netdev - Re: [PATCH] net-packet: tx timestamping on tpacket ring

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Date:	Mon, 15 Apr 2013 18:56:03 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Paul Chavent <Paul.Chavent@...ra.fr>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	daniel.borkmann@....ee.ethz.ch, xemul@...allels.com,
	ebiederm@...ssion.com, netdev@...r.kernel.org
Subject: Re: [PATCH] net-packet: tx timestamping on tpacket ring

On Mon, Apr 15, 2013 at 09:37:30AM +0200, Paul Chavent wrote:
> On 04/14/2013 03:07 PM, Richard Cochran wrote:
> >As it stand now, it is fairly useless, since there is no way for user
> >space to tell which kind of time stamp has been reported. In fact, the
> >code will silently intermingle hardware and software time stamps. That
> >is surely a mean trick to play on the users.
> 
> Isn't it the one that the user ask with setsockopt(fd, SOL_PACKET,
> PACKET_TIMESTAMP, &timestamping, sizeof(timestamping)) ?

No, not necessarily. Look at the code in net/packet/af_packet.c.

		if ((po->tp_tstamp & SOF_TIMESTAMPING_SYS_HARDWARE)
				&& shhwtstamps->syststamp.tv64)
			ts = ktime_to_timespec(shhwtstamps->syststamp);
		else if ((po->tp_tstamp & SOF_TIMESTAMPING_RAW_HARDWARE)
				&& shhwtstamps->hwtstamp.tv64)
			ts = ktime_to_timespec(shhwtstamps->hwtstamp);
		else if (skb->tstamp.tv64)
			ts = ktime_to_timespec(skb->tstamp);
		else
			getnstimeofday(&ts);

What happens if RAW is requested, but no HW time stamp is available?

 
> However, i wonder why you added an other sockopt that do the same
> thing as SOL_SOCKET/SO_TIMESTAMPING sockopt ?

Not sure what you mean. I did not add the SOL_PACKET socket option.

Thanks,
Richard
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