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------------------------------------------------------------------- 60 61This option is identical to SO_TIMESTAMP except for the returned data type. 62Its struct timespec allows for higher resolution (ns) timestamps than the 63timeval of SO_TIMESTAMP (ms). 64 65Always use SO_TIMESTAMPNS_NEW timestamp to always get timestamp in 66struct __kernel_timespec format. 67 68SO_TIMESTAMPNS_OLD returns incorrect timestamps after the year 2038 69on 32 bit machines. 70 711.3 SO_TIMESTAMPING (also SO_TIMESTAMPING_OLD and SO_TIMESTAMPING_NEW) 72---------------------------------------------------------------------- 73 74Supports multiple types of timestamp requests. As a result, this 75socket option takes a bitmap of flags, not a boolean. In:: 76 77 err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val)); 78 79val is an integer with any of the following bits set. Setting other 80bit returns EINVAL and does not change the current state. 81 82The socket option configures timestamp generation for individual 83sk_buffs (1.3.1), timestamp reporting to the socket's error 84queue (1.3.2) and options (1.3.3). Timestamp generation can also 85be enabled for individual sendmsg calls using cmsg (1.3.4). 86 87 881.3.1 Timestamp Generation 89^^^^^^^^^^^^^^^^^^^^^^^^^^ 90 91Some bits are requests to the stack to try to generate timestamps. Any 92combination of them is valid. Changes to these bits apply to newly 93created packets, not to packets already in the stack. As a result, it 94is possible to selectively request timestamps for a subset of packets 95(e.g., for sampling) by embedding an send() call within two setsockopt 96calls, one to enable timestamp generation and one to disable it. 97Timestamps may also be generated for reasons other than being 98requested by a particular socket, such as when receive timestamping is 99enabled system wide, as explained earlier. 100 101SOF_TIMESTAMPING_RX_HARDWARE: 102 Request rx timestamps generated by the network adapter. 103 104SOF_TIMESTAMPING_RX_SOFTWARE: 105 Request rx timestamps when data enters the kernel. These timestamps 106 are generated just after a device driver hands a packet to the 107 kernel receive stack. 108 109SOF_TIMESTAMPING_TX_HARDWARE: 110 Request tx timestamps generated by the network adapter. This flag 111 can be enabled via both socket options and control messages. 112 113SOF_TIMESTAMPING_TX_SOFTWARE: 114 Request tx timestamps when data leaves the kernel. These timestamps 115 are generated in the device driver as close as possible, but always 116 prior to, passing the packet to the network interface. Hence, they 117 require driver support and may not be available for all devices. 118 This flag can be enabled via both socket options and control messages. 119 120SOF_TIMESTAMPING_TX_SCHED: 121 Request tx timestamps prior to entering the packet scheduler. Kernel 122 transmit latency is, if long, often dominated by queuing delay. The 123 difference between this timestamp and one taken at 124 SOF_TIMESTAMPING_TX_SOFTWARE will expose this latency independent 125 of protocol processing. The latency incurred in protocol 126 processing, if any, can be computed by subtracting a userspace 127 timestamp taken immediately before send() from this timestamp. On 128 machines with virtual devices where a transmitted packet travels 129 through multiple devices and, hence, multiple packet schedulers, 130 a timestamp is generated at each layer. This allows for fine 131 grained measurement of queuing delay. This flag can be enabled 132 via both socket options and control messages. 133 134SOF_TIMESTAMPING_TX_ACK: 135 Request tx timestamps when all data in the send buffer has been 136 acknowledged. This only makes sense for reliable protocols. It is 137 currently only implemented for TCP. For that protocol, it may 138 over-report measurement, because the timestamp is generated when all 139 data up to and including the buffer at send() was acknowledged: the 140 cumulative acknowledgment. The mechanism ignores SACK and FACK. 141 This flag can be enabled via both socket options and control messages. 142 143 1441.3.2 Timestamp Reporting 145^^^^^^^^^^^^^^^^^^^^^^^^^ 146 147The other three bits control which timestamps will be reported in a 148generated control message. Changes to the bits take immediate 149effect at the timestamp reporting locations in the stack. Timestamps 150are only reported for packets that also have the relevant timestamp 151generation request set. 152 153SOF_TIMESTAMPING_SOFTWARE: 154 Report any software timestamps when available. 155 156SOF_TIMESTAMPING_SYS_HARDWARE: 157 This option is deprecated and ignored. 