HDMI audio on SOCs looks like it's not terribly hard. VC4's HDMI driver needs to instantiate a platform device for audio when it gets loaded, and attach a function table to it for calls to be made when starting/stopping audio. Then I write a sound driver that will bind to the node that VC4 created, accepting a parameter of which register to DMA into. That sound driver will tell ALSA about what formats it can acccept and actually drive the DMA engine to write audio frames into HDMI. On the VC4 HDMI side, we do a bit of register setup (clocks, channel mappings, and the HDMI audio infoframe) when the sound driver tells us it wants to start.
It all sounds pretty straightforward so far. I'm currently at the "lots of typing" stage of development of this.
Second, I did a little poking at the DSI branch again. I've rebased onto 4.9 and tested an idea I had for how DSI might be failing. Still no luck. There's code on drm-vc4-dsi-panel-restart-cleanup-4.9, and the length of that name suggests how long I've been bashing my head against DSI. I would love for anyone out there with the inclination to do display debug to spend some time looking into it. I know the history there is terrible, but I haven't prioritized cleanup while trying to get the driver working in the first place.
I also finished the simulator cleanup from last week, found a regression in the major functional improvement in the cleanup, and landed what worked. Dave Airlie pointed out that he took a different approach for testing with virgl called vtest, and I took a long look at that. Now that I've got my partial cleanup in place, the vtest-style simulator wrapper looks a lot more doable, and if it doesn't have the major failing that swrast did with visuals then it should actually make vc4 simulation more correct.
I landed a very tiny optimization to Mesa that I wrote while debugging DEQP failures.
Finally, I put together the -next branches for upstream bcm2835 now that 4.9-rc1 is out. So far I've landed a major cleanup of pinctrl setup in the DT (started by me and finished by Gerd Hoffmann from Red Hat, to whom I'm very grateful) and a little USB setup fix. I did a lot of review of VCHIQ patches for staging, and sort of rewrote Linus Walleij's RFC patch for getting descriptive names for the GPIOs in lsgpio.