mlx5: TX error for multi-seg packet where first segment has 14~18 bytes

https://bugs.dpdk.org/show_bug.cgi?id=96

Bug ID: 96
Summary: mlx5: TX error for multi-seg packet where first
segment has 14~18 bytes
Product: DPDK
Version: 18.08
Hardware: x86
OS: Linux
Status: CONFIRMED
Severity: normal
Priority: Normal
Component: ethdev
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---

Created attachment 14
--> https://bugs.dpdk.org/attachment.cgi?id=14&action=edit
program to reproduce bug

mlx5 driver fails to transmit a 2-segment packet where the first segment has 14, 15, 16, or 17 bytes.
mlx5 driver reports "mlx5_tx_complete(): unexpected error CQE, Tx stopped" error when transmitting a 2-segment packet where the first segment has 18 bytes.

Steps to Reproduce:

1) Use a fiber to connect two ConnectX-5 or ConnectX-4 Ethernet adapters together.
2) Run ip link set NETDEV mtu 9000 to increase MTU.
3) Run tcpdump to observe traffic on the second Ethernet adapter.
4) Execute the attached tx-firstseg18.c program on the first Ethernet adapter.

Actual Results:

1) tcpdump does not receive any packets.
2) The program reports "mlx5_tx_complete(): unexpected error CQE, Tx stopped".

Expected Results:

1) tcpdump receives packets.

Build Date & Hardware:

1) Ubuntu 18.04 Server, with gcc 7.3.0
2) DPDK 18.08, with the following config:
CONFIG_RTE_LIBRTE_MLX5_PMD=y CONFIG_RTE_LIBRTE_MLX5_DEBUG=y CONFIG_RTE_LIBRTE_MBUF_DEBUG=y
3) Mellanox Technologies MT27800 Family [ConnectX-5] Mellanox Technologies MT27700 Family [ConnectX-4]

Additional Information:

In the attached tx-firstseg18.c program, there's a line "#define SIZE_OF_FIRST_SEG (sizeof(ethhdr) + 4)". Changing the number "4" to 0, 1, 2, or 3 makes the "mlx5_tx_complete(): unexpected error CQE, Tx stopped" error disappear, but tcpdump still does not receive packets. Changing the number "4" to 5 or greater causes everything to work, i.e. no error and tcpdump receives packets.

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