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:   Thu, 18 Jan 2018 09:42:27 +0100
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
Cc:     netdev@...r.kernel.org, john.stultz@...aro.org,
        Richard Cochran <rcochran@...utronix.de>, jiri@...nulli.us,
        ivan.briano@...el.com, richardcochran@...il.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.

On Wed, Jan 17, 2018 at 03:06:12PM -0800, Jesus Sanchez-Palencia wrote:
> From: Richard Cochran <rcochran@...utronix.de>
> 
> This patch introduces SO_TXTIME.  User space enables this option in
> order to pass a desired future transmit time in a CMSG when calling
> sendmsg(2).
> 
> A new field is added to struct sockcm_cookie, and the tstamp from
> skbuffs will be used later on.

In the discussion about the v1 patchset, there was a question if the
cmsg should include a clockid_t. Without that, how can an application
prevent the packet from being sent using an incorrect clock, e.g.
the system clock when it expects it to be a PHC, or a different PHC
when the socket is not bound to a specific interface?

At least in some applications it would be preferred to not sent a
packet at all instead of sending it at a wrong time.

Please keep in mind that the PHCs and the system clock don't have to
be synchronized to each other. If I understand the rest of the series
correctly, there is an assumption that the PHCs are keeping time in
TAI and CLOCK_TAI can be used as a fallback.

-- 
Miroslav Lichvar