Documentation/networking/timestamping.rst

1 .. SPDX-License-Identifier: GPL-2.0
43 -------------------------------------------------------------
59 -------------------------------------------------------------------
72 ----------------------------------------------------------------------
117   require driver support and may not be available for all devices.
138   over-report measurement, because the timestamp is generated when all
189   is derived from a per-socket u32 counter (that wraps). For datagram
212     cmsg->cmsg_level		 = SOL_SOCKET;
213     cmsg->cmsg_type		 = SCM_TS_OPT_ID;
214     cmsg->cmsg_len		 = CMSG_LEN(sizeof(__u32));
230   a timestamp with counter N-1. SOF_TIMESTAMPING_OPT_ID_TCP
250   Support recv() cmsg for all timestamped packets. Control messages
280   received the packet and its length at layer 2. A valid (non-zero)
305   ignore the unexpected non-zero value. But it makes behavior subtly
332   cmsg->cmsg_level	       = SOL_SOCKET;
333   cmsg->cmsg_type	       = SO_TIMESTAMPING;
334   cmsg->cmsg_len	       = CMSG_LEN(sizeof(__u32));
352 -------------------------
362 correlating a timestamp with data is non-trivial. A range of bytes
366 byte range can coexist in multiple segments for protocols that
391 relevant sequence number in skb_shinfo(skb)->tskey. Because an skbuff
400 autocork. After linux-4.7, a better way to prevent coalescing is
422 ----------------------------
445 feature. At least one field is non-zero at any time. Most timestamps
452 as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst.
489 is the first if ts[2] is non-zero, the second otherwise, in which
517 Reading from the error queue is always a non-blocking operation. To
562 support HWTSTAMP_FILTER_PTP_V2_EVENT, then it should generally always upscale
582 If the requested fine-grained filtering for incoming packets is not
589     /* possible values for hwtstamp_config->tx_type */
607     /* possible values for hwtstamp_config->rx_filter */
627 --------------------------------------------------------
629 A driver which supports hardware time stamping must support the
632 the section on SIOCSHWTSTAMP. It should also support ndo_hwtstamp_get or
648 - In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
649   is set no-zero. If yes, then the driver is expected to do hardware time
651 - If this is possible for the skb and requested, then declare
653   SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with::
655       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
661 - Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware
664 - As soon as the driver has sent the packet and/or obtained a
676 ----------------------------------------------------------
717 - ``.port_txtstamp()``: a hook called prior to the transmission of
719   This is required for two-step timestamping, since the hardware
722   packet so that it can re-enqueue the packet back into the socket's
725   in skb->cb and enqueue a tx skb queue. Typically, a switch will have a
728   key-value pairs of PTP sequence ID/message type/domain number and the
736   One-step TX timestamping do not require packet cloning, since there is
737   no follow-up message required by the PTP protocol (because the
740   to be re-enqueued into its socket's error queue.
742 - ``.port_rxtstamp()``: On RX, the BPF classifier is run by DSA to
747   timestamps might either be available in-band (through metadata in the
748   DSA header, or attached in other ways to the packet), or out-of-band
764 mii_timestamper`` and add a pointer to it in ``phydev->mii_ts``. The presence
770 to each individual MAC driver for PHY timestamping support. This entails:
772 - Checking, in ``.ndo_eth_ioctl``, whether ``phy_has_hwtstamp(netdev->phydev)``
776 - On RX, special intervention may or may not be needed, depending on the
779   ``skb_defer_rx_timestamp(skb)`` is necessary or not - and if it is, don't
781   enabled, and ``skb->dev->phydev->mii_ts`` exists, its ``.rxtstamp()`` hook
792 - On TX, again, special intervention might or might not be needed.  The
793   function that calls the ``mii_ts->txtstamp()`` hook is named
795   (case in which explicit MAC driver support is indeed needed), but the
806 any PHY even if it doesn't support timestamping. In Linux, they are
818       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
829    the ``.ndo_eth_ioctl`` ("``priv->hwtstamp_tx_enabled == true``") and the
830    current skb requires a TX timestamp ("``skb_shinfo(skb)->tx_flags &
832    "``skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS``" flag. Note: as
840    "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``" was set. With a stacked
847 "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``", but also for
848 "``priv->hwtstamp_tx_enabled == true``". Because the rest of the system ensures