linux-kernel - Re: [alsa-devel] [PATCH v2 3/5] ASoC: core: add support to snd_soc_dai_get_sdw

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Message-ID: <2326a155-332e-fda0-b7a2-b48f348e1911@linux.intel.com>
Date:   Wed, 9 Oct 2019 13:53:51 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Vinod Koul <vkoul@...nel.org>, Mark Brown <broonie@...nel.org>
Cc:     devicetree@...r.kernel.org, alsa-devel@...a-project.org,
        bgoswami@...eaurora.org, plai@...eaurora.org,
        linux-kernel@...r.kernel.org, lgirdwood@...il.com,
        robh+dt@...nel.org, spapothi@...eaurora.org
Subject: Re: [alsa-devel] [PATCH v2 3/5] ASoC: core: add support to
 snd_soc_dai_get_sdw_stream()

On 10/9/19 11:01 AM, Srinivas Kandagatla wrote:
> 
> 
> On 09/10/2019 15:29, Pierre-Louis Bossart wrote:
>>
>>
>> On 10/9/19 3:32 AM, Srinivas Kandagatla wrote:
>>> Hi Pierre,
>>>
>>> On 14/08/2019 15:09, Pierre-Louis Bossart wrote:
>>>>
>>>>
>>>> On 8/13/19 11:11 PM, Vinod Koul wrote:
>>>>> On 13-08-19, 20:58, Mark Brown wrote:
>>>>>> On Tue, Aug 13, 2019 at 02:38:53PM -0500, Pierre-Louis Bossart wrote:
>>>>>>
>>>>>>> Indeed. I don't have a full understanding of that part to be 
>>>>>>> honest, nor why
>>>>>>> we need something SoundWire-specific. We already abused the 
>>>>>>> set_tdm_slot API
>>>>>>> to store an HDaudio stream, now we have a rather confusing stream
>>>>>>> information for SoundWire and I have about 3 other 'stream' 
>>>>>>> contexts in
>>>>>>> SOF... I am still doing basic cleanups but this has been on my 
>>>>>>> radar for a
>>>>>>> while.
>>>>>>
>>>>>> There is something to be said for not abusing the TDM slot API if 
>>>>>> it can
>>>>>> make things clearer by using bus-idiomatic mechanisms, but it does 
>>>>>> mean
>>>>>> everything needs to know about each individual bus :/ .
>>>>>
>>>>> Here ASoC doesn't need to know about sdw bus. As Srini explained, this
>>>>> helps in the case for him to get the stream context and set the stream
>>>>> context from the machine driver.
>>>>>
>>>>> Nothing else is expected to be done from this API. We already do a set
>>>>> using snd_soc_dai_set_sdw_stream(). Here we add the 
>>>>> snd_soc_dai_get_sdw_stream() to query
>>>>
>>>> I didn't see a call to snd_soc_dai_set_sdw_stream() in Srini's code?
>>>
>>>
>>> There is a snd_soc_dai_get_sdw_stream() to get stream context and we 
>>> add slave streams(amplifier in this case) to that context using 
>>> sdw_stream_add_slave() in machine driver[1].
>>>
>>> Without this helper there is no way to link slave streams to stream 
>>> context in non dai based setup like smart speaker amplifiers.
>>>
>>> Currently this driver is blocked on this patch, If you think there 
>>> are other ways to do this, am happy to try them out.
>>
>> So to be clear, you are *not* using snd_soc_dai_set_sdw_stream?
> Yes, am not using snd_soc_dai_set_sdw_stream().

It's been a while since this thread started, and I still don't quite get 
the concepts or logic.

First, I don't understand what the problem with "aux" devices is. All 
the SoundWire stuff is based on the concept of DAI, so I guess I am 
missing why introducing the "aux" device changes anything.

Next, a 'stream' connects multiple DAIs to transmit information from 
sources to sinks. A DAI in itself is created without having any notion 
of which stream it will be associated with. This can only be done with 
machine level information.

If you query a DAI to get a stream context, then how is this stream 
context allocated in the first place? It looks like a horse and cart 
problem. Or you are assuming that a specific DAI provides the context, 
and that all other DAIs do not expose this .get_sdw_stream()? What if 
more that 1 DAI provide a stream context?

And last, I am even more lost since w/ the existing codec drivers we 
have, sdw_stream_add_slave() is called from the dai .hw_params callback, 
once you have information on formats/rates, etc. using 
sdw_stream_add_slave() from a machine driver looks like an even bigger 
stretch. I really thought the machine driver would only propagate the 
notion of stream to all DAIs that are part of the same dailink.

I am not trying to block the Qualcomm implementation, just would like to 
make sure the assumptions are clear and that we are not using the same 
API in completely different ways.