net: add a helper to avoid issues with HW TX timestamping and SO_TXTIME

As explained in commit 29d98f5 ("net: enetc: allow hardware
timestamping on TX queues with tc-etf enabled"), hardware TX
timestamping requires an skb with skb->tstamp = 0. When a packet is sent
with SO_TXTIME, the skb->skb_mstamp_ns corrupts the value of skb->tstamp,
so the drivers need to explicitly reset skb->tstamp to zero after
consuming the TX time.

Create a helper named skb_txtime_consumed() which does just that. All
drivers which offload TC_SETUP_QDISC_ETF should implement it, and it
would make it easier to assess during review whether they do the right
thing in order to be compatible with hardware timestamping or not.

Suggested-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/enetc/enetc.c

@@ -5,6 +5,7 @@
 #include <linux/tcp.h>
 #include <linux/udp.h>
 #include <linux/vmalloc.h>
+#include <net/pkt_sched.h>
 
 /* ENETC overhead: optional extension BD + 1 BD gap */
 #define ENETC_TXBDS_NEEDED(val)	((val) + 2)
@@ -344,12 +345,7 @@ static void enetc_tstamp_tx(struct sk_buff *skb, u64 tstamp)
 	if (skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS) {
 		memset(&shhwtstamps, 0, sizeof(shhwtstamps));
 		shhwtstamps.hwtstamp = ns_to_ktime(tstamp);
-		/* Ensure skb_mstamp_ns, which might have been populated with
-		 * the txtime, is not mistaken for a software timestamp,
-		 * because this will prevent the dispatch of our hardware
-		 * timestamp to the socket.
-		 */
-		skb->tstamp = ktime_set(0, 0);
+		skb_txtime_consumed(skb);
 		skb_tstamp_tx(skb, &shhwtstamps);
 	}
 }

drivers/net/ethernet/intel/igb/igb_main.c

@@ -5856,7 +5856,7 @@ static void igb_tx_ctxtdesc(struct igb_ring *tx_ring,
 	 */
 	if (tx_ring->launchtime_enable) {
 		ts = ktime_to_timespec64(first->skb->tstamp);
-		first->skb->tstamp = ktime_set(0, 0);
+		skb_txtime_consumed(first->skb);
 		context_desc->seqnum_seed = cpu_to_le32(ts.tv_nsec / 32);
 	} else {
 		context_desc->seqnum_seed = 0;

drivers/net/ethernet/intel/igc/igc_main.c

@@ -941,7 +941,7 @@ static void igc_tx_ctxtdesc(struct igc_ring *tx_ring,
 		struct igc_adapter *adapter = netdev_priv(tx_ring->netdev);
 		ktime_t txtime = first->skb->tstamp;
 
-		first->skb->tstamp = ktime_set(0, 0);
+		skb_txtime_consumed(first->skb);
 		context_desc->launch_time = igc_tx_launchtime(adapter,
 							      txtime);
 	} else {

include/net/pkt_sched.h

@@ -188,4 +188,13 @@ struct tc_taprio_qopt_offload *taprio_offload_get(struct tc_taprio_qopt_offload
 						  *offload);
 void taprio_offload_free(struct tc_taprio_qopt_offload *offload);
 
+/* Ensure skb_mstamp_ns, which might have been populated with the txtime, is
+ * not mistaken for a software timestamp, because this will otherwise prevent
+ * the dispatch of hardware timestamps to the socket.
+ */
+static inline void skb_txtime_consumed(struct sk_buff *skb)
+{
+	skb->tstamp = ktime_set(0, 0);
+}
+
 #endif