Re: Hardware Timestamping Problem

[Guenter Ebermann <guenter.ebermann () googlemail com>]

> Am 10.06.2016 um 00:13 schrieb Guy Harris <guy () alum mit edu>:
>
> But that doesn't mean that the packets time stamped by the hardware when transmitted will be delivered to the 
> PF_PACKET sockets used by libpcap *with the hardware time stamp as the time stamp*.
>
> In order make that happen, if hardware transmit time stamping is enabled for a PF_PACKET socket:
>
>       dev_queue_xmit_nit() will *NOT* deliver, to that socket, packets queued for transmission;
>
>       when the hardware says "I've transmitted these packets" (and time-stamped them), the driver will take those 
> packets and deliver them to all PF_PACKET sockets with hardware transmit time stamping enabled?
>
> If those aren't done, then code processing packets from a PF_PACKET socket will get a mix of packets with software 
> time stamps (packets sent by the machine on which that code is running) and hardware time stamps (packets received by 
> that machine).
>
> I don't see any obvious code in dev_queue_xmit_nit() to avoid delivering packets to sockets that have requested 
> hardware time stamping, so the first of those statements doesn't appear to be true; is the second one true?

Perhaps I am wrong, but it seems the drivers deliver the sent packets, including the hardware-timestamp, to the error 
queue of the socket.
They all call skb_tstamp_tx() to do this, after the hardware has transmitted the packet and time-stamped it.

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