linux-kernel - Re: [PATCH v9 09/34] ASoC: qcom: qdsp6: Introduce USB AFE port to q6dsp

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Message-ID: <ec25bb67-6c83-430b-bc79-234c03801250@linux.intel.com>
Date:   Wed, 18 Oct 2023 20:00:00 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Wesley Cheng <quic_wcheng@...cinc.com>, mathias.nyman@...el.com,
        gregkh@...uxfoundation.org, lgirdwood@...il.com,
        broonie@...nel.org, perex@...ex.cz, tiwai@...e.com,
        agross@...nel.org, andersson@...nel.org, konrad.dybcio@...aro.org,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        conor+dt@...nel.org, srinivas.kandagatla@...aro.org,
        bgoswami@...cinc.com, Thinh.Nguyen@...opsys.com
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        alsa-devel@...a-project.org, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v9 09/34] ASoC: qcom: qdsp6: Introduce USB AFE port to
 q6dsp



>>>>> Specifically, the QC ADSP can support all potential endpoints that are
>>>>> exposed by the audio data interface.  This includes, feedback
>>>>> endpoints
>>>>> (both implicit and explicit) as well as the isochronous (data)
>>>>> endpoints.
>>>>
>>>> implicit feedback means support for capture. This is confusing...
>>>>
>>>
>>> I mean, a USB device can expose a capture path, but as of now, we won't
>>> enable the offloading to the audio DSP for it.  However, if we're
>>> executing playback, and device does support implicit feedback, we will
>>> pass that along to the audio DSP to utilize.
>>
>> Not following. Implicit feedback means a capture stream *SHALL* be
>> started. Are you saying this capture stream is hidden and handled at the
>> DSP level only? If yes, what prevents you from exposing the capture
>> stream to userspace as well?
>>
>> I must be missing something.
>>
> 
> My understanding is that with implicit feedback endpoints, it allows for
> another data endpoint in the opposite direction to be utilized as a
> feedback endpoint (versus having to expose another EP, such as in the
> case of explicit feedback).  For example, if we are enabling the
> playback path (and the device does have a capture data ep) then the data
> ep used for the capture path can be used.

That's right, so all the plumbing is enabled for the capture path...
Making a decision to discard the data is very odd, all the work has
already been done at lower levels, so why not expose the captured data?