[Bug 1367] net/mlx5 Tx stuck if mbuf has too many segments

[bugzilla at dpdk.org]

https://bugs.dpdk.org/show_bug.cgi?id=1367

            Bug ID: 1367
           Summary: net/mlx5 Tx stuck if mbuf has too many segments
           Product: DPDK
           Version: 23.11
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: ethdev
          Assignee: dev at dpdk.org
          Reporter: andrew.rybchenko at oktetlabs.ru
  Target Milestone: ---

net/mlx5 Tx stuck if mbuf has too many segments

net/mlx5 reports maximum number of Tx segments in device info, but it does not
check it on Tx prepare and simply do not send it on Tx burst.

As the result if such packet is encountered, app does not know (without extra
efforts) why it fails to send the packet after successful Tx prepare. In theory
the reason could be fully occupied Tx queue and application should simply retry
forever.

Found by test run at UNH IOL:

1. Test checks reported maximum segments count: tx_desc_lim.nb_mtu_seg_max=40
https://ts-factory.io/bublik/v2/log/477842?focusId=479564&mode=treeAndinfoAndlog&experimental=true&lineNumber=1_25

2. Test splits the packet into 60 segments:
https://ts-factory.io/bublik/v2/log/477842?focusId=479564&mode=treeAndinfoAndlog&experimental=true&lineNumber=1_49

3. Test logs expectations for the packet to be rejected by Tx prepare:
https://ts-factory.io/bublik/v2/log/477842?focusId=479564&mode=treeAndinfoAndlog&experimental=true&lineNumber=1_60

4. Tx prepare accepts the packet and the test logs error:
https://ts-factory.io/bublik/v2/log/477842?focusId=479564&mode=treeAndinfoAndlog&experimental=true&lineNumber=1_62

5. Test tries to Tx burst the packet, but it returns 0. One more error is
logged.
https://ts-factory.io/bublik/v2/log/477842?focusId=479564&mode=treeAndinfoAndlog&experimental=true&lineNumber=1_62

IMHO better behaviour here would be to accept the packet bug simply drop it in
SW on Tx before passing to HW. Just avoid Tx stuck.

Of course Tx prepare should reject the packet at step 4.

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