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
180   is derived from a per-socket u32 counter (that wraps). For datagram
207   a timestamp with counter N-1. SOF_TIMESTAMPING_OPT_ID_TCP
227   Support recv() cmsg for all timestamped packets. Control messages
257   received the packet and its length at layer 2. A valid (non-zero)
292   cmsg->cmsg_level	       = SOL_SOCKET;
293   cmsg->cmsg_type	       = SO_TIMESTAMPING;
294   cmsg->cmsg_len	       = CMSG_LEN(sizeof(__u32));
312 -------------------------
322 correlating a timestamp with data is non-trivial. A range of bytes
326 byte range can coexist in multiple segments for protocols that
351 relevant sequence number in skb_shinfo(skb)->tskey. Because an skbuff
360 autocork. After linux-4.7, a better way to prevent coalescing is
382 ----------------------------
405 feature. At least one field is non-zero at any time. Most timestamps
412 as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst.
449 is the first if ts[2] is non-zero, the second otherwise, in which
477 Reading from the error queue is always a non-blocking operation. To
513 the requested fine-grained filtering for incoming packets is not
520 support HWTSTAMP_FILTER_PTP_V2_EVENT, then it should generally always upscale
540     /* possible values for hwtstamp_config->tx_type */
558     /* possible values for hwtstamp_config->rx_filter */
578 --------------------------------------------------------
580 A driver which supports hardware time stamping must support the
583 should also support SIOCGHWTSTAMP.
598 - In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
599   is set no-zero. If yes, then the driver is expected to do hardware time
601 - If this is possible for the skb and requested, then declare
603   SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with::
605       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
611 - Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware
614 - As soon as the driver has sent the packet and/or obtained a
626 ----------------------------------------------------------
667 - ``.port_txtstamp()``: a hook called prior to the transmission of
669   This is required for two-step timestamping, since the hardware
672   packet so that it can re-enqueue the packet back into the socket's
675   in skb->cb and enqueue a tx skb queue. Typically, a switch will have a
678   key-value pairs of PTP sequence ID/message type/domain number and the
686   One-step TX timestamping do not require packet cloning, since there is
687   no follow-up message required by the PTP protocol (because the
690   to be re-enqueued into its socket's error queue.
692 - ``.port_rxtstamp()``: On RX, the BPF classifier is run by DSA to
697   timestamps might either be available in-band (through metadata in the
698   DSA header, or attached in other ways to the packet), or out-of-band
714 mii_timestamper`` and add a pointer to it in ``phydev->mii_ts``. The presence
720 to each individual MAC driver for PHY timestamping support. This entails:
722 - Checking, in ``.ndo_eth_ioctl``, whether ``phy_has_hwtstamp(netdev->phydev)``
726 - On RX, special intervention may or may not be needed, depending on the
729   ``skb_defer_rx_timestamp(skb)`` is necessary or not - and if it is, don't
731   enabled, and ``skb->dev->phydev->mii_ts`` exists, its ``.rxtstamp()`` hook
742 - On TX, again, special intervention might or might not be needed.  The
743   function that calls the ``mii_ts->txtstamp()`` hook is named
745   (case in which explicit MAC driver support is indeed needed), but the
756 any PHY even if it doesn't support timestamping. In Linux, they are
764 Stacked PHCs, especially DSA (but not only) - since that doesn't require any
766 all possible code paths - is that they uncover bugs which were impossible to
770       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
781    the ``.ndo_eth_ioctl`` ("``priv->hwtstamp_tx_enabled == true``") and the
782    current skb requires a TX timestamp ("``skb_shinfo(skb)->tx_flags &
784    "``skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS``" flag. Note: as
792    "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``" was set. With a stacked
799 "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``", but also for
800 "``priv->hwtstamp_tx_enabled == true``". Because the rest of the system ensures