netdev - Re: [RFC PATCH 3/7] net: add option to get information about timestamped packets

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Message-ID: <CAF=yD-+tJpY7XJrqxga=0_kn-FjS0M=yuu59m_ZRRrrcpJCbZw@mail.gmail.com>
Date:   Thu, 13 Apr 2017 12:16:09 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Denny Page <dennypage@...com>, Jiri Benc <jbenc@...hat.com>
Subject: Re: [RFC PATCH 3/7] net: add option to get information about
 timestamped packets

On Thu, Apr 13, 2017 at 11:18 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> On Thu, Apr 13, 2017 at 10:37:07AM -0400, Willem de Bruijn wrote:
>> On Wed, Apr 12, 2017 at 10:17 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
>> > Extend the skb_shared_hwtstamps structure with the index of the
>> > real interface which received or transmitted the packet and the length
>> > of the packet at layer 2.
>>
>> The original packet is received along with the timestamp.
>
> But only outgoing packets, right?

Timestamps for incoming packets are also passed alongside the original packet.

>> Why is this L2 length needed?
>
> It's needed for incoming packets to allow converting of preamble
> timestamps to trailer timestamps.

Receiving the mac length of a packet sounds like a feature independent
from timestamping. Either an ioctl similar to SIOCGIFMTU or, if it may
vary due to existince of vlan headers, a new independent cmsg at the
SOL_SOCKET layer.

>> > Add a SOF_TIMESTAMPING_OPT_PKTINFO flag to
>> > the SO_TIMESTAMPING option to allow applications to get this information
>> > as struct scm_ts_pktinfo in SCM_TIMESTAMPING_PKTINFO control message.
>>
>> This patch saves skb->dev->ifindex, which is the same as existing
>> SOF_TIMESTAMPING_OPT_CMSG. See also the bug fix for that
>> feature I sent yesterday: http://patchwork.ozlabs.org/patch/750197/
>
> The main point is that it provides the index of the device which
> received the packet. It does duplicate the functionality of OPT_CMSG +
> IP_PKTINFO for outgoing packets, but I thought it might be useful with
> the TSONLY option.

Agreed. I'd prefer to reuse the existing option for that and just extend it
to work together with TSONLY.

We will have to set serr->header.h4.iif from something other than skb->dev
if the skb was allocated fresh in __skb_tstamp_tx without the device
association.