1.. SPDX-License-Identifier: GPL-2.0 2 3============ 4Timestamping 5============ 6 7 81. Control Interfaces 9===================== 10 11The interfaces for receiving network packages timestamps are: 12 13SO_TIMESTAMP 14 Generates a timestamp for each incoming packet in (not necessarily 15 monotonic) system time. Reports the timestamp via recvmsg() in a 16 control message in usec resolution. 17 SO_TIMESTAMP is defined as SO_TIMESTAMP_NEW or SO_TIMESTAMP_OLD 18 based on the architecture type and time_t representation of libc. 19 Control message format is in struct __kernel_old_timeval for 20 SO_TIMESTAMP_OLD and in struct __kernel_sock_timeval for 21 SO_TIMESTAMP_NEW options respectively. 22 23SO_TIMESTAMPNS 24 Same timestamping mechanism as SO_TIMESTAMP, but reports the 25 timestamp as struct timespec in nsec resolution. 26 SO_TIMESTAMPNS is defined as SO_TIMESTAMPNS_NEW or SO_TIMESTAMPNS_OLD 27 based on the architecture type and time_t representation of libc. 28 Control message format is in struct timespec for SO_TIMESTAMPNS_OLD 29 and in struct __kernel_timespec for SO_TIMESTAMPNS_NEW options 30 respectively. 31 32IP_MULTICAST_LOOP + SO_TIMESTAMP[NS] 33 Only for multicast:approximate transmit timestamp obtained by 34 reading the looped packet receive timestamp. 35 36SO_TIMESTAMPING 37 Generates timestamps on reception, transmission or both. Supports 38 multiple timestamp sources, including hardware. Supports generating 39 timestamps for stream sockets. 40 41 421.1 SO_TIMESTAMP (also SO_TIMESTAMP_OLD and SO_TIMESTAMP_NEW) 43------------------------------------------------------------- 44 45This socket option enables timestamping of datagrams on the reception 46path. Because the destination socket, if any, is not known early in 47the network stack, the feature has to be enabled for all packets. The 48same is true for all early receive timestamp options. 49 50For interface details, see `man 7 socket`. 51 52Always use SO_TIMESTAMP_NEW timestamp to always get timestamp in 53struct __kernel_sock_timeval format. 54 55SO_TIMESTAMP_OLD returns incorrect timestamps after the year 2038 56on 32 bit machines. 57 581.2 SO_TIMESTAMPNS (also SO_TIMESTAMPNS_OLD and SO_TIMESTAMPNS_NEW): 59 60This option is identical to SO_TIMESTAMP except for the returned data type. 61Its struct timespec allows for higher resolution (ns) timestamps than the 62timeval of SO_TIMESTAMP (ms). 63 64Always use SO_TIMESTAMPNS_NEW timestamp to always get timestamp in 65struct __kernel_timespec format. 66 67SO_TIMESTAMPNS_OLD returns incorrect timestamps after the year 2038 68on 32 bit machines. 69 701.3 SO_TIMESTAMPING (also SO_TIMESTAMPING_OLD and SO_TIMESTAMPING_NEW) 71---------------------------------------------------------------------- 72 73Supports multiple types of timestamp requests. As a result, this 74socket option takes a bitmap of flags, not a boolean. In:: 75 76 err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val)); 77 78val is an integer with any of the following bits set. Setting other 79bit returns EINVAL and does not change the current state. 80 81The socket option configures timestamp generation for individual 82sk_buffs (1.3.1), timestamp reporting to the socket's error 83queue (1.3.2) and options (1.3.3). Timestamp generation can also 84be enabled for individual sendmsg calls using cmsg (1.3.4). 85 86 871.3.1 Timestamp Generation 88^^^^^^^^^^^^^^^^^^^^^^^^^^ 89 90Some bits are requests to the stack to try to generate timestamps. Any 91combination of them is valid. Changes to these bits apply to newly 92created packets, not to packets already in the stack. As a result, it 93is possible to selectively request timestamps for a subset of packets 94(e.g., for sampling) by embedding an send() call within two setsockopt 95calls, one to enable timestamp generation and one to disable it. 96Timestamps may also be generated for reasons other than being 97requested by a particular socket, such as when receive timestamping is 98enabled system wide, as explained earlier. 99 100SOF_TIMESTAMPING_RX_HARDWARE: 101 Request rx timestamps generated by the network adapter. 102 103SOF_TIMESTAMPING_RX_SOFTWARE: 104 Request rx timestamps when data enters the kernel. These timestamps 105 are generated just after a device driver hands a packet to the 106 kernel receive stack. 107 108SOF_TIMESTAMPING_TX_HARDWARE: 109 Request tx timestamps generated by the network adapter. This flag 110 can be enabled via both socket options and control messages. 