It also doesn’t make any sense that you can’t type an underscore to add to the grub command line. You must not be doing something correctly.
@ricklinux I promise you the whole underscore thing is strange o_0
I tried the iommu=soft and the amdgpu.dpm=0 and neither made a difference.
The only thing that made a difference is removing the Radeon RS 5600 XT.
I put in an old Nvidia Quadro card and I am right now booted into the Live USB of EndeavourOS typing this reply in Firefox.
The only thing I can conclude is that the AMD GPU driver in the Linux kernel has some kind of bug or incompatibility with the way this old Dell workstation handles using a newer generation PCIe card. Works fine in Windows. 
You guys know a developer that works on the open source AMD drivers for Linux by chance?
What is the motherboard that you are using? Also I’m interested in how you update the vbios on the graphics card? I have an RX590 and I’d like to check if it has any update for that also. I do a lot of firmware updates but i have never updated a video card with a newer vbios.
Also if you put in an older Quadro card is it supported by the latest nvidia drivers? What card is it? You can install with nvidia option then.
Edit: I think if it were me i would try emailing AMD and asking for their input. They have hired a lot of linux developers as of late. You could explain that the card won’t boot and give all the particulars such as the card, vbios version your motherboard and bios version and what you are trying to install. You can explain it’s Pci-e 3.0 and the card is Pci-e 4.0 and maybe they have some information?
The computer is a Dell workstation. You can get all the hardware specs of the motherboard from the Dell support site with this service tag: 3V7WR22
When I first got the machine from Ebay I updated the motherboard BIOS to the latest Dell had available which was from 2019.
The video card is a Quadro K600 I do not believe it supports the latest drivers.
I downloaded the vBIOS updater from ASUS for my card. The SKU is TUF 3-RX5600XT-T6G-EVO-GAMING. They have a utility that installs it for the card that I have. I assume a different card would use similar software from the manufacturer.
I feel like reaching out to AMD is a long shot but could be something… I feel like I might have to compile a custom kernel LoL 
These cards should work on amdgpu but i do know in the early stages the 5000 series cards had issues so I’m just suggesting it as AMD knows their product best. My RX590 is a Gigabyte and I’m really happy with it but I’m also using it on a newer chipset X570 MSI board. I have zero issues with it.
Edit: Also the RX590 is before the 5000 series card which most now use a type of firmware on them.
The quadro K600 is supported by the 470.xx drivers. They are available to install on EOS. I think i would probably install it with the default menu entry and then it would install with the nouveau open source drivers. Then you could install the nvidia 470xx version after as they are in the AUR.
OH well yeah but then I would not be able to game on the old Quadro 
I’ll have to look into possible workarounds for the 5000 series cards.
I was going to get a Radeon RX Vega 64 which those are PCIe 3.0 but are the same price as the one I got and are older. Ugh
Thanks for the time you spent thinking about this.
If I find a solution I’ll make another post for sure.
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I’m having the same issue with my newly built PC. It has a RX 7600 though. Did you solve the issue?
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Welcome to the Forum!
I am running a rx 6600 on my newly build PC without any issues. It would be desirable if you open a new topic. This one is already a year old.