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

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 5/23/2020 6:21 PM, Vivien Didelot 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).

Overall good idea and patch looks good.

Only concern is change in the libpcap dependency. Do you know which libpcap
version supports 'PCAP_TSTAMP_PRECISION_NANO'?
If a user of pcap PMD updates to latest DPDK and the environment doesn't have
new version of the libpcap, this change will require an environment update and
this may or may not be easy to do. That is why not sure if the updates require
dependency change should wait for the LTS or not.

Another things is the backward compatibility, as far as I understand the pcap
file recorded with nanosecond precision can be read and parsed without problem
by old application that doesn't know the nanosecond precision, but can you
please confirm this?

Thanks,
ferruh

> 
> 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;
> +	}
>  }
>  
>  /*
> @@ -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;
>