ASoC: usb: Rediscover USB SND devices on USB port add

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.

Signed-off-by: Wesley Cheng <[e-mail]>
Acked-by: Mark Brown <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

sound/soc/soc-usb.c

···
 	mutex_lock(&ctx_mutex);
 	list_add_tail(&usb->list, &usb_ctx_list);
 	mutex_unlock(&ctx_mutex);
+
+	snd_usb_rediscover_devices();
 }
 EXPORT_SYMBOL_GPL(snd_soc_usb_add_port);