158 159SOF_TIMESTAMPING_RAW_HARDWARE: 160 Report hardware timestamps as generated by 161 SOF_TIMESTAMPING_TX_HARDWARE or SOF_TIMESTAMPING_RX_HARDWARE 162 when available. 163 164 1651.3.3 Timestamp Options 166^^^^^^^^^^^^^^^^^^^^^^^ 167 168The interface supports the options 169 170SOF_TIMESTAMPING_OPT_ID: 171 Generate a unique identifier along with each packet. A process can 172 have multiple concurrent timestamping requests outstanding. Packets 173 can be reordered in the transmit path, for instance in the packet 174 scheduler. In that case timestamps will be queued onto the error 175 queue out of order from the original send() calls. It is not always 176 possible to uniquely match timestamps to the original send() calls 177 based on timestamp order or payload inspection alone, then. 178 179 This option associates each packet at send() with a unique 180 identifier and returns that along with the timestamp. The identifier 181 is derived from a per-socket u32 counter (that wraps). For datagram 182 sockets, the counter increments with each sent packet. For stream 183 sockets, it increments with every byte. For stream sockets, also set 184 SOF_TIMESTAMPING_OPT_ID_TCP, see the section below. 185 186 The counter starts at zero. It is initialized the first time that 187 the socket option is enabled. It is reset each time the option is 188 enabled after having been disabled. Resetting the counter does not 189 change the identifiers of existing packets in the system. 190 191 This option is implemented only for transmit timestamps. There, the 192 timestamp is always looped along with a struct sock_extended_err. 193 The option modifies field ee_data to pass an id that is unique 194 among all possibly concurrently outstanding timestamp requests for 195 that socket. 196 197SOF_TIMESTAMPING_OPT_ID_TCP: 198 Pass this modifier along with SOF_TIMESTAMPING_OPT_ID for new TCP 199 timestamping applications. SOF_TIMESTAMPING_OPT_ID defines how the 200 counter increments for stream sockets, but its starting point is 201 not entirely trivial. This option fixes that. 202 203 For stream sockets, if SOF_TIMESTAMPING_OPT_ID is set, this should 204 always be set too. On datagram sockets the option has no effect. 205 206 A reasonable expectation is that the counter is reset to zero with 207 the system call, so that a subsequent write() of N bytes generates 208 a timestamp with counter N-1. SOF_TIMESTAMPING_OPT_ID_TCP 209 implements this behavior under all conditions. 210 211 SOF_TIMESTAMPING_OPT_ID without modifier often reports the same, 212 especially when the socket option is set when no data is in 213 transmission. If data is being transmitted, it may be off by the 214 length of the output queue (SIOCOUTQ). 215 216 The difference is due to being based on snd_una versus write_seq. 217 snd_una is the offset in the stream acknowledged by the peer. This 218 depends on factors outside of process control, such as network RTT. 219 write_seq is the last byte written by the process. This offset is 220 not affected by external inputs. 221 222 The difference is subtle and unlikely to be noticed when configured 223 at initial socket creation, when no data is queued or sent. But 224 SOF_TIMESTAMPING_OPT_ID_TCP behavior is more robust regardless of 225 when the socket option is set. 226 227SOF_TIMESTAMPING_OPT_CMSG: 228 Support recv() cmsg for all timestamped packets. Control messages 229 are already supported unconditionally on all packets with receive 230 timestamps and on IPv6 packets with transmit timestamp. This option 231 extends them to IPv4 packets with transmit timestamp. One use case 232 is to correlate packets with their egress device, by enabling socket 233 option IP_PKTINFO simultaneously. 234 235 236SOF_TIMESTAMPING_OPT_TSONLY: 237 Applies to transmit timestamps only. Makes the kernel return the 238 timestamp as a cmsg alongside an empty packet, as opposed to 239 alongside the original packet. This reduces the amount of memory 240 charged to the socket's receive budget (SO_RCVBUF) and delivers 241 the timestamp even if sysctl net.core.tstamp_allow_data is 0. 242 This option disables SOF_TIMESTAMPING_OPT_CMSG. 243 244SOF_TIMESTAMPING_OPT_STATS: 245 Optional stats that are obtained along with the transmit timestamps. 246 It must be used together with SOF_TIMESTAMPING_OPT_TSONLY. When the 247 transmit timestamp is available, the stats are available in a 248 separate control message of type SCM_TIMESTAMPING_OPT_STATS, as a 249 list of TLVs (struct nlattr) of types. These stats allow the 250 application to associate various transport layer stats with 251 the transmit timestamps, such as how long a certain block of 252 data was limited by peer's receiver window. 