linux-kernel - Re: [PATCH v21 22/39] ALSA: usb-audio: Prevent starting of audio stream if in use

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Date: Thu, 9 May 2024 08:01:04 -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 22/39] ALSA: usb-audio: Prevent starting of audio
 stream if in use



On 5/8/24 18:40, Wesley Cheng wrote:
> Hi Pierre,
> 
> On 5/7/2024 2:20 PM, Pierre-Louis Bossart wrote:
>>
>>> If a PCM device is already in use, the check will return an error to
>>> userspace notifying that the stream is currently busy.  This ensures
>>> that
>>> only one path is using the USB substream.
>>
>> What was the point of having a "USB Mixer" then?
> 
> The USB mixer is intended to enable/route the USB offloading path to the
> audio DSP, and is for controlling the ASoC specific entities.  This
> change is needed to resolve any contention between the USB SND PCM
> device (non offload path) and the ASoC USB BE DAI (offload path).

Not following, sorry. Is the "USB Mixer" some sort of hardware entity
related to USB offload or just a pure DAPM processing widget handling
volume and actual mixing between streams?

I was trying to get clarity on whether there can be multiple streams
mixed before going to the USB endpoint. The commit message "only one
path is using the USB substream" is ambiguous, not sure if you are
referring to mutual exclusion between offloaded and non-offloaded paths,
or number of streams when offloaded is supported. Different concepts/levels.