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 .. SPDX-License-Identifier: GPL-2.0
 
 ============
 Timestamping
 ============
 
 
 1. Control Interfaces
 =====================
 
 The interfaces for receiving network packages timestamps are:
 
 SO_TIMESTAMP
   Generates a timestamp for each incoming packet in (not necessarily
   monotonic) system time. Reports the timestamp via recvmsg() in a
   control message in usec resolution.
   SO_TIMESTAMP is defined as SO_TIMESTAMP_NEW or SO_TIMESTAMP_OLD
   based on the architecture type and time_t representation of libc.
   Control message format is in struct __kernel_old_timeval for
   SO_TIMESTAMP_OLD and in struct __kernel_sock_timeval for
   SO_TIMESTAMP_NEW options respectively.
 
 SO_TIMESTAMPNS
   Same timestamping mechanism as SO_TIMESTAMP, but reports the
   timestamp as struct timespec in nsec resolution.
   SO_TIMESTAMPNS is defined as SO_TIMESTAMPNS_NEW or SO_TIMESTAMPNS_OLD
   based on the architecture type and time_t representation of libc.
   Control message format is in struct timespec for SO_TIMESTAMPNS_OLD
   and in struct __kernel_timespec for SO_TIMESTAMPNS_NEW options
   respectively.
 
 IP_MULTICAST_LOOP + SO_TIMESTAMP[NS]
   Only for multicast:approximate transmit timestamp obtained by
   reading the looped packet receive timestamp.
 
 SO_TIMESTAMPING
   Generates timestamps on reception, transmission or both. Supports
   multiple timestamp sources, including hardware. Supports generating
   timestamps for stream sockets.
 
 
 1.1 SO_TIMESTAMP (also SO_TIMESTAMP_OLD and SO_TIMESTAMP_NEW)
 -------------------------------------------------------------
 
 This socket option enables timestamping of datagrams on the reception
 path. Because the destination socket, if any, is not known early in
 the network stack, the feature has to be enabled for all packets. The
 same is true for all early receive timestamp options.
 
 For interface details, see `man 7 socket`.
 
 Always use SO_TIMESTAMP_NEW timestamp to always get timestamp in
 struct __kernel_sock_timeval format.
 
 SO_TIMESTAMP_OLD returns incorrect timestamps after the year 2038
 on 32 bit machines.
 
 1.2 SO_TIMESTAMPNS (also SO_TIMESTAMPNS_OLD and SO_TIMESTAMPNS_NEW)
 -------------------------------------------------------------------
 
 This option is identical to SO_TIMESTAMP except for the returned data type.
 Its struct timespec allows for higher resolution (ns) timestamps than the
 timeval of SO_TIMESTAMP (ms).
 
 Always use SO_TIMESTAMPNS_NEW timestamp to always get timestamp in
 struct __kernel_timespec format.
 
 SO_TIMESTAMPNS_OLD returns incorrect timestamps after the year 2038
 on 32 bit machines.
 
 1.3 SO_TIMESTAMPING (also SO_TIMESTAMPING_OLD and SO_TIMESTAMPING_NEW)
 ----------------------------------------------------------------------
 
 Supports multiple types of timestamp requests. As a result, this
 socket option takes a bitmap of flags, not a boolean. In::
 
   err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val));
 
 val is an integer with any of the following bits set. Setting other
 bit returns EINVAL and does not change the current state.
 
 The socket option configures timestamp generation for individual
 sk_buffs (1.3.1), timestamp reporting to the socket's error
 queue (1.3.2) and options (1.3.3). Timestamp generation can also
 be enabled for individual sendmsg calls using cmsg (1.3.4).
 
 
 1.3.1 Timestamp Generation
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Some bits are requests to the stack to try to generate timestamps. Any
 combination of them is valid. Changes to these bits apply to newly
 created packets, not to packets already in the stack. As a result, it
 is possible to selectively request timestamps for a subset of packets
 (e.g., for sampling) by embedding an send() call within two setsockopt
 calls, one to enable timestamp generation and one to disable it.
 Timestamps may also be generated for reasons other than being
 requested by a particular socket, such as when receive timestamping is
 enabled system wide, as explained earlier.
 
 SOF_TIMESTAMPING_RX_HARDWARE:
   Request rx timestamps generated by the network adapter.
 
