Re: [RFC PATCH v2 09/22] ASoC: qcom: qdsp6: Introduce USB AFE port to q6dsp

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

On 1/31/23 20:40, Wesley Cheng wrote:
> Hi Pierre,
> 
> On 1/30/2023 3:59 PM, Pierre-Louis Bossart wrote:
>>
>>
>> On 1/30/23 16:54, Wesley Cheng wrote:
>>> Hi Pierre,
>>>
>>> On 1/26/2023 7:38 AM, Pierre-Louis Bossart wrote:
>>>>
>>>>
>>>> On 1/25/23 21:14, Wesley Cheng wrote:
>>>>> The QC ADSP is able to support USB playback endpoints, so that the
>>>>> main
>>>>> application processor can be placed into lower CPU power modes.  This
>>>>> adds
>>>>> the required AFE port configurations and port start command to
>>>>> start an
>>>>> audio session.
>>>>>
>>>>> 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.
>>>>> The size of audio samples sent per USB frame (microframe) will be
>>>>> adjusted
>>>>> based on information received on the feedback endpoint.
>>>>
>>>> I think you meant "support all potential endpoint types"
>>>>
>>>> It's likely that some USB devices have more endpoints than what the DSP
>>>> can handle, no?
>>>>
>>>
>>> True, as we discussed before, we only handle the endpoints for the audio
>>> interface.  Other endpoints, such as HID, or control is still handled by
>>> the main processor.
>>
>> The number of isoc/audio endpoints can be larger than 1 per direction,
>> it's not uncommon for a USB device to have multiple connectors on the
>> front side for instruments, mics, monitor speakers, you name it. Just
>> google 'motu' or 'rme usb' and you'll see examples of USB devices that
>> are very different from plain vanilla headsets.
>>
> 
> Thanks for the reference.
> 
> I tried to do some research on the RME USB audio devices, and they
> mentioned that they do have a "class compliant mode," which is for
> compatibility w/ Linux hosts.  I didn't see a vendor specific USB SND
> driver matching the USB VID/PID either, so I am assuming that it uses
> the USB SND driver as is.(and that Linux doesn't currently support their
> vendor specific mode)  In that case, the device should conform to the
> UAC2.0 spec (same statement seen on UAC3.0), which states in Section
> 4.9.1 Standard AS Interface Descriptor Table 4-26:
> 
> "4 bNumEndpoints 1 Number Number of endpoints used by this
> interface (excluding endpoint 0). Must be
> either 0 (no data endpoint), 1 (data
> endpoint) or 2 (data and explicit feedback
> endpoint)."
> 
> So each audio streaming interface should only have 1 data and
> potentially 1 feedback.  However, this device does expose a large number
> of channels (I saw up to 18 channels), which the USB backend won't be
> able to support.  I still need to check how ASoC behaves if I pass in a
> profile that the backend can't support.
> 
> Maybe in the non-class compliant/vendor based class driver, they have
> the support for multiple EPs per data interface?  I don't have one of
> these devices on hand, so I can't confirm that.

Look at Figure 3-1 in the UAC2 spec, it shows it's perfectly legal to
have multiple Audio Streaming interfaces - but one Audio Control
interface only.

The fact that there is a restriction to 1 or 2 endpoints per Audio
Streaming interface does not really matter if in the end there are
multiple endpoints and concurrent isoc transfers happening to/from the
same USB device.