linux-kernel - Re: [PATCH v21 34/39] ALSA: usb-audio: Add USB offloading capable kcontrol

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Date: Thu, 9 May 2024 15:05:02 -0700
From: Wesley Cheng <quic_wcheng@...cinc.com>
To: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        <srinivas.kandagatla@...aro.org>, <mathias.nyman@...el.com>,
        <perex@...ex.cz>, <conor+dt@...nel.org>, <corbet@....net>,
        <lgirdwood@...il.com>, <andersson@...nel.org>, <krzk+dt@...nel.org>,
        <gregkh@...uxfoundation.org>, <Thinh.Nguyen@...opsys.com>,
        <broonie@...nel.org>, <bgoswami@...cinc.com>, <tiwai@...e.com>,
        <bagasdotme@...il.com>, <robh@...nel.org>, <konrad.dybcio@...aro.org>
CC: <linux-kernel@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-sound@...r.kernel.org>, <linux-usb@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <alsa-devel@...a-project.org>
Subject: Re: [PATCH v21 34/39] ALSA: usb-audio: Add USB offloading capable
 kcontrol

Hi Pierre,

On 5/9/2024 6:11 AM, Pierre-Louis Bossart wrote:
> 
>>>>> 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?

Its part of this patchset.  Please refer to patch#34.  The 
mixer_usb_offload is initialized by the offload entity residing in USB 
SND (qc_usb_audio_offload), and will add it to the sound card associated 
with the USB device.

Thanks
Wesley Cheng