 SOF_TIMESTAMPING_RX_SOFTWARE:
   Request rx timestamps when data enters the kernel. These timestamps
   are generated just after a device driver hands a packet to the
   kernel receive stack.
 
 SOF_TIMESTAMPING_TX_HARDWARE:
   Request tx timestamps generated by the network adapter. This flag
   can be enabled via both socket options and control messages.
 
 SOF_TIMESTAMPING_TX_SOFTWARE:
   Request tx timestamps when data leaves the kernel. These timestamps
   are generated in the device driver as close as possible, but always
   prior to, passing the packet to the network interface. Hence, they
   require driver support and may not be available for all devices.
   This flag can be enabled via both socket options and control messages.
 
 SOF_TIMESTAMPING_TX_SCHED:
   Request tx timestamps prior to entering the packet scheduler. Kernel
   transmit latency is, if long, often dominated by queuing delay. The
   difference between this timestamp and one taken at
   SOF_TIMESTAMPING_TX_SOFTWARE will expose this latency independent
   of protocol processing. The latency incurred in protocol
   processing, if any, can be computed by subtracting a userspace
   timestamp taken immediately before send() from this timestamp. On
   machines with virtual devices where a transmitted packet travels
   through multiple devices and, hence, multiple packet schedulers,
   a timestamp is generated at each layer. This allows for fine
   grained measurement of queuing delay. This flag can be enabled
   via both socket options and control messages.
 
 SOF_TIMESTAMPING_TX_ACK:
   Request tx timestamps when all data in the send buffer has been
   acknowledged. This only makes sense for reliable protocols. It is
   currently only implemented for TCP. For that protocol, it may
   over-report measurement, because the timestamp is generated when all
   data up to and including the buffer at send() was acknowledged: the
   cumulative acknowledgment. The mechanism ignores SACK and FACK.
   This flag can be enabled via both socket options and control messages.
 
 
 1.3.2 Timestamp Reporting
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 The other three bits control which timestamps will be reported in a
 generated control message. Changes to the bits take immediate
 effect at the timestamp reporting locations in the stack. Timestamps
 are only reported for packets that also have the relevant timestamp
 generation request set.
 
 SOF_TIMESTAMPING_SOFTWARE:
   Report any software timestamps when available.
 
 SOF_TIMESTAMPING_SYS_HARDWARE:
   This option is deprecated and ignored.
 
 SOF_TIMESTAMPING_RAW_HARDWARE:
   Report hardware timestamps as generated by
   SOF_TIMESTAMPING_TX_HARDWARE when available.
 
 
 1.3.3 Timestamp Options
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 The interface supports the options
 
 SOF_TIMESTAMPING_OPT_ID:
   Generate a unique identifier along with each packet. A process can
   have multiple concurrent timestamping requests outstanding. Packets
   can be reordered in the transmit path, for instance in the packet
   scheduler. In that case timestamps will be queued onto the error
   queue out of order from the original send() calls. It is not always
   possible to uniquely match timestamps to the original send() calls
   based on timestamp order or payload inspection alone, then.
 
   This option associates each packet at send() with a unique
   identifier and returns that along with the timestamp. The identifier
   is derived from a per-socket u32 counter (that wraps). For datagram
   sockets, the counter increments with each sent packet. For stream
   sockets, it increments with every byte.
 
   The counter starts at zero. It is initialized the first time that
   the socket option is enabled. It is reset each time the option is
   enabled after having been disabled. Resetting the counter does not
   change the identifiers of existing packets in the system.
 
   This option is implemented only for transmit timestamps. There, the
   timestamp is always looped along with a struct sock_extended_err.
   The option modifies field ee_data to pass an id that is unique
   among all possibly concurrently outstanding timestamp requests for
   that socket.
 
 
 SOF_TIMESTAMPING_OPT_CMSG:
   Support recv() cmsg for all timestamped packets. Control messages
   are already supported unconditionally on all packets with receive
   timestamps and on IPv6 packets with transmit timestamp. This option
   extends them to IPv4 packets with transmit timestamp. One use case
   is to correlate packets with their egress device, by enabling socket
   option IP_PKTINFO simultaneously.
 
 
 SOF_TIMESTAMPING_OPT_TSONLY:
   Applies to transmit timestamps only. Makes the kernel return the
   timestamp as a cmsg alongside an empty packet, as opposed to
   alongside the original packet. This reduces the amount of memory
   charged to the socket's receive budget (SO_RCVBUF) and delivers
   the timestamp even if sysctl net.core.tstamp_allow_data is 0.
   This option disables SOF_TIMESTAMPING_OPT_CMSG.
 
