No video output from APU when GPU present

Good catch on those BIOS notes. I'm running 3.30, though. Perhaps 3.10 was on a different branch and those options aren't available in 3.30...

mizzack wrote: Hi, I've got a X370 Gaming K4 with A8-9600 APU installed. The HDMI output on the board works correctly and outputs video as expected when all PCIE slots are empty. However, when a dGPU is installed, the onboard graphics gets disabled completely in favor of the dGPU (no output from mobo HDMI and lspci no longer shows the onboard VGA controller). There doesn't seem to be any option to prefer onboard video within the UEFI either. This is a problem because I need the onboard to work for PCI passthrough. Is this by design? The manual makes no mention of this crippling limitation.

Which UEFI/BIOS version do you have installed?

I cannot be certain this would make a difference, since I don't have a Ryzen APU processor installed in my different ASRock X370 board, but the 3.10 UEFI version for your board has this description:

Add AMD PBS Primary Video Adapter to control Bristol Ridge CPU display order.

http://www.asrock.com/mb/AMD/Fatal1ty%20X370%20Gaming%20K4/index.asp#BIOS

That sounds like an option for selecting the video source, but I could be wrong. I have the equivalent UEFI version installed in my X370 board, but use a Ryzen 7 1700X. I haven't noticed an option in the UEFI for video source selection, but since that option would not apply with my CPU, it may not be shown in that situation.

Intel mother boards, with most of their processors having an iGPU, have had UEFI options to select the Primary Graphics Adapter, as well as enable the iGPU while the dGPU is in use, for years. I assume the same would be possible with AM4 boards.

I don't know where this UEFI option would be located in an AM4 board's UEFI. Possibly in the NB (North Bridge) Configuration screen in the Advanced section, or buried somewhere in the many Advanced\AMD CBS screens, such as NBIO Common Options.

The Ryzen manuals don't reflect the current state of a Ryzen board's UEFI options or features, which have been evolving along with things like new AGESA versions from AMD.