Re: [dpdk-dev] [PATCH v2 0/3] Add rte_eth_read_clock API

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 27 Mar 2019 07:19:32 +0100
Tom Barbette <barbette@kth.se> wrote:

> Some NICs allow to timestamp packets, but do not support the full
> PTP synchronization process. Hence, the value set in the mbuf
> timestamp field is only the raw value of an internal clock.
> 
> To make sense of this value, one at least needs to be able to query
> the current hardware clock value. As with the TSC, from there
> a frequency can be derieved by querying multiple time the current value of the
> internal clock with some known delay between the queries (example
> provided in the API doc).
> 
> This patch series adds support for MLX5.
> 
> An example app is provided in the rxtx_callback application.
> It has been updated to display, on top of the software latency
> in cycles, the total latency since the packet was received in hardware.
> The API is used to compute a delta in the TX callback. The raw amount of
> ticks is converted to cycles using a variation of the technique describe above.
> 
> Aside from offloading timestamping, which relieve the
> software from a few operations, this allows to get much more precision
> when studying the source of the latency in a system.
> Eg. in our 100G, CX5 setup the rxtx callback application shows
> SW latency is around 74 cycles (TSC is 3.2Ghz), but the latency
> including NIC processing, PCIe, and queuing is around 196 cycles.
> 
> One may think at first this API is overlapping with te_eth_timesync_read_time.
> rte_eth_timesync_read_time is clearly identified as part of a set of functions
> to use PTP synchronization.
> The device raw clock is not "sync" in any way. More importantly, the returned
> value is not a timeval, but an amount of ticks. We could have a cast-based
> solution, but on top of being an ugly solution, some people seeing the timeval
> type of rte_eth_timesync_read_time could use it blindly.
> 
> Change in v2:
>   - Rebase on current master
> 
> Tom Barbette (3):
>   rte_ethdev: Add API function to read dev clock
>   mlx5: Implement support for read_clock
>   rxtx_callbacks: Add support for HW timestamp
> 
>  doc/guides/nics/features.rst                |  1 +
>  doc/guides/sample_app_ug/rxtx_callbacks.rst |  9 ++-
>  drivers/net/mlx5/mlx5.c                     |  1 +
>  drivers/net/mlx5/mlx5.h                     |  1 +
>  drivers/net/mlx5/mlx5_ethdev.c              | 29 +++++++
>  drivers/net/mlx5/mlx5_glue.c                |  8 ++
>  drivers/net/mlx5/mlx5_glue.h                |  2 +
>  examples/rxtx_callbacks/Makefile            |  2 +
>  examples/rxtx_callbacks/main.c              | 86 ++++++++++++++++++++-
>  examples/rxtx_callbacks/meson.build         |  1 +
>  lib/librte_ethdev/rte_ethdev.c              | 13 ++++
>  lib/librte_ethdev/rte_ethdev.h              | 44 +++++++++++
>  lib/librte_ethdev/rte_ethdev_core.h         |  6 ++
>  lib/librte_ethdev/rte_ethdev_version.map    |  1 +
>  lib/librte_mbuf/rte_mbuf.h                  |  2 +
>  15 files changed, 201 insertions(+), 5 deletions(-)


I like this approach but would like to see the same API supported
on multiple devices.

The current timestamp API is a mess because not all devices behave the
same way. Trying to write an application that uses timestamping is therefore
very difficult.