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
  45struct scm_timestamping {
  46        struct timespec systime;
  47        struct timespec hwtimetrans;
  48        struct timespec hwtimeraw;
  49};
  50
  51recvmsg() can be used to get this control message for regular incoming
  52packets. For send time stamps the outgoing packet is looped back to
  53the socket's error queue with the send time stamp(s) attached. It can
  54be received with recvmsg(flags=MSG_ERRQUEUE). The call returns the
  55original outgoing packet data including all headers preprended down to
  56and including the link layer, the scm_timestamping control message and
  57a sock_extended_err control message with ee_errno==ENOMSG and
  58ee_origin==SO_EE_ORIGIN_TIMESTAMPING. A socket with such a pending
  59bounced packet is ready for reading as far as select() is concerned.
  60If the outgoing packet has to be fragmented, then only the first
  61fragment is time stamped and returned to the sending socket.
  62
  63All three values correspond to the same event in time, but were
  64generated in different ways. Each of these values may be empty (= all
  65zero), in which case no such value was available. If the application
  66is not interested in some of these values, they can be left blank to
  67avoid the potential overhead of calculating them.
  68
  69systime is the value of the system time at that moment. This
  70corresponds to the value also returned via SO_TIMESTAMP[NS]. If the
  71time stamp was generated by hardware, then this field is
  72empty. Otherwise it is filled in if SOF_TIMESTAMPING_SOFTWARE is
  73set.
  74
  75hwtimeraw is the original hardware time stamp. Filled in if
  76SOF_TIMESTAMPING_RAW_HARDWARE is set. No assumptions about its
  77relation to system time should be made.
  78
  79hwtimetrans is the hardware time stamp transformed so that it
  80corresponds as good as possible to system time. This correlation is
  81not perfect; as a consequence, sorting packets received via different
  82NICs by their hwtimetrans may differ from the order in which they were
  83received. hwtimetrans may be non-monotonic even for the same NIC.
  84Filled in if SOF_TIMESTAMPING_SYS_HARDWARE is set. Requires support
  85by the network device and will be empty without that support.
  86
  87
  88SIOCSHWTSTAMP:
  89
  90Hardware time stamping must also be initialized for each device driver
  91that is expected to do hardware time stamping. The parameter is defined in
  92/include/linux/net_tstamp.h as:
  93
  94struct hwtstamp_config {
  95        int flags;      /* no flags defined right now, must be zero */
  96        int tx_type;    /* HWTSTAMP_TX_* */
  97        int rx_filter;  /* HWTSTAMP_FILTER_* */
  98};
  99
 100Desired behavior is passed into the kernel and to a specific device by
 101calling ioctl(SIOCSHWTSTAMP) with a pointer to a struct ifreq whose
 102ifr_data points to a struct hwtstamp_config. The tx_type and
 103rx_filter are hints to the driver what it is expected to do. If
 104the requested fine-grained filtering for incoming packets is not
 105supported, the driver may time stamp more than just the requested types
 106of packets.
 107
 108A driver which supports hardware time stamping shall update the struct
 109with the actual, possibly more permissive configuration. If the
 110requested packets cannot be time stamped, then nothing should be
 111changed and ERANGE shall be returned (in contrast to EINVAL, which
 112indicates that SIOCSHWTSTAMP is not supported at all).
 113
 114Only a processes with admin rights may change the configuration. User
 115space is responsible to ensure that multiple processes don't interfere
 116with each other and that the settings are reset.
 117
 118/* possible values for hwtstamp_config->tx_type */
 119enum {
 120        /*
 121         * no outgoing packet will need hardware time stamping;
 122         * should a packet arrive which asks for it, no hardware
 123         * time stamping will be done
 124         */
 125        HWTSTAMP_TX_OFF,
 126
 127        /*
 128         * enables hardware time stamping for outgoing packets;
 129         * the sender of the packet decides which are to be
 130         * time stamped by setting SOF_TIMESTAMPING_TX_SOFTWARE
 131         * before sending the packet
 132         */
 133        HWTSTAMP_TX_ON,
 134};
 135
 136/* possible values for hwtstamp_config->rx_filter */
 137enum {
 138        /* time stamp no incoming packet at all */
 139        HWTSTAMP_FILTER_NONE,
 140
 141        /* time stamp any incoming packet */
 142        HWTSTAMP_FILTER_ALL,
 143
 144        /* return value: time stamp all packets requested plus some others */
 145        HWTSTAMP_FILTER_SOME,
 146
 147        /* PTP v1, UDP, any kind of event packet */
 148        HWTSTAMP_FILTER_PTP_V1_L4_EVENT,
 149
 150        /* for the complete list of values, please check
 151         * the include file /include/linux/net_tstamp.h
 152         */
 153};
 154
 155
 156DEVICE IMPLEMENTATION
 157
 158A driver which supports hardware time stamping must support the
 159SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with
 160the actual values as described in the section on SIOCSHWTSTAMP.
 161
 162Time stamps for received packets must be stored in the skb. To get a pointer
 163to the shared time stamp structure of the skb call skb_hwtstamps(). Then
 164set the time stamps in the structure:
 165
 166struct skb_shared_hwtstamps {
 167        /* hardware time stamp transformed into duration
 168         * since arbitrary point in time
 169         */
 170        ktime_t hwtstamp;
 171        ktime_t syststamp; /* hwtstamp transformed to system time base */
 172};
 173
 174Time stamps for outgoing packets are to be generated as follows:
 175- In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
 176  is set no-zero. If yes, then the driver is expected to do hardware time
 177  stamping.
 178- If this is possible for the skb and requested, then declare
 179  that the driver is doing the time stamping by setting the flag
 180  SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with
 181
 182      skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
 183
 184  You might want to keep a pointer to the associated skb for the next step
 185  and not free the skb. A driver not supporting hardware time stamping doesn't
 186  do that. A driver must never touch sk_buff::tstamp! It is used to store
 187  software generated time stamps by the network subsystem.
 188- As soon as the driver has sent the packet and/or obtained a
 189  hardware time stamp for it, it passes the time stamp back by
 190  calling skb_hwtstamp_tx() with the original skb, the raw
 191  hardware time stamp. skb_hwtstamp_tx() clones the original skb and
 192  adds the timestamps, therefore the original skb has to be freed now.
 193  If obtaining the hardware time stamp somehow fails, then the driver
 194  should not fall back to software time stamping. The rationale is that
 195  this would occur at a later time in the processing pipeline than other
 196  software time stamping and therefore could lead to unexpected deltas
 197  between time stamps.
 198- If the driver did not set the SKBTX_IN_PROGRESS flag (see above), then
 199  dev_hard_start_xmit() checks whether software time stamping
 200  is wanted as fallback and potentially generates the time stamp.
 201
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