 SOF_TIMESTAMPING_OPT_STATS:
   Optional stats that are obtained along with the transmit timestamps.
   It must be used together with SOF_TIMESTAMPING_OPT_TSONLY. When the
   transmit timestamp is available, the stats are available in a
   separate control message of type SCM_TIMESTAMPING_OPT_STATS, as a
   list of TLVs (struct nlattr) of types. These stats allow the
   application to associate various transport layer stats with
   the transmit timestamps, such as how long a certain block of
   data was limited by peer's receiver window.
 
 SOF_TIMESTAMPING_OPT_PKTINFO:
   Enable the SCM_TIMESTAMPING_PKTINFO control message for incoming
   packets with hardware timestamps. The message contains struct
   scm_ts_pktinfo, which supplies the index of the real interface which
   received the packet and its length at layer 2. A valid (non-zero)
   interface index will be returned only if CONFIG_NET_RX_BUSY_POLL is
   enabled and the driver is using NAPI. The struct contains also two
   other fields, but they are reserved and undefined.
 
 SOF_TIMESTAMPING_OPT_TX_SWHW:
   Request both hardware and software timestamps for outgoing packets
   when SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE
   are enabled at the same time. If both timestamps are generated,
   two separate messages will be looped to the socket's error queue,
   each containing just one timestamp.
 
 New applications are encouraged to pass SOF_TIMESTAMPING_OPT_ID to
 disambiguate timestamps and SOF_TIMESTAMPING_OPT_TSONLY to operate
 regardless of the setting of sysctl net.core.tstamp_allow_data.
 
 An exception is when a process needs additional cmsg data, for
 instance SOL_IP/IP_PKTINFO to detect the egress network interface.
 Then pass option SOF_TIMESTAMPING_OPT_CMSG. This option depends on
 having access to the contents of the original packet, so cannot be
 combined with SOF_TIMESTAMPING_OPT_TSONLY.
 
 
 1.3.4. Enabling timestamps via control messages
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 In addition to socket options, timestamp generation can be requested
 per write via cmsg, only for SOF_TIMESTAMPING_TX_* (see Section 1.3.1).
 Using this feature, applications can sample timestamps per sendmsg()
 without paying the overhead of enabling and disabling timestamps via
 setsockopt::
 
   struct msghdr *msg;
   ...
   cmsg                         = CMSG_FIRSTHDR(msg);
   cmsg->cmsg_level             = SOL_SOCKET;
   cmsg->cmsg_type              = SO_TIMESTAMPING;
   cmsg->cmsg_len               = CMSG_LEN(sizeof(__u32));
   *((__u32 *) CMSG_DATA(cmsg)) = SOF_TIMESTAMPING_TX_SCHED |
                                  SOF_TIMESTAMPING_TX_SOFTWARE |
                                  SOF_TIMESTAMPING_TX_ACK;
   err = sendmsg(fd, msg, 0);
 
 The SOF_TIMESTAMPING_TX_* flags set via cmsg will override
 the SOF_TIMESTAMPING_TX_* flags set via setsockopt.
 
 Moreover, applications must still enable timestamp reporting via
 setsockopt to receive timestamps::
 
   __u32 val = SOF_TIMESTAMPING_SOFTWARE |
               SOF_TIMESTAMPING_OPT_ID /* or any other flag */;
   err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val));
 
 
 1.4 Bytestream Timestamps
 -------------------------
 
 The SO_TIMESTAMPING interface supports timestamping of bytes in a
 bytestream. Each request is interpreted as a request for when the
 entire contents of the buffer has passed a timestamping point. That
 is, for streams option SOF_TIMESTAMPING_TX_SOFTWARE will record
 when all bytes have reached the device driver, regardless of how
 many packets the data has been converted into.
 
 In general, bytestreams have no natural delimiters and therefore
 correlating a timestamp with data is non-trivial. A range of bytes
 may be split across segments, any segments may be merged (possibly
 coalescing sections of previously segmented buffers associated with
 independent send() calls). Segments can be reordered and the same
 byte range can coexist in multiple segments for protocols that
 implement retransmissions.
 
