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 14:38:01 -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 5/9/2024 6:01 AM, Pierre-Louis Bossart wrote:
> 
> 
> 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?
> 

It controls which Multimedia (ASM) stream can be routed to the USB BE DAI.

> 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.

Ideally we shouldn't.  Only one ASM stream should be allowed to open the 
USB AFE port at a time.

Thanks
Wesley Cheng