HDMI audio support on the open-source driver for the Radeon HD 5000 series and newer is a feature many Linux desktop customers have sought after, but AMD hasn't yet published any code to make this happen. It was believed it would land for the Linux 3.2 kernel, but that's too late now and the next merge target would be the Linux 3.3 kernel. That is assuming the code clears AMD's IP legal review process in the coming weeks.
Evergreen HDMI audio has been discussed at length in the Phoronix Forums.
Going back to August there's been comments by AMD's John Bridgman about the support code going through several rounds of review, but as of yet it hasn't been cleared. Bridgman was hoping it would have been ready for the Linux 3.2 kernel.
AMD's Alex Deucher commented in another Phoronix Forums post that this Evergreen HDMI audio support code also covers the Northern Islands (Radeon HD 6000 series) ASICs too and presumably the latest Radeon HD 6900 (Cayman) series, but it seems the show-stopper right now is on clearing the legal review process before the code can be made public.
Here's another comment by Bridgman that he wrote on Friday.
The way IP release works is that we try to release, we hit "can't be done" roadblocks, then the proposed release sits until one of us has an idea for either a different set of information to release, a different approach to how we trace the IP back to original licenses/standards, or a different way to "make the release safe" in the case of things where DRM lurks nearby, then we try again and usually hit a different roadblock, rinse and repeat. Sometimes we get lucky and are able to release the info, other times we are not.
We obviously try to release the most broadly useful info first, but in practice what gets released is a function of both our perception of importance and our ability to make a compelling case for being able to release... so to a certain extent we "pushed the easy stuff out first" and are dealing with the harder stuff now.
HDMI audio is one of those things that seems "stupidly simple" at first glance but in practice is fairly complicated. I think this will be our fourth attempt.
Let's hope the fourth round of changes and legal review is successful as there's only a few weeks to get the code ready and it's complicated by the holiday season.
At least for owners of the Radeon HD 4000 series and earlier, there is already open-source HDMI audio support in place via the Radeon DRM driver and xf86-video-ati DDX. The proprietary Catalyst Linux driver also exposes HDMI audio support in suitable configurations.