111 112SOF_TIMESTAMPING_TX_SOFTWARE: 113 Request tx timestamps when data leaves the kernel. These timestamps 114 are generated in the device driver as close as possible, but always 115 prior to, passing the packet to the network interface. Hence, they 116 require driver support and may not be available for all devices. 117 This flag can be enabled via both socket options and control messages. 118 119SOF_TIMESTAMPING_TX_SCHED: 120 Request tx timestamps prior to entering the packet scheduler. Kernel 121 transmit latency is, if long, often dominated by queuing delay. The 122 difference between this timestamp and one taken at 123 SOF_TIMESTAMPING_TX_SOFTWARE will expose this latency independent 124 of protocol processing. The latency incurred in protocol 125 processing, if any, can be computed by subtracting a userspace 126 timestamp taken immediately before send() from this timestamp. On 127 machines with virtual devices where a transmitted packet travels 128 through multiple devices and, hence, multiple packet schedulers, 129 a timestamp is generated at each layer. This allows for fine 130 grained measurement of queuing delay. This flag can be enabled 131 via both socket options and control messages. 132 133SOF_TIMESTAMPING_TX_ACK: 134 Request tx timestamps when all data in the send buffer has been 135 acknowledged. This only makes sense for reliable protocols. It is 136 currently only implemented for TCP. For that protocol, it may 137 over-report measurement, because the timestamp is generated when all 138 data up to and including the buffer at send() was acknowledged: the 139 cumulative acknowledgment. The mechanism ignores SACK and FACK. 140 This flag can be enabled via both socket options and control messages. 141 142 1431.3.2 Timestamp Reporting 144^^^^^^^^^^^^^^^^^^^^^^^^^ 145 146The other three bits control which timestamps will be reported in a 147generated control message. Changes to the bits take immediate 148effect at the timestamp reporting locations in the stack. Timestamps 149are only reported for packets that also have the relevant timestamp 150generation request set. 151 152SOF_TIMESTAMPING_SOFTWARE: 153 Report any software timestamps when available. 154 155SOF_TIMESTAMPING_SYS_HARDWARE: 156 This option is deprecated and ignored. 157 158SOF_TIMESTAMPING_RAW_HARDWARE: 159 Report hardware timestamps as generated by 160 SOF_TIMESTAMPING_TX_HARDWARE when available. 161 162 1631.3.3 Timestamp Options 164^^^^^^^^^^^^^^^^^^^^^^^ 165 166The interface supports the options 167 168SOF_TIMESTAMPING_OPT_ID: 169 Generate a unique identifier along with each packet. A process can 170 have multiple concurrent timestamping requests outstanding. Packets 171 can be reordered in the transmit path, for instance in the packet 172 scheduler. In that case timestamps will be queued onto the error 173 queue out of order from the original send() calls. It is not always 174 possible to uniquely match timestamps to the original send() calls 175 based on timestamp order or payload inspection alone, then. 176 177 This option associates each packet at send() with a unique 178 identifier and returns that along with the timestamp. The identifier 179 is derived from a per-socket u32 counter (that wraps). For datagram 180 sockets, the counter increments with each sent packet. For stream 181 sockets, it increments with every byte. 182 183 The counter starts at zero. It is initialized the first time that 184 the socket option is enabled. It is reset each time the option is 185 enabled after having been disabled. Resetting the counter does not 186 change the identifiers of existing packets in the system. 187 188 This option is implemented only for transmit timestamps. There, the 189 timestamp is always looped along with a struct sock_extended_err. 190 The option modifies field ee_data to pass an id that is unique 191 among all possibly concurrently outstanding timestamp requests for 192 that socket. 193 194 195SOF_TIMESTAMPING_OPT_CMSG: 196 Support recv() cmsg for all timestamped packets. Control messages 197 are already supported unconditionally on all packets with receive 198 timestamps and on IPv6 packets with transmit timestamp. This option 199 extends them to IPv4 packets with transmit timestamp. One use case 200 is to correlate packets with their egress device, by enabling socket 201 option IP_PKTINFO simultaneously. 