I came here to ask why it matters to the GPU how the host boots. Does it need to load some drivers or firmware before the host OS? The only thing I found in the article was "... may lose access to important and necessary features of your motherboard,"
This is actually a huge problem with DDR5 memory training. Since memory training must happen before the GPU is brought up, the screen stays blank for that duration (which can take several minutes). The only indication that it's happening is in debug LEDs if your motherboard has them.
How many good boards get RMA'd because they provide no feedback that you're in the middle of memory training, versus the $1 BOM addition to put a seven-segment display-- or hell, one of those 1" OLED modules-- on boards other than the $300+ "Autonomous Collective of Joystick Monkeys" X870E Torpedo Grenade ExplodyThingy Max?
That makes sense. I also have a newer host that uses DDR5 RAM. The first time I powered it up, I thought it was not working. While I was sitting there trying to figure out what to try nest, It finally came up. It gets better with time (or BIOS updates.)
Only if you want some display output before the host OS starts. Like maybe firmware menus, or boot prompts. Generally, this functionality is enabled by a ROM with code for the system firmware to call into: VGA BIOS for CSM and the UEFI GOP for UEFI.
I think all of the IBM video standards involve semi-fixed memory and i/o addresses, so a video card operating in CSM mode needs to listen on those addresses or CSM software is unlikely to work.