[alsa-devel] Question about Component and Card

[Lars-Peter Clausen]

On 08/03/2015 10:23 AM, Kuninori Morimoto wrote:
> 
> Hi Lars
> 
>>> I noticed my sound card (which is not upstreamed) can't probe in next branch.
>>> I think it is because this patch
>>> 1b7c12316982f74a5b8e7704c24cf5524d0723a3
>>> (ASoC: Prevent components from being bound to multiple cards)
>>>
>>> In my case, my sound device (= CPU) has some DAIs,
>>> and these are connected to some codecs.
>>> It is shown as below. And I would like to use these as 2 sound cards.
>>>  ex)	aplay -D hw:0 xxx
>>> 	aplay -D hw:1 xxx
>>> But, 2nd sound card can't probe because of above patch now.
>>> (component->card indicates 1st card)
>>> It can be probeed if I used DPCM card (= 1 CPU + 2 Codec), but...
>>> But, is my understanding wrong ?
>>
>> ASoC always had the restriction that you can only bind a component to a
>> single card. Otherwise the internal data structures will get corrupted,
>> which may result in undefined behavior at a later point. The patch made this
>> restriction explicit to make sure that issue is caught when it happens and
>> you don't have dig through crashes from random memory corruption.
> 
> Hmm...
> Current my CPU driver is probing all channels in 1 driver.
> Then, I can use multi sound card if CPU driver probes limited channels.
> (ex driver0 probes ch0, ch1
>     driver1 probes ch2, ch3, or something like this)
> 
> This means I need to re-consider / change my CPU driver for it.
> Do you think this is good idea ? or do I have other solution ?

Yeah, it's difficult. I guess this is a case where the control path (e.g.
memory mapped register region, irq, etc.) is shared, but the data paths are
independent. In theory this means you can use each DAI in a different sound
card. But in practice they still share the same DAPM context, even though
there are no connections in the DAPM graph between them. I don't know how to
solve this issue. The easiest way out would be to register a different
component for each DAI. But that might not be possible depending on how the
system is structured.