igb: correctly setting default value TX_WTHRESH (due to performance warning during PMD init)

[Matthew Hall <mhall@mhcomputing.net>]

I get this error when running DPDK with the IGB driver in pktgen-dpdk 
and other code:

PMD: WARNING: eth_igb_tx_queue_setup(): To improve 1G driver 
performance, consider setting the TX WTHRESH value to 4, 8, or 16.

Internally, the code is checking "if (tx_conf->tx_thresh.wthresh == 0)".

But, I am confused now because a while ago I thought we made a bunch of 
DPDK usability changes to get rid of this headache. I remember reading / 
learning that if you set zero for most rx_conf and tx_conf parameters, 
the PMD is supposed to auto-select sane default values.

Given that, why are you forced to set TX_WTHRESH by hand for igb?
What sane default value one should pick for this setting?

Could we do something better to auto-select this like we did for most 
rx_conf / tx_conf settings previously? Or did I miss something about the 
usability changes or do something wrong?

Sincerely,
Matthew Hall.