docs: networking: convert timestamping.txt to ReST

- add SPDX header;
- add a document title;
- adjust titles and chapters, adding proper markups;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 591 insertions, 0 deletions

diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
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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_V2_EVENT, then it should generally always upscale
+HWTSTAMP_FILTER_V2_L2_SYNC_MESSAGE, and so forth, as HWTSTAMP_FILTER_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_hwtstamp_tx() with the original skb, the raw
+  hardware time stamp. skb_hwtstamp_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.