I bought one of these Cavium CN8890 based blades. Assuming it turns up and it isn't totally broken I will detail how to get it working here.
20210926 BMC lights up when standby is supplied. I thought it might be +5V but there is a +5V LDO on the distribution board connected to that rail so that seemed weird. A similar 18 pin power connector on a similar Gigabyte server motherboard was +12V for standby. Based on the regulator and the other board I tried +12V on the standby rail and the BMC LED started blinking and nothing exploded.
20210926.1 BMC JTAG 4 pin header is the uart for the BMC
20210929 Unlocking the BMC The BMC password has been set and it's impossible to login. It should be possible to update the BMC firmware from u-boot and erase the settings at the same time but the updater command complains. I unlocked the BMC by pulling the flash chip out (SOP16 SPI NOR), erased it and flashed 773.bin to it.
20211005 Memory,.. Non ECC DIMMs do not work. Current registered ECC DIMMs I have don't complete the memory setup most of the time. Usually the second channel on the second CPU. Moving them around changes nothing so it's possible the board is broken or it just doesn't like these DIMMs. DIMMs listed as working in gigabytes list on order.
20211016 New DIMMs are apparently working. No memory errors now. To get a working keyboard in the debian installer you must pass acpi=force otherwise devicetree is used and USB doesn't work.
20211017 console=tty0 acpi=force is the secret sauce. They allow the debian installer to run perfectly. You need a USB ethernet adapter unless you have a QFP+ switch. Running an NVMe drive in the PCIe slot might be no go. It boots fine but then the starts returning garbage data or timing out and the system goes nuts. Same NVMe drive in a USB adapter is fine.
https://www.servethehome.com/gigabyte-r120-t30-overview-first-cavium-thunderx-system/