 It is essential that all timestamps implement the same semantics,
 regardless of these possible transformations, as otherwise they are
 incomparable. Handling "rare" corner cases differently from the
 simple case (a 1:1 mapping from buffer to skb) is insufficient
 because performance debugging often needs to focus on such outliers.
 
 In practice, timestamps can be correlated with segments of a
 bytestream consistently, if both semantics of the timestamp and the
 timing of measurement are chosen correctly. This challenge is no
 different from deciding on a strategy for IP fragmentation. There, the
 definition is that only the first fragment is timestamped. For
 bytestreams, we chose that a timestamp is generated only when all
 bytes have passed a point. SOF_TIMESTAMPING_TX_ACK as defined is easy to
 implement and reason about. An implementation that has to take into
 account SACK would be more complex due to possible transmission holes
 and out of order arrival.
 
 On the host, TCP can also break the simple 1:1 mapping from buffer to
 skbuff as a result of Nagle, cork, autocork, segmentation and GSO. The
 implementation ensures correctness in all cases by tracking the
 individual last byte passed to send(), even if it is no longer the
 last byte after an skbuff extend or merge operation. It stores the
 relevant sequence number in skb_shinfo(skb)->tskey. Because an skbuff
 has only one such field, only one timestamp can be generated.
 
 In rare cases, a timestamp request can be missed if two requests are
 collapsed onto the same skb. A process can detect this situation by
 enabling SOF_TIMESTAMPING_OPT_ID and comparing the byte offset at
 send time with the value returned for each timestamp. It can prevent
 the situation by always flushing the TCP stack in between requests,
 for instance by enabling TCP_NODELAY and disabling TCP_CORK and
 autocork.
 
 These precautions ensure that the timestamp is generated only when all
 bytes have passed a timestamp point, assuming that the network stack
 itself does not reorder the segments. The stack indeed tries to avoid
 reordering. The one exception is under administrator control: it is
 possible to construct a packet scheduler configuration that delays
 segments from the same stream differently. Such a setup would be
 unusual.
 
 
 2 Data Interfaces
 ==================
 
 Timestamps are read using the ancillary data feature of recvmsg().
 See `man 3 cmsg` for details of this interface. The socket manual
 page (`man 7 socket`) describes how timestamps generated with
 SO_TIMESTAMP and SO_TIMESTAMPNS records can be retrieved.
 
 
 2.1 SCM_TIMESTAMPING records
 ----------------------------
 
 These timestamps are returned in a control message with cmsg_level
 SOL_SOCKET, cmsg_type SCM_TIMESTAMPING, and payload of type
 
 For SO_TIMESTAMPING_OLD::
 
         struct scm_timestamping {
                 struct timespec ts[3];
         };
 
 For SO_TIMESTAMPING_NEW::
 
         struct scm_timestamping64 {
                 struct __kernel_timespec ts[3];
 
 Always use SO_TIMESTAMPING_NEW timestamp to always get timestamp in
 struct scm_timestamping64 format.
 
 SO_TIMESTAMPING_OLD returns incorrect timestamps after the year 2038
 on 32 bit machines.
 
 The structure can return up to three timestamps. This is a legacy
 feature. At least one field is non-zero at any time. Most timestamps
 are passed in ts[0]. Hardware timestamps are passed in ts[2].
 
 ts[1] used to hold hardware timestamps converted to system time.
 Instead, expose the hardware clock device on the NIC directly as
 a HW PTP clock source, to allow time conversion in userspace and
 optionally synchronize system time with a userspace PTP stack such
 as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst.
 
 Note that if the SO_TIMESTAMP or SO_TIMESTAMPNS option is enabled
 together with SO_TIMESTAMPING using SOF_TIMESTAMPING_SOFTWARE, a false
 software timestamp will be generated in the recvmsg() call and passed
 in ts[0] when a real software timestamp is missing. This happens also
 on hardware transmit timestamps.
 
 2.1.1 Transmit timestamps with MSG_ERRQUEUE
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 For transmit timestamps the outgoing packet is looped back to the
 socket's error queue with the send timestamp(s) attached. A process
 receives the timestamps by calling recvmsg() with flag MSG_ERRQUEUE
 set and with a msg_control buffer sufficiently large to receive the
 relevant metadata structures. The recvmsg call returns the original
 outgoing data packet with two ancillary messages attached.
 
