i40e: add statistic indicating number of skipped Tx timestamps

The i40e driver can only handle one Tx timestamp request at a time.
This means it is possible for an application timestamp request to be
ignored.

There is no easy way for an administrator to determine if this occurred.
Add a new statistic which tracks this, tx_hwtstamp_skipped.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e.h

@@ -506,6 +506,7 @@ struct i40e_pf {
 	struct mutex tmreg_lock; /* Used to protect the SYSTIME registers. */
 	u64 ptp_base_adj;
 	u32 tx_hwtstamp_timeouts;
+	u32 tx_hwtstamp_skipped;
 	u32 rx_hwtstamp_cleared;
 	u32 latch_event_flags;
 	spinlock_t ptp_rx_lock; /* Used to protect Rx timestamp registers. */

drivers/net/ethernet/intel/i40e/i40e_ethtool.c

@@ -147,6 +147,7 @@ static const struct i40e_stats i40e_gstrings_stats[] = {
 	I40E_PF_STAT("VF_admin_queue_requests", vf_aq_requests),
 	I40E_PF_STAT("arq_overflows", arq_overflows),
 	I40E_PF_STAT("rx_hwtstamp_cleared", rx_hwtstamp_cleared),
+	I40E_PF_STAT("tx_hwtstamp_skipped", tx_hwtstamp_skipped),
 	I40E_PF_STAT("fdir_flush_cnt", fd_flush_cnt),
 	I40E_PF_STAT("fdir_atr_match", stats.fd_atr_match),
 	I40E_PF_STAT("fdir_atr_tunnel_match", stats.fd_atr_tunnel_match),

drivers/net/ethernet/intel/i40e/i40e_txrx.c

@@ -2630,6 +2630,7 @@ static int i40e_tsyn(struct i40e_ring *tx_ring, struct sk_buff *skb,
 		skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
 		pf->ptp_tx_skb = skb_get(skb);
 	} else {
+		pf->tx_hwtstamp_skipped++;
 		return 0;
 	}