Re: [PATCH v2 6/8] ASoC: cs42l42: Add Soundwire support

[Pierre-Louis Bossart]

>>> It's nicer to do the check in startup() because then the application
>>> open() will fail cleanly. We could delay until prepare - which is the
>>> point we really need the hardware to be accessible - and hope the
>>> hardware enumerated and initialized by that time. But that's not so
>>> nice from the app point of view.
>>
>> Another way to avoid problems is to rely on the codec component .probe
>> to check if the SoundWire device is initialized before registering a
>> card.
>>
>> I just tried with a system where the ACPI info exposes a codec which is
>> not connected, it fails nicely. That avoids the pitfalls of creating a
>> card which isn't functional since all dependencies are not met.
>>
>> [   64.616530] snd_soc_sof_sdw:mc_probe: sof_sdw sof_sdw: Entry
>> [   64.616549] snd_soc_sof_sdw:log_quirks: sof_sdw sof_sdw: quirk
>> SOF_SDW_PCH_DMIC enabled
>> [   64.616559] snd_soc_sof_sdw:sof_card_dai_links_create: sof_sdw
>> sof_sdw: sdw 2, ssp 0, dmic 2, hdmi 0
>> [   64.616587] snd_soc_sof_sdw:init_dai_link: sof_sdw sof_sdw: create
>> dai link SDW0-Playback, id 0
>> [   64.616600] snd_soc_sof_sdw:init_dai_link: sof_sdw sof_sdw: create
>> dai link SDW0-Capture, id 1
>> [   64.616607] snd_soc_sof_sdw:init_dai_link: sof_sdw sof_sdw: create
>> dai link dmic01, id 2
>> [   64.616614] snd_soc_sof_sdw:init_dai_link: sof_sdw sof_sdw: create
>> dai link dmic16k, id 3
>> [   69.757115] rt5682 sdw:0:025d:5682:00: Initialization not complete,
>> timed out
>> [   69.757128] rt5682 sdw:0:025d:5682:00: ASoC: error at
>> snd_soc_component_probe on sdw:0:025d:5682:00: -110
>> [   69.757224] sof_sdw sof_sdw: ASoC: failed to instantiate card -110
>> [   69.757734] sof_sdw sof_sdw: snd_soc_register_card failed -110
>>
>> see
>> https://elixir.bootlin.com/linux/latest/source/sound/soc/codecs/rt5682.c#L2927
>>
>> I think this is compatible with the device model and bind/unbind, but it
>> could be improved with the removal of the wait if we had a way to return
>> -EPROBEDEFER, and have a mechanism to force the deferred probe work to
>> be triggered when a device actually shows up. It's a generic problem
>> that the probe cannot always be a synchronous function but may complete
>> 'later'.
> 
> I see what you've done in your patch, but I had already experimented
> with this idea and found that the wait_for_completion() can deadlock the
> Soundwire core.

That's not good. Do you have any logs or explanation on what the
root-cause of this deadlock might be? If something's broken, we might as
well fix it.