Re: [dpdk-dev] mlx5 & pdump: convert HW timestamps to nanoseconds

From: PATRICK KEROULAS <patrick.keroulas@radio-canada.ca>
To: Slava Ovsiienko <viacheslavo@mellanox.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>,
	Vivien Didelot <vivien.didelot@gmail.com>,
	 Shahaf Shuler <shahafs@mellanox.com>,
	Raslan Darawsheh <rasland@mellanox.com>,
	Matan Azrad <matan@mellanox.com>
Subject: Re: [dpdk-dev] mlx5 & pdump: convert HW timestamps to nanoseconds
Date: Fri, 29 May 2020 16:56:00 -0400	[thread overview]
Message-ID: <CALEF-=D-qtmO-urE4WB3xJCPO1d086a4T=0isk3xhd=90RpJcQ@mail.gmail.com> (raw)
In-Reply-To: <AM4PR05MB3265F9A5814408F1E961ECDED2B00@AM4PR05MB3265.eurprd05.prod.outlook.com>

On Tue, May 26, 2020 at 12:00 PM Slava Ovsiienko <viacheslavo@mellanox.com>
wrote:
>> Hi, Patrick
>
> ConnectX HW timestamp is the captured value of internal 64-bit counter
running at the frequency,
> reported in the device_frequency_khz field of struct
mlx5_ifc_cmd_hca_cap_bits{}.
> This structure is queried in mlx5_devx_cmd_query_hca_attr() routine.
> So, with known frequency it is possible to recalculate timestamp ticks to
desired units.

Hello Slava,

Assuming that the NIC clock is already synced thanks to a PTP client,
does the bit counter give an absolute time value (0 => 1 January 1970
00:00:00)? Or do I need to calculate a time duration from the process
start time?

I just want to validate the path from mlx5 eth dev(Rx) to eth pcap (Tx) :
- query the oscillator frequency at the mlx5_eth_dev init step
  (mlx5_devx_cmd_query_hca_attr())
- store the freq with other hca_attr, carried by dev config which should
  be shared with the secondary process
- in eth_pcap_tx_dumper(), retrieve the freq from the dev given by
  mbuf->port
- convert all the incoming mbuf->timestamp using this freq whose
  variation should be negligible over the capture duration

Last question: what is your opinion about this other method?
https://github.com/linux-rdma/rdma-core/blob/7af01c79e00555207dee6132d72e7bfc1bb5485e/providers/mlx5/mlx5dv.h#L1201

Thanks a lot!