202 203 204SOF_TIMESTAMPING_OPT_TSONLY: 205 Applies to transmit timestamps only. Makes the kernel return the 206 timestamp as a cmsg alongside an empty packet, as opposed to 207 alongside the original packet. This reduces the amount of memory 208 charged to the socket's receive budget (SO_RCVBUF) and delivers 209 the timestamp even if sysctl net.core.tstamp_allow_data is 0. 210 This option disables SOF_TIMESTAMPING_OPT_CMSG. 211 212SOF_TIMESTAMPING_OPT_STATS: 213 Optional stats that are obtained along with the transmit timestamps. 214 It must be used together with SOF_TIMESTAMPING_OPT_TSONLY. When the 215 transmit timestamp is available, the stats are available in a 216 separate control message of type SCM_TIMESTAMPING_OPT_STATS, as a 217 list of TLVs (struct nlattr) of types. These stats allow the 218 application to associate various transport layer stats with 219 the transmit timestamps, such as how long a certain block of 220 data was limited by peer's receiver window. 221 222SOF_TIMESTAMPING_OPT_PKTINFO: 223 Enable the SCM_TIMESTAMPING_PKTINFO control message for incoming 224 packets with hardware timestamps. The message contains struct 225 scm_ts_pktinfo, which supplies the index of the real interface which 226 received the packet and its length at layer 2. A valid (non-zero) 227 interface index will be returned only if CONFIG_NET_RX_BUSY_POLL is 228 enabled and the driver is using NAPI. The struct contains also two 229 other fields, but they are reserved and undefined. 230 231SOF_TIMESTAMPING_OPT_TX_SWHW: 232 Request both hardware and software timestamps for outgoing packets 233 when SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE 234 are enabled at the same time. If both timestamps are generated, 235 two separate messages will be looped to the socket's error queue, 236 each containing just one timestamp. 237 238New applications are encouraged to pass SOF_TIMESTAMPING_OPT_ID to 239disambiguate timestamps and SOF_TIMESTAMPING_OPT_TSONLY to operate 240regardless of the setting of sysctl net.core.tstamp_allow_data. 241 242An exception is when a process needs additional cmsg data, for 243instance SOL_IP/IP_PKTINFO to detect the egress network interface. 244Then pass option SOF_TIMESTAMPING_OPT_CMSG. This option depends on 245having access to the contents of the original packet, so cannot be 246combined with SOF_TIMESTAMPING_OPT_TSONLY. 247 248 2491.3.4. Enabling timestamps via control messages 250^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 251 252In addition to socket options, timestamp generation can be requested 253per write via cmsg, only for SOF_TIMESTAMPING_TX_* (see Section 1.3.1). 254Using this feature, applications can sample timestamps per sendmsg() 255without paying the overhead of enabling and disabling timestamps via 256setsockopt:: 257 258 struct msghdr *msg; 259 ... 260 cmsg = CMSG_FIRSTHDR(msg); 261 cmsg->cmsg_level = SOL_SOCKET; 262 cmsg->cmsg_type = SO_TIMESTAMPING; 263 cmsg->cmsg_len = CMSG_LEN(sizeof(__u32)); 264 *((__u32 *) CMSG_DATA(cmsg)) = SOF_TIMESTAMPING_TX_SCHED | 265 SOF_TIMESTAMPING_TX_SOFTWARE | 266 SOF_TIMESTAMPING_TX_ACK; 267 err = sendmsg(fd, msg, 0); 268 269The SOF_TIMESTAMPING_TX_* flags set via cmsg will override 270the SOF_TIMESTAMPING_TX_* flags set via setsockopt. 271 272Moreover, applications must still enable timestamp reporting via 273setsockopt to receive timestamps:: 274 275 __u32 val = SOF_TIMESTAMPING_SOFTWARE | 276 SOF_TIMESTAMPING_OPT_ID /* or any other flag */; 277 err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val)); 278 279 2801.4 Bytestream Timestamps 281------------------------- 282 283The SO_TIMESTAMPING interface supports timestamping of bytes in a 284bytestream. Each request is interpreted as a request for when the 285entire contents of the buffer has passed a timestamping point. That 286is, for streams option SOF_TIMESTAMPING_TX_SOFTWARE will record 287when all bytes have reached the device driver, regardless of how 288many packets the data has been converted into. 289 290In general, bytestreams have no natural delimiters and therefore 291correlating a timestamp with data is non-trivial. A range of bytes 292may be split across segments, any segments may be merged (possibly 293coalescing sections of previously segmented buffers associated with 294independent send() calls). Segments can be reordered and the same 295byte range can coexist in multiple segments for protocols that 296implement retransmissions. 