253 254SOF_TIMESTAMPING_OPT_PKTINFO: 255 Enable the SCM_TIMESTAMPING_PKTINFO control message for incoming 256 packets with hardware timestamps. The message contains struct 257 scm_ts_pktinfo, which supplies the index of the real interface which 258 received the packet and its length at layer 2. A valid (non-zero) 259 interface index will be returned only if CONFIG_NET_RX_BUSY_POLL is 260 enabled and the driver is using NAPI. The struct contains also two 261 other fields, but they are reserved and undefined. 262 263SOF_TIMESTAMPING_OPT_TX_SWHW: 264 Request both hardware and software timestamps for outgoing packets 265 when SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE 266 are enabled at the same time. If both timestamps are generated, 267 two separate messages will be looped to the socket's error queue, 268 each containing just one timestamp. 269 270SOF_TIMESTAMPING_OPT_RX_FILTER: 271 Filter out spurious receive timestamps: report a receive timestamp 272 only if the matching timestamp generation flag is enabled. 273 274 Receive timestamps are generated early in the ingress path, before a 275 packet's destination socket is known. If any socket enables receive 276 timestamps, packets for all socket will receive timestamped packets. 277 Including those that request timestamp reporting with 278 SOF_TIMESTAMPING_SOFTWARE and/or SOF_TIMESTAMPING_RAW_HARDWARE, but 279 do not request receive timestamp generation. This can happen when 280 requesting transmit timestamps only. 281 282 Receiving spurious timestamps is generally benign. A process can 283 ignore the unexpected non-zero value. But it makes behavior subtly 284 dependent on other sockets. This flag isolates the socket for more 285 deterministic behavior. 286 287New applications are encouraged to pass SOF_TIMESTAMPING_OPT_ID to 288disambiguate timestamps and SOF_TIMESTAMPING_OPT_TSONLY to operate 289regardless of the setting of sysctl net.core.tstamp_allow_data. 290 291An exception is when a process needs additional cmsg data, for 292instance SOL_IP/IP_PKTINFO to detect the egress network interface. 293Then pass option SOF_TIMESTAMPING_OPT_CMSG. This option depends on 294having access to the contents of the original packet, so cannot be 295combined with SOF_TIMESTAMPING_OPT_TSONLY. 296 297 2981.3.4. Enabling timestamps via control messages 299^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 300 301In addition to socket options, timestamp generation can be requested 302per write via cmsg, only for SOF_TIMESTAMPING_TX_* (see Section 1.3.1). 303Using this feature, applications can sample timestamps per sendmsg() 304without paying the overhead of enabling and disabling timestamps via 305setsockopt:: 306 307 struct msghdr *msg; 308 ... 309 cmsg = CMSG_FIRSTHDR(msg); 310 cmsg->cmsg_level = SOL_SOCKET; 311 cmsg->cmsg_type = SO_TIMESTAMPING; 312 cmsg->cmsg_len = CMSG_LEN(sizeof(__u32)); 313 *((__u32 *) CMSG_DATA(cmsg)) = SOF_TIMESTAMPING_TX_SCHED | 314 SOF_TIMESTAMPING_TX_SOFTWARE | 315 SOF_TIMESTAMPING_TX_ACK; 316 err = sendmsg(fd, msg, 0); 317 318The SOF_TIMESTAMPING_TX_* flags set via cmsg will override 319the SOF_TIMESTAMPING_TX_* flags set via setsockopt. 320 321Moreover, applications must still enable timestamp reporting via 322setsockopt to receive timestamps:: 323 324 __u32 val = SOF_TIMESTAMPING_SOFTWARE | 325 SOF_TIMESTAMPING_OPT_ID /* or any other flag */; 326 err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val)); 327 328 3291.4 Bytestream Timestamps 330------------------------- 331 332The SO_TIMESTAMPING interface supports timestamping of bytes in a 333bytestream. Each request is interpreted as a request for when the 334entire contents of the buffer has passed a timestamping point. That 335is, for streams option SOF_TIMESTAMPING_TX_SOFTWARE will record 336when all bytes have reached the device driver, regardless of how 337many packets the data has been converted into. 338 339In general, bytestreams have no natural delimiters and therefore 340correlating a timestamp with data is non-trivial. A range of bytes 341may be split across segments, any segments may be merged (possibly 342coalescing sections of previously segmented buffers associated with 343independent send() calls). Segments can be reordered and the same 344byte range can coexist in multiple segments for protocols that 345implement retransmissions. 346 347It is essential that all timestamps implement the same semantics, 348regardless of these possible transformations, as otherwise they are 349incomparable. Handling "rare" corner cases differently from the 350simple case (a 1:1 mapping from buffer to skb) is insufficient 351because performance debugging often needs to focus on such outliers. 