[dpdk-dev] [PATCH v2] net/pcap: support software Tx nanosecond timestamps

[Vivien Didelot <vivien.didelot@gmail.com>]

When capturing packets into a PCAP file, DPDK currently uses
microseconds for the timestamps. But libpcap supports interpreting
tv_usec as nanoseconds depending on the file timestamp precision,
as of commit ba89e4a18e8b ("Make timestamps precision configurable").

To support this, use PCAP_TSTAMP_PRECISION_NANO when creating the
empty PCAP file as specified by PCAP_OPEN_DEAD(3PCAP) and implement
nanosecond timeval addition. This also ensures that the precision
reported by capinfos is nanoseconds (9).

Note that NSEC_PER_SEC is defined as 1000000000L instead of 1e9 since
the latter might be interpreted as floating point.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
---
 doc/guides/rel_notes/release_20_08.rst |  6 ++++++
 drivers/net/pcap/rte_eth_pcap.c        | 15 ++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/doc/guides/rel_notes/release_20_08.rst b/doc/guides/rel_notes/release_20_08.rst
index dee4ccbb5..7a67c960c 100644
--- a/doc/guides/rel_notes/release_20_08.rst
+++ b/doc/guides/rel_notes/release_20_08.rst
@@ -56,6 +56,12 @@ New Features
      Also, make sure to start the actual text at the margin.
      =========================================================
 
+* **Updated PCAP driver.**
+
+  Updated PCAP driver with new features and improvements, including:
+
+  * Support software Tx nanosecond timestamps precision.
+
 * **Updated Mellanox mlx5 driver.**
 
   Updated Mellanox mlx5 driver with new features and improvements, including:
diff --git a/drivers/net/pcap/rte_eth_pcap.c b/drivers/net/pcap/rte_eth_pcap.c
index b4c79d174..13a3d0ac7 100644
--- a/drivers/net/pcap/rte_eth_pcap.c
+++ b/drivers/net/pcap/rte_eth_pcap.c
@@ -287,6 +287,8 @@ eth_null_rx(void *queue __rte_unused,
 	return 0;
 }
 
+#define NSEC_PER_SEC	1000000000L
+
 static inline void
 calculate_timestamp(struct timeval *ts) {
 	uint64_t cycles;
@@ -294,8 +296,14 @@ calculate_timestamp(struct timeval *ts) {
 
 	cycles = rte_get_timer_cycles() - start_cycles;
 	cur_time.tv_sec = cycles / hz;
-	cur_time.tv_usec = (cycles % hz) * 1e6 / hz;
-	timeradd(&start_time, &cur_time, ts);
+	cur_time.tv_usec = (cycles % hz) * NSEC_PER_SEC / hz;
+
+	ts->tv_sec = start_time.tv_sec + cur_time.tv_sec;
+	ts->tv_usec = start_time.tv_usec + cur_time.tv_usec;
+	if (ts->tv_usec >= NSEC_PER_SEC) {
+		ts->tv_usec -= NSEC_PER_SEC;
+		ts->tv_sec += 1;
+	}
 }
 
 /*
@@ -475,7 +483,8 @@ open_single_tx_pcap(const char *pcap_filename, pcap_dumper_t **dumper)
 	 * with pcap_dump_open(). We create big enough an Ethernet
 	 * pcap holder.
 	 */
-	tx_pcap = pcap_open_dead(DLT_EN10MB, RTE_ETH_PCAP_SNAPSHOT_LEN);
+	tx_pcap = pcap_open_dead_with_tstamp_precision(DLT_EN10MB,
+			RTE_ETH_PCAP_SNAPSHOT_LEN, PCAP_TSTAMP_PRECISION_NANO);
 	if (tx_pcap == NULL) {
 		PMD_LOG(ERR, "Couldn't create dead pcap");
 		return -1;
-- 
2.27.0