linux-kernel - 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 16:23:27 +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 Fri, Jan 21, 2022 at 07:28:20AM -0800, Richard Cochran wrote:
> On Fri, Jan 21, 2022 at 02:50:36PM +0000, Vladimir Oltean wrote:
> > 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.
> 
> This is not possible without making a brand new CMSG to accommodate
> time stamps from all the various layers.
> 
> That is completely out of scope for this series.
> 
> The only practical use case of this series is to switch from PHY back to MAC.

I don't think my proposal is out of scope. It deals with the same thing
as what you propose: the kernel makes a selection by default, user space
can change it. The need for PHC identification in the cmsg arises as a
direct consequence of the fact that there are multiple PHCs in the path.
It isn't a requirement that I introduced artificially. Your solution has
a need to deal with that problem too, it's just that it omits to do so:

|               When changing the value, some packets in the kernel
|               networking stack may still be delivered with time
|               stamps from the previous provider.