Documentation/networking/timestamping.rst

1 .. SPDX-License-Identifier: GPL-2.0
43 -------------------------------------------------------------
59 -------------------------------------------------------------------
72 ----------------------------------------------------------------------
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
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 ----------------------------
408 		struct timespec ts[3];
414 		struct __kernel_timespec ts[3];
423 feature. At least one field is non-zero at any time. Most timestamps
424 are passed in ts[0]. Hardware timestamps are passed in ts[2].
426 ts[1] used to hold hardware timestamps converted to system time.
430 as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst.
435 in ts[0] when a real software timestamp is missing. This happens also
442 socket's error queue with the send timestamp(s) attached. A process
446 outgoing data packet with two ancillary messages attached.
467 is the first if ts[2] is non-zero, the second otherwise, in which
468 case the timestamp is stored in ts[0].
495 Reading from the error queue is always a non-blocking operation. To
510 implicitly defined. ts[0] holds a software timestamp if set, ts[1]
511 is again deprecated and ts[2] holds a hardware timestamp if set.
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 ----------------------------------------------------------
663 When a DSA switch is attached to a host port, PTP synchronization has to
675 handling in the driver for the host port it is attached to.  However, when the
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
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
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