linux/Documentation/networking/timestamping.txt
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   1The existing interfaces for getting network packages time stamped are:
   2
   3* SO_TIMESTAMP
   4  Generate time stamp for each incoming packet using the (not necessarily
   5  monotonous!) system time. Result is returned via recv_msg() in a
   6  control message as timeval (usec resolution).
   7
   8* SO_TIMESTAMPNS
   9  Same time stamping mechanism as SO_TIMESTAMP, but returns result as
  10  timespec (nsec resolution).
  11
  12* IP_MULTICAST_LOOP + SO_TIMESTAMP[NS]
  13  Only for multicasts: approximate send time stamp by receiving the looped
  14  packet and using its receive time stamp.
  15
  16The following interface complements the existing ones: receive time
  17stamps can be generated and returned for arbitrary packets and much
  18closer to the point where the packet is really sent. Time stamps can
  19be generated in software (as before) or in hardware (if the hardware
  20has such a feature).
  21
  22SO_TIMESTAMPING:
  23
  24Instructs the socket layer which kind of information is wanted. The
  25parameter is an integer with some of the following bits set. Setting
  26other bits is an error and doesn't change the current state.
  27
  28SOF_TIMESTAMPING_TX_HARDWARE:  try to obtain send time stamp in hardware
  29SOF_TIMESTAMPING_TX_SOFTWARE:  if SOF_TIMESTAMPING_TX_HARDWARE is off or
  30                               fails, then do it in software
  31SOF_TIMESTAMPING_RX_HARDWARE:  return the original, unmodified time stamp
  32                               as generated by the hardware
  33SOF_TIMESTAMPING_RX_SOFTWARE:  if SOF_TIMESTAMPING_RX_HARDWARE is off or
  34                               fails, then do it in software
  35SOF_TIMESTAMPING_RAW_HARDWARE: return original raw hardware time stamp
  36SOF_TIMESTAMPING_SYS_HARDWARE: return hardware time stamp transformed to
  37                               the system time base
  38SOF_TIMESTAMPING_SOFTWARE:     return system time stamp generated in
  39                               software
  40
  41SOF_TIMESTAMPING_TX/RX determine how time stamps are generated.
  42SOF_TIMESTAMPING_RAW/SYS determine how they are reported in the
  43following control message:
  44    struct scm_timestamping {
  45           struct timespec systime;
  46           struct timespec hwtimetrans;
  47           struct timespec hwtimeraw;
  48    };
  49
  50recvmsg() can be used to get this control message for regular incoming
  51packets. For send time stamps the outgoing packet is looped back to
  52the socket's error queue with the send time stamp(s) attached. It can
  53be received with recvmsg(flags=MSG_ERRQUEUE). The call returns the
  54original outgoing packet data including all headers preprended down to
  55and including the link layer, the scm_timestamping control message and
  56a sock_extended_err control message with ee_errno==ENOMSG and
  57ee_origin==SO_EE_ORIGIN_TIMESTAMPING. A socket with such a pending
  58bounced packet is ready for reading as far as select() is concerned.
  59If the outgoing packet has to be fragmented, then only the first
  60fragment is time stamped and returned to the sending socket.
  61
  62All three values correspond to the same event in time, but were
  63generated in different ways. Each of these values may be empty (= all
  64zero), in which case no such value was available. If the application
  65is not interested in some of these values, they can be left blank to
  66avoid the potential overhead of calculating them.
  67
  68systime is the value of the system time at that moment. This
  69corresponds to the value also returned via SO_TIMESTAMP[NS]. If the
  70time stamp was generated by hardware, then this field is
  71empty. Otherwise it is filled in if SOF_TIMESTAMPING_SOFTWARE is
  72set.
  73
  74hwtimeraw is the original hardware time stamp. Filled in if
  75SOF_TIMESTAMPING_RAW_HARDWARE is set. No assumptions about its
  76relation to system time should be made.
  77
  78hwtimetrans is the hardware time stamp transformed so that it
  79corresponds as good as possible to system time. This correlation is
  80not perfect; as a consequence, sorting packets received via different
  81NICs by their hwtimetrans may differ from the order in which they were
  82received. hwtimetrans may be non-monotonic even for the same NIC.