 A message of cm_level SOL_IP(V6) and cm_type IP(V6)_RECVERR
 embeds a struct sock_extended_err. This defines the error type. For
 timestamps, the ee_errno field is ENOMSG. The other ancillary message
 will have cm_level SOL_SOCKET and cm_type SCM_TIMESTAMPING. This
 embeds the struct scm_timestamping.
 
 
 2.1.1.2 Timestamp types
 ~~~~~~~~~~~~~~~~~~~~~~~
 
 The semantics of the three struct timespec are defined by field
 ee_info in the extended error structure. It contains a value of
 type SCM_TSTAMP_* to define the actual timestamp passed in
 scm_timestamping.
 
 The SCM_TSTAMP_* types are 1:1 matches to the SOF_TIMESTAMPING_*
 control fields discussed previously, with one exception. For legacy
 reasons, SCM_TSTAMP_SND is equal to zero and can be set for both
 SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE. It
 is the first if ts[2] is non-zero, the second otherwise, in which
 case the timestamp is stored in ts[0].
 
 
 2.1.1.3 Fragmentation
 ~~~~~~~~~~~~~~~~~~~~~
 
 Fragmentation of outgoing datagrams is rare, but is possible, e.g., by
 explicitly disabling PMTU discovery. If an outgoing packet is fragmented,
 then only the first fragment is timestamped and returned to the sending
 socket.
 
 
 2.1.1.4 Packet Payload
 ~~~~~~~~~~~~~~~~~~~~~~
 
 The calling application is often not interested in receiving the whole
 packet payload that it passed to the stack originally: the socket
 error queue mechanism is just a method to piggyback the timestamp on.
 In this case, the application can choose to read datagrams with a
 smaller buffer, possibly even of length 0. The payload is truncated
 accordingly. Until the process calls recvmsg() on the error queue,
 however, the full packet is queued, taking up budget from SO_RCVBUF.
 
 
 2.1.1.5 Blocking Read
 ~~~~~~~~~~~~~~~~~~~~~
 
 Reading from the error queue is always a non-blocking operation. To
 block waiting on a timestamp, use poll or select. poll() will return
 POLLERR in pollfd.revents if any data is ready on the error queue.
 There is no need to pass this flag in pollfd.events. This flag is
 ignored on request. See also `man 2 poll`.
 
 
 2.1.2 Receive timestamps
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 On reception, there is no reason to read from the socket error queue.
 The SCM_TIMESTAMPING ancillary data is sent along with the packet data
 on a normal recvmsg(). Since this is not a socket error, it is not
 accompanied by a message SOL_IP(V6)/IP(V6)_RECVERROR. In this case,
 the meaning of the three fields in struct scm_timestamping is
 implicitly defined. ts[0] holds a software timestamp if set, ts[1]
 is again deprecated and ts[2] holds a hardware timestamp if set.
 
 
 3. Hardware Timestamping configuration: SIOCSHWTSTAMP and SIOCGHWTSTAMP
 =======================================================================
 
 Hardware time stamping must also be initialized for each device driver
 that is expected to do hardware time stamping. The parameter is defined in
 include/uapi/linux/net_tstamp.h as::
 
         struct hwtstamp_config {
                 int flags;      /* no flags defined right now, must be zero */
                 int tx_type;    /* HWTSTAMP_TX_* */
                 int rx_filter;  /* HWTSTAMP_FILTER_* */
         };
 
 Desired behavior is passed into the kernel and to a specific device by
 calling ioctl(SIOCSHWTSTAMP) with a pointer to a struct ifreq whose
 ifr_data points to a struct hwtstamp_config. The tx_type and
 rx_filter are hints to the driver what it is expected to do. If
 the requested fine-grained filtering for incoming packets is not
 supported, the driver may time stamp more than just the requested types
 of packets.
 
 Drivers are free to use a more permissive configuration than the requested
 configuration. It is expected that drivers should only implement directly the
 most generic mode that can be supported. For example if the hardware can
 support HWTSTAMP_FILTER_PTP_V2_EVENT, then it should generally always upscale
 HWTSTAMP_FILTER_PTP_V2_L2_SYNC, and so forth, as HWTSTAMP_FILTER_PTP_V2_EVENT
 is more generic (and more useful to applications).
 
