netdev - Re: [PATCH] docs: networking: timestamping: add section for stacked PHC devices

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Message-ID: <87mu48ho9z.fsf@osv.gnss.ru>
Date:   Thu, 09 Jul 2020 15:21:44 +0300
From:   Sergey Organov <sorganov@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        richardcochran@...il.com, andrew@...n.ch
Subject: Re: [PATCH] docs: networking: timestamping: add section for stacked
 PHC devices

Vladimir Oltean <olteanv@...il.com> writes:

> The concept of timestamping DSA switches / Ethernet PHYs is becoming
> more and more popular, however the Linux kernel timestamping code has
> evolved quite organically and there's layers upon layers of new and old
> code that need to work together for things to behave as expected.
>
> Add this chapter to explain what the overall goals are.

Nice job! That'd definitely save me a day or two if it were there a
month ago.

Please see one minor doubt below.

>
> Loosely based upon this email discussion plus some more info:
> https://lkml.org/lkml/2020/7/6/481
>
> Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> ---
>  Documentation/networking/timestamping.rst | 149 ++++++++++++++++++++++
>  1 file changed, 149 insertions(+)
>
> diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
> index 1adead6a4527..14df58c24e8c 100644
> --- a/Documentation/networking/timestamping.rst
> +++ b/Documentation/networking/timestamping.rst
> @@ -589,3 +589,152 @@ Time stamps for outgoing packets are to be generated as follows:
>    this would occur at a later time in the processing pipeline than other
>    software time stamping and therefore could lead to unexpected deltas
>    between time stamps.
> +
> +3.2 Special considerations for stacked PTP Hardware Clocks
> +----------------------------------------------------------
> +
> +There are situations when there may be more than one PHC (PTP Hardware Clock)
> +in the data path of a packet. The kernel has no explicit mechanism to allow the
> +user to select which PHC to use for timestamping Ethernet frames. Instead, the
> +assumption is that the outermost PHC is always the most preferable, and that
> +kernel drivers collaborate towards achieving that goal. Currently there are 3
> +cases of stacked PHCs, detailed below:
> +
> +- DSA (Distributed Switch Architecture) switches. These are Ethernet switches
> +  which have one of their ports connected to an (otherwise completely unaware)
> +  host Ethernet interface, and perform the role of a port multiplier with
> +  optional forwarding acceleration features.  Each DSA switch port is visible
> +  to the user as a standalone (virtual) network interface, however network I/O
> +  is performed under the hood indirectly through the host interface.

Here, "however" somehow makes me feel uneasy. "even though" maybe?

[...]

Thanks,
-- Sergey