netdev - Re: [Intel-wired-lan] [RFC v2 net-next 01/10] net: Add a new socket option for a future transmit time.

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Date:   Wed, 24 Jan 2018 14:46:24 -0800
From:   Vinicius Costa Gomes <vinicius.gomes@...el.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Miroslav Lichvar <mlichvar@...hat.com>,
        Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>,
        netdev@...r.kernel.org, john.stultz@...aro.org,
        Richard Cochran <rcochran@...utronix.de>, jiri@...nulli.us,
        ivan.briano@...el.com, henrik@...tad.us, jhs@...atatu.com,
        levi.pearson@...man.com, intel-wired-lan@...ts.osuosl.org,
        xiyou.wangcong@...il.com, tglx@...utronix.de,
        anna-maria@...utronix.de
Subject: Re: [Intel-wired-lan] [RFC v2 net-next 01/10] net: Add a new socket option for a future transmit time.

Hi Richard,

Richard Cochran <richardcochran@...il.com> writes:

> On Tue, Jan 23, 2018 at 01:22:37PM -0800, Vinicius Costa Gomes wrote:
>> What I think would be the ideal scenario would be if the clockid
>> parameter to the TBS Qdisc would not be necessary (if offload was
>> enabled), but that's not quite possible right now, because there's no
>> support for using the hrtimer infrastructure with dynamic clocks
>> (/dev/ptp*).
>
> We don't need hrtimer for HW offloading.  Just enqueue the packets.  I
> thought we agreed that user space get the ordering correct.  In fact,
> davem insisted on it, IIRC.

About the ordering of packets, From here [1], there are 3 clear points
(in my understanding):

1. Re-ordering of TX descriptors on the device queue should/must not
   happen;

2. Out of order requests are an error;

3. Timestamps in the past are an error;

The only robust way that we could think of about keeping the the packets
in order for the device queue is re-ordering packets in the Qdisc.

We tried to reach out for confirmation [2] of this understanding but
didn't receive any word.

Even if we reach a decision that the Qdisc should not re-order packets
(we wouldn't have any dependency on hrtimers in the offload case, as you
pointed out), we still need hrtimers for the software implementation.

So, I guess, the problem remains, if it's possible for the user to
express a /dev/ptp* clock, what should we do? 

>
> Thanks,
> Richard

Cheers,
--
Vinicius

[1] https://patchwork.ozlabs.org/comment/1770302/

[2] https://patchwork.ozlabs.org/comment/1816492/q