Documentation/networking/timestamping.rst

1 .. SPDX-License-Identifier: GPL-2.0
32 IP_MULTICAST_LOOP + SO_TIMESTAMP[NS]
43 -------------------------------------------------------------
59 -------------------------------------------------------------------
62 Its struct timespec allows for higher resolution (ns) timestamps than the
72 ----------------------------------------------------------------------
102   Request rx timestamps generated by the network adapter.
105   Request rx timestamps when data enters the kernel. These timestamps
122   transmit latency is, if long, often dominated by queuing delay. The
131   grained measurement of queuing delay. This flag can be enabled
138   over-report measurement, because the timestamp is generated when all
181   is derived from a per-socket u32 counter (that wraps). For datagram
208   a timestamp with counter N-1. SOF_TIMESTAMPING_OPT_ID_TCP
258   received the packet and its length at layer 2. A valid (non-zero)
283   ignore the unexpected non-zero value. But it makes behavior subtly
303 Using this feature, applications can sample timestamps per sendmsg()
310   cmsg->cmsg_level	       = SOL_SOCKET;
311   cmsg->cmsg_type	       = SO_TIMESTAMPING;
312   cmsg->cmsg_len	       = CMSG_LEN(sizeof(__u32));
330 -------------------------
340 correlating a timestamp with data is non-trivial. A range of bytes
369 relevant sequence number in skb_shinfo(skb)->tskey. Because an skbuff
378 autocork. After linux-4.7, a better way to prevent coalescing is
400 ----------------------------
423 feature. At least one field is non-zero at any time. Most timestamps
430 as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst.
467 is the first if ts[2] is non-zero, the second otherwise, in which
495 Reading from the error queue is always a non-blocking operation. To
531 the requested fine-grained filtering for incoming packets is not
558     /* possible values for hwtstamp_config->tx_type */
576     /* possible values for hwtstamp_config->rx_filter */
596 --------------------------------------------------------
616 - In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
617   is set no-zero. If yes, then the driver is expected to do hardware time
619 - If this is possible for the skb and requested, then declare
621   SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with::
623       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
629 - Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware
632 - As soon as the driver has sent the packet and/or obtained a
644 ----------------------------------------------------------
661 interface (redirecting to the host port on TX, and intercepting frames on RX).
664 suffer, since the switch's variable queuing delay introduces a path delay
685 - ``.port_txtstamp()``: a hook called prior to the transmission of
687   This is required for two-step timestamping, since the hardware
690   packet so that it can re-enqueue the packet back into the socket's
693   in skb->cb and enqueue a tx skb queue. Typically, a switch will have a
696   key-value pairs of PTP sequence ID/message type/domain number and the
704   One-step TX timestamping do not require packet cloning, since there is
705   no follow-up message required by the PTP protocol (because the
708   to be re-enqueued into its socket's error queue.
710 - ``.port_rxtstamp()``: On RX, the BPF classifier is run by DSA to
715   timestamps might either be available in-band (through metadata in the
716   DSA header, or attached in other ways to the packet), or out-of-band
717   (through another RX timestamping FIFO). Deferral on RX is typically
732 mii_timestamper`` and add a pointer to it in ``phydev->mii_ts``. The presence
740 - Checking, in ``.ndo_eth_ioctl``, whether ``phy_has_hwtstamp(netdev->phydev)``
744 - On RX, special intervention may or may not be needed, depending on the
747   ``skb_defer_rx_timestamp(skb)`` is necessary or not - and if it is, don't
749   enabled, and ``skb->dev->phydev->mii_ts`` exists, its ``.rxtstamp()`` hook
751   deferral for RX timestamping is necessary.  Again like DSA, it becomes the
760 - On TX, again, special intervention might or might not be needed.  The
761   function that calls the ``mii_ts->txtstamp()`` hook is named
782 Stacked PHCs, especially DSA (but not only) - since that doesn't require any
784 all possible code paths - is that they uncover bugs which were impossible to
788       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
799    the ``.ndo_eth_ioctl`` ("``priv->hwtstamp_tx_enabled == true``") and the
800    current skb requires a TX timestamp ("``skb_shinfo(skb)->tx_flags &
802    "``skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS``" flag. Note: as
810    "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``" was set. With a stacked
817 "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``", but also for
818 "``priv->hwtstamp_tx_enabled == true``". Because the rest of the system ensures