352 353In practice, timestamps can be correlated with segments of a 354bytestream consistently, if both semantics of the timestamp and the 355timing of measurement are chosen correctly. This challenge is no 356different from deciding on a strategy for IP fragmentation. There, the 357definition is that only the first fragment is timestamped. For 358bytestreams, we chose that a timestamp is generated only when all 359bytes have passed a point. SOF_TIMESTAMPING_TX_ACK as defined is easy to 360implement and reason about. An implementation that has to take into 361account SACK would be more complex due to possible transmission holes 362and out of order arrival. 363 364On the host, TCP can also break the simple 1:1 mapping from buffer to 365skbuff as a result of Nagle, cork, autocork, segmentation and GSO. The 366implementation ensures correctness in all cases by tracking the 367individual last byte passed to send(), even if it is no longer the 368last byte after an skbuff extend or merge operation. It stores the 369relevant sequence number in skb_shinfo(skb)->tskey. Because an skbuff 370has only one such field, only one timestamp can be generated. 371 372In rare cases, a timestamp request can be missed if two requests are 373collapsed onto the same skb. A process can detect this situation by 374enabling SOF_TIMESTAMPING_OPT_ID and comparing the byte offset at 375send time with the value returned for each timestamp. It can prevent 376the situation by always flushing the TCP stack in between requests, 377for instance by enabling TCP_NODELAY and disabling TCP_CORK and 378autocork. After linux-4.7, a better way to prevent coalescing is 379to use MSG_EOR flag at sendmsg() time. 380 381These precautions ensure that the timestamp is generated only when all 382bytes have passed a timestamp point, assuming that the network stack 383itself does not reorder the segments. The stack indeed tries to avoid 384reordering. The one exception is under administrator control: it is 385possible to construct a packet scheduler configuration that delays 386segments from the same stream differently. Such a setup would be 387unusual. 388 389 3902 Data Interfaces 391================== 392 393Timestamps are read using the ancillary data feature of recvmsg(). 394See `man 3 cmsg` for details of this interface. The socket manual 395page (`man 7 socket`) describes how timestamps generated with 396SO_TIMESTAMP and SO_TIMESTAMPNS records can be retrieved. 397 398 3992.1 SCM_TIMESTAMPING records 400---------------------------- 401 402These timestamps are returned in a control message with cmsg_level 403SOL_SOCKET, cmsg_type SCM_TIMESTAMPING, and payload of type 404 405For SO_TIMESTAMPING_OLD:: 406 407 struct scm_timestamping { 408 struct timespec ts[3]; 409 }; 410 411For SO_TIMESTAMPING_NEW:: 412 413 struct scm_timestamping64 { 414 struct __kernel_timespec ts[3]; 415 416Always use SO_TIMESTAMPING_NEW timestamp to always get timestamp in 417struct scm_timestamping64 format. 418 419SO_TIMESTAMPING_OLD returns incorrect timestamps after the year 2038 420on 32 bit machines. 421 422The structure can return up to three timestamps. This is a legacy 423feature. At least one field is non-zero at any time. Most timestamps 424are passed in ts[0]. Hardware timestamps are passed in ts[2]. 425 426ts[1] used to hold hardware timestamps converted to system time. 427Instead, expose the hardware clock device on the NIC directly as 428a HW PTP clock source, to allow time conversion in userspace and 429optionally synchronize system time with a userspace PTP stack such 430as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst. 431 432Note that if the SO_TIMESTAMP or SO_TIMESTAMPNS option is enabled 433together with SO_TIMESTAMPING using SOF_TIMESTAMPING_SOFTWARE, a false 434software timestamp will be generated in the recvmsg() call and passed 435in ts[0] when a real software timestamp is missing. This happens also 436on hardware transmit timestamps. 437 4382.1.1 Transmit timestamps with MSG_ERRQUEUE 439^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 440 441For transmit timestamps the outgoing packet is looped back to the 442socket's error queue with the send timestamp(s) attached. A process 443receives the timestamps by calling recvmsg() with flag MSG_ERRQUEUE 444set and with a msg_control buffer sufficiently large to receive the 445relevant metadata structures. The recvmsg call returns the original 446outgoing data packet with two ancillary messages attached. 447 448A message of cm_level SOL_IP(V6) and cm_type IP(V6)_RECVERR 449embeds a struct sock_extended_err. This defines the error type. For 450timestamps, the ee_errno field is ENOMSG. The other ancillary message 451will have cm_level SOL_SOCKET and cm_type SCM_TIMESTAMPING. This 452embeds the struct scm_timestamping. 