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

[Richard Fitzgerald]

On 20/01/2023 19:55, Pierre-Louis Bossart wrote:
> 
> 
> On 1/19/23 09:35, Richard Fitzgerald wrote:
>> On 19/1/23 14:48, Pierre-Louis Bossart wrote:
>>>
>>>>>> +static int cs42l42_sdw_dai_startup(struct snd_pcm_substream
>>>>>> *substream,
>>>>>> +                   struct snd_soc_dai *dai)
>>>>>> +{
>>>>>> +    struct cs42l42_private *cs42l42 =
>>>>>> snd_soc_component_get_drvdata(dai->component);
>>>>>> +
>>>>>> +    if (!cs42l42->init_done)
>>>>>> +        return -ENODEV;
>>>>>
>>>>> Can this happen? IIRC the ASoC framework would use
>>>>> pm_runtime_resume_and_get() before .startup, which would guarantee that
>>>>> the device is initialized, no?
>>>>>
>>>>
>>>> Yes, this can happen. Because of the way that the SoundWire enumeration
>>>> was implemented in the core code, it isn't a probe event so we cannot
>>>> call snd_soc_register_component() on enumeration because -EPROBE_DEFER
>>>> wouldn't be handled. So the snd_soc_register_component() must be called
>>>> from probe(). This leaves a limbo situation where we've registered the
>>>> driver but in fact don't yet have any hardware. ALSA/ASoC doesn't know
>>>> that we've registered before we are functional so they are happy to
>>>> go ahead and try to use the soundcard. If for some reason the hardware
>>>> failed to enumerate we can get here without having enumerated.
>>>
>>> Humm, yes, but you've also made the regmap cache-only, so is there
>>> really a problem?
>>>
>>
>> It's true that normally we go past these stages in cache-only, but that
>> is because normally (non-Soundwire) we already initialized the hardware
>> to good state during probe().
>> If we just carry on when it hasn't enumerated and we haven't initialized
>> it yet, who knows what will happen if it enumerates some time later.
>>
>> We could just ignore it and see if anyone has a problem but for the sake
>> of a couple of lines of code I feel like I'd rather check for it.
>>
>>> FWIW I don't see a startup callback in any other codec driver. It may be
>>> wrong but it's also a sign that this isn't a problem we've seen so far
>>> on existing Intel-based platforms.
>>>
>>
>> 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.