I'm using Rtnet with 82583 ethernet card.
It takes about 2us to send AF_PACKET using rt_dev_sendmsg(), about 200 bytes of 
data.

It is pretty fast, but I want to find what takes so long.
At last I find that "writel(tx_ring->tail)" spend above 1us to complete.

This is just MMIO write to 82583, of course MMIO is uncache.
And PCIE ASPM is disabled, so L0s is not the problem.

I think PCIE Memory Write TLPs is fast enough, but why spend so much time?

thanks,
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