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+The existing interfaces for getting network packages time stamped are:
+
+* SO_TIMESTAMP
+ Generate time stamp for each incoming packet using the (not necessarily
+ monotonous!) system time. Result is returned via recv_msg() in a
+ control message as timeval (usec resolution).
+
+* SO_TIMESTAMPNS
+ Same time stamping mechanism as SO_TIMESTAMP, but returns result as
+ timespec (nsec resolution).
+
+* IP_MULTICAST_LOOP + SO_TIMESTAMP[NS]
+ Only for multicasts: approximate send time stamp by receiving the looped
+ packet and using its receive time stamp.
+
+The following interface complements the existing ones: receive time
+stamps can be generated and returned for arbitrary packets and much
+closer to the point where the packet is really sent. Time stamps can
+be generated in software (as before) or in hardware (if the hardware
+has such a feature).
+
+SO_TIMESTAMPING:
+
+Instructs the socket layer which kind of information is wanted. The
+parameter is an integer with some of the following bits set. Setting
+other bits is an error and doesn't change the current state.
+
+SOF_TIMESTAMPING_TX_HARDWARE: try to obtain send time stamp in hardware
+SOF_TIMESTAMPING_TX_SOFTWARE: if SOF_TIMESTAMPING_TX_HARDWARE is off or
+ fails, then do it in software
+SOF_TIMESTAMPING_RX_HARDWARE: return the original, unmodified time stamp
+ as generated by the hardware
+SOF_TIMESTAMPING_RX_SOFTWARE: if SOF_TIMESTAMPING_RX_HARDWARE is off or
+ fails, then do it in software
+SOF_TIMESTAMPING_RAW_HARDWARE: return original raw hardware time stamp
+SOF_TIMESTAMPING_SYS_HARDWARE: return hardware time stamp transformed to
+ the system time base
+SOF_TIMESTAMPING_SOFTWARE: return system time stamp generated in
+ software
+
+SOF_TIMESTAMPING_TX/RX determine how time stamps are generated.
+SOF_TIMESTAMPING_RAW/SYS determine how they are reported in the
+following control message:
+
+struct scm_timestamping {
+ struct timespec systime;
+ struct timespec hwtimetrans;
+ struct timespec hwtimeraw;
+};
+
+recvmsg() can be used to get this control message for regular incoming
+packets. For send time stamps the outgoing packet is looped back to
+the socket's error queue with the send time stamp(s) attached. It can
+be received with recvmsg(flags=MSG_ERRQUEUE). The call returns the
+original outgoing packet data including all headers preprended down to
+and including the link layer, the scm_timestamping control message and
+a sock_extended_err control message with ee_errno==ENOMSG and
+ee_origin==SO_EE_ORIGIN_TIMESTAMPING. A socket with such a pending
+bounced packet is ready for reading as far as select() is concerned.
+If the outgoing packet has to be fragmented, then only the first
+fragment is time stamped and returned to the sending socket.
+
+All three values correspond to the same event in time, but were
+generated in different ways. Each of these values may be empty (= all
+zero), in which case no such value was available. If the application
+is not interested in some of these values, they can be left blank to
+avoid the potential overhead of calculating them.
+
+systime is the value of the system time at that moment. This
+corresponds to the value also returned via SO_TIMESTAMP[NS]. If the
+time stamp was generated by hardware, then this field is
+empty. Otherwise it is filled in if SOF_TIMESTAMPING_SOFTWARE is
+set.
+
+hwtimeraw is the original hardware time stamp. Filled in if
+SOF_TIMESTAMPING_RAW_HARDWARE is set. No assumptions about its
+relation to system time should be made.
+
+hwtimetrans is the hardware time stamp transformed so that it
+corresponds as good as possible to system time. This correlation is
+not perfect; as a consequence, sorting packets received via different
+NICs by their hwtimetrans may differ from the order in which they were
+received. hwtimetrans may be non-monotonic even for the same NIC.
+Filled in if SOF_TIMESTAMPING_SYS_HARDWARE is set. Requires support
+by the network device and will be empty without that support.
+
+
+SIOCSHWTSTAMP:
+
+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/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.
+
+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.
+
+/* 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/linux/net_tstamp.h
+ */
+};
+
+
+DEVICE IMPLEMENTATION
+
+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.
+
+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;
+ ktime_t syststamp; /* hwtstamp transformed to system time base */
+};
+
+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.
+- 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.
+- If the driver did not set the SKBTX_IN_PROGRESS flag (see above), then
+ dev_hard_start_xmit() checks whether software time stamping
+ is wanted as fallback and potentially generates the time stamp.