Some NICs allows 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. This RFC patch series adds support for MLX5. An example 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 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. The RFC lacks the documentation update for the sample application. I wanted further validation before doing so. I'm not sure if it's fine to update an example to show a "double feature". At the same time a full app for this would be overkill, and the information nicely complements the one in rxtx_callback. 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 + drivers/net/mlx5/mlx5.c | 1 + drivers/net/mlx5/mlx5.h | 1 + drivers/net/mlx5/mlx5_ethdev.c | 31 +++++++++ drivers/net/mlx5/mlx5_glue.c | 8 +++ drivers/net/mlx5/mlx5_glue.h | 2 + examples/rxtx_callbacks/Makefile | 2 + examples/rxtx_callbacks/main.c | 87 ++++++++++++++++++++++-- examples/rxtx_callbacks/meson.build | 1 + lib/librte_ethdev/rte_ethdev.c | 13 ++++ lib/librte_ethdev/rte_ethdev.h | 23 +++++++ lib/librte_ethdev/rte_ethdev_core.h | 6 ++ lib/librte_ethdev/rte_ethdev_version.map | 1 + lib/librte_mbuf/rte_mbuf.h | 2 + 14 files changed, 175 insertions(+), 4 deletions(-) -- 2.17.1
