linux-kernel - Re: [PATCH v21 27/39] ASoC: Introduce SND kcontrols to select sound card and PCM device

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Date: Wed, 8 May 2024 17:10:27 -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 27/39] ASoC: Introduce SND kcontrols to select sound
 card and PCM device

Hi Pierre,

On 5/7/2024 2:26 PM, Pierre-Louis Bossart wrote:
> 
> 
> On 5/7/24 14:51, Wesley Cheng wrote:
>> Add SND kcontrol to SOC USB, which will allow for userpsace to determine
>> which USB card number and PCM device to offload.  This allows for userspace
>> to potentially tag an alternate path for a specific USB SND card and PCM
>> device.  Previously, control was absent, and the offload path would be
>> enabled on the last USB SND device which was connected.  This logic will
>> continue to be applicable if no mixer input is received for specific device
>> selection.
>>
>> An example to configure the offload device using tinymix:
>> tinymix -D 0 set 'USB Offload Playback Route Select' 1 0
>>
>> The above command will configure the offload path to utilize card#1 and PCM
>> stream#0.
> 
> I don't know how this is usable in practice. Using card indices is
> really hard to do, it depends on the order in which devices are
> plugged-in...

How are the existing mechanisms handling USB audio devices, or what is 
the identifier being used?

Thanks
Wesley Cheng