Hang on boot

[Kurt Hackenberg]

Hello people,

So here's a question.  I'm trying to run FreeBSD 12.0 on a new AMD
desktop PC, and it's not working.  I've read the relevant parts of the
handbook and release notes, installed the package drm-kmod-g20190710,
and added

    kld_list="/boot/modules/amdgpu.ko"

to rc.conf.  Booting ends up in a silent hang: screen goes dark,
keyboard and mouse get no response, the system doesn't answer pings.
The only way out is to pull the plug.

Without loading that module, booting succeeds, but /dev/dri doesn't
exist and the X server refuses to run.

I've read the wiki page on graphics, and saw the statement that the
AMD drivers conflict with the EFI frame buffer.  That doesn't apply; I
boot FreeBSD the "legacy" way, not through UEFI.

Have I missed something?

The computer is an HP Pavilion Desktop 590-p0025, with an AMD Ryzen 3
2200G processor and Radeon Vega 8 Graphics.  The boot messages say
it's

CPU: AMD Ryzen 3 2200G with Radeon Vega Graphics     (3493.52-MHz K8-class CPU)
  Origin="AuthenticAMD"  Id=0x810f10  Family=0x17  Model=0x11  Stepping=0

plus a long list of features.

In /boot/modules, I see files whose names include amdgpu_vega10,
vega12, vega20, and vegam, but not vega8.  Could that be the problem?

How do I debug this?  Preferably without pulling the plug over and
over.