Re: [dpdk-dev] [PATCH 1/2] net/pcap: support software Tx nanosecond timestamp

[Stephen Hemminger <stephen@networkplumber.org>]

On Sat, 23 May 2020 13:21:29 -0400
Vivien Didelot <vivien.didelot@gmail.com> wrote:

> When capturing packets into a PCAP file, DPDK currently uses
> microseconds for the timestamp. But libpcap supports interpreting
> tv_usec as nanoseconds depending on the file timestamp precision.
> 
> 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).
> 
> Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
> ---
>  drivers/net/pcap/rte_eth_pcap.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/pcap/rte_eth_pcap.c b/drivers/net/pcap/rte_eth_pcap.c
> index b4c79d174..68588c3d7 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	1e9
> +
>  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;
> +	}

Please no floating point math in the fast path of capturing packets!