netdev - Re: [PATCH] net-timestamp: Fix a documentation typo

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Date:	Mon, 24 Nov 2014 17:38:21 -0500
From:	Willem de Bruijn <willemb@...gle.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Network Development <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] net-timestamp: Fix a documentation typo

>>> Is user code supposed to rely on this and, further, on the fact that the
>>> counter starts at zero?  If not, how else is user code supposed to match
>>> outgoing data to timestamps?
>>
>> That is correct. The per-socket counter is reset when
>> SOF_TIMESTAMPING_OPT_ID is set. On datagram sockets, it returns the
>> packet number since the reset. On stream sockets, it returns the byte
>> offset since the reset.
>>
>
> It might be worth tweaking the docs at some point to make this clearer.

Good point. The commit message is apparently more informative than the
actual documentation.

>>> Also, is it intentional that the payload data associated with the tx
>>> timestamp is (I think) the full outgoing packet including lower-layer
>>> headers?
>>
>> Absolutely not. I'll look into that right away. It doesn't on ACK, and
>> should certainly not expose this info in the other cases, either.
>
> Then I won't start trying to decode it :)

The datagram feature existed before I added the counter and stream
support, so returning the entire packet in that case is legacy
behavior, I suppose. I did not intend to expose network headers for
the new stream socket interface, though.

> TBH, all I looked at was the packet size, which matched the full
> link-layer packet.
>
> Also, the address returned by recvmsg appeared to be garbage instead
> of 0.0.0.0 (or something meaningful, whatever that would be).
>
>>
>>> And, finally, would it be possible to attach IP_PKTINFO to the looped
>>> timestamp?  That way I could finally update my fancy ping program to
>>> track which outgoing interface was used for a request.
>>
>> If socket option IP_PKTINFO is set, you want to receive in_pktinfo for
>> any packet that happens to be queued onto the error queue? Both
>> SKB_EXT_ERR(skb) and PKTINFO_SKB_CB(skb) use the control block to
>> store data that is later encoded in a cmsg, so there may not be enough
>> room to hold both. I'll take a look.
>
> I don't really care what the mechanism is, but it would be really nice
> if I could see what interface the send timestamp is associated with.

Okay. I'll see if I can cook something up.

> Also, thanks for this new feature.  It's great!

Thanks!

> --Andy
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