netdev - Re: Problem with frame time stamping

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Date:	Tue, 13 Nov 2007 11:27:24 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Antoine Zen-Ruffinen <antoine.zen@...il.com>
Cc:	netdev@...r.kernel.org, linux-net@...r.kernel.org,
	netfilter-devel@...r.kernel.org, patrik.arlos@....se
Subject: Re: Problem with frame time stamping

Antoine Zen-Ruffinen a écrit :
> What does it bring me to have a nanosecond precision if it is not
> related to the actual arrival of frame time ? As it seem I can feel
> skb->tstamp with whatever I want, I always become something else using
> ioctl(). (I'm using kernel 2.6.23).
>
>   
I guess you misunderstood kernel source, because it is related to 
arrival time, more exactly when it was processed by network stack.
(Beware  modern NICS can delay the rx interrupt by some us (ethtool -c 
eth0), so that an interrupt can feed more than one packet to the OS)

Check net/core/dev.c function netif_rx()

{
...
   if (!skb->tstamp.tv64)
       net_timestamp(skb);

}

So as soon your system as at least one socket 'asking for tsamps', 
netstamp_needed is not null, and net_timestamp() will call __net_timestamp()
wich does :

skb->tstamp = ktime_get_real();

So you should not 'feed' tstamp.tv64 and let it being 0, so that 
netif_rx() can do its job.





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