net: introduce ptp one step time stamp mode for sync packets

The IEEE 1588 standard (PTP) has a provision for a "one step" mode, where
time stamps on outgoing event packets are inserted into the packet by the
hardware on the fly. This patch adds a new flag for the SIOCSHWTSTAMP
ioctl that lets user space programs request this mode.
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/net_tstamp.h

@@ -60,6 +60,15 @@ enum {
 	 * before sending the packet.
 	 */
 	HWTSTAMP_TX_ON,
+
+	/*
+	 * Enables time stamping for outgoing packets just as
+	 * HWTSTAMP_TX_ON does, but also enables time stamp insertion
+	 * directly into Sync packets. In this case, transmitted Sync
+	 * packets will not received a time stamp via the socket error
+	 * queue.
+	 */
+	HWTSTAMP_TX_ONESTEP_SYNC,
 };
 
 /* possible values for hwtstamp_config->rx_filter */