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

[Stephen Hemminger]

On Tue,  9 Jun 2020 15:07:19 -0400
Vivien Didelot <vivien.didelot at gmail.com> wrote:

>  
> +#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;
> +	}
>  }
>  

You may want to pre-compute the reciprocal here to save the expensive
cost of divide in the fast path. See rte_reciprocal.