297 298It is essential that all timestamps implement the same semantics, 299regardless of these possible transformations, as otherwise they are 300incomparable. Handling "rare" corner cases differently from the 301simple case (a 1:1 mapping from buffer to skb) is insufficient 302because performance debugging often needs to focus on such outliers. 303 304In practice, timestamps can be correlated with segments of a 305bytestream consistently, if both semantics of the timestamp and the 306timing of measurement are chosen correctly. This challenge is no 307different from deciding on a strategy for IP fragmentation. There, the 308definition is that only the first fragment is timestamped. For 309bytestreams, we chose that a timestamp is generated only when all 310bytes have passed a point. SOF_TIMESTAMPING_TX_ACK as defined is easy to 311implement and reason about. An implementation that has to take into 312account SACK would be more complex due to possible transmission holes 313and out of order arrival. 314 315On the host, TCP can also break the simple 1:1 mapping from buffer to 316skbuff as a result of Nagle, cork, autocork, segmentation and GSO. The 317implementation ensures correctness in all cases by tracking the 318individual last byte passed to send(), even if it is no longer the 319last byte after an skbuff extend or merge operation. It stores the 320relevant sequence number in skb_shinfo(skb)->tskey. Because an skbuff 321has only one such field, only one timestamp can be generated. 322 323In rare cases, a timestamp request can be missed if two requests are 324collapsed onto the same skb. A process can detect this situation by 325enabling SOF_TIMESTAMPING_OPT_ID and comparing the byte offset at 326send time with the value returned for each timestamp. It can prevent 327the situation by always flushing the TCP stack in between requests, 328for instance by enabling TCP_NODELAY and disabling TCP_CORK and 329autocork. 330 331These precautions ensure that the timestamp is generated only when all 332bytes have passed a timestamp point, assuming that the network stack 333itself does not reorder the segments. The stack indeed tries to avoid 334reordering. The one exception is under administrator control: it is 335possible to construct a packet scheduler configuration that delays 336segments from the same stream differently. Such a setup would be 337unusual. 338 339 3402 Data Interfaces 341================== 342 343Timestamps are read using the ancillary data feature of recvmsg(). 344See `man 3 cmsg` for details of this interface. The socket manual 345page (`man 7 socket`) describes how timestamps generated with 346SO_TIMESTAMP and SO_TIMESTAMPNS records can be retrieved. 347 348 3492.1 SCM_TIMESTAMPING records 350---------------------------- 351 352These timestamps are returned in a control message with cmsg_level 353SOL_SOCKET, cmsg_type SCM_TIMESTAMPING, and payload of type 354 355For SO_TIMESTAMPING_OLD:: 356 357 struct scm_timestamping { 358 struct timespec ts[3]; 359 }; 360 361For SO_TIMESTAMPING_NEW:: 362 363 struct scm_timestamping64 { 364 struct __kernel_timespec ts[3]; 365 366Always use SO_TIMESTAMPING_NEW timestamp to always get timestamp in 367struct scm_timestamping64 format. 368 369SO_TIMESTAMPING_OLD returns incorrect timestamps after the year 2038 370on 32 bit machines. 371 372The structure can return up to three timestamps. This is a legacy 373feature. At least one field is non-zero at any time. Most timestamps 374are passed in ts[0]. Hardware timestamps are passed in ts[2]. 375 376ts[1] used to hold hardware timestamps converted to system time. 377Instead, expose the hardware clock device on the NIC directly as 378a HW PTP clock source, to allow time conversion in userspace and 379optionally synchronize system time with a userspace PTP stack such 380as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst. 381 382Note that if the SO_TIMESTAMP or SO_TIMESTAMPNS option is enabled 383together with SO_TIMESTAMPING using SOF_TIMESTAMPING_SOFTWARE, a false 384software timestamp will be generated in the recvmsg() call and passed 385in ts[0] when a real software timestamp is missing. This happens also 386on hardware transmit timestamps. 