14:47 +0100, Will Deacon wrote:

> 
> I don't think we want an skb_reserve at all, since the hardware shifts the
> data in the RX FIFO, meaning that we will read two bytes of 0 anyway before
> valid data.
> 
> > 		skb_put(skb, pktlength - 4);
> 
> I can move the put here if you like, but we need to use pktwords << 2 to
> make sure that we read the leading and trailing zeroes inserted by the
> hardware.

before calling linux stack, you'll have to skip those 2 bytes.

This is skb_reserve() purpose.