[alsa-devel] Timing issues between ALSA and i915 drivers

[Pierre-Louis Bossart]

>> 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