387 3882.1.1 Transmit timestamps with MSG_ERRQUEUE 389^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 390 391For transmit timestamps the outgoing packet is looped back to the 392socket's error queue with the send timestamp(s) attached. A process 393receives the timestamps by calling recvmsg() with flag MSG_ERRQUEUE 394set and with a msg_control buffer sufficiently large to receive the 395relevant metadata structures. The recvmsg call returns the original 396outgoing data packet with two ancillary messages attached. 397 398A message of cm_level SOL_IP(V6) and cm_type IP(V6)_RECVERR 399embeds a struct sock_extended_err. This defines the error type. For 400timestamps, the ee_errno field is ENOMSG. The other ancillary message 401will have cm_level SOL_SOCKET and cm_type SCM_TIMESTAMPING. This 402embeds the struct scm_timestamping. 403 404 4052.1.1.2 Timestamp types 406~~~~~~~~~~~~~~~~~~~~~~~ 407 408The semantics of the three struct timespec are defined by field 409ee_info in the extended error structure. It contains a value of 410type SCM_TSTAMP_* to define the actual timestamp passed in 411scm_timestamping. 412 413The SCM_TSTAMP_* types are 1:1 matches to the SOF_TIMESTAMPING_* 414control fields discussed previously, with one exception. For legacy 415reasons, SCM_TSTAMP_SND is equal to zero and can be set for both 416SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE. It 417is the first if ts[2] is non-zero, the second otherwise, in which 418case the timestamp is stored in ts[0]. 419 420 4212.1.1.3 Fragmentation 422~~~~~~~~~~~~~~~~~~~~~ 423 424Fragmentation of outgoing datagrams is rare, but is possible, e.g., by 425explicitly disabling PMTU discovery. If an outgoing packet is fragmented, 426then only the first fragment is timestamped and returned to the sending 427socket. 428 429 4302.1.1.4 Packet Payload 431~~~~~~~~~~~~~~~~~~~~~~ 432 433The calling application is often not interested in receiving the whole 434packet payload that it passed to the stack originally: the socket 435error queue mechanism is just a method to piggyback the timestamp on. 436In this case, the application can choose to read datagrams with a 437smaller buffer, possibly even of length 0. The payload is truncated 438accordingly. Until the process calls recvmsg() on the error queue, 439however, the full packet is queued, taking up budget from SO_RCVBUF. 440 441 4422.1.1.5 Blocking Read 443~~~~~~~~~~~~~~~~~~~~~ 444 445Reading from the error queue is always a non-blocking operation. To 446block waiting on a timestamp, use poll or select. poll() will return 447POLLERR in pollfd.revents if any data is ready on the error queue. 448There is no need to pass this flag in pollfd.events. This flag is 449ignored on request. See also `man 2 poll`. 450 451 4522.1.2 Receive timestamps 453^^^^^^^^^^^^^^^^^^^^^^^^ 454 455On reception, there is no reason to read from the socket error queue. 456The SCM_TIMESTAMPING ancillary data is sent along with the packet data 457on a normal recvmsg(). Since this is not a socket error, it is not 458accompanied by a message SOL_IP(V6)/IP(V6)_RECVERROR. In this case, 459the meaning of the three fields in struct scm_timestamping is 460implicitly defined. ts[0] holds a software timestamp if set, ts[1] 461is again deprecated and ts[2] holds a hardware timestamp if set. 462 463 4643. Hardware Timestamping configuration: SIOCSHWTSTAMP and SIOCGHWTSTAMP 465======================================================================= 466 467Hardware time stamping must also be initialized for each device driver 468that is expected to do hardware time stamping. The parameter is defined in 469include/uapi/linux/net_tstamp.h as:: 470 471 struct hwtstamp_config { 472 int flags; /* no flags defined right now, must be zero */ 473 int tx_type; /* HWTSTAMP_TX_* */ 474 int rx_filter; /* HWTSTAMP_FILTER_* */ 475 }; 476 477Desired behavior is passed into the kernel and to a specific device by 478calling ioctl(SIOCSHWTSTAMP) with a pointer to a struct ifreq whose 479ifr_data points to a struct hwtstamp_config. The tx_type and 480rx_filter are hints to the driver what it is expected to do. If 481the requested fine-grained filtering for incoming packets is not 482supported, the driver may time stamp more than just the requested types 483of packets. 