 A driver which supports hardware time stamping shall update the struct
 with the actual, possibly more permissive configuration. If the
 requested packets cannot be time stamped, then nothing should be
 changed and ERANGE shall be returned (in contrast to EINVAL, which
 indicates that SIOCSHWTSTAMP is not supported at all).
 
 Only a processes with admin rights may change the configuration. User
 space is responsible to ensure that multiple processes don't interfere
 with each other and that the settings are reset.
 
 Any process can read the actual configuration by passing this
 structure to ioctl(SIOCGHWTSTAMP) in the same way.  However, this has
 not been implemented in all drivers.
 
 ::
 
     /* possible values for hwtstamp_config->tx_type */
     enum {
             /*
             * no outgoing packet will need hardware time stamping;
             * should a packet arrive which asks for it, no hardware
             * time stamping will be done
             */
             HWTSTAMP_TX_OFF,
 
             /*
             * enables hardware time stamping for outgoing packets;
             * the sender of the packet decides which are to be
             * time stamped by setting SOF_TIMESTAMPING_TX_SOFTWARE
             * before sending the packet
             */
             HWTSTAMP_TX_ON,
     };
 
     /* possible values for hwtstamp_config->rx_filter */
     enum {
             /* time stamp no incoming packet at all */
             HWTSTAMP_FILTER_NONE,
 
             /* time stamp any incoming packet */
             HWTSTAMP_FILTER_ALL,
 
             /* return value: time stamp all packets requested plus some others */
             HWTSTAMP_FILTER_SOME,
 
             /* PTP v1, UDP, any kind of event packet */
             HWTSTAMP_FILTER_PTP_V1_L4_EVENT,
 
             /* for the complete list of values, please check
             * the include file include/uapi/linux/net_tstamp.h
             */
     };
 
 3.1 Hardware Timestamping Implementation: Device Drivers
 --------------------------------------------------------
 
 A driver which supports hardware time stamping must support the
 SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with
 the actual values as described in the section on SIOCSHWTSTAMP.  It
 should also support SIOCGHWTSTAMP.
 
 Time stamps for received packets must be stored in the skb. To get a pointer
 to the shared time stamp structure of the skb call skb_hwtstamps(). Then
 set the time stamps in the structure::
 
     struct skb_shared_hwtstamps {
             /* hardware time stamp transformed into duration
             * since arbitrary point in time
             */
             ktime_t     hwtstamp;
     };
 
 Time stamps for outgoing packets are to be generated as follows:
 
 - In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
   is set no-zero. If yes, then the driver is expected to do hardware time
   stamping.
 - If this is possible for the skb and requested, then declare
   that the driver is doing the time stamping by setting the flag
   SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with::
 
       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
 
   You might want to keep a pointer to the associated skb for the next step
   and not free the skb. A driver not supporting hardware time stamping doesn't
   do that. A driver must never touch sk_buff::tstamp! It is used to store
   software generated time stamps by the network subsystem.
 - Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware
   as possible. skb_tx_timestamp() provides a software time stamp if requested
   and hardware timestamping is not possible (SKBTX_IN_PROGRESS not set).
 - As soon as the driver has sent the packet and/or obtained a
   hardware time stamp for it, it passes the time stamp back by
   calling skb_tstamp_tx() with the original skb, the raw
   hardware time stamp. skb_tstamp_tx() clones the original skb and
   adds the timestamps, therefore the original skb has to be freed now.
   If obtaining the hardware time stamp somehow fails, then the driver
   should not fall back to software time stamping. The rationale is that
   this would occur at a later time in the processing pipeline than other
   software time stamping and therefore could lead to unexpected deltas
   between time stamps.
 
 3.2 Special considerations for stacked PTP Hardware Clocks
 ----------------------------------------------------------
 
 There are situations when there may be more than one PHC (PTP Hardware Clock)
 in the data path of a packet. The kernel has no explicit mechanism to allow the
 user to select which PHC to use for timestamping Ethernet frames. Instead, the
 assumption is that the outermost PHC is always the most preferable, and that
 kernel drivers collaborate towards achieving that goal. Currently there are 3
 cases of stacked PHCs, detailed below:
 
 3.2.1 DSA (Distributed Switch Architecture) switches
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 These are Ethernet switches which have one of their ports connected to an
 (otherwise completely unaware) host Ethernet interface, and perform the role of
 a port multiplier with optional forwarding acceleration features.  Each DSA
 switch port is visible to the user as a standalone (virtual) network interface,
 and its network I/O is performed, under the hood, indirectly through the host
 interface (redirecting to the host port on TX, and intercepting frames on RX).
 
