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Commit fd91e12f authored by Soheil Hassas Yeganeh's avatar Soheil Hassas Yeganeh Committed by David S. Miller
Browse files

sock: document timestamping via cmsg in Documentation 



Update docs and add code snippet for using cmsg for timestamping.

Signed-off-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: default avatarWillem de Bruijn <willemb@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent c14ac945

Documentation/networking/timestamping.txt

+45 −3
Original line number Diff line number Diff line
@@ -44,11 +44,17 @@ timeval of SO_TIMESTAMP (ms). 
Supports multiple types of timestamp requests. As a result, this

socket option takes a bitmap of flags, not a boolean. In



  err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, (void *) val, &val);

  err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, (void *) val,

                   sizeof(val));



val is an integer with any of the following bits set. Setting other

bit returns EINVAL and does not change the current state.



The socket option configures timestamp generation for individual

sk_buffs (1.3.1), timestamp reporting to the socket's error

queue (1.3.2) and options (1.3.3). Timestamp generation can also

be enabled for individual sendmsg calls using cmsg (1.3.4).





1.3.1 Timestamp Generation
@@ -71,13 +77,16 @@ SOF_TIMESTAMPING_RX_SOFTWARE: 
  kernel receive stack.



SOF_TIMESTAMPING_TX_HARDWARE:

  Request tx timestamps generated by the network adapter.

  Request tx timestamps generated by the network adapter. This flag

  can be enabled via both socket options and control messages.



SOF_TIMESTAMPING_TX_SOFTWARE:

  Request tx timestamps when data leaves the kernel. These timestamps

  are generated in the device driver as close as possible, but always

  prior to, passing the packet to the network interface. Hence, they

  require driver support and may not be available for all devices.

  This flag can be enabled via both socket options and control messages.





SOF_TIMESTAMPING_TX_SCHED:

  Request tx timestamps prior to entering the packet scheduler. Kernel
@@ -90,7 +99,8 @@ SOF_TIMESTAMPING_TX_SCHED: 
  machines with virtual devices where a transmitted packet travels

  through multiple devices and, hence, multiple packet schedulers,

  a timestamp is generated at each layer. This allows for fine

  grained measurement of queuing delay.

  grained measurement of queuing delay. This flag can be enabled

  via both socket options and control messages.



SOF_TIMESTAMPING_TX_ACK:

  Request tx timestamps when all data in the send buffer has been
@@ -99,6 +109,7 @@ SOF_TIMESTAMPING_TX_ACK: 
  over-report measurement, because the timestamp is generated when all

  data up to and including the buffer at send() was acknowledged: the

  cumulative acknowledgment. The mechanism ignores SACK and FACK.

  This flag can be enabled via both socket options and control messages.





1.3.2 Timestamp Reporting
@@ -183,6 +194,37 @@ having access to the contents of the original packet, so cannot be 
combined with SOF_TIMESTAMPING_OPT_TSONLY.





1.3.4. Enabling timestamps via control messages



In addition to socket options, timestamp generation can be requested

per write via cmsg, only for SOF_TIMESTAMPING_TX_* (see Section 1.3.1).

Using this feature, applications can sample timestamps per sendmsg()

without paying the overhead of enabling and disabling timestamps via

setsockopt:



  struct msghdr *msg;

  ...

  cmsg			       = CMSG_FIRSTHDR(msg);

  cmsg->cmsg_level	       = SOL_SOCKET;

  cmsg->cmsg_type	       = SO_TIMESTAMPING;

  cmsg->cmsg_len	       = CMSG_LEN(sizeof(__u32));

  *((__u32 *) CMSG_DATA(cmsg)) = SOF_TIMESTAMPING_TX_SCHED |

				 SOF_TIMESTAMPING_TX_SOFTWARE |

				 SOF_TIMESTAMPING_TX_ACK;

  err = sendmsg(fd, msg, 0);



The SOF_TIMESTAMPING_TX_* flags set via cmsg will override

the SOF_TIMESTAMPING_TX_* flags set via setsockopt.



Moreover, applications must still enable timestamp reporting via

setsockopt to receive timestamps:



  __u32 val = SOF_TIMESTAMPING_SOFTWARE |

	      SOF_TIMESTAMPING_OPT_ID /* or any other flag */;

  err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, (void *) val,

                   sizeof(val));





1.4 Bytestream Timestamps



The SO_TIMESTAMPING interface supports timestamping of bytes in a