netdev - Re: [RFC] net : add tx timestamp to packet mmap.

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Date:	Sat, 13 Apr 2013 14:33:49 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	Paul Chavent <Paul.Chavent@...ra.fr>
Cc:	Richard Cochran <richardcochran@...il.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	daniel.borkmann@....ee.ethz.ch, xemul@...allels.com,
	ebiederm@...ssion.com, netdev@...r.kernel.org
Subject: Re: [RFC] net : add tx timestamp to packet mmap.

On Thu, Dec 13, 2012 at 11:13 AM, Paul Chavent <Paul.Chavent@...ra.fr> wrote:
> Hello.
>
>
> On 12/13/2012 02:29 PM, Richard Cochran wrote:
>>
>> On Wed, Dec 12, 2012 at 04:29:25PM +0100, Paul Chavent wrote:
>>>
>>> This patch allow to generate tx timestamps of packets sent by the packet
>>> mmap interface.
>>>
>>> Actually, you can't get tx timestamps with the sample code below.
>>>
>>> I wonder if my current implementation is good. And if not, how should i
>>> get the timestamps ?
>>
>>
>> In order for time stamps to appear, somebody has to call
>> skb_tx_timestamp() ...
>
> Yes. "Somebody" means "the hardware driver" after completing xmit. That's
> true ?
>
>
>>
>>> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
>>> index e639645..948748b 100644
>>> --- a/net/packet/af_packet.c
>>> +++ b/net/packet/af_packet.c
>>> @@ -1857,6 +1857,10 @@ static int tpacket_fill_skb(struct packet_sock
>>> *po, struct sk_buff *skb,
>>>         void *data;
>>>         int err;
>>>
>>> +       err = sock_tx_timestamp(&po->sk, &skb_shinfo(skb)->tx_flags);
>
>>
>> and this call is only setting some flags.
>
> Yes, it only sets some flags. I thought that those flags was required by the
> skb_tx_timestamp() in order to make the appropriate timestamping (hardware,
> software, etc).
>
> So in order to have tx timestamp that work, both calls are needed ?

Yes.

> Why sock_tx_timestamp is called in packet_fill_skb and packet_sendmsg_spkt
> and not in tpacket_fill_skb ?

Good point. From what I can tell, if you add it, the existing error
queue mechanism should work fine. At a glance, your code looks fine,
too, although I haven't tested it.

> Why i can retrieve timestamps when i add this call ?

I actually wondered it if would be possible to return the timestamps
in the ring itself. It is, but requires two additional changes:
writing the timestamp to orig_skb->tstamp in skb_tstamp_tx and then
writing it into the ring in tpacket_destruct_skb. The first change is
a bit odd, as the orig_skb is usually freed shortly after
skb_tstamp_tx is called. Still, I verified that it works and the patch
should also make your errorqueue-based code work, as it inserts the
sock_tx_timestamp. Will send it for review following this.

>>
>> HTH,
>> Richard
>>
> Thank for your help.
>
> Paul.
>
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