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Date: Thu, 9 May 2024 14:38:01 -0700
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Subject: Re: [PATCH v21 22/39] ALSA: usb-audio: Prevent starting of audio
 stream if in use
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To: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>,
        <srinivas.kandagatla@linaro.org>, <mathias.nyman@intel.com>,
        <perex@perex.cz>, <conor+dt@kernel.org>, <corbet@lwn.net>,
        <lgirdwood@gmail.com>, <andersson@kernel.org>, <krzk+dt@kernel.org>,
        <gregkh@linuxfoundation.org>, <Thinh.Nguyen@synopsys.com>,
        <broonie@kernel.org>, <bgoswami@quicinc.com>, <tiwai@suse.com>,
        <bagasdotme@gmail.com>, <robh@kernel.org>, <konrad.dybcio@linaro.org>
CC: <linux-kernel@vger.kernel.org>, <devicetree@vger.kernel.org>,
        <linux-sound@vger.kernel.org>, <linux-usb@vger.kernel.org>,
        <linux-arm-msm@vger.kernel.org>, <linux-doc@vger.kernel.org>,
        <alsa-devel@alsa-project.org>
References: <20240507195116.9464-1-quic_wcheng@quicinc.com>
 <20240507195116.9464-23-quic_wcheng@quicinc.com>
 <1e98935e-e35a-49e0-bbbf-ff326d40b581@linux.intel.com>
 <066fe096-a9d3-2498-275d-185f709e9a02@quicinc.com>
 <c97a0563-c4ce-45cd-8141-ee9be1e01899@linux.intel.com>
From: Wesley Cheng <quic_wcheng@quicinc.com>
In-Reply-To: <c97a0563-c4ce-45cd-8141-ee9be1e01899@linux.intel.com>
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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