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

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

On 6/4/2020 11:52 AM, Ferruh Yigit wrote:
> 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.
> 

But it may be good to mention from the change in the release notes, can you
please update the release notes (doc/guides/rel_notes/release_20_08.rst) in next
version of the patch.