453 454 4552.1.1.2 Timestamp types 456~~~~~~~~~~~~~~~~~~~~~~~ 457 458The semantics of the three struct timespec are defined by field 459ee_info in the extended error structure. It contains a value of 460type SCM_TSTAMP_* to define the actual timestamp passed in 461scm_timestamping. 462 463The SCM_TSTAMP_* types are 1:1 matches to the SOF_TIMESTAMPING_* 464control fields discussed previously, with one exception. For legacy 465reasons, SCM_TSTAMP_SND is equal to zero and can be set for both 466SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE. It 467is the first if ts[2] is non-zero, the second otherwise, in which 468case the timestamp is stored in ts[0]. 469 470 4712.1.1.3 Fragmentation 472~~~~~~~~~~~~~~~~~~~~~ 473 474Fragmentation of outgoing datagrams is rare, but is possible, e.g., by 475explicitly disabling PMTU discovery. If an outgoing packet is fragmented, 476then only the first fragment is timestamped and returned to the sending 477socket. 478 479 4802.1.1.4 Packet Payload 481~~~~~~~~~~~~~~~~~~~~~~ 482 483The calling application is often not interested in receiving the whole 484packet payload that it passed to the stack originally: the socket 485error queue mechanism is just a method to piggyback the timestamp on. 486In this case, the application can choose to read datagrams with a 487smaller buffer, possibly even of length 0. The payload is truncated 488accordingly. Until the process calls recvmsg() on the error queue, 489however, the full packet is queued, taking up budget from SO_RCVBUF. 490 491 4922.1.1.5 Blocking Read 493~~~~~~~~~~~~~~~~~~~~~ 494 495Reading from the error queue is always a non-blocking operation. To 496block waiting on a timestamp, use poll or select. poll() will return 497POLLERR in pollfd.revents if any data is ready on the error queue. 498There is no need to pass this flag in pollfd.events. This flag is 499ignored on request. See also `man 2 poll`. 500 501 5022.1.2 Receive timestamps 503^^^^^^^^^^^^^^^^^^^^^^^^ 504 505On reception, there is no reason to read from the socket error queue. 506The SCM_TIMESTAMPING ancillary data is sent along with the packet data 507on a normal recvmsg(). Since this is not a socket error, it is not 508accompanied by a message SOL_IP(V6)/IP(V6)_RECVERROR. In this case, 509the meaning of the three fields in struct scm_timestamping is 510implicitly defined. ts[0] holds a software timestamp if set, ts[1] 511is again deprecated and ts[2] holds a hardware timestamp if set. 512 513 5143. Hardware Timestamping configuration: SIOCSHWTSTAMP and SIOCGHWTSTAMP 515======================================================================= 516 517Hardware time stamping must also be initialized for each device driver 518that is expected to do hardware time stamping. The parameter is defined in 519include/uapi/linux/net_tstamp.h as:: 520 521 struct hwtstamp_config { 522 int flags; /* no flags defined right now, must be zero */ 523 int tx_type; /* HWTSTAMP_TX_* */ 524 int rx_filter; /* HWTSTAMP_FILTER_* */ 525 }; 526 527Desired behavior is passed into the kernel and to a specific device by 528calling ioctl(SIOCSHWTSTAMP) with a pointer to a struct ifreq whose 529ifr_data points to a struct hwtstamp_config. The tx_type and 530rx_filter are hints to the driver what it is expected to do. If 531the requested fine-grained filtering for incoming packets is not 532supported, the driver may time stamp more than just the requested types 533of packets. 534 535Drivers are free to use a more permissive configuration than the requested 536configuration. It is expected that drivers should only implement directly the 537most generic mode that can be supported. For example if the hardware can 538support HWTSTAMP_FILTER_PTP_V2_EVENT, then it should generally always upscale 539HWTSTAMP_FILTER_PTP_V2_L2_SYNC, and so forth, as HWTSTAMP_FILTER_PTP_V2_EVENT 540is more generic (and more useful to applications). 541 542A driver which supports hardware time stamping shall update the struct 543with the actual, possibly more permissive configuration. If the 544requested packets cannot be time stamped, then nothing should be 545changed and ERANGE shall be returned (in contrast to EINVAL, which 546indicates that SIOCSHWTSTAMP is not supported at all). 547 548Only a processes with admin rights may change the configuration. User 549space is responsible to ensure that multiple processes don't interfere 550with each other and that the settings are reset. 551 552Any process can read the actual configuration by passing this 553structure to ioctl(SIOCGHWTSTAMP) in the same way. However, this has 554not been implemented in all drivers. 