Implement HW/SW TX timestamp feature

[tribela]

Even if failed to set HW TX timestamp, SW TX timestamp is still can be used.

This PR produces these functions:

• TsnSocket.enable_tx_timestamp()
  Enables TX timestamp feature
  Even if failed to set HW TX timestamp, SW(kernel) TX timestamp is still can be used on get_tx_timestamp() which is better than application level SW timestamp

• TsnSocket.get_tx_timestamp()
  Use this function right after TsnSock.send() on TX timestamp enabled socket. It returns tsn::time::Timespec

example code:

use std::env;

fn main() {
    let args = env::args().collect::<Vec<String>>();
    if args.len() != 2 {
        println!("Usage: {} <iface>", args[0]);
        return;
    }
    let iface = &args[1];

    let mut sock = match tsn::sock_open(iface, 10, 3, 0x0003) {
        Ok(sock) => sock,
        Err(e) => {
            eprintln!("Error: {}", e);
            return;
        }
    };

    if sock.enable_tx_timestamp().is_err() {
        eprintln!("Error: failed to enable tx timestamp");
        return;
    }

    let buf = [0u8; 1024];

    sock.send(&buf);
    let ts = sock.get_tx_timestamp().unwrap();

    println!("tx timestamp: {:?}", ts);

    if let Err(err) = sock.close() {
        eprintln!("Error: {}", err);
    };
}

[SikYoon7]

I wonder how much time difference will occur between HW and SW(Kernel) timestamp roughly?
Could you give some description or example measurement with an available target board?

[tribela]

I wonder how much time difference will occur between HW and SW(Kernel) timestamp roughly? Could you give some description with an available target board?

I never seen that both HW and SW timestamping support in one NIC.
AIK, SW(kernel) timestamp is when the kernel sends packet to NIC(HW), HW timestamp is when the NIC sends packet to outer port.

Kontron NIC and the NIC used on NXP switch(IDK what vendor used on this) has HW timestamping support without SW timestamping
My Realtek NIC has SW timestaming support without HW timstamping