[alsa-devel] Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

[Caleb Crome]

On Fri, Oct 9, 2015 at 2:34 AM, Liam Girdwood
<liam.r.girdwood at linux.intel.com> wrote:
> On Thu, 2015-10-08 at 08:51 -0700, Caleb Crome wrote:
>> Hi All,
>>    I'm in a constant struggle to bring up many channel audio on each
>> separate SoC.
>>
>> I can easily put a microcontroller in place that will collect and
>> distrubute all the TDM channels to the codecs, and connect the
>> hardware via an SPI interface to the SoC.
>>
>> So, instead of:
>>
>> CODECS <---TDM--->  SoC
>>
>> It would be
>>
>> CODECS <---TDM---> uC <---SPI---> SoC
>>
>> So, my questions are:
>>
>> * I suspect the SPI interface could be used more universally than each
>> individual I2S/TDM interface (like FSL SSI vs. Ti McBSP vs. Ti McASP,
>> etc).  and the SPI port would provide a very common API regardless of
>> SoC.   Is that true?
>
> Some SPI ports could probably be used for audio, but this depends on the
> SPI port HW capabilities. e.g. the SSP port on minnowboard can be
> configured for TDM, I2S and SPI (afaik). I don't think any advantage
> could be gained from running in SPI mode unless your HW permits some
> special features ?

Sorry, I wasn't clear.  The point is to use the 'generic' SPI API in
the linux kernel to stream the data, and *not* use an audio format.
So, the idea is, the external micro would buffer up a block of data,
(in our case, maybe 160 samples * 32 channels  = 10 kBytes), then use
the SPI port to read and write to the micro as if it were a memory or
something like that to transfer the data.  So the external micro would
appear to the CPU as an external register bank, and would do all the
audio aggregation.

I was hoping that would provide a pretty much universal means of
getting many channels of data into and out of pretty much any SoC that
has an SPI port capable of the requisite speed.

Does that make sense?

I guess there would be an interrupt line from the micro->SoC that
says, "data ready".   When that's triggered, the SPI bus just goes and
reads 160 samples x 32 channels x 2 bytes, and writes as many as
needed to satisfy the given application.


>
>>
>> * Is it feasible to integrate an SPI transport into the SoC/alsa
>> layer?  What are the difficulties involved with that?
>>
>
> Yes, that's doable providing the SPI port is capable of supporting audio
> formats and has good DMA connectivity. A DAI driver would need to be
> written for your SPI port to allow it to send audio data within ASoC.

Right.  That's the bit I'm pretty clueless on.

>
> Liam
>>
>> Cheers,
>>   -Caleb
>
>