HDMI signal in Fedora/Linux

I’ m facing weird issue and since i have use only Fedora/Linux i’ m posting here.
I bought a new PC monitor with HDMI (old one was with VGA). Old BIOS PC.
I didn’t have video signal during POST, but only when OS was taking over (switch root).
The above problem was present with external gpu and igpu.

I bought new PC (new mobo, CPU, UEFI, etc). Again the same problem as above.

To install Fedora, i took a TV monitor (with HDMI) and i was able to see POST and enter UEFI.

So i guess the above indicate that the problem is the monitor.

Here is the tricky part. If i’ m logged in to Gnome desktop with the TV monitor with HDMI and i un-plugg it, i can’t get back video signal. With any monitor. I have to reboot.

Hi knstn,

did you have a look in your BIOS to see what is configured as your primary display?

Just for closure, i went into UEFI bios and disabled the CSM mode. It’s a compatibility mode for hardware that doesn’t support UEFI but only the old BIOS.
That did the trick. I am getting signal now from POST.
The only thing i can’t understand is why the TV monitor was getting signal with the same GPU but the PC monitor not and it requires UEFI mode only.