netdev - Re: [PATCH RFC V1 net-next 3/4] net: Let the active time stamping layer be selectable.

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Date:   Fri, 21 Jan 2022 14:50:36 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Richard Cochran <richardcochran@...il.com>
CC:     Andrew Lunn <andrew@...n.ch>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Jakub Kicinski <kuba@...nel.org>,
        Joakim Zhang <qiangqing.zhang@....com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH RFC V1 net-next 3/4] net: Let the active time stamping
 layer be selectable.

On Thu, Jan 20, 2022 at 08:05:08PM -0800, Richard Cochran wrote:
> On Fri, Jan 21, 2022 at 04:38:09AM +0100, Andrew Lunn wrote:
> 
> > So in the extreme case, you have 7 time stamps, 3 from MACs and 4 from
> > PHYs!
> 
> :^)
>  
> > I doubt we want to support this, is there a valid use case for it?
> 
> Someday, someone will surely say it is important, but with any luck
> I'll be dead by then...

So as I mentioned earlier, the use case would be hardware performance
testing and diagnosing. You may consider that as not that important, but
this is basically what I had to do for several months, and even wrote
a program for that, that collects packet timestamps at all possible points.
And to your point, the more complex a system is, the more important it
becomes to be able to diagnose it precisely.