 When a DSA switch is attached to a host port, PTP synchronization has to
 suffer, since the switch's variable queuing delay introduces a path delay
 jitter between the host port and its PTP partner. For this reason, some DSA
 switches include a timestamping clock of their own, and have the ability to
 perform network timestamping on their own MAC, such that path delays only
 measure wire and PHY propagation latencies. Timestamping DSA switches are
 supported in Linux and expose the same ABI as any other network interface (save
 for the fact that the DSA interfaces are in fact virtual in terms of network
 I/O, they do have their own PHC).  It is typical, but not mandatory, for all
 interfaces of a DSA switch to share the same PHC.
 
 By design, PTP timestamping with a DSA switch does not need any special
 handling in the driver for the host port it is attached to.  However, when the
 host port also supports PTP timestamping, DSA will take care of intercepting
 the ``.ndo_eth_ioctl`` calls towards the host port, and block attempts to enable
 hardware timestamping on it. This is because the SO_TIMESTAMPING API does not
 allow the delivery of multiple hardware timestamps for the same packet, so
 anybody else except for the DSA switch port must be prevented from doing so.
 
 In the generic layer, DSA provides the following infrastructure for PTP
 timestamping:
 
 - ``.port_txtstamp()``: a hook called prior to the transmission of
   packets with a hardware TX timestamping request from user space.
   This is required for two-step timestamping, since the hardware
   timestamp becomes available after the actual MAC transmission, so the
   driver must be prepared to correlate the timestamp with the original
   packet so that it can re-enqueue the packet back into the socket's
   error queue. To save the packet for when the timestamp becomes
   available, the driver can call ``skb_clone_sk`` , save the clone pointer
   in skb->cb and enqueue a tx skb queue. Typically, a switch will have a
   PTP TX timestamp register (or sometimes a FIFO) where the timestamp
   becomes available. In case of a FIFO, the hardware might store
   key-value pairs of PTP sequence ID/message type/domain number and the
   actual timestamp. To perform the correlation correctly between the
   packets in a queue waiting for timestamping and the actual timestamps,
   drivers can use a BPF classifier (``ptp_classify_raw``) to identify
   the PTP transport type, and ``ptp_parse_header`` to interpret the PTP
   header fields. There may be an IRQ that is raised upon this
   timestamp's availability, or the driver might have to poll after
   invoking ``dev_queue_xmit()`` towards the host interface.
   One-step TX timestamping do not require packet cloning, since there is
   no follow-up message required by the PTP protocol (because the
   TX timestamp is embedded into the packet by the MAC), and therefore
   user space does not expect the packet annotated with the TX timestamp
   to be re-enqueued into its socket's error queue.
 
 - ``.port_rxtstamp()``: On RX, the BPF classifier is run by DSA to
   identify PTP event messages (any other packets, including PTP general
   messages, are not timestamped). The original (and only) timestampable
   skb is provided to the driver, for it to annotate it with a timestamp,
   if that is immediately available, or defer to later. On reception,
   timestamps might either be available in-band (through metadata in the
   DSA header, or attached in other ways to the packet), or out-of-band
   (through another RX timestamping FIFO). Deferral on RX is typically
   necessary when retrieving the timestamp needs a sleepable context. In
   that case, it is the responsibility of the DSA driver to call
   ``netif_rx()`` on the freshly timestamped skb.
 
 3.2.2 Ethernet PHYs
 ^^^^^^^^^^^^^^^^^^^
 
 These are devices that typically fulfill a Layer 1 role in the network stack,
 hence they do not have a representation in terms of a network interface as DSA
 switches do. However, PHYs may be able to detect and timestamp PTP packets, for
 performance reasons: timestamps taken as close as possible to the wire have the
 potential to yield a more stable and precise synchronization.
 
 A PHY driver that supports PTP timestamping must create a ``struct
 mii_timestamper`` and add a pointer to it in ``phydev->mii_ts``. The presence
 of this pointer will be checked by the networking stack.
 
