Hardware Timestamping

[Arne Vogel]

Hi,

I am looking into getting the hardware timestamp of when a packet is 
received by the NIC inside of my DPDK program. In a regular program I 
can get the timestamp the packet is received by the NIC and the kernel 
using SO_TIMESTAMPING [0]. Is there a way to get this timestamp in dpdk?

What I found when looking for this, it looks like `mbuf->timestamp` was 
deprecated [1] and was replaced in this [2] example by 
`rte_mbuf_timestamp_t` (in hwts_field). When looking at the values I get 
from `hwts_field` it cycles around 50244192 though, going up _and_ down?

Looking at the output I am unsure what is actually being measured by the 
rxtx_callback example. With a hardware freq. of 156024300 and about 40 
cycles the latency is about 262ns. But what is this latency? From the 
time the packet is received by the NIC to the first time dpdk interacts 
with it? Or when I get the packet in my dpdk program?

Thanks

Arne

[0] https://docs.kernel.org/networking/timestamping.html

[1] 
https://github.com/DPDK/dpdk/commit/c7e857e16fda217cb9fd3644b66abe19daa3afb3

[2] 
https://github.com/DPDK/dpdk/commit/13ad83f18aaab6e0b5ddf18b3be829254a36eb00#diff-066094c5e52eeca7b5bbcc3f60cea0c8da30bf39bf039bcd809968fe3aeb9dc1L79-R89

[1] 
https://github.com/DPDK/dpdk/blob/main/examples/rxtx_callbacks/main.c#L101
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