On Jun 9, 2016, at 1:19 PM, Guenter Ebermann <guenter.eberm...@googlemail.com>
wrote:
>> Am 09.06.2016 um 15:47 schrieb Michael Richardson <m...@sandelman.ca>:
>>
>> Guenter Ebermann <guenter.eberm...@googlemail.com> wrote:
>>> Hardware timestamping of sending/receiving buffer descriptors is done
>>> by NIC.
>>
>> Receiving I understand.
>>
>> Are you sure that the hardware is going to timestamp sent packets, and then
>> turn around and send the back to the kernel?
>
> Yes, the driver must monitor tx descriptor consumption by hardware anyways.
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?
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