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

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Date: Wed, 8 May 2024 16:40:19 -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 22/39] ALSA: usb-audio: Prevent starting of audio
 stream if in use

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).

Thanks
Wesley Cheng