[dpdk-dev] [PATCH v4 0/3] Add rte_eth_read_clock API

Ferruh Yigit ferruh.yigit at intel.com
Fri May 31 09:46:27 CEST 2019


On 5/2/2019 1:11 PM, Tom Barbette 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. This patch series adds a new API to do
> so, rte_eth_read_clock. 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 of read_clock 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
> 
> Change in v3:
>   - Address comments from Ferruh Yigit
> 
> Changes in v4:
>   - Address comments from Keith Wiles and Andrew Rybchenko
>   - Use "clock" as argunment name everywhere.
>   - Expand the API description to make clear that read_clock gives an
>     amount in ticks, and that it has no unit.
> 
> 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

Series applied to dpdk-next-net/master, thanks.


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