https://bugs.freedesktop.org/show_bug.cgi?id=90777
--- Comment #10 from James Le Cuirot chewi@gentoo.org --- (In reply to Shimi Chen from comment #9)
My HDMI audio also does not work since 4.0. I tried installing the mainline 4.2rc3 kernel and it is not fixed. Reverting to 3.17.6 fixes the issue. I'm using Arch Linux and a laptop with Radeon HD 4570.
Should I report a separate bug?
I think you should. Sound was still working for me thanks to this fix as of 4.2rc1. It's worth noting that multichannel PCM only started working for me around 3.18 so userspace may be seeing your card a little differently. My system gets confused between the EDID sent by my AV unit vs the EDID sent by my stereo monitor so I had to add this line to /etc/modprobe.d/alsa.conf to get surround working reliably.
options snd-hda-codec-hdmi static_hdmi_pcm=1