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

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Date: Thu, 9 May 2024 08:07:36 -0500
From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To: Wesley Cheng <quic_wcheng@...cinc.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



On 5/8/24 19:10, Wesley Cheng wrote:
> 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?

Well it's a mess, that's why I asked.

There are configuration work-arounds to make sure that 'local'
accessories are handled first and get repeatable card indices.

But between USB devices I guess the rule is 'anything goes'. Even if
there are two devices connected at boot, the index allocation will
depend on probe order. The card names are not necessarily super-useful
either, i.e. yesterday I was confused by an USB card named "CODEC"
without any details.