mlx5 does not reflect lpbk_mode setting

https://bugs.dpdk.org/show_bug.cgi?id=645

Bug ID: 645
Summary: mlx5 does not reflect lpbk_mode setting
Product: DPDK
Version: unspecified
Hardware: All
OS: All
Status: UNCONFIRMED
Severity: normal
Priority: Normal
Component: ethdev
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---

The mlx5 PMD does not seem to reflect rte_eth_conf.lpbk_mode=1 settings to a port when calling rte_eth_dev_configure(). It is expected that after this setting everything sent on TX of that port would be returned on RX of the same port but no packet is received.

System: CentOS 7, DPDK 20.11.0, OFED-5.2-1.0.4, rdma-core 1fdf0b375818823ead89cd39ff2ff4c1279a60f4.

Tested NICs: MT2892 Family [ConnectX-6 Dx] 101d (fw 22.28.1002), MT27800 Family [ConnectX-5] 1017 (fw 16.27.2008)

Reproduce:

```
  > port stop all
  > set fwd txonly
  > port config all loopback 1
  > port start all
  > start 
  > show port stats all
  ######################## NIC statistics for port 0  ########################
  RX-packets: 0          RX-missed: 0          RX-bytes:  0
  RX-errors: 0
  RX-nombuf:  0         
  TX-packets: 959424928  TX-errors: 0          TX-bytes:  61403195392

  Throughput (since last show)
  Rx-pps:            0          Rx-bps:            0
  Tx-pps:      9542270          Tx-bps:   4885642712
  ############################################################################

  ######################## NIC statistics for port 1  ########################
  RX-packets: 0          RX-missed: 0          RX-bytes:  0
  RX-errors: 0
  RX-nombuf:  0         
  TX-packets: 959429568  TX-errors: 0          TX-bytes:  61403492352

  Throughput (since last show)
  Rx-pps:            0          Rx-bps:            0
  Tx-pps:      9541256          Tx-bps:   4885110968
  ############################################################################
```

The same steps work with 82599ES/ixgbe with results like (RX-missed roughly quals to TX-packets):

```
  ######################## NIC statistics for port 0  ########################
  RX-packets: 255        RX-missed: 149026638  RX-bytes:  16320
  RX-errors: 0
  RX-nombuf:  0         
  TX-packets: 149033632  TX-errors: 0          TX-bytes:  9538149184

  Throughput (since last show)
  Rx-pps:            0          Rx-bps:            0
  Tx-pps:     14204421          Tx-bps:   7272664256
  ############################################################################
```

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