Help troubleshooting Vega 56: Lights are on, no one's home. Why does this fix work!?

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So I started this conversation over in the PSU section as I though my issue was related to PSU.

So here’s the background info (feel free to skip to last line):

I had a Asrock Z77 Pro4-M mobo with 2500k and R9 380X GPU. I decided it would be a good idea to upgrade to Vega 56. I install it and I can’t POST. Fans power on, lights are on, but no POST codes or display signal. I’ve cleared CMOS, switched out RAM, etc. Most frustrating, is that I install a PC speaker (for POST codes) and it’s silent (confirmed the speaker works). So at this point, I’m not POSTing, no POST codes, and no display signal. Alright, I figure I will go back the original config - neither my 380X or the iGPU will work in the system now, and now POST codes!

So in a panic, I buy a Ryzen 5 2400G with B450M (MSI gaming plus), and I have the same issue! However, this time the mobo has debug LEDs to let me know where it’s hanging in the process. First CPU, it cycles, then Mem. I reseat the RAM and sure enough, I’m back at no POST codes, no display signal, but the LEDs indicate it’s hanging at VGA. I remove the GPU, and I’m able to boot with integrated Vega 11 graphics. I’m also able to boot with 380X as well (POST codes are working now). I thought it was power supply issue, but confirmed it wasn’t, and at my wit’s end, I return the GPU. It also dawns on me that since I switched the RAM on my Z77 mobo, the RAM also probably needed to be reseated, so I upgraded to Ryzen for nothing.

Now this brand new replacement Vega 56 has the same issue. So after some more googling, I found if you switch the graphics settings from PEG to IGD (external to integrated), it will worked! I haven’t confirmed this fix with my Z77 platform, BUT WHY DOES THIS WORK!?

TL;DR: Couldn’t POST with Vega 56 until I switched the display mode in BIOS from external PCI-e to integrated - why could I boot with my old GPUs without this fix; in other words, why does Vega 56 need this fix and why does it work!?

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Have you confirmed that you are running on the Vega 56 when booted and not just passing the igpu through the Vegas outputs?

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I’ve confirmed as not outputs were connected to the Vega 56. After some more tinkering I finally got it to work (also confirmed with 65 FPS on Very High 2560x1080 (probably a little low from stuttering from loading from 5400RPM HDD). I suppose there was a conflict in BIOS settings by default between the Vega 11 and Vega 56 that didn’t occur with my Tonga GPU. As I mull this over some more it makes sense, but what’s still puzzling is that it happened on the Z77 system.

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Could be a BIOS bug with regards to what the setting is labeled vs. what it actually DOES.

I’m a firm believer that motherboard BIOS developers are generally little better than a pack of moderately trained monkeys with a compiler.

I’d flick the setting and move on. Chalk it up to bios developer muppetry.