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@xxxxxxxxx> --- 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@xxxxxxxxxxxxxxx. 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. + +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``. -- 2.25.1
