I tried to narrow down the issue further and my current understanding is that the Skylake driver performs link reset operations without the display power turned on - which does not look like a very smart thing to do in hindsight.
In other words, it's not really when snd_hdac_i915_init() is called that matters as I assumed initially, but more when snd_hdac_display_power() is invoked. There are two cases where this happens, and for each of them turning the display power on results in HDMI detection. The attached diffs split the initialization from the power on, which provides a better understanding of the issue.
OK, this makes some sense, and that's the very reason we have HDA_CODEC_IDX_CONTROLLER for snd_hdac_display_power(). IIRC, we needed to power on the display for probing of the legacy HDA, too. Once after that, for the normal operation, the display power is needed only when you output the HDMI stream.
What would be really useful at this point is a confirmation that snd_hdac_i915_init() cannot be called in the initial probe but does need to be executed in a work queue. That would really impact the way the initialization sequence is reworked on the Skylake side as well as modify the way the SOF driver deals with i915 initialization.
It's needed to be called in a work queue, yes.
Basically you shouldn't call request_module() in the driver's probe callback. When the probe callback is called from the module loading, it blocks the module loading itself, hence loading yet another module can't work. A situation might be easier than the past (which deadlocked), but still it's advised to use either the request_module_nowait() with the callback or call request_module() asynchronously from probe.
Thanks Takashi, this is very useful. I guess that will require a complete rework of the Skylake initialization sequence then, my simple code translation isn't enough indeed and the current partition between probe/work queue can't comply with both requirements (request module asynchronously from probe, display turned on before mucking with links).
We also need this changed for SOF, the i915_init is done in the probe.
-Pierre