Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

[Caleb Crome]

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?

* Is it feasible to integrate an SPI transport into the SoC/alsa
layer?  What are the difficulties involved with that?


Cheers,
  -Caleb

Re: Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

[Liam Girdwood]

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 ?

> 
> * 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.

Liam
> 
> Cheers,
>   -Caleb

Re: 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@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
>
>

Re: Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

[Liam Girdwood]

+ Mark

On Fri, 2015-10-09 at 09:11 -0700, Caleb Crome wrote:
> On Fri, Oct 9, 2015 at 2:34 AM, Liam Girdwood
> <liam.r.girdwood@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.
> 

Ok, so this sounds like a burst based DAI where the host sends audio
data in bursts then sleeps. I think this has been done by some codec
vendors, but I dont know if any code is upstream.

> 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?

Yes, just need to make sure that your SPI port has low latency and
enough bandwidth and I think you should be OK.

Liam

> 
> 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
> >
> >
> _______________________________________________
> Alsa-devel mailing list
> Alsa-devel@alsa-project.org
> http://mailman.alsa-project.org/mailman/listinfo/alsa-devel

Re: Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

[Lars-Peter Clausen]

On 10/13/2015 11:57 AM, Liam Girdwood wrote:
> + Mark
> 
> On Fri, 2015-10-09 at 09:11 -0700, Caleb Crome wrote:
>> On Fri, Oct 9, 2015 at 2:34 AM, Liam Girdwood
>> <liam.r.girdwood@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.
>>
> 
> Ok, so this sounds like a burst based DAI where the host sends audio
> data in bursts then sleeps. I think this has been done by some codec
> vendors, but I dont know if any code is upstream.

The upstream tlv320dac33 driver does something like this, but it is not
necessarily an implementation I'd recommend to mimic. The whole burst access
mode probably wants some support at the framework level.

Re: Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

[Mark Brown]

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On Tue, Oct 13, 2015 at 10:57:45AM +0100, Liam Girdwood wrote:
> On Fri, 2015-10-09 at 09:11 -0700, Caleb Crome wrote:

> > 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.

> Ok, so this sounds like a burst based DAI where the host sends audio
> data in bursts then sleeps. I think this has been done by some codec
> vendors, but I dont know if any code is upstream.

Well, it's not triggerd from the CODEC side which is a bit different to
what the CODEC vendors were trying - it sounds more like what some of
the DSP vendors are doing with bursting data into the DSP over a message
based interface which maps pretty well onto the existing copy()
operation.

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Re: Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

[Caleb Crome]

On Tue, Oct 13, 2015 at 5:32 AM, Mark Brown <broonie@kernel.org> wrote:
> On Tue, Oct 13, 2015 at 10:57:45AM +0100, Liam Girdwood wrote:
>> On Fri, 2015-10-09 at 09:11 -0700, Caleb Crome wrote:
>
>> > 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.
>
>> Ok, so this sounds like a burst based DAI where the host sends audio
>> data in bursts then sleeps. I think this has been done by some codec
>> vendors, but I dont know if any code is upstream.
>
> Well, it's not triggerd from the CODEC side which is a bit different to
> what the CODEC vendors were trying - it sounds more like what some of
> the DSP vendors are doing with bursting data into the DSP over a message
> based interface which maps pretty well onto the existing copy()
> operation.

That sounds right.  Send over a messaging interface, rather than an
audio interface.  Which copy() operation do you mean?

Thanks,
 - Caleb

Re: Feasibility of adding alternative audio transport besides I2S/PWM/SPDIF, etc

[Mark Brown]

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On Tue, Oct 13, 2015 at 08:33:26AM -0700, Caleb Crome wrote:
> On Tue, Oct 13, 2015 at 5:32 AM, Mark Brown <broonie@kernel.org> wrote:

> > Well, it's not triggerd from the CODEC side which is a bit different to
> > what the CODEC vendors were trying - it sounds more like what some of
> > the DSP vendors are doing with bursting data into the DSP over a message
> > based interface which maps pretty well onto the existing copy()
> > operation.

> That sounds right.  Send over a messaging interface, rather than an
> audio interface.  Which copy() operation do you mean?

The one in snd_pcm_ops.

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