Hello list,

I read that 82599ES family does support hardware timestamping, but when I
tried to read from it, it returned -EINVAL. Below is my code:

```
// port initialization excised
    struct timespec tmp;
    ret = rte_eth_timesync_enable(0);
    printf("Enable tx timestamp: status %d\n", ret);
    ret = rte_eth_timesync_read_tx_timestamp(0, &tmp);
    printf("Get tx timestamp: status %d\n", ret);
```

And the output is:

```
Enable tx timestamp: status 0
Get tx timestamp: status -22
```

Why does it happen? I looked into `examples/ptpclient/ptpclient.c` and
found that no other requirement is needed to read the hardware timestamp.


FYI, I'm using Ubuntu 20.04, DPDK 21.05, and below is what devbind returns:

```
Network devices using DPDK-compatible driver
============================================
0000:04:00.0 '82599ES 10-Gigabit SFI/SFP+ Network Connection 10fb'
drv=uio_pci_generic unused=ixgbe,vfio-pci
0000:04:00.1 '82599ES 10-Gigabit SFI/SFP+ Network Connection 10fb'
drv=uio_pci_generic unused=ixgbe,vfio-pci
```

Thank you in advance!

--
Best Regards,
Fengkai