(In reply to cosiekvfj from comment #54)
> Created attachment 144558 [details]
> dmesg log
> 
> [   14.303343] [drm] radeon: 1 quad pipes, 1 z pipes initialized.

This is really interesting! It tells us why it works in your case (the 1 quad
pipe is reported "properly"), but I have no idea why it is reported properly.

I see you are using 4.x kernel so there might be changes - or for some reason
your card returns proper values from the registers the driver read from and
mine does not. Thank you for the information!