net: add driver hook for tx time stamping.

This patch adds a hook for transmit time stamps. The transmit hook
allows a software fallback for transmit time stamps, for MACs
lacking time stamping hardware. Using the hook will still require
adding an inline function call to each MAC driver.
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1947,6 +1947,27 @@ static inline ktime_t net_invalid_timestamp(void)
 extern void skb_tstamp_tx(struct sk_buff *orig_skb,
 			struct skb_shared_hwtstamps *hwtstamps);
 
+static inline void sw_tx_timestamp(struct sk_buff *skb)
+{
+	union skb_shared_tx *shtx = skb_tx(skb);
+	if (shtx->software && !shtx->in_progress)
+		skb_tstamp_tx(skb, NULL);
+}
+
+/**
+ * skb_tx_timestamp() - Driver hook for transmit timestamping
+ *
+ * Ethernet MAC Drivers should call this function in their hard_xmit()
+ * function as soon as possible after giving the sk_buff to the MAC
+ * hardware, but before freeing the sk_buff.
+ *
+ * @skb: A socket buffer.
+ */
+static inline void skb_tx_timestamp(struct sk_buff *skb)
+{
+	sw_tx_timestamp(skb);
+}
+
 extern __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len);
 extern __sum16 __skb_checksum_complete(struct sk_buff *skb);