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

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Date:	Mon, 24 Nov 2014 14:18:39 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] net-timestamp: Fix a documentation typo

On Mon, Nov 24, 2014 at 2:11 PM, Willem de Bruijn <willemb@...gle.com> wrote:
> On Mon, Nov 24, 2014 at 3:02 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> SOF_TIMESTAMPING_OPT_ID puts the id in ee_data, not ee_info.
>>
>> Cc: Willem de Bruijn <willemb@...gle.com>
>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>
> Acked-by: Willem de Bruijn <willemb@...gle.com>
>
>> ---
>
> Thanks for sending a fix.
>
>> While I'm here, the docs say:
>>
>>     In practice, it [ee_data] is a monotonically increasing u32 (that wraps).
>>
>> 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.

>> 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 :)

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.

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

--Andy
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