  83Filled in if SOF_TIMESTAMPING_SYS_HARDWARE is set. Requires support
  84by the network device and will be empty without that support.
  85
  86
  87SIOCSHWTSTAMP:
  88
  89Hardware time stamping must also be initialized for each device driver
  90that is expected to do hardware time stamping. The parameter is:
  91
  92struct hwtstamp_config {
  93    int flags;           /* no flags defined right now, must be zero */
  94    int tx_type;         /* HWTSTAMP_TX_* */
  95    int rx_filter;       /* HWTSTAMP_FILTER_* */
  96};
  97
  98Desired behavior is passed into the kernel and to a specific device by
  99calling ioctl(SIOCSHWTSTAMP) with a pointer to a struct ifreq whose
 100ifr_data points to a struct hwtstamp_config. The tx_type and
 101rx_filter are hints to the driver what it is expected to do. If
 102the requested fine-grained filtering for incoming packets is not
 103supported, the driver may time stamp more than just the requested types
 104of packets.
 105
 106A driver which supports hardware time stamping shall update the struct
 107with the actual, possibly more permissive configuration. If the
 108requested packets cannot be time stamped, then nothing should be
 109changed and ERANGE shall be returned (in contrast to EINVAL, which
 110indicates that SIOCSHWTSTAMP is not supported at all).
 111
 112Only a processes with admin rights may change the configuration. User
 113space is responsible to ensure that multiple processes don't interfere
 114with each other and that the settings are reset.
 115
 116/* possible values for hwtstamp_config->tx_type */
 117enum {
 118        /*
 119         * no outgoing packet will need hardware time stamping;
 120         * should a packet arrive which asks for it, no hardware
 121         * time stamping will be done
 122         */
 123        HWTSTAMP_TX_OFF,
 124
 125        /*
 126         * enables hardware time stamping for outgoing packets;
 127         * the sender of the packet decides which are to be
 128         * time stamped by setting SOF_TIMESTAMPING_TX_SOFTWARE
 129         * before sending the packet
 130         */
 131        HWTSTAMP_TX_ON,
 132};
 133
 134/* possible values for hwtstamp_config->rx_filter */
 135enum {
 136        /* time stamp no incoming packet at all */
 137        HWTSTAMP_FILTER_NONE,
 138
 139        /* time stamp any incoming packet */
 140        HWTSTAMP_FILTER_ALL,
 141
 142        /* return value: time stamp all packets requested plus some others */
 143        HWTSTAMP_FILTER_SOME,
 144
 145        /* PTP v1, UDP, any kind of event packet */
 146        HWTSTAMP_FILTER_PTP_V1_L4_EVENT,
 147
 148        ...
 149};
 150
 151
 152DEVICE IMPLEMENTATION
 153
 154A driver which supports hardware time stamping must support the
 155SIOCSHWTSTAMP ioctl. Time stamps for received packets must be stored
 156in the skb with skb_hwtstamp_set().
 157
 158Time stamps for outgoing packets are to be generated as follows:
 159- In hard_start_xmit(), check if skb_hwtstamp_check_tx_hardware()
 160  returns non-zero. If yes, then the driver is expected
 161  to do hardware time stamping.
 162- If this is possible for the skb and requested, then declare
 163  that the driver is doing the time stamping by calling
 164  skb_hwtstamp_tx_in_progress(). A driver not supporting
 165  hardware time stamping doesn't do that. A driver must never
 166  touch sk_buff::tstamp! It is used to store how time stamping
 167  for an outgoing packets is to be done.
 168- As soon as the driver has sent the packet and/or obtained a
 169  hardware time stamp for it, it passes the time stamp back by
 170  calling skb_hwtstamp_tx() with the original skb, the raw
 171  hardware time stamp and a handle to the device (necessary
 172  to convert the hardware time stamp to system time). If obtaining
 173  the hardware time stamp somehow fails, then the driver should
 174  not fall back to software time stamping. The rationale is that
 175  this would occur at a later time in the processing pipeline
 176  than other software time stamping and therefore could lead
 177  to unexpected deltas between time stamps.
 178- If the driver did not call skb_hwtstamp_tx_in_progress(), then
 179  dev_hard_start_xmit() checks whether software time stamping
 180  is wanted as fallback and potentially generates the time stamp.
 181
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