484 485Drivers are free to use a more permissive configuration than the requested 486configuration. It is expected that drivers should only implement directly the 487most generic mode that can be supported. For example if the hardware can 488support HWTSTAMP_FILTER_V2_EVENT, then it should generally always upscale 489HWTSTAMP_FILTER_V2_L2_SYNC_MESSAGE, and so forth, as HWTSTAMP_FILTER_V2_EVENT 490is more generic (and more useful to applications). 491 492A driver which supports hardware time stamping shall update the struct 493with the actual, possibly more permissive configuration. If the 494requested packets cannot be time stamped, then nothing should be 495changed and ERANGE shall be returned (in contrast to EINVAL, which 496indicates that SIOCSHWTSTAMP is not supported at all). 497 498Only a processes with admin rights may change the configuration. User 499space is responsible to ensure that multiple processes don't interfere 500with each other and that the settings are reset. 501 502Any process can read the actual configuration by passing this 503structure to ioctl(SIOCGHWTSTAMP) in the same way. However, this has 504not been implemented in all drivers. 505 506:: 507 508 /* possible values for hwtstamp_config->tx_type */ 509 enum { 510 /* 511 * no outgoing packet will need hardware time stamping; 512 * should a packet arrive which asks for it, no hardware 513 * time stamping will be done 514 */ 515 HWTSTAMP_TX_OFF, 516 517 /* 518 * enables hardware time stamping for outgoing packets; 519 * the sender of the packet decides which are to be 520 * time stamped by setting SOF_TIMESTAMPING_TX_SOFTWARE 521 * before sending the packet 522 */ 523 HWTSTAMP_TX_ON, 524 }; 525 526 /* possible values for hwtstamp_config->rx_filter */ 527 enum { 528 /* time stamp no incoming packet at all */ 529 HWTSTAMP_FILTER_NONE, 530 531 /* time stamp any incoming packet */ 532 HWTSTAMP_FILTER_ALL, 533 534 /* return value: time stamp all packets requested plus some others */ 535 HWTSTAMP_FILTER_SOME, 536 537 /* PTP v1, UDP, any kind of event packet */ 538 HWTSTAMP_FILTER_PTP_V1_L4_EVENT, 539 540 /* for the complete list of values, please check 541 * the include file include/uapi/linux/net_tstamp.h 542 */ 543 }; 544 5453.1 Hardware Timestamping Implementation: Device Drivers 546-------------------------------------------------------- 547 548A driver which supports hardware time stamping must support the 549SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with 550the actual values as described in the section on SIOCSHWTSTAMP. It 551should also support SIOCGHWTSTAMP. 552 553Time stamps for received packets must be stored in the skb. To get a pointer 554to the shared time stamp structure of the skb call skb_hwtstamps(). Then 555set the time stamps in the structure:: 556 557 struct skb_shared_hwtstamps { 558 /* hardware time stamp transformed into duration 559 * since arbitrary point in time 560 */ 561 ktime_t hwtstamp; 562 }; 563 564Time stamps for outgoing packets are to be generated as follows: 565 566- In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) 567 is set no-zero. If yes, then the driver is expected to do hardware time 568 stamping. 569- If this is possible for the skb and requested, then declare 570 that the driver is doing the time stamping by setting the flag 571 SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with:: 572 573 skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS; 574 575 You might want to keep a pointer to the associated skb for the next step 576 and not free the skb. A driver not supporting hardware time stamping doesn't 577 do that. A driver must never touch sk_buff::tstamp! It is used to store 578 software generated time stamps by the network subsystem. 579- Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware 580 as possible. skb_tx_timestamp() provides a software time stamp if requested 581 and hardware timestamping is not possible (SKBTX_IN_PROGRESS not set). 582- As soon as the driver has sent the packet and/or obtained a 583 hardware time stamp for it, it passes the time stamp back by 584 calling skb_hwtstamp_tx() with the original skb, the raw 585 hardware time stamp. skb_hwtstamp_tx() clones the original skb and 586 adds the timestamps, therefore the original skb has to be freed now. 587 If obtaining the hardware time stamp somehow fails, then the driver 588 should not fall back to software time stamping. The rationale is that 589 this would occur at a later time in the processing pipeline than other 590 software time stamping and therefore could lead to unexpected deltas 591 between time stamps. 592