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Date: Thu, 9 May 2024 08:11:33 -0500
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Subject: Re: [PATCH v21 34/39] ALSA: usb-audio: Add USB offloading capable
 kcontrol
To: Wesley Cheng <quic_wcheng@quicinc.com>, srinivas.kandagatla@linaro.org,
 mathias.nyman@intel.com, perex@perex.cz, conor+dt@kernel.org,
 corbet@lwn.net, lgirdwood@gmail.com, andersson@kernel.org,
 krzk+dt@kernel.org, gregkh@linuxfoundation.org, Thinh.Nguyen@synopsys.com,
 broonie@kernel.org, bgoswami@quicinc.com, tiwai@suse.com,
 bagasdotme@gmail.com, robh@kernel.org, konrad.dybcio@linaro.org
Cc: linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
 linux-sound@vger.kernel.org, linux-usb@vger.kernel.org,
 linux-arm-msm@vger.kernel.org, linux-doc@vger.kernel.org,
 alsa-devel@alsa-project.org
References: <20240507195116.9464-1-quic_wcheng@quicinc.com>
 <20240507195116.9464-35-quic_wcheng@quicinc.com>
 <4ce8ee3b-21d3-4aa3-8fd5-7dcccc2b8abe@linux.intel.com>
 <a602e121-28e0-3255-87bb-c75355926125@quicinc.com>
 <a0d19e7c-8fb0-e93a-d79b-319d8339504a@quicinc.com>
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From: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
In-Reply-To: <a0d19e7c-8fb0-e93a-d79b-319d8339504a@quicinc.com>
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>>>> It is expected for the USB offloading driver to add the kcontrol to the
>>>> sound card associated with the USB audio device.  An example output
>>>> would
>>>> look like:
>>>>
>>>> tinymix -D 1 get 'USB Offload Playback Capable Card'
>>>> 0 (range -1->32)
>>>
>>> You already gave the following examples in patch 29:
>>>
>>> "
>>> USB offloading idle:
>>> tinymix -D 0 get 'USB Offload Playback Route Status'
>>> -->-1, -1 (range -1->32)
>>>
>>> USB offloading active(USB card#1 pcm#0):
>>> tinymix -D 0 get 'USB Offload Playback Route Status'
>>> -->1, 0 (range -1->32)
>>> "
>>>
>>> Can you clarify how many controls there would be in the end?
>>
>> For USB offload situations, there will be a set of controls for
>> playback status and playback select.  The offload jack will also be
>> there to tell us if there is an offload path available for the
>> platform ASoC sound card.
>>
>>> Also isn't tinymix -D N going to give you the controls for card N?
>>>
>>
>> Yes, since the offload portion is handled as a DPCM DAI link to the
>> platform ASoC card, it will be included as a kcontrol for that.
>>
>> Thanks
>> Wesley Cheng
>>
>>
> 
> Sorry for responding again.  I read your email again, and wanted to also
> add that aside from the above, which are all within the ASoC layer, as
> we discussed previously, we should have a kcontrol in the USB SND card
> to determine if there is an ASoC platform card capable of offloading.
> This is also available from the SND card created by the USB audio device.

That makes sense: if the application wanted to use a given endpoint, it
could check if there is a 'better' path exposed by another card. It'd be
a lot easier than reading controls from random cards.

Was this part of this patchset or more of an idea for a future addition?