 Since PHYs do not have network interface representations, the timestamping and
 ethtool ioctl operations for them need to be mediated by their respective MAC
 driver.  Therefore, as opposed to DSA switches, modifications need to be done
 to each individual MAC driver for PHY timestamping support. This entails:
 
 - Checking, in ``.ndo_eth_ioctl``, whether ``phy_has_hwtstamp(netdev->phydev)``
   is true or not. If it is, then the MAC driver should not process this request
   but instead pass it on to the PHY using ``phy_mii_ioctl()``.
 
 - On RX, special intervention may or may not be needed, depending on the
   function used to deliver skb's up the network stack. In the case of plain
   ``netif_rx()`` and similar, MAC drivers must check whether
   ``skb_defer_rx_timestamp(skb)`` is necessary or not - and if it is, don't
   call ``netif_rx()`` at all.  If ``CONFIG_NETWORK_PHY_TIMESTAMPING`` is
   enabled, and ``skb->dev->phydev->mii_ts`` exists, its ``.rxtstamp()`` hook
   will be called now, to determine, using logic very similar to DSA, whether
   deferral for RX timestamping is necessary.  Again like DSA, it becomes the
   responsibility of the PHY driver to send the packet up the stack when the
   timestamp is available.
 
   For other skb receive functions, such as ``napi_gro_receive`` and
   ``netif_receive_skb``, the stack automatically checks whether
   ``skb_defer_rx_timestamp()`` is necessary, so this check is not needed inside
   the driver.
 
 - On TX, again, special intervention might or might not be needed.  The
   function that calls the ``mii_ts->txtstamp()`` hook is named
   ``skb_clone_tx_timestamp()``. This function can either be called directly
   (case in which explicit MAC driver support is indeed needed), but the
   function also piggybacks from the ``skb_tx_timestamp()`` call, which many MAC
   drivers already perform for software timestamping purposes. Therefore, if a
   MAC supports software timestamping, it does not need to do anything further
   at this stage.
 
 3.2.3 MII bus snooping devices
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 These perform the same role as timestamping Ethernet PHYs, save for the fact
 that they are discrete devices and can therefore be used in conjunction with
 any PHY even if it doesn't support timestamping. In Linux, they are
 discoverable and attachable to a ``struct phy_device`` through Device Tree, and
 for the rest, they use the same mii_ts infrastructure as those. See
 Documentation/devicetree/bindings/ptp/timestamper.txt for more details.
 
 3.2.4 Other caveats for MAC drivers
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Stacked PHCs, especially DSA (but not only) - since that doesn't require any
 modification to MAC drivers, so it is more difficult to ensure correctness of
 all possible code paths - is that they uncover bugs which were impossible to
 trigger before the existence of stacked PTP clocks.  One example has to do with
 this line of code, already presented earlier::
 
       skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
 
 Any TX timestamping logic, be it a plain MAC driver, a DSA switch driver, a PHY
 driver or a MII bus snooping device driver, should set this flag.
 But a MAC driver that is unaware of PHC stacking might get tripped up by
 somebody other than itself setting this flag, and deliver a duplicate
 timestamp.
 For example, a typical driver design for TX timestamping might be to split the
 transmission part into 2 portions:
 
 1. "TX": checks whether PTP timestamping has been previously enabled through
    the ``.ndo_eth_ioctl`` ("``priv->hwtstamp_tx_enabled == true``") and the
    current skb requires a TX timestamp ("``skb_shinfo(skb)->tx_flags &
    SKBTX_HW_TSTAMP``"). If this is true, it sets the
    "``skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS``" flag. Note: as
    described above, in the case of a stacked PHC system, this condition should
    never trigger, as this MAC is certainly not the outermost PHC. But this is
    not where the typical issue is.  Transmission proceeds with this packet.
 
 2. "TX confirmation": Transmission has finished. The driver checks whether it
    is necessary to collect any TX timestamp for it. Here is where the typical
    issues are: the MAC driver takes a shortcut and only checks whether
    "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``" was set. With a stacked
    PHC system, this is incorrect because this MAC driver is not the only entity
    in the TX data path who could have enabled SKBTX_IN_PROGRESS in the first
    place.
 
 The correct solution for this problem is for MAC drivers to have a compound
 check in their "TX confirmation" portion, not only for
 "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``", but also for
 "``priv->hwtstamp_tx_enabled == true``". Because the rest of the system ensures
 that PTP timestamping is not enabled for anything other than the outermost PHC,
 this enhanced check will avoid delivering a duplicated TX timestamp to user
 space.

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