netdev - Re: [PATCH net-next 3/3] docs: networking: timestamping: add a set of frequently asked questions

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Date:   Fri, 17 Jul 2020 16:12:07 -0700
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Vladimir Oltean <olteanv@...il.com>, kuba@...nel.org,
        davem@...emloft.net, netdev@...r.kernel.org
Cc:     richardcochran@...il.com, sorganov@...il.com,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] docs: networking: timestamping: add a set of
 frequently asked questions



On 7/17/2020 9:10 AM, Vladimir Oltean wrote:
> These are some questions I had while trying to explain the behavior of
> some drivers with respect to software timestamping. Answered with the
> help of Richard Cochran.
> 
> Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> ---
>  Documentation/networking/timestamping.rst | 26 +++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
> index 4004c5d2771d..e01ec01179fe 100644
> --- a/Documentation/networking/timestamping.rst
> +++ b/Documentation/networking/timestamping.rst
> @@ -791,3 +791,29 @@ The correct solution to this problem is to implement the PHY timestamping
>  requirements in the MAC driver found broken, and submit as a bug fix patch to
>  netdev@...r.kernel.org. See :ref:`Documentation/process/stable-kernel-rules.rst
>  <stable_kernel_rules>` for more details.
> +
> +3.4 Frequently asked questions
> +------------------------------
> +
> +Q: When should drivers set SKBTX_IN_PROGRESS?
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +When the interface they represent offers both ``SOF_TIMESTAMPING_TX_HARDWARE``
> +and ``SOF_TIMESTAMPING_TX_SOFTWARE``.
> +Originally, the network stack could deliver either a hardware or a software
> +time stamp, but not both. This flag prevents software timestamp delivery.
> +This restriction was eventually lifted via the ``SOF_TIMESTAMPING_OPT_TX_SWHW``
> +option, but still the original behavior is preserved as the default.
> +

So, this implies that we set this only if both are supported? I thought
the intention was to set this flag whenever we start a HW timestamp.

> +Q: Should drivers that don't offer SOF_TIMESTAMPING_TX_SOFTWARE call skb_tx_timestamp()?
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The ``skb_clone_tx_timestamp()`` function from its body helps with propagation
> +of TX timestamps from PTP PHYs, and the required placement of this call is the
> +same as for software TX timestamping.
> +Additionally, since PTP is broken on ports with timestamping PHYs and unmet
> +requirements, the consequence is that any driver which may be ever coupled to
> +a timestamping-capable PHY in ``netdev->phydev`` should call at least
> +``skb_clone_tx_timestamp()``. However, calling the higher-level
> +``skb_tx_timestamp()`` instead achieves the same purpose, but also offers
> +additional compliance to ``SOF_TIMESTAMPING_TX_SOFTWARE``.
> 

This makes sense.

Thanks,
Jake