sock: document timestamping via cmsg in Documentation

Update docs and add code snippet for using cmsg for timestamping.
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/timestamping.txt

@@ -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