Hello @Lost_N_BIOS & @davidm71
Today I received my new M2 adapter card which consists 4 M2 slots and looks it works greatly.
Even the issue I mentioned before seems somehow disappear (still need verification). I have 3 M2 nvme devices installed on that card and the performance is quite good. e.g. simutanously test Samsung SM961 & Intel P4510, both can hit 3000MB/s+ in reading test.
Previously I was using Asus M2 Hyper kit which is nothing but a regular PCIe 4X M2 adapter. I think the issue I mentioned before might be related to some circuits issue that pluging a PCIe 4x device into a x4x4x4x4 PCIe x16 slot. However, I have no deeper knowledge regarding the area thus hard to understand the full story.
Anyway, I will still check if the setting is working stable.
To whom might search this topic further, what I have done is only to change the corresponding option in IOU0 of first IIO to x4x4x4x4 mode. In my particular case, it means set value at 0x533 to 0x00 and that’s all.