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

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

On 6/3/2020 9:26 PM, Vivien Didelot wrote:
> On Wed, 3 Jun 2020 13:11:39 -0700, Stephen Hemminger <stephen@networkplumber.org> wrote:
>> On Wed, 3 Jun 2020 18:50:51 +0100
>> Ferruh Yigit <ferruh.yigit@intel.com> wrote:
>>
>>> 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?
>>
>> We should do pcapng instead of doing the pcap with nano timestamp.
>> My impression is that libpcap is a dormant project at this point, and the
>> finer grain timestamp is a hack that is only in some verisions.
>> That was one of the reasons pcapng started.
> 
> Reading tv_usec as nanoseconds has been supported for years in libpcap.
> 
> For instance PCAP_TSTAMP_PRECISION_NANO was introduced in ba89e4a18e8b
> ("Make timestamps precision configurable") on May 17, 2013.

Thanks for the commit info, it looks like this support exists since
libpcap-1.5.0, and even CentOS 7 has libpcap-1.5.3-xx so looks safe to proceed.