netdev - Re: [PATCH net-next 2/3] docs: networking: timestamping: add one more known issue

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Date:   Sat, 18 Jul 2020 14:36:37 +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 2/3] docs: networking: timestamping: add one
 more known issue

On Fri, Jul 17, 2020 at 04:08:03PM -0700, Jacob Keller wrote:
> 
> 
> On 7/17/2020 9:10 AM, Vladimir Oltean wrote:
> > Document the fact that Ethernet PHY timestamping has a fundamentally
> > flawed corner case (which in fact hits the majority of networking
> > drivers): a PHY for which its host MAC driver doesn't forward the
> > phy_mii_ioctl for timestamping is still going to be presented to user
> > space as functional.
> > 
> > Fixing this inconsistency would require moving the phy_has_tsinfo()
> > check inside all MAC drivers which are capable of PHY timestamping, to
> > be in harmony with the existing design for phy_has_hwtstamp() checks.
> > Instead of doing that, document the preferable solution which is that
> > offending MAC drivers be fixed instead.
> 
> This statement feels weird. Aren't you documenting that the preferable
> solution is? I.e. "Document this preferable solution: Fix the offending
> MAC driver"
> 
> Or am I misunderstanding what the issue is here?
> 

You're right, it looks like I wasn't thinking in full sentences at that
particular time of day.

> > 
> > Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> > ---
> >  Documentation/networking/timestamping.rst | 37 +++++++++++++++++++++++
> >  1 file changed, 37 insertions(+)
> > 
> > diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
> > index 9a1f4cb4ce9e..4004c5d2771d 100644
> > --- a/Documentation/networking/timestamping.rst
> > +++ b/Documentation/networking/timestamping.rst
> > @@ -754,3 +754,40 @@ check in their "TX confirmation" portion, not only for
> >  that PTP timestamping is not enabled for anything other than the outermost PHC,
> >  this enhanced check will avoid delivering a duplicated TX timestamp to user
> >  space.
> > +
> > +Another known limitation is the design of the ``__ethtool_get_ts_info``
> > +function::
> > +
> > +  int __ethtool_get_ts_info(struct net_device *dev, struct ethtool_ts_info *info)
> > +  {
> > +          const struct ethtool_ops *ops = dev->ethtool_ops;
> > +          struct phy_device *phydev = dev->phydev;
> > +
> > +          memset(info, 0, sizeof(*info));
> > +          info->cmd = ETHTOOL_GET_TS_INFO;
> > +
> > +          if (phy_has_tsinfo(phydev))
> > +                  return phy_ts_info(phydev, info);
> > +          if (ops->get_ts_info)
> > +                  return ops->get_ts_info(dev, info);
> > +
> > +          info->so_timestamping = SOF_TIMESTAMPING_RX_SOFTWARE |
> > +                                  SOF_TIMESTAMPING_SOFTWARE;
> > +          info->phc_index = -1;
> > +
> > +          return 0;
> > +  }
> > +
> > +Because the generic function searches first for the timestamping capabilities
> > +of the attached PHY, and returns them directly without consulting the MAC
> > +driver, no checking is being done whether the requirements described in `3.2.2
> > +Ethernet PHYs`_ are implemented or not. Therefore, if the MAC driver does not
> > +satisfy the requirements for PHY timestamping, and
> > +``CONFIG_NETWORK_PHY_TIMESTAMPING`` is enabled, then a non-functional PHC index
> > +(the one corresponding to the PHY) will be reported to user space, via
> > +``ethtool -T``.
> > +
> > +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.
> > 

Thanks,
-Vladimir