Re: [PATCH v9 34/34] ASoC: usb: Rediscover USB SND devices on USB port add

[Pierre-Louis Bossart]

On 10/17/23 15:01, Wesley Cheng wrote:
> In case the USB backend device has not been initialized/probed, USB SND
> device connections can still occur.  When the USB backend is eventually
> made available, previous USB SND device connections are not communicated to
> the USB backend.  Call snd_usb_rediscover_devices() to generate the connect
> callbacks for all USB SND devices connected.  This will allow for the USB
> backend to be updated with the current set of devices available.
> 
> The chip array entries are all populated and removed while under the
> register_mutex, so going over potential race conditions:
> 
> Thread#1:
>   q6usb_component_probe()
>     --> snd_soc_usb_add_port()
>       --> snd_usb_rediscover_devices()
>         --> mutex_lock(register_mutex)
> 
> Thread#2
>   --> usb_audio_disconnect()
>     --> mutex_lock(register_mutex)
> 
> So either thread#1 or thread#2 will complete first.  If
> 
> Thread#1 completes before thread#2:
>   SOC USB will notify DPCM backend of the device connection.  Shortly
>   after, once thread#2 runs, we will get a disconnect event for the
>   connected device.
> 
> Thread#2 completes before thread#1:
>   Then during snd_usb_rediscover_devices() it won't notify of any
>   connection for that particular chip index.
Looks like you are assuming the regular USB audio stuff is probed first?

What if it's not the case? Have you tested with a manual 'blacklist' and
"modprobe" sequence long after all the DSP stuff is initialized?

It really reminds me of audio+display issues, and the same opens apply IMHO.