Mellanox SN2410: 100G link over a 2 m QSFP28 copper cable to a Chelsio adapter never comes up with FEC off

On a Mellanox SN2410 (MLNX-OS 3.6.4112) a 100G port connected to a Chelsio adapter through a 2 m unequalised QSFP copper cable would not link when FEC was disabled; no HSS lane signal was detected on the adapter side. The poster required FEC-off operaation and asked whether the switch supports it at 100G and how to tell whether the switch or the adapter was at fault.

• Hardware: Mellanox SN2410, Chelsio 100G adapter
• Transceivers: QSFP28 passive copper 2 m
• Form facttor: QSFP28, DAC

Symptoms / messages:

no link with FEC off
no HSS lane signal