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

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Date:   Sat, 18 Jul 2020 14:35:19 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Jacob Keller <jacob.e.keller@...el.com>
Cc:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        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 Fri, Jul 17, 2020 at 04:12:07PM -0700, Jacob Keller wrote:
> 
> 
> 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.
> 

It's only _required_ when SOF_TIMESTAMPING_TX_SOFTWARE is used, it
seems. I had also thought of setting 'SKBTX_IN_PROGRESS' as good
practice, but there are many situations where it can do more harm than
good.

> > +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

Thanks,
-Vladimir