Re: [PATCH v24 23/34] ALSA: usb-audio: Prevent starting of audio stream if in use

Wesley Cheng
2024-08-08T01:20:03Z

Hi Amadeusz,

On 8/6/2024 7:51 AM, Amadeusz Sławiński wrote:
> On 8/1/2024 3:17 AM, Wesley Cheng wrote:
>> With USB audio offloading, an audio session is started from the ASoC
>> platform sound card and PCM devices.  Likewise, the USB SND path is still
>> readily available for use, in case the non-offload path is desired.  In
>> order to prevent the two entities from attempting to use the USB bus,
>> introduce a flag that determines when either paths are in use.
>>
>
> How can this happen? Can you provide some example with list of devices and which one should block the other? If I recall correctly devices are already exclusive unless you support substreams which ASoC does not at the moment.
>
From past discussions, I think so far everyone is on board with the idea of having both the USB sound card and PCM devices exist in conjunction w/ the USB offload path, which is going to be done over the ASoC platform card.  So for example,

/ # cat /proc/asound/cards
0 [SM8250MTPWCD938]: sm8250 - SM8250-MTP-WCD9380-WSA8810-VA-D
SM8250-MTP-WCD9380-WSA8810-VA-DMIC
1 [C320M          ]: USB-Audio - Plantronics C320-M
Plantronics Plantronics C320-M at usb-xhci-hcd.1.auto-1.2, full speed

This device currently has the following sound cards within the system:

- card#0 - ASoC platform card: handles USB offload, speaker, etc...

- card#1 - USB SND card: card created for interacting with the connected USB device.

So now, with USB offloading in the picture, there are basically two paths that can start attempting to utilize the same USB device endpoints.  Let's keep it simple and assume the device only has one playback substream (which means only one PCM device)

/proc/asound/card1 # cat stream0
Plantronics Plantronics C320-M at usb-xhci-hcd.1.auto-1.2, full speed : USB Audio

Playback:
Status: Stop
Interface 2
Altset 1
Format: S16_LE
Channels: 2
Endpoint: 0x01 (1 OUT) (ADAPTIVE)
Rates: 8000, 16000, 24000, 32000, 44100, 48000
Bits: 16
Channel map: FL FR

So the patch here will prevent transfers from happening from both the offload path and directly over the USB SND PCM device, which correlates to the following paths:

- offload: card#0 pcm#0

- USB SND: card#1 pcm#0

Thanks

Wesley Cheng