555 556:: 557 558 /* possible values for hwtstamp_config->tx_type */ 559 enum { 560 /* 561 * no outgoing packet will need hardware time stamping; 562 * should a packet arrive which asks for it, no hardware 563 * time stamping will be done 564 */ 565 HWTSTAMP_TX_OFF, 566 567 /* 568 * enables hardware time stamping for outgoing packets; 569 * the sender of the packet decides which are to be 570 * time stamped by setting SOF_TIMESTAMPING_TX_SOFTWARE 571 * before sending the packet 572 */ 573 HWTSTAMP_TX_ON, 574 }; 575 576 /* possible values for hwtstamp_config->rx_filter */ 577 enum { 578 /* time stamp no incoming packet at all */ 579 HWTSTAMP_FILTER_NONE, 580 581 /* time stamp any incoming packet */ 582 HWTSTAMP_FILTER_ALL, 583 584 /* return value: time stamp all packets requested plus some others */ 585 HWTSTAMP_FILTER_SOME, 586 587 /* PTP v1, UDP, any kind of event packet */ 588 HWTSTAMP_FILTER_PTP_V1_L4_EVENT, 589 590 /* for the complete list of values, please check 591 * the include file include/uapi/linux/net_tstamp.h 592 */ 593 }; 594 5953.1 Hardware Timestamping Implementation: Device Drivers 596-------------------------------------------------------- 597 598A driver which supports hardware time stamping must support the 599SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with 600the actual values as described in the section on SIOCSHWTSTAMP. It 601should also support SIOCGHWTSTAMP. 602 603Time stamps for received packets must be stored in the skb. To get a pointer 604to the shared time stamp structure of the skb call skb_hwtstamps(). Then 605set the time stamps in the structure:: 606 607 struct skb_shared_hwtstamps { 608 /* hardware time stamp transformed into duration 609 * since arbitrary point in time 610 */ 611 ktime_t hwtstamp; 612 }; 613 614Time stamps for outgoing packets are to be generated as follows: 615 616- In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) 617 is set no-zero. If yes, then the driver is expected to do hardware time 618 stamping. 619- If this is possible for the skb and requested, then declare 620 that the driver is doing the time stamping by setting the flag 621 SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with:: 622 623 skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS; 624 625 You might want to keep a pointer to the associated skb for the next step 626 and not free the skb. A driver not supporting hardware time stamping doesn't 627 do that. A driver must never touch sk_buff::tstamp! It is used to store 628 software generated time stamps by the network subsystem. 629- Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware 630 as possible. skb_tx_timestamp() provides a software time stamp if requested 631 and hardware timestamping is not possible (SKBTX_IN_PROGRESS not set). 632- As soon as the driver has sent the packet and/or obtained a 633 hardware time stamp for it, it passes the time stamp back by 634 calling skb_tstamp_tx() with the original skb, the raw 635 hardware time stamp. skb_tstamp_tx() clones the original skb and 636 adds the timestamps, therefore the original skb has to be freed now. 637 If obtaining the hardware time stamp somehow fails, then the driver 638 should not fall back to software time stamping. The rationale is that 639 this would occur at a later time in the processing pipeline than other 640 software time stamping and therefore could lead to unexpected deltas 641 between time stamps. 642 6433.2 Special considerations for stacked PTP Hardware Clocks 644---------------------------------------------------------- 645 646There are situations when there may be more than one PHC (PTP Hardware Clock) 647in the data path of a packet. The kernel has no explicit mechanism to allow the 648user to select which PHC to use for timestamping Ethernet frames. Instead, the 649assumption is that the outermost PHC is always the most preferable, and that 650kernel drivers collaborate towards achieving that goal. Currently there are 3 651cases of stacked PHCs, detailed below: 652 6533.2.1 DSA (Distributed Switch Architecture) switches 654^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 655 656These are Ethernet switches which have one of their ports connected to an 657(otherwise completely unaware) host Ethernet interface, and perform the role of 658a port multiplier with optional forwarding acceleration features. Each DSA 659switch port is visible to the user as a standalone (virtual) network interface, 660and its network I/O is performed, under the hood, indirectly through the host 661interface (redirecting to the host port on TX, and intercepting frames on RX). 