Documentation/networking/timestamping.rst

1 .. SPDX-License-Identifier: GPL-2.0
32 IP_MULTICAST_LOOP + SO_TIMESTAMP[NS]
43 -------------------------------------------------------------
61 Its struct timespec allows for higher resolution (ns) timestamps than the
71 ----------------------------------------------------------------------
101   Request rx timestamps generated by the network adapter.
104   Request rx timestamps when data enters the kernel. These timestamps
121   transmit latency is, if long, often dominated by queuing delay. The
130   grained measurement of queuing delay. This flag can be enabled
137   over-report measurement, because the timestamp is generated when all
179   is derived from a per-socket u32 counter (that wraps). For datagram
226   received the packet and its length at layer 2. A valid (non-zero)
254 Using this feature, applications can sample timestamps per sendmsg()
261   cmsg->cmsg_level	       = SOL_SOCKET;
262   cmsg->cmsg_type	       = SO_TIMESTAMPING;
263   cmsg->cmsg_len	       = CMSG_LEN(sizeof(__u32));
281 -------------------------
291 correlating a timestamp with data is non-trivial. A range of bytes
320 relevant sequence number in skb_shinfo(skb)->tskey. Because an skbuff
350 ----------------------------
373 feature. At least one field is non-zero at any time. Most timestamps
380 as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst.
417 is the first if ts[2] is non-zero, the second otherwise, in which
445 Reading from the error queue is always a non-blocking operation. To
481 the requested fine-grained filtering for incoming packets is not
508     /* possible values for hwtstamp_config->tx_type */
526     /* possible values for hwtstamp_config->rx_filter */
546 --------------------------------------------------------
566 - In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
567   is set no-zero. If yes, then the driver is expected to do hardware time
569 - If this is possible for the skb and requested, then declare
571   SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with::
573       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
579 - Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware
582 - As soon as the driver has sent the packet and/or obtained a
594 ----------------------------------------------------------
611 interface (redirecting to the host port on TX, and intercepting frames on RX).
614 suffer, since the switch's variable queuing delay introduces a path delay
637 - ``.port_txtstamp()``: The driver is passed a clone of the timestampable skb
645   a pointer to its clone, in ``DSA_SKB_CB(skb)->clone``, to ease the driver's
648 - ``.port_rxtstamp()``: The original (and only) timestampable skb is provided
651   available in-band (through metadata in the DSA header, or attached in other
652   ways to the packet), or out-of-band (through another RX timestamping FIFO).
653   Deferral on RX is typically necessary when retrieving the timestamp needs a
667 mii_timestamper`` and add a pointer to it in ``phydev->mii_ts``. The presence
675 - Checking, in ``.ndo_do_ioctl``, whether ``phy_has_hwtstamp(netdev->phydev)``
679 - On RX, special intervention may or may not be needed, depending on the
682   ``skb_defer_rx_timestamp(skb)`` is necessary or not - and if it is, don't
684   enabled, and ``skb->dev->phydev->mii_ts`` exists, its ``.rxtstamp()`` hook
686   deferral for RX timestamping is necessary.  Again like DSA, it becomes the
695 - On TX, again, special intervention might or might not be needed.  The
696   function that calls the ``mii_ts->txtstamp()`` hook is named
717 Stacked PHCs, especially DSA (but not only) - since that doesn't require any
719 all possible code paths - is that they uncover bugs which were impossible to
723       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
734    the ``.ndo_do_ioctl`` ("``priv->hwtstamp_tx_enabled == true``") and the
735    current skb requires a TX timestamp ("``skb_shinfo(skb)->tx_flags &
737    "``skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS``" flag. Note: as
745    "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``" was set. With a stacked
752 "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``", but also for
753 "``priv->hwtstamp_tx_enabled == true``". Because the rest of the system ensures