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Date:   Fri, 20 Jan 2023 13:55:00 -0600
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Subject: Re: [PATCH v2 6/8] ASoC: cs42l42: Add Soundwire support
To:     Richard Fitzgerald <rf@opensource.cirrus.com>,
        Stefan Binding <sbinding@opensource.cirrus.com>,
        Mark Brown <broonie@kernel.org>
Cc:     alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org,
        patches@opensource.cirrus.com
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From:   Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
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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'.