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

[Thomas Monjalon <thomas@monjalon.net>]

27/03/2019 15:41, Stephen Hemminger:
> 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
> 
> 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.

So what do you suggest?