I tried to test HDMI audio on Up^2 and hdac_hdmi_set_hw_params() is failing with -ENODEV because no port is assigned. Some very light analysis and debugging showed, that hdac_hdmi_pcm_open() hasn't been called, but it might wrong. Are there any non-trivial requirements to use HDMI audio on Up^2? Can it be, that my monitor or my cable aren't good enough?
You need to set the PortN-PinM mixers, otherwise the hw_params will fail. It's super painful and I don't know why it's needed but that's what it is for now. I typically look for the dmesg, find out which port is not disconnected, and usually set Port6Pin0 to 1, then use hw:0,1 to get things to play. I don't have the hardware with me so can't provide better advice. Also please use the dapm graph tool to make use everything is setup correctly.