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

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.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
---
 Documentation/networking/timestamping.rst | 37 +++++++++++++++++++++++
 1 file changed, 37 insertions(+)

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?

> 
> Signed-off-by: Vladimir Oltean <olteanv@gmail.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@vger.kernel.org. See :ref:`Documentation/process/stable-kernel-rules.rst
> +<stable_kernel_rules>` for more details.
>

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@gmail.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@vger.kernel.org. See :ref:`Documentation/process/stable-kernel-rules.rst
> > +<stable_kernel_rules>` for more details.
> > 

Thanks,
-Vladimir