netdev - Re: [PATCH net-next v6 4/7] net: dsa: hellcreek: Add support for hardware timestamping

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Date:   Wed, 14 Oct 2020 13:37:47 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Richard Cochran <richardcochran@...il.com>,
        Vladimir Oltean <olteanv@...il.com>
Cc:     Kamil Alkhouri <kamil.alkhouri@...offenburg.de>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        ilias.apalodimas@...aro.org
Subject: Re: [PATCH net-next v6 4/7] net: dsa: hellcreek: Add support for hardware timestamping

On Wed Oct 14 2020, Richard Cochran wrote:
> On Wed, Oct 14, 2020 at 12:57:47PM +0300, Vladimir Oltean wrote:
>> So the discussion is about how to have the cake and eat it at the same
>> time.
>
> And I wish for a pony.  With sparkles.  And a unicorn.  And a rainbow.
>
>> Silicon vendors eager to follow the latest trends in standards are
>> implementing hybrid PTP clocks, where an unsynchronizable version of the
>> clock delivers MAC timestamps to the application stack, and a
>> synchronizable wrapper over that same clock is what gets fed into the
>> offloading engines, like the ones behind the tc-taprio and tc-gate
>> offload. Some of these vendors perform cross-timestamping (they deliver
>> a timestamp from the MAC with 2, or 3, or 4, timestamps, depending on
>> how many PHCs that MAC has wired to it), some don't, and just deliver a
>> single timestamp from a configurable source.
>
> Sounds like it will be nearly impossible to make a single tc-taprio
> framework that fits all the hardware variants.

Why? All the gate operations work on the synchronized clock. I assume
all Qbv capable switches have a synchronized clock?

It's just that some switches have multiple PHCs instead of a single
one. It seems to be quite common to have a free-running as well as a
synchronized clock. In order for a better(?) or more accurate(?) ptp
implementation they expose not a single but rather multiple timestamps
from all PHCs (-> cross-timestamping) to user space for the ptp event
messages. That's at least my very limited understanding.

>
>> The operating system is supposed to ??? in order to synchronize the
>> synchronizable clock to the virtual time retrieved via TIME_STATUS_NP
>> that you're talking about. The question is what to replace that ???
>> with, of course.
>
> You have a choice.  Either you synchronize the local PHC to the global
> TAI time base or not.  If you do synchronize the PHC, then everything
> (like the globally scheduled time slots) just works.  If you decide to
> follow the nonsensical idea (following 802.1-AS) and leave the PHC
> free running, then you will have a difficult time scheduling those
> time windows.
>
> So it is all up to you.
>
>> I'm not an expert in kernel implementation either, but perhaps in the
>> light of this, you can revisit the idea that kernel changes will not be
>> needed (or explain more, if you still think they aren't).
>
> I am not opposed to kernel changes, but there must be:
>
> - A clear statement of the background context, and
> - an explanation of the issue to solved, and
> - a realistic solution that will support the wide variety of HW. 

Agreed.

Thanks,
Kurt

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