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From: Vladimir Oltean <olteanv@gmail.com>
To: Jacob Keller <jacob.e.keller@intel.com>
Cc: kuba@kernel.org, davem@davemloft.net, netdev@vger.kernel.org,
	richardcochran@gmail.com, sorganov@gmail.com,
	linux-doc@vger.kernel.org
Subject: Re: [PATCH net-next 3/3] docs: networking: timestamping: add a set of frequently asked questions
Date: Sat, 18 Jul 2020 14:35:19 +0300
Message-ID: <20200718113519.htopj6tgfvimaywn@skbuf> (raw)
In-Reply-To: <e6b6f240-c2b2-b57c-7334-4762f034aae3@intel.com>

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@gmail.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@vger.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

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-17 16:10 [PATCH net-next 0/3] Document more PTP timestamping known quirks Vladimir Oltean
2020-07-17 16:10 ` [PATCH net-next 1/3] docs: networking: timestamping: rename last section to "Known bugs" Vladimir Oltean
2020-07-17 22:05   ` Jacob Keller
2020-07-17 16:10 ` [PATCH net-next 2/3] docs: networking: timestamping: add one more known issue Vladimir Oltean
2020-07-17 23:08   ` Jacob Keller
2020-07-18 11:36     ` Vladimir Oltean
2020-07-17 16:10 ` [PATCH net-next 3/3] docs: networking: timestamping: add a set of frequently asked questions Vladimir Oltean
2020-07-17 23:12   ` Jacob Keller
2020-07-18 11:35     ` Vladimir Oltean [this message]
2020-07-20 18:54       ` Jacob Keller
2020-07-20 21:05         ` Vladimir Oltean
2020-07-20 21:45           ` Jacob Keller
2020-07-20 22:13             ` Vladimir Oltean
2020-07-21  0:21               ` Richard Cochran
2020-07-21 19:51                 ` Vladimir Oltean
2020-07-22  3:25                   ` Richard Cochran
2020-07-25 21:32                     ` Vladimir Oltean
2020-07-21 17:16               ` Jacob Keller
2020-07-21  0:15           ` Richard Cochran
2020-07-17 21:13 ` [PATCH net-next 0/3] Document more PTP timestamping known quirks Sergey Organov
2020-07-17 21:57   ` Vladimir Oltean
2020-07-17 23:13     ` Jacob Keller
2020-07-18 10:54     ` Sergey Organov
2020-07-18 11:30       ` Vladimir Oltean
2020-07-18 13:35         ` Sergey Organov
2020-07-18 11:21 ` Sergey Organov

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