662 663When a DSA switch is attached to a host port, PTP synchronization has to 664suffer, since the switch's variable queuing delay introduces a path delay 665jitter between the host port and its PTP partner. For this reason, some DSA 666switches include a timestamping clock of their own, and have the ability to 667perform network timestamping on their own MAC, such that path delays only 668measure wire and PHY propagation latencies. Timestamping DSA switches are 669supported in Linux and expose the same ABI as any other network interface (save 670for the fact that the DSA interfaces are in fact virtual in terms of network 671I/O, they do have their own PHC). It is typical, but not mandatory, for all 672interfaces of a DSA switch to share the same PHC. 673 674By design, PTP timestamping with a DSA switch does not need any special 675handling in the driver for the host port it is attached to. However, when the 676host port also supports PTP timestamping, DSA will take care of intercepting 677the ``.ndo_eth_ioctl`` calls towards the host port, and block attempts to enable 678hardware timestamping on it. This is because the SO_TIMESTAMPING API does not 679allow the delivery of multiple hardware timestamps for the same packet, so 680anybody else except for the DSA switch port must be prevented from doing so. 681 682In the generic layer, DSA provides the following infrastructure for PTP 683timestamping: 684 685- ``.port_txtstamp()``: a hook called prior to the transmission of 686 packets with a hardware TX timestamping request from user space. 687 This is required for two-step timestamping, since the hardware 688 timestamp becomes available after the actual MAC transmission, so the 689 driver must be prepared to correlate the timestamp with the original 690 packet so that it can re-enqueue the packet back into the socket's 691 error queue. To save the packet for when the timestamp becomes 692 available, the driver can call ``skb_clone_sk`` , save the clone pointer 693 in skb->cb and enqueue a tx skb queue. Typically, a switch will have a 694 PTP TX timestamp register (or sometimes a FIFO) where the timestamp 695 becomes available. In case of a FIFO, the hardware might store 696 key-value pairs of PTP sequence ID/message type/domain number and the 697 actual timestamp. To perform the correlation correctly between the 698 packets in a queue waiting for timestamping and the actual timestamps, 699 drivers can use a BPF classifier (``ptp_classify_raw``) to identify 700 the PTP transport type, and ``ptp_parse_header`` to interpret the PTP 701 header fields. There may be an IRQ that is raised upon this 702 timestamp's availability, or the driver might have to poll after 703 invoking ``dev_queue_xmit()`` towards the host interface. 704 One-step TX timestamping do not require packet cloning, since there is 705 no follow-up message required by the PTP protocol (because the 706 TX timestamp is embedded into the packet by the MAC), and therefore 707 user space does not expect the packet annotated with the TX timestamp 708 to be re-enqueued into its socket's error queue. 709 710- ``.port_rxtstamp()``: On RX, the BPF classifier is run by DSA to 711 identify PTP event messages (any other packets, including PTP general 712 messages, are not timestamped). The original (and only) timestampable 713 skb is provided to the driver, for it to annotate it with a timestamp, 714 if that is immediately available, or defer to later. On reception, 715 timestamps might either be available in-band (through metadata in the 716 DSA header, or attached in other ways to the packet), or out-of-band 717 (through another RX timestamping FIFO). Deferral on RX is typically 718 necessary when retrieving the timestamp needs a sleepable context. In 719 that case, it is the responsibility of the DSA driver to call 720 ``netif_rx()`` on the freshly timestamped skb. 721 7223.2.2 Ethernet PHYs 723^^^^^^^^^^^^^^^^^^^ 724 725These are devices that typically fulfill a Layer 1 role in the network stack, 726hence they do not have a representation in terms of a network interface as DSA 727switches do. However, PHYs may be able to detect and timestamp PTP packets, for 728performance reasons: timestamps taken as close as possible to the wire have the 729potential to yield a more stable and precise synchronization. 730 731A PHY driver that supports PTP timestamping must create a ``struct 732mii_timestamper`` and add a pointer to it in ``phydev->mii_ts``. The presence 733of this pointer will be checked by the networking stack. 734 735Since PHYs do not have network interface representations, the timestamping and 736ethtool ioctl operations for them need to be mediated by their respective MAC 737driver. Therefore, as opposed to DSA switches, modifications need to be done 738to each individual MAC driver for PHY timestamping support. This entails: 739 740- Checking, in ``.ndo_eth_ioctl``, whether ``phy_has_hwtstamp(netdev->phydev)`` 741 is true or not. If it is, then the MAC driver should not process this request 742 but instead pass it on to the PHY using ``phy_mii_ioctl()``. 743 744- On RX, special intervention may or may not be needed, depending on the 745 function used to deliver skb's up the network stack. In the case of plain 746 ``netif_rx()`` and similar, MAC drivers must check whether 747 ``skb_defer_rx_timestamp(skb)`` is necessary or not - and if it is, don't 748 call ``netif_rx()`` at all. If ``CONFIG_NETWORK_PHY_TIMESTAMPING`` is 749 enabled, and ``skb->dev->phydev->mii_ts`` exists, its ``.rxtstamp()`` hook 750 will be called now, to determine, using logic very similar to DSA, whether 751 deferral for RX timestamping is necessary. Again like DSA, it becomes the 752 responsibility of the PHY driver to send the packet up the stack when the 753 timestamp is available. 754 755 For other skb receive functions, such as ``napi_gro_receive`` and 756 ``netif_receive_skb``, the stack automatically checks whether 757 ``skb_defer_rx_timestamp()`` is necessary, so this check is not needed inside 758 the driver. 759 760- On TX, again, special intervention might or might not be needed. The 761 function that calls the ``mii_ts->txtstamp()`` hook is named 762 ``skb_clone_tx_timestamp()``. This function can either be called directly 763 (case in which explicit MAC driver support is indeed needed), but the 764 function also piggybacks from the ``skb_tx_timestamp()`` call, which many MAC 765 drivers already perform for software timestamping purposes. Therefore, if a 766 MAC supports software timestamping, it does not need to do anything further 767 at this stage. 768 7693.2.3 MII bus snooping devices 770^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 771 772These perform the same role as timestamping Ethernet PHYs, save for the fact 773that they are discrete devices and can therefore be used in conjunction with 774any PHY even if it doesn't support timestamping. In Linux, they are 775discoverable and attachable to a ``struct phy_device`` through Device Tree, and 776for the rest, they use the same mii_ts infrastructure as those. See 777Documentation/devicetree/bindings/ptp/timestamper.txt for more details. 778 7793.2.4 Other caveats for MAC drivers 780^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 781 782Stacked PHCs, especially DSA (but not only) - since that doesn't require any 783modification to MAC drivers, so it is more difficult to ensure correctness of 784all possible code paths - is that they uncover bugs which were impossible to 785trigger before the existence of stacked PTP clocks. One example has to do with 786this line of code, already presented earlier:: 787 788 skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS; 789 790Any TX timestamping logic, be it a plain MAC driver, a DSA switch driver, a PHY 791driver or a MII bus snooping device driver, should set this flag. 792But a MAC driver that is unaware of PHC stacking might get tripped up by 793somebody other than itself setting this flag, and deliver a duplicate 794timestamp. 795For example, a typical driver design for TX timestamping might be to split the 796transmission part into 2 portions: 797 7981. "TX": checks whether PTP timestamping has been previously enabled through 799 the ``.ndo_eth_ioctl`` ("``priv->hwtstamp_tx_enabled == true``") and the 800 current skb requires a TX timestamp ("``skb_shinfo(skb)->tx_flags & 801 SKBTX_HW_TSTAMP``"). If this is true, it sets the 802 "``skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS``" flag. Note: as 803 described above, in the case of a stacked PHC system, this condition should 804 never trigger, as this MAC is certainly not the outermost PHC. But this is 805 not where the typical issue is. Transmission proceeds with this packet. 806 8072. "TX confirmation": Transmission has finished. The driver checks whether it 808 is necessary to collect any TX timestamp for it. Here is where the typical 809 issues are: the MAC driver takes a shortcut and only checks whether 810 "``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``" was set. With a stacked 811 PHC system, this is incorrect because this MAC driver is not the only entity 812 in the TX data path who could have enabled SKBTX_IN_PROGRESS in the first 813 place. 814 815The correct solution for this problem is for MAC drivers to have a compound 816check in their "TX confirmation" portion, not only for 817"``skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS``", but also for 818"``priv->hwtstamp_tx_enabled == true``". Because the rest of the system ensures 819that PTP timestamping is not enabled for anything other than the outermost PHC, 820this enhanced check will avoid delivering a